Aircraft Radio System

ORDER NUMBER EA-356

AIRCRAFT RADIO SYSTEMS By J. Powell BA, C Eng, MIERE, GradIMA

Reprinted in India by

HIMALAYAN BOOKS °

New Delhi-110001. Distributed by: The English Book Store The Aviation People

17-L, Connaught Circus New Delhi-110001 (India)

Contents

5

Preface 1

Historical, technical and legal context 1 Introduction 1 Historical background 1 Basic principles of radio 2 Digital systems 8 Categorization of airborne radio equipments Navigation nomenclature 13 Interference 13 Maintenance 17 Regulating and advisory bodies 18

2

3

4

Communication systems 20 Introduction 20 V.h.f. communications 20 H.f. communications 29 Selcal 35 Audio integrating systems - (Intercom) 37 Testing and trouble shooting the audio systems 43 Automatic direction finding 45 Introduction 45 Basic principles 45 Simplified block diagram operation 4.7 Block diagram detail 47 Sources of system error 49 Installation 52 Controls and operation 54 Characteristics 55 Calibration and testing on the ramp 55 V.h.f. omnidirectional range (VOR) 58 Introduction 58 Basic principles 58 Doppler VOR (DVOR) Aircraft installation

61 63

Controls and operation 65 Simplified block diagram operation 65 Characteristics 65 Ramp testing 67

Instrument landing system Introduction 69 Basic principles 69

69

Simplified block diagram operation Installation 74 Controls and operation 76 Characteristics 77 Ramp testing 78 6

Hyperbolic navigation systems General principles

79

79

Omega navigation system 7

72

Distance measuring equipment Introduction 105 Basic principles 105 X and Y channel arrangements

83 105

110

The link with v.h.f. navigation 110 Installation 111 Controls and operation 112 Simplified block diagram operation 112 Range measuring and mode control 114 8 ATC transponder 121 Introduction 121 Basic principles 121 Installation 126 Controls and operation 127 128 Simplified block diagram operation Block diagram details 128 Characteristics 135 Ramp testing 136 9 Weather avoidance 139 Introduction 139 Weather radar 140 Choice of characteristics and features 140 Installation 146 Controls 147 Operation 148

Block diagram operation Scanner stabilization

150

Aircraft installation delay Interface 198

157

Other applications for weather radar 160 Weather radar characteristics 162 Maintenance and testing 164 Ryan storniscope 168 Appendix Factors affecting weather radar performance 169 10

11

197

Multiple installations 199 Characteristics '00 Ramp testing and maintenance 20 0 Appendix Sinusoidal frequency modulation 201 12 Area navigation 202 Development of airspace organization 202 Generalized area navigation system 202 VOR/DME based RNAV. principles 204 Bendix nav. computer programmer NP-2041A 206 King KDE 566 210 Standardization 213 Testing RNAV 215

Doppler navigation 172 Introduction 172 Doppler effect 173 Antenna mechanization 174 Doppler spectrum 175 ` Beam geometry 175 Transmitter frequency 176 Modulation 177 Over-water errors 178 Navigation calculations 179 Block diagram operation 179 Installation 182 Controls and operation 184 Characteristics 185 Testing 185 Appendix Relationships between aircraft and earth co-ordinates 186 The Doppler shifts for a four-beam Janus configuration 187 The aircraft velocity in earth co-ordinates expressed in terms of Doppler shifts 188

13 Current and future developments 216 Introduction 216 The state of the art 216 The flight deck 217 Multi-systern packages 219 Data link 219 ADSEL/DABS 2 21 Satcom and satnav 223 Microwave landing system 2 24 Microwave aircraft digital guidance equipment 225 Collision avoidance 2 29 The current generation of ARINC characteristics 230 Concluding remarks 231

Radio altimeter 189

Recommended reading 232

Introduction 189 Basic principles 189 Factors affecting performance Block diagram operation Monitoring and self-test Indicat or 196 Installation 196

191 192 195

Glossary

234

Exercises

246

Index 252

Preface

The cockpit and equipment racks of modern aircraft, large and small, are becoming filled with ever more sophisticated systems . This book attempts to describe a certain class of such systems, namely those which rely for their operation on electromagnetic radiation. The subject matter is complex and wide-ranging, hence not all aspects can be covered in one volume In deciding where the treatment should be light or perhaps non-existent, I have asked myself two questions: (1) which aspects can most usefully be covered in a book; and ( 2 ) at which group of people involved in aviation should a book covering such aspects be aimed? The answer to (1) must be `describe the theory'. One can, and indeed must, read or be told about how to operate the systems: how to navigate using the systems; how to solder, crimp and change items; how to use test equipment, etc. but proficiency is impossible without practice. On the other hand gaining an understanding of how a particular system works is more of a mental exercise which can be guided in a book such as this. This is not to say that more practical matters are neglected, since it would not help one's understanding of the theory of operation not to see, at least in words and pictures, how a particular system is controlled, presents its information, reacts to the environment, etc. Having decided the main line of attack the more difficult question of depth of treatment must be answered: in other words which group should be satisfied'.' Pilots need a superficial knowledge of how all the systems work; maintenance engineers on the ramp and in the hangar a more detailed knowledge; workshop engineers must have an understanding of the circuitry for perhaps a limited range of equipments; while designers should have the greatest depth of knowledge of a11. It is virtually impossible to draw dividing lines, but it is hoped that if enough theory is given to satisfy the aircraft radio maintenance engineer then the book might be useful to all groups mentioned. The depth of treatment varies, it being impossible to cover everything, or indeed anything, to the depth I would have liked. In particular few details of

circuitry are given since I feel most readers will be more interested in the operation of the system as a whole. Nevertheless, some circuits are given purely as examples. Should the reader need circuit knowledge, the equipment maintenance manual is the best place to find it, assuming he knows the system and he has a basic knowledge of electronics. The state of the art of the equipment described is also varied. 1 did not see the point of describing only equipment containing microprocessors, since the vast majority of systems in service do not use them as yet. On the other hand if the life of this book is not to be too severely restricted, the latest techniques must be described. Within the pages that follow, analogue, analogue/digital, hardwired digital and programmable digital equipments all find a place. As stated previously, the book is aimed primarily at the maintenance engineer. However, I hope several groups might be interested. This poses problems concerning the background knowledge required. For what I hope is a fairly substantial part of the book, any reasonably intelligent technically minded person with a basic knowledge of mathematics and a familiarity with aircraft will have no difficulty that two or perhaps three readings will not overcome. There are parts, however, where some knowledge of electronics, radio theory or more sophisticated mathematics is needed. In three chapters where the going gets a bit tough, I have relegated the offending material to an appendix. Some background material is covered in Chapter 1, in particular, basic radio theory and a discussion of digital systems in so far as coding and computers are concerned. If you are one of the few people who plough all the way through the Preface to a book, you may have decided by now that this book is concerned with theory and little else. That this is not so may be clear if I outline briefly the contents of each chapter. An introduction saying a few words about the history and function of the system is followed by a fairly thorough coverage of the basic principles. In some chapters the next item is a discussion of the installation, i.e. the units, how they are interconnected, which other systems they interface

with and any special considerations such as cooling, positioning, type of antennas and feeders, etc. This, together with a description of controls and operation, puts some practical meat on to the bare bones of the theory which continues with a consideration of the block diagram operation. In certain chapters the order: installation - controls and operation -- block diagram, is reversed where 1 thought it was perhaps to the reader's disadvantage to break up the flow of the more theoretical aspects. A brief look at characteristics, in practically all cases based on ARINC publications, and -testing / maintenance concludes each chapter. Most chapters deal with one system; none of them is exclusively military. The exceptions are, in reverse order, Chapter 13 where I look at the current scene and review some systems we should see in the next few years; Chapter 12 which is a bringing-together of some of the previously covered systems: Chapter 6 covering Omega, Decca Navigator and Loran C'; Chapter 2 which covers both radio and non -radio communications; and Chapter 1 where some chosen background material is given. I should point out that this is not a textbook in the sense that everything is examinable in accordance with some syllabus. The reader will take from the book however big a chunk he desires, depend ing on his background knowledge, his profession, the examinations he hopes to take and, of course, his inclination. Some will have, or end up with, an understanding of all that is included herein, in which case I hope the book may be seen as a source of reference.

Acknowledgements A number of manufacturers have given valuable assistance including the supplying of material and granting permission to reproduce data and illustrations. Without the generosity of the following, this book would have been of very limited use. Bendix Avionics Division Boeing Commercial Aero plane Company British Aerospace

Communications Components Corporation The Decca Navigator Company Limited Field Tech Limited Hazeltine Corporation IFR Electronics Inc King Radio Corporation Litton Systems International Inc., Aero Products Division

Marconi avionics Limited MEL Equipment Company Limited RCA Limited Rockwell-Collins (UK) Limited Ryan Storm scope Tel-Instrument Electronics Corporation (TIC) Although I am grateful to all the above, I must reserve a special word of thanks to Mr Wayne Brown of Bendix, Mr. A. E. Crawford of King and Mr. T. C. Wood of RCA, who arranged for the dispatch of several expensive and heavy maintenance manuals in reply to my request for information. These manuals, and indeed all other information received, were used in the preparation of this book and continue to be used in the training of students at Brunel Technical College, Bristol, England. I also wish to thank all my colleagues at Br unel who have helped, often unwittingly, in conversation. In particular my thanks go to John Stokes, Clive Stratton and Peter Kemp for proof-reading some of the chapters and also Leighton Fletcher for helping with the illustrations. May I add that, although I received technical assistance from the above, any mistakes which remain are obviously mine. I would be grateful to any reader who might take the trouble to point out any errors. Finally, my thanks to Pauline Rickards, whose fingers must be sore from typing; to the publishers who displayed great patience as the deadline for the submission of the typescript came and went; and, most of all, to my wife Pat and son Adam who showed even more patience and understanding than Pitman’s. Bristol, England

J. P.

I Historical, technical and legal context Introduction

This book deals with airborne systems that depend for their operation on the generation and detection of that intangible discovery the radio wave. Such systems split naturally into two parts: communications and navigation. The former provide two-way radio contact between air and ground, while the latter enables an aircraft to be flown safely from A to B along a prescribed route with a landing safely executed at B. An understanding of such systems requires a working knowledge of basic electronics, radio, computer systems and other topics. A book of this length cannot provide all that is necessary but it was thought that some readers might appreciate a review of selected background material. This is the objective of Chapter 1. It may be that on consulting the list of contents, the reader will decide to omit all or part of this chapter. On the other hand, some readers may decide that more basic information is needed, in which case, the list of recommended books will help point the way to sources of such material. Historical Background

In 1864 James Clerk Maxwell, Professor of Experimental Physics at Cambridge, proved mathematically that any electrical disturbance co uld produce an effect at a considerable distance from the point at which it occurred. He predicted that electromagnetic (e.m.) energy would travel outward from a source as waves moving at the speed of light. In 1888 Hertz, a German physicist, demonstrated ,hat Maxwell's theory was correct, at least over distances within the confines of a laboratory. It was left to the Italian physicist Marconi to generate e.m. waves and detect them at a remote receiver, as he did by bridging the Atlantic in 1901. Other notable landmarks in the development of radio include: 1897

First commercial company incorporated for the manufacture of radio apparatus: the

1936

Wireless Telegraph and Signal Company Limited (England), later the Marconi Wireless Telegraph Company Limited. Fleming's (British) discovery of the thermionic valve - the diode. First patent for a radar-like system to a German engineer, Hulsmeyer. Workable but not accepted. De Forest's (American) invention of an amplifying thermionic valve (triode). Direction-finding properties of radio waves investigated. Discovery of the oscillating properties of De Forest's valve. The first workable pulse radar.

1939

Invention of the magnetron in Britain.

1904 1904 1906 1911 1912

1948

Invention of the transistor by Bardeen, Brattain and Shockley (Bell Telephone Laboratories, USA). To bring us up to date, in the early 1970s the first microprocessor appeared from Intel (USA) leading directly to present-day microcomputers. Paralleling the progress of radio was the second of the three great developments of the twentieth century, i.e. powered flight in heavier-than-air machines. (The other two developments referred to are electronics and applications of nuclear physics; the reader is concerned with two out of three.) There can be few people who have not heard of Wilbur and Orville Wright; who designed and built the first successful powered aircraft which Orville flew for the first time at 10.35 on 17 December 1903, making a landing without damage after 12 seconds airborne. Since then landmarks in aviation, with particular reference to civil aviation, include: 1907 First fatality: Lieut. T. E. Selfridge, a passenger in a Wright Fly er. 1909 Bleriot (French) flies the English Channel. 1912 Sikorsky (Russian) builds first multi-engined (four), passenger (sixteen) aircraft. 1914 World War I. The years 1914-18 saw

advances in performance and a vast increase in number of aircraft, engines and pilots. 1919 Sustained daily scheduled flights begin in Europe. 1928 Whittle (British) publishes thesis on jet engine. 1929 First blind landing by Doolittle (American) using only aircraft instruments. 1937 Flying-boat service inaugurated from Britain to the Far East. Britain to Australia took 8 days in 1938, either by KLM or Imperial Airways. 1939 First jet -powered flight by He 178 (German). 1939 Inaugural air-mail service between Britain 1939 World War II. The years 1939 -45 saw the growth of world-wide military air transport services, and the USA established as the postwar leader in civil aviation. 1944 1945

1952 1953 1954

1958

1965 1970 1970

International Civil Aviation Organisation formed at Chicago conference. American Overseas Airlines operate scheduled flights over North Atlantic with landplane (DC 4). First civil jet aircraft, the Comet 1, goes into service with BOAC. First civil turboprop aircraft, the Viscount, goes into service with BEA. Previously unknown problem of metal fatigue discovered in Comet 1. Withdrawn. 1956 Tu 104 first jet aircraft to commence sustained commercial service. First transatlantic jet service by BOAC with the Comet 4. (PAA's Boeing 707 -120 follows three weeks later.) First short -haul jet to enter service, the BAC 1 -11. Boeing 747 introduced; the first of the Jumbo Jets. First civil aircraft supersonic flights, Concorde and the Tu 144.

From the time of the Wright brothers to the present day, the non -commercial side of civil aviation, known as general aviation (business and private) has grown with less spectacular firsts than its big brother, so that now by far the largest number of civil aircraft are in this category. It was inevitable that the new toys of radio and aircraft should be married early on in their history. Later the vast increase in air traffic made it essential that radio aids, in both communication and navigation, should be made full use of, to cope safely

with the crowded skies. In 1910 the first transmission of e.m. waves from air to ground occurred. Speech was conveyed to an aircraft flying near Brook lands Airfield (England) by means of an e.m. wave in 1916. By the 1920s, radio was being used for aircraft navigation by employing rudimentary direction -finding techniques (Chapter 3). The introduction of four -course low-frequency range equipment in 1929 provided the pilot with directional guidance without the need for a direction -finder on the aircraft. Steady progress was made up to 1939, but it was

world War I I which gave the impetus to airborne Radio innovations . Apart fr om very high frequency (v.h.f.) communications, introduced during the war, a number of radio navigation aids saw the first light of day in the period since 1939. These systems are described in the following chapters.

Basic Principles of Radio Radiation of Electromagnetic (e.m.) Waves and Antennas If a wire is fed with an alternating current, some of the power will be radiated into space. A similar wire parallel to and remote from the first will intercept some of the radiated power and as a consequence an alternating current will be induced, so that using an appropriate detector, the characteristics of the original current may be measured. This is the basis of all radio systems. The above involves a transfer of energy from one point to another by means of an e.m. wave. The wave consists of two oscillating fields mutually perpendicular t o each other and to the direction of propagation. The electric field (E) will be parallel to the wire from which the wave was transmitted, while the magnetic field (H) will be at right angles. A 'snapshot' of such a wave is shown in Fig. 1.1 where the distance shown between successive peaks is known as the wavelength. The velocity and wavelength of an e.m. wave are

Fig. 1.1

An electromagnetic wave

directly related through the frequency of the alternating current generating the wave. The law is: c= Y`f where : c is the speed of light (3 X 108 m/s). Y` is the wavelength in metres. f is the frequency in Hertz (cycles/s). A radiating wire is most efficient when its length is equal to half a wavelength. Thus for a frequency of 100 MHz the wire should be (3 X 10 8)/(2 X 100 X 106 ) = 1.5m long, in which case it is known as a dipole. In practice many airborne radio systems do not make use of dipole antennas since their size is prohibitively large, except at very high frequencies, and the radiation patt ern is not suited to applications where energy needs to be transmitted in or received from a certain direction. A close relative of the dipole is the unipole antenna which is a X/4 length conductor mounted vertically on the metal fuselage which acts as a ground plane in which a reflection of the unipole is `seen' to form a dipole. Thus a v.h.f. communication (comm.) unipole would be less than 60 cm long (centre frequency of the band is 127 MHz). Two unipoles are sometimes mounted back to back on the vertical stabilizer to function as a dipole antenna for use with VOR (Chapter 4) or ILS (Chapter 5). At frequencies in the region of 2-30 MHz (h.f.) a dipole would be between 5 and 75 m. Since the dimensions of aircraft fall, roughly speaking, within this range of lengths it is possible to use the aircraft as the radiating or receiving element. A notch or slot cut in a suitable part of the airframe (e.g. base of vertical stabilizer) has a large oscillating voltage applied across it, so driving current through the fuselage which in turn radiates. The notch/airframe load must be `tuned' to the correct frequency for efficient transmission. Without tuning, little energy would be radiated and a large standing wave would be set up on the connector feeding the notch. This is due to the interaction of incident and reflected energy to and from the antenna. An alternative type of antenna for this band of frequencies is a long length of wire similarly tuned, i.e. with variable reactive components. F or frequencies within the range 10-100 kHz the maximum dimension of even large aircraft is only a small fraction of a wavelength. At these frequencies capacitive type antennas maybe used. One plate of the capacitor is the airframe; the other a horizont al tube, vertical blade or a mesh (sometimes a solid plate). The aircraft causes the field to become

intensified over a limited region near its surface. The resulting comparatively strong oscillating E field between the capacitor's plates causes a current to flow in twin feeder or coaxial cable connected across the antenna. The airborne systems operating in the relevant frequency band are the receive-only systems covered in Chapter 6 (Omega, Decca and Loran C). Although ADF (Chapter 3) receives sign als in the band of frequencies immediately above those considered in this paragraph, one of its two antennas (sense) utilizes the principles discussed. An alternative to the capacitance antenna is the loop antenna which is basically a loop of wire which cuts the H field component of the e.m. wave. The field is intensified by use of a ferrite core on which several turns are wound. Use of two loops mounted at right angles provides a means of ascertaining the direction of arrival (ambiguous) of an e.m. wave. Such antennas are used for ADF (loop) and may also be used for Omega. At frequencies above, say, 3000 MHz the properties of waveguides may be used. A waveguide is a hollow metal tube, usually of rectangular cross section, along which an e.m. w ave can propagate. If the end of a waveguide is left open some energy will be radiated. To improve the efficiency, the walls of the waveguide are flared out, so providing matching to free space and hence little or no reflected energy back down the guide. Such an antenna is called a horn and may be used for radio altimeters (Chapter 11). Associated with the wave propagated along a waveguide are wall currents which flow in specific directions. A slot, about 1 em in length, cut in the waveguide so as to interrupt the current flow will act as a radiator. If several slots are cut the energy from them will combine several wavelengths from the antenna to form a directional beam. The direction depends on the spacing of the slots. Such antennas may be used for Doppler radar (Chapter 10) and weather radar (Chapter 9). The theory of some of the more esoteric antennas used on aircraft is a little sketchy and design is finalized, if not based, on empirical data. However the antenna is designed, it will only s ee service if it performs its function of transmitting and/or receiving e.m. waves in and/or from required directions. The directivity of an antenna, or the lack of directivity, is most clearly defined by means of a polar diagram. If we take a transmitting antenna and plot points of equal field strength (one value only) we have such a diagram. The same antenna used for receiving would, of course, have the same polar diagram. If the diagram is a circle centred on the antenna, as would be the case if the plot were in the plane perpendicular

to a dipole, then the antenna is said to be omnidirectional in the plane in which the measurements were made. A practical antenna cannot be omnidirectional in all planes, i.e. in three dimensions.

Table 1.3 Approximate bands for microwave frequencies Letter designation

Frequency range (GHz)

The e.m. Spectrum and Propagation

L

1-3

As can be seen from the previous paragraph, the frequency of the radio wave is an important consideration when considering antenna design. In addition the behavior of the wave as it propagates through the earth's atmosphere is also very much dependent on the frequency. However, before considering propagation, we will place radio waves in the spectrum of all e.m. waves (Table 1.1). In doing so we see that the range of frequencies we are concerned with is small when

S C X K Q

2-5-4

Table 1.1 The electro magnetic spectrum Hz Region 10 25

Cosmic rays

10 21

Gamma rays

10' 9 X rays 10 17 Ultraviolet 10 15 Visible 10 14 Infra-red 10 11 Radio waves

Abbreviation Frequency

Very low frequency Low frequency Medium frequency High frequency Very high frequency Ultrahigh frequency Superhigh frequency Extremely high frequency

VIE l.f. m.f. h.f. v.h.f. u.h.f. s.h.f. e.h.f.

3-30 30-300 300-3000 3-30 30-300 300-3000 3-30

Frequency band

Omega Decca Loran C ADF h.f. comm. Marker ILS (Localizer) VOR v.h.f. comm.

10-14 kHz

Weather radar (X) Doppler (K)

Radio frequency categorization

Name

System

ILS (Glideslope) DME SSR Radio altimeter Weather radar (C) Doppler (X)

compared with the complete spectrum. By general agreement radio frequencies are categorized as in Table 1.2. There is less agreement about the letter designations used for the higher radio frequencies which are tabulated with approximate frequency ranges in Table 1.3. Finally, Table 1.4 lists the frequencies used for airborne radio systems by international agreement. Table 1.2

3.5-7.5 6-12.5 12.5 -40 33-50 Table 1.4 Airborne radio frequency utilization (exact frequencies given in relevant chapters)

kHz kHz kHz MHz MHz MHz GHz

30-300 GHz

70-130 kHz 2-25kHz 100 MHz 75 MHz 108-112 MHz 108-118 MHz 118-136 MHz 320-340 MHz 960-1215 MHz 1030 and 1090 MHz 4.2-4.4 GHz

In free space, all radio waves travel in straight lines at the speed of light. Such a mode of propagation is known as the space wave. In addition, two other modes of propagation are used with airborne radio equipment: the ground wave and the sky wave. A fourth mode known as tropospheric scatter is used only for fixed ground stations since elaborate and expensive. equipment must be used at both ends of the link due to the poor transmission efficiency. The ground wave follows the surface of the earth partly because of diffraction, a phenomenon associated with all wave motion which causes the wave to bend around any obstacle it passes. In addition, the wave H field cuts the earth's surface, so causing currents to flow. The required power for these currents must come from the wave, thus a flow

of energy from wave to earth takes place causing bending and attenuation. The attenuation is a limiting factor on the range of frequencies which can be used. The higher the frequency the greater the rate of change of field strength, so more attenuation is experienced in maintaining the higher currents. Ground waves are used for v.l.f. and l.f. systems. Radio waves striking the ionosphere (a s et of ionized layers lying between 50 and 500 km above the earth's surface) are refracted by an amount depending on the frequency of the incident wave. Under favourable circumstances the wave will return to the earth. The distance between the transmit ter and point of return (one hop) is known as the skip distance. Multiple hops may occur giving a very long range. Above about 30 MHz there is no sky wave since insufficient refraction occurs. Sky wave propagation is useful for h.f. comm. but can caus e problems with l.f. and m.f. navigation aids since the sky wave and ground wave may combine at the receiver in such a way as to cause fading, false direction of arrival or false propagation time measurements. At v.l.f. the ionosphere reflects, rather than refracts, with little loss; thus v.l.f. navigation aids of extremely long range may be used. Above 30 MHz, space waves, sometimes called line of sight waves, are utilized. From about 100 MHz to 3 GHz the transmission path is highly predictable and reliable, and little atmospheric attenuation occurs. Above 3 GHz attenuation and scattering occur, which become limiting factors above about 10 GHz. The fact that space waves travel in a straight line at a known speed and, furthermore, are reflected from certain objects (including thunderstorms and aircraft) makes the detection and determination of range and bearing of such objects possible. Modulation Being able to receive a remotely transmitted e.m. wave and measure its characteristics is not in itself of much use. To form a useful link, information must

be superimposed on the e.m. wave carrier. There are several ways in which the wave can carry information and all of them involve varying some characteristic of the carrier (amplitude or frequency modulation) or interrupting the carrier (pulse modulation). The simplest, and earliest, way in which a radio wave is made to carry information is by use of Morse Code. Switching the transmitter on for a short time -interval, corresponding to a dot, o r a longer time -interval, corresponding to a dash, enables a message to be transmitted. Figure 1.2 illustrates the transmission of SOS, the time-intervals shown being typical. In radar the information which must be superimposed is simply the time of t ransmission. This can easily be achieved by switching on the transmitter for a very short time to produce a pulse of e.m. energy. When transmitting complex information, such as speech, we effectively have the problem of transmitting an extremely large number of sine waves. Since the effect of each modulating sine wave on the radio frequency (r.f.) carrier is similar, we need only consider a single sine wave modulating frequency. The characteristics of the modulating signal which must be transmitted are the frequency and amplitude. Figure 1.3 shows three ways in which a pulsed carrier may be modulated by a sine wave while Figs 1.4 and 1.5 show amplitude and frequency modulation of a continuous wave (c.w.) carrier. Both amplitude modulated (a.m.) and frequency modulated (f.m.) carriers are commonly used for airborne systems. With a.m. the amplitude of the carrier represents the amplitude of the modulating signal, while the rate of change of amplitude represents the frequency. With f.m. the amplitude and frequency of the modulating signal is represented by the frequency deviation and rate of change of frequency of the carrier respectively. Both a.m. and f.m. waves have informative parameters associated with them. With a.m. if the

, Time y

x = 0 .1s, y=0 -3 s Fig. 1.2 Morse code: SOS

Radio frequency transmitted

--, Time

Fig. 1.4 Amplitude modulation

Fig. 1.3 Pulse modulation - from top to bottom: unmodulated carrier, modulating waveform, pulse amplitude modulation, pulse width modulation and pulse position modulation

carrier amplitude is Y"e and the modulating signal amplitude is V„, then the modulation factor is Vt,-,, , 'C. This fraction can be expressed as a percentage, in which case it is known as the percentage modulation or depth of modulation (note sometimes depth of modulation is quoted as a decimal fraction). Figure 1.4 shows 100 per cent modulation. With f.m. the parameter is the deviation ratio which is given by the ratio of maximum frequency deviation (fd max) to maximum modulating frequency (fm max). The ratio fd/fr„ is called the modulation index and will only be constant and equal to the deviation ratio if the modulating signal is fixed in frequency and amplitude. In Figs 1.3, 1.4 and 1.5 the modulated signal is illustrated in the time domain, i.e. with time along the horizontal axis. It is instructive to look at the frequency domain representations as shown in

Fig. 1.5 Frequency modulation

Fig. 1.6 where a single sine wave of frequency fT , is the modulating signal. It can be seen that several frequencies are present, so giving rise to the idea of bandwidth of a radio information channel. The most significant difference between a.m. and f.m. is that the a.m. bandwidth is finite whereas, in theory, the f.m. bandwidth is infinite. In practice the f.m. bandwidth is regarded as finite, being limited by those extreme sidebands which are regarded as significant, say 10 per cent of amplitude of the largest frequency

frequency component is 3000 Hz we need only transmit a sample of the instantaneous amplitude every 1/6000 = 0-000 166 7 s (= 166.7 ps). Thus we have time-intervals during which we can transmit samples of other signals. The number of signals we can time multiplex on one carrier link depends on the duration and frequency of each sample. The shorter the sample duration the greater the bandwidth required, confirming the statement made earlier that more information requires wider bandwidths. Carrier (fc ) Fig. 1.6 Amplitude modulation and frequency modulation spectrums for a pure sine wave modulating signal of

Basic Receivers and Transmitters A much simplified transmitter block diagram is shown in Fig. 1.7. This could be called the all-purpose block diagram since it could easily be converted to a

frequency fm

component. The relative amplitudes of the carrier and sidebands depend on modulation factor and index for a.m. and f.m. respectively. In any information link there is a relationship between the bandwidth and the amount of information which can be carried, hence high-fidelity stereo broadcasts occupy a wide bandwidth. It is not, however, desirable to have as wide a bandwidth as possible since (a) the number of available channels is reduced; (b) electrical noise, generated at all frequencies by electrical equipment and components, and by atmospheric effects, will be present in the receiver channel at a greater power level the wider the bandwidth. The signal power to noise power ratio is a limiting factor in the performance of receiving equipment. The information in an a.m. wave is repeated in each of the sidebands; the carrier frequency component has no information content. As a consequence, at the expense of more complicated transmitting and receiving equipment, we need transmit only one sideband. Single sideband (s.s.b.) transmission conserves bandwidth, with attendant advantages, and is found in airborne h.f. comm. systems. Multiplexing In most airborne systems the required number of channels is obtained by allocating non-overlapping bands of frequencies centred on specified discrete carriers. This is known as frequency multiplexing. Shannon's sampling theory shows that a sine wave of frequency fm can be completely specified by a series of samples spaced at no more than 1/2 f m second (s). To transmit speech where the highest

Fig. 1.7

Basic radio transmitter block diagram

low-level a.m. transmitter, (little if any amplification of the carrier before modulation), a high-level a.m. transmitter (little if any amplification of the carrier after modulation), an s.s.b. transmitter (introduce a band pass filter after the modulator) or an f.m. transmitter (introduce a frequency multiplifer after the modulator). Obviously in the above examples the circuit details would vary greatly, particularly in the modulators, and if detailed block diagrams were drawn the underlying similarities in structure would be less obvious. The most basic type of receiver is a tuned radio frequency (t.r.f.), however this is rarely used. The standard receiver configuration is the superheterodyne (superhet) shown in Fig. 1.8. The desired r.f. is converted to a constant intermediate frequency by taking the difference frequency after mixing the received signal with the output from a local oscillator (Lo.). Since most of the amplification and selectivity is provided by constant frequency and bandwidth stages the design problem is eased. In both the transmitter and the receiver, r.f. oscillators have to b e tuned to different frequencies. In the transmitter it is the m.o. (master oscillator), while in the receiver it is the Lo. Modern practice is

Fig. 1.8 Basic superhetrodyne receiver block diagram

to usea frequencysynthesizerwith a singlecrystal to providestability dnd accuracy.

In all of the abovethe logic may be reversed(positive and negativelogic). Thus we can representa binary digit (bit) by an electricalsignal,but if the number to be representedis largerthan l, we must combinebits into someintelligiblecode. DigitalSystems Binary code hasbeenmentioned;this is simply counting to the base2 rather than the basel0 Coding (decimalcode) aswe do normally. Unfortunately, Most of the airborne systemsin use are basically binary numberssoon becomevery large,for example analogue,i.e. they dealwith signalswhich represent 9 l r o = I 0 I I 0 I 1 2( t h e s u b s c r i p t s i n d i c a t i n g t h e variousquantitiescontinuouslyand smoothly. For = examplein DME a very smallincreasein rangeresults base),so octal (base8 23) and hexadecimal = 24) may be used. The machinemay still (base l6 in a correspondingincreasein time;we say time is an dealwith a 1/0 situationbut the numbersaremore analogueof distance. With a digital system, when written down, for example manageable information is representedby a numberencodedin 91 ro = l33s = 5816. Note, in the examplesgiven,if somesuitableway. Sinceit is difficult to detectmany different voltage we split the binary number into groupsof three from the right (leastsignificantbit, l.s.b.)we have or current levelsonly two are used,and this leads l , 0 l l , 0 1 1 2 = 1 , 3 , 3 s , i . e .e a c hg r o u pi s t h e b i n a r y naturallyto expressingnumbersto the base2 (binary code)wherethe only digits are 0 and l. It remainsto code for an octal digit. Similarly =5,816. defineelectronicrepresentations r 1 0 1 ,l 0 l l 2 of 0 and I in an and hexadecimalcodesare usedin digital Binary unambiguousway. Various methodsare usedwith code is usedfor the ATC octal computers, (a) beingby far the most common,in the (Chapter 8). The task of frequency transponder non-exhaustive list which follows. selectionis one which lendsitself to coding,and amongseveralwhich havebeenused,the two most - 0 commonarebinarycodeddecimal(b.c.d.)and two (a) Voltagelevel no voltage - t from five (2/5). Both of thesecodesretain the high voltage = l decimaldigit 'flavour' of the number to be encoded (b) Pulsepolarity positive = Q at the expenseof usingextra bits. To represent9l negative 16 (c) Pulseposition a time interval is split in we considerthe decimaldigits 9 and I separatelyto two halves: give: = | pulse in first half 9lro=1001 00016.9.6., pulsein secondhalf = 0 (d) Phasechange at specified read time a 9l ro=10001 11000275 sinewave: changesphase Equivalentsfor all the codesmentionedare givenfor = (180"C) | numbers0 to l5 in Table 1.5. decimal . doesnot change It can be seenfrom the abovethat more bits than = Q are absolutelynecessary phase are usedfor b.c.d. and2l5.

4. conversionfrom binarYto b.c.d.; 5 . b.c.d. fed to frequencysynthesizer; 6 . conversionfrom b.c.d. to specialcode; 7 . specialcode fed to readoutdevice.

Tabh 1.5 Variouscodeequivalents Code

Base

1 0 2

8 1 6

0 0000 0 0 I 000r I I 2 0010 2 2 3 00ll 3 3 4 0100 4 4 5 0l0l 5 5 6 0 1 1 06 6 ? 0lll 7 I 8 1000 l0 8 9 1 0 0 1I I 9 l0 l0l0 l2 A ll l0ll l3 E 12 I 100 14 c 13 ll01 15 D 14 lll0 l6 E 15 llll l7 F

2ls

BCD

0001 0001 0001 0001 0001 0001

0000 0001 0010 001I 0100 0l0l 0l 10 0lll 1000 l00l 0000 0001 0010 00lt 0100 0101

11000 11000 11000 11000 11000 u000

0l001 l 1000 10100 0l 100 0 l0 l q 0 0 1l 0 00101 0001I 10010 10001 01001 11000 10100 01100 01010 00110

If the bits are transmitted serially, one after the other in time down a line, then more time is neededfor the of a number than would be neededif transmission. binary codewere used. If the bits are transmittedin parallel,one bit per line, then more lines are needed. This hasa certain advantagein that the redundancy may be usedto detect transmissionerrors, for e x a m p l eI 0 I I O c o u l d n o t b e a 2 / 5 c o d e a n d I 0 I 0 could not be b.c.d. Error checking can also be used with binary codes. We will alwaysbe restrictedto a certainmaximum numberof bits, one of which can be designateda parity bit usedsolely for error detecting. Supposewe had eightbits available,eachgroup bf eight-bitswould be call-eda word of length 8 (commonly calleda byte). Ttrefirst sevenbits of the word would be used to encodethe decimal digit (0 to 127) while the eighthwould be the parity bit. For odd parity we set the parity bit to 0 or I so as to make the total numberof onesin the word odd; similarly for even parity. Thus61e= 00001l0l odd parity or j 3,o oooot 100 evenparity. Error correcting(as opposedto detecting)codesexist but do not find use in airborneequiPmentasYet. To considera practicalapplicationqf the above Srpposea particular frequencyis selectedon a control unit, we may havethe following sequenceof events: l. information from controller: 215 code', 2. conversionfrom2l5 to binary; binary data; 3. microcomputerprocesses

So far we haveonly discussedthe coding of numericaldata. The ISO (lnternationalStandards Organisation)alphabetNo. 5 is a seven-bitword code which can be usedto encodeupper and lower case letters,punctuationmarks,decimaldigits and various other charactersand control symbols' The full code may be found in most of the latest ARINC and will not be repeatedhere,however' characteristics examplesare A = I 0 0 0 0 0 I' O I 0 0 10,etc. Aparitybitmaybeadded n=i to give a byte. Wh"r. . limited numberof actualwords needto be 'distance','speed','heading',etc' special encoded,e.g. codesmay be designated'Suchcodesare describedin AR INC specification 429'2 digital in fo rm ation transfersystem(DITS) which is discussedin Chapter13. Microcomputers The microprocessorhas brought powerful computers on to aircraft to perform a number of functions, includingthe solution of navigationequations,in a more sophisticatedway than before' A microcomputerconsistsof a microprocessorand severalperipheralintegratedcircuits(chips),to help the microprocessorperform its function' There are four basicparts to computers,micro or otherwise:memory, arithmeticlogic unit (ALU)' control unit and the input/output unit (l/O)' In a microcomputerthe ALU and control unit are usually combinedon a singlechip, the microprocessoror centralprocessingunit (CPU)' Figure l '9 illustratesa basicsystem. The memory containsboth instructionsand data in the form of binary words' Memory is of two basic types, ,ead only (ROM) and random access(RAM)' The ROM doesnot rememberany previousstate which may haveexisted;it merely definesa functional relationshrpbetweenits input lines and its output lines. The RAM could be termed readand write memory; sincedata can be both readfrom memory urd written into memory, i.e. its statemay change' trnformation in RAM is usually lost when power is switched off. The ALU contains the necessarycircuitry to allow it to carry out arithmeticoperations,such asaddition and subtiaction,and logicalfunctions such as Boolean algebraoperations(combinationsof NANDs and NORsetc.).

f':

J

FA. t.9 Basicmicrocomputer organization The control unit providestiming instructionsand from memory, on the data bus, to the control unit synchronizationfor all other units. The control whereit is decoded. The programcounter signalscausethe other units to move data, manip_ulate automaticallyincrementsby one count, and after the numbers,input and output information. All this current instruction has been executedthe next instructionis fetched. This basiccvcle of: activity dependson a set of step-by-stepinstructions (known as the program)which residein memory. The l/O unit is the computer'sinterfacewith the fetch outsideworld. decode From Fig. 1.9 it can be seenthat the units are increment interconnectedby three main buses. A bus is several execute electricalconnectionsdedicatedto a particular task. A unidirectionalbus allowsdata flow in one direction only, unlike a bidirectionalbus where flow is two-way. is repeatedcontinuously. During the executionof an instructiondata may have to be fetched from ln a microcomputerwe usuallyhave: memory, for exampleto add two numbersthe instructionwill need to tell the CPU not only that an l. address bus: sixteenunidirectionallirfes; additionoperationis necessary, but the location.in 2. databus: eightor sixteenbidirectionallines; 3. control bus: the numberof linesvarieswith the the memory,of the numbersto be added. The rate at which instructionsareexecuted systemand may haveboth unidirectionaland dependson the complexityof the instrtrctionand the bidirectionallines. frequencyof the systernclock. Eachpulsefrom the .: clock initiatesthe next actionof the system;several To operate,eachstep-by-step instructionmust be actionsper instructionareneeded.Often the clock fetched,in order,from memoryand executedby the CPU. To keeptrack of the next stepin the program, circuitis on the CPU chip. the only external a programcounter is usedwhich incremelts eachtime componentbeinga crystal. The I/O data flows via logicalcircuits calledporrs. an hstruction is fetched. Before an instruction can be executed.it must be decodedin the CPU to Theseports may be openedin a similar way to that in determinehow it is to be accomplished. which memory is addressed.In somesystemsthe I/O On switch-on,the programcounter is set to the ports are treated as if they were RAM - an address first storedinstruction. The address(location) of this opensa particularport and data flows in or out of first instruction is placedon the addressbus by the that port dependingon whether a read or write signal pro$am counter causingthe instruction to be fetched is present. A variety of chips are used for l/O, some 10

of whi_chare very basic; others (programmable ports) more flexible. The program which is resident in ROM is srbdividedinto routines. Someroutineswill be runningcontinuouslyunlessstopped;others may only be called for when the need ariseJ. For example, a navigationcomputer will continuously compute the aircraft position by running the mainioufini (or bop) which instructs the ALU as to which calculationsmust be carried out using data available in memory. This data must be updated periodically by acceptinginformation from, siy, u ,"dio navigationsensor. When data is available from the extemalequipment,an interrupt sigral is generated and fed to the microcomputer on an interiupt line. Such a signalcausesthe computer to abandon the main routine and commencea serviceroutine which will supervisethe transfer of the new data into rrrmory. After transfer the main routine will rccommenceat the next step, rememberedby a CpU register. The topics discussedin the paragraphsabove can all be classifiedashardware or softwaie. The hardwareis the sum total of actual components up the computer: chips,active and passive T"king discretecomponents,and interwiring. Software comprises_programs, proceduresand the languagesor codesusedfor internal and external commu-nication. Softwaredeterminesthe stateof the hardwarear any particulartime. In an airbornecomputer both the software and hardware are fixed Uy itre designer. The operatordoesnot haveto program the computer in the sensethat he must write a routine; however, he plays his part in how the computer will function b1,,for example,selectinga switChposition which will causecertain data to be preseniedto him by the computer,insertingatard (hardware),on which codedinstructionsor data (software)havebeen *Titten, into a cardreader,etc. Examplesof the use of microcomputersare coruideredin someof the chaptersto follow. These applications,and the abovebrief discussion.should givethe readera basicidea on how computerswork; for detailsof circuitry and programminj consult the readily availablespecialistliteriture.

CaGgorization of Airborne Radio Eqripments Frcqucy and Modulation T* 9" techniquesinvolvedvary greatlywith the r.f. andtypeof modulationused,itls ofien usefulto crtegonze equipmentasto the bandof frequencies in

which it operates(seeTables1.2 and 1.3) and as being pulsed, a.m. or f.m. From both the desigr and maintenancepoint of view, the frequency at which equipmentoperatesis perhapsmore important than the modulation used, at least in so far ai the choice of componentsand test equipmentis concerned. The higher the frequency the greaterthe effect of stray capacitanceand inductance, sigral transit time and skin effect in conductors. In thi microwave region(s.h.f. and the high end of u.h.f.) wavezuide replacesco-axialcable,certainly above5 GHz] and specialcomponentswhosedimensionsplay a critical part in their operationareintroduced(klystrons, magnetrons,etc.). Analogue-Digital Theseterms have alreadybeen mentioned and certain aspectsof digital systemshavebeendiscussed.In modern airborne systemsthe information in the radio and intermediatefrequencystages,including the 'wireless' r.f. link, is usually in analogueform (the exception being secondarysurveillance rcdar (see Chapter8), to be joined in future by microwave gtail8 systems,data link and the replacementfor SSR(seeChapter l3)). In addition Commonlyused transducerssuch as synchros,potentiometers, microphones,telephonesand speakersare all analogue devices.Not all transducersarein the analogue Tlegory, a shaft angleencoderused in encoding altimetersis basicallyan analogueto digital converter. With the exception of the above almost everything . elsein current equipmentis digital, whereas previouslysystemswere all analogue.There is a further subdivisionwithin digitaliquipment into thoseusinga combinationof hardwareand software (computer-controlled)and thoseusingonly hardware (hardwiredlogic). The trend is towardsthl former. Function The two basic categorieswith regardto function are communicationsand navigation. If navigationis definedin its widest senseassafe,economicalpassage from A to B via selectedpoints (waypoints) then communicationssystemseould be consideredas belongingto the navigationcategory. If, however, communicationssystemsare regardedas those systemscapableof transmittingspeechover radio or wire links, and all other systemsas navigation,we iue obeyinga sensibleconvention. The introduction of data links will requiresomeamendmentto the definition of communicationssystems,since , non-navigationaldata will be transmittedbut not as a speechpattern. Navigationsystemsmay be subdividedinto radio

t1

category; landing-aids landingsystemsbelongto the category the of subdivisions these itfr*t?i',vpes of in will be considered Chapters systems lf nuuigution ui9: systems r'ndlnc, Position-fixing 111 "^.,. S. f f uiO 5 respectively' ffi; ;;rfieiiht-nndrng, d and the latter we nave ntaine For self'co into d environment-monitoring' I ^"' u.'i" t,ft.i subtlivide a]tim,etlrs radio dead while uses former systems' The -l.i-ghlri"olng weatheravoidance categorv andinstrument ilffi;;,i"t-tutta'

systemsconcemls andnon'radio,but only the radio posit i on-t txmg is on si i iv d ;;;":'-A;;ih.i possiblesub

fi "* ; ;it

(@er4''l.j$*

(VOR/DME/RNAV/ILS) SYSTEM NAVIGATION re{S80 IIITEGRATED

<-NAV & GS RECEIVER

\COI,IPUTER

navigationsystem Fh. l.l0 KNS 80 integrated CorP') Radio (JurtesY King

12

BOARD

reckoning to compute the aircraft's position while the latter usesa variety of methods:rho'theta, rho-rho, rho-rho-rho,theta-thetaand hyperbolic. The Greeklettersp (rho) and 0 (theta) areusedto representdistance(range)and angle(bearing)to a fixed point of known location. The pilot can determine(fix) his positionif he knows: (a) p and 0 to one fixed Point: (b) p to three distinct fixed Points; (c) 0 to two distinct fixed Points. A rho-rho systemgivesan ambiguousfix unlessthe aircraft is at the midpoint of the line joining the two stationsto which the rangeis known. With hyperbolicsystemsposition-fixingis achievedby measuringdifferencesin range;ambiguitymay be avoidedby varioustechniques(Chapter6). One item of navigationequipmentoverlapsthe boundhriesbetweenthe different methodsof positionfixing, namelyOmega;this usesdeadreckoningin conjunctionwith rho-rho, rho-rho'rhoor hyperbolic methods. Two systems,VOR (Chapter4) andDME (Chapter7) are usedtogetherto gfuea rho-thetafix. With miniaturizationof circuitry, it is now possible to houseseveralsystems,which were previously physicallyseparate,into one box' It is still possible to categorizeby function, but we must bearin mind that the circuit implementationmay be intimately connected.Suchan exampleis givenby the King KNS 80 integratednavigationsystem(Fig. I ' l0) VOR, DME and ILS (Chapters4, which incorporates 7 and 5) aswell as areanavigationfacilities(Chapter l2). Other equipmentmay grouptogethersystems operatingwithin the sameband of frequenciessuch asv.h.f.comm.and v.h.f.nav.(VOR and ILS). Figure l.l I illustratesthe navigationsystemsin use on a Boeing747 with a typical fit; different operators may takeop different options. This diagramincludes non-radio(mainly in the top half) aswell as radio systems,and illustratesthe interrelationships betweenthem especiallywith regardto display (right-handside). A similardiagramfor the communicationssystemsis includedin Chapter2 (Fig. 2.15). The largenumberof radionavigation systems,someduplicatedor eventriplicated for safety,presentthe problem of whereto position the antennas.The solution for the Boeing747 is shown i n F i g .1 . 1 2 .

NavigationNomenclature Figure I .13 and Table I .6 definethe most commonly

usedterms in aircraft navigation. All of the quantities defined,with the exceptionof heading,can be found usingradio systems,or areinput by the pilot at some stageof the flight, usually prior to take off.

Interference The e.m. environment of an aircraft radio system is suchthat it may suffer from interferingsignals-and/or noise,man-madeor natural,ind causeinterference itself to other systems.Interferencemay be either radiatedor conducted. As the aircraft fliei through the atmosphere,it picks up electricalchargedue to frictional contact with atmosphericparticles(precipitationstatic) and alsowhile flying through cloud formations,within which very strongelectricfieldsexist (electrostatic induction). An unevendistribution of chargewill causecurrentsto flow in the aircraft skin; possiblyin the form of a spark,betweenparts of unequal potential. Any sparkresultsin a wide band of radiatedr.f. which will be picked up by radio systems asnoiseand possiblymaskwantedsigrals. To avoid this type ofinterference,a bondingsystemis used comprisingnumerousmetal stripswhich presentvery low iesistancelinks betweenall parts of the aircraft. within the aitcraft,a In addition to discharges if a sufficiently to atmosphere occur dischargewill largedifferencein potential exists. The discharge cannotbe avoided,but in an attempt to keep the activity as far from antennasas possible,static dischaigersare fitted to the trailing edgeof the mainplane,tailplaneand verticalstabilizerin order to providean easypath for it. By providinga numberof dischargepoints at eachdischargerthe voltageis kept low. The bondingsystemcarriesthe largecurrents involvedto thoseparts of the airframewherethe are fitted. Lightningconductors, static dischargers such ason the insidesurfaceof the non-conducting connectedto noseradome,and lightningdischargers the lead-inof wire antennasand somenotch antennas,help conduct any strike to the bulk of the airframe,so preventingdamageto equipment. A wire path between antennawill alsohavea high resistance static of any leakage allow to airframe and the it build-up on the antenna. enginc Sparksoccur in d.c. motors and generators, igrition systems,etc. Capacitorsareusedto provide a low resistancer.f. path acrossbrushes,commutators and contacts, a form of protection known as nrppression. Another form ofinterferenceis capacitiveand inductive pick-up and cross-talkbetween adjacent

13

Wcathcr radar systcm

instrurnant

Loran indicetors Radio rnag indic6tors

dircctor in.

lbcrqr

lffic'_l

3yst. I

lbcacon ird.l

Fig. l.l I Boeing747: typical navigationsystemsfit (courtesyBoeingComntrcial AeroplaneCo.)

cables. Pick-up is the term usedwhen the interfering sourceis a.c.power (400 Hz in aircraft),while cross-talkis interferencefrom a nearby signal-carryingcable. The problem arisesout of the capacitanceand mutual inductancewhich edsts betweenthe cables.A pair of wiresmay be twisted togetherto reduceboth types ofinterference- the pick-up or cross-talkon adjacentloops,formed by the twist, tendingto cancelout. An earthedmetallic screenor shield will provide an effective reduction in capacitiveinterferencebut low-frequency inductive 14

pick-up is not appreciably affected by the non-magneticscreen. At high frequenciesskin effect confines the magneticfields of co-axial cablesto their cablesareboth screened interior. Most signal-carrying and twisted;some,whereintegrity is especially important, e.g. radio altimeteroutput, may have double screening. The screenaround a wire must be earthedin order to be effective. Howeverif both endsof the screen are earthed,an earth loop may be formed sincethe completecircuit through the screen,remoteearth

Wirthor ndu Locrlizrr No. Locrlizcr No. 1 end tlo. 2

Loft tnd right AFT nos. 9e8r door glidc slopo tracl end capturr antcnna sy3tams Ccntor .furolegcJ aqulpment cantcr

Low rmgc rrdio altimotrr ADf loop No. 1

l{rrtrr batcon ADF loop llo. 2 ADF srnse entcnnaNo. 2

ADF rns. antoma t{o. 1

VORNo.1 VOR No. 2

Fig.l.l2 Boeing747.. typicalnavigation systems aerial (courtesy locations BoeingCommercial Aeroplane Co.) points and the airframe is of non-zero resistance. As a consequence, interferingsourcesmay causea potential differenceto edst betweenthe endsof the screen.The resultingcurrent flow and its associated H field would causeinterferencein the inner conductor. Earth loops are a particularproblem in audio systemsand must be avoided. The earth pointsfor screenedcablesand a.c.power must be remote from one another. If a screenwereto be connecteddirectly to an a.c. power earth, conductedmainsinterferencemay result. Another form of conductedinterferenceis cross-talkwhere a

number of signalcarryiqg wires are brought together, e.g.audio signalsbeingfed to an interphoneamplifier. Suitably designedpotential divider networkskeep this conducted cross-talkto a minimum (Chapter 2). Adequateseparationof antennasoperatingwithin the samefrequencyband is necessary to prevent mutual interferenceby radiation. Frequency and time domain filtering may be usedin helping to avoid suchinterference,the former in c.w. systems,the latter in pulsedsystems.Different polarization(E field direction) will assistin preventing cross-coupling betweenantennas.

15

,/ WPTO

\Mnd dircction dnd velocatY

On tnck \Mnd dircction and velocity

(

Fit; l.l3 Navigationnomenclature(courtesyLitton Systems lnternational lnc., Aero hoducts Division)

16

Navigation nomenclature - abbreviations electrical equipment will interfere with the magnetic compass. Units are marked with their 'compasssafe distance' as appropriate but care should also be taken Abbreviation Meaning with cables,particularlyfor d.c. power. Table 1.6

HDG

TK

Heading - angle,measuredclockwise betweenNorth and the direction in which the aircraft is pointing, Track - direction in which the aircraft is moving. Desired'Track- direction in which the pilot wishesthe aircraft to move. Drift Angle - anglebetweenhcadingand track measuredto port (left) or starboard (right). Track Angle Error - angle bctween track and desiredtrack, usually quoted as left or rght. Ground Speed - spdedof the aircraft in the directionof the track.in the'plane' parallel to the earth's surface(map speed). . Comparewith air speedwhich is the speed of the aircraft relative to the air mass through which it is moving. Position. Waypoint - a significant point on the route which may t c usedfor reporting to Air Traffic Control, turning or landing. Distanceto go from position to waypoint, CrossTrack - the perpendiculardistance from the aircraft to the line joining the two waypoints betweenwhich the aircraft is flying. Estimated time of arrival

Maintenance

This is not the placeto go into greatdetail on this important practicaltopic but somenotesof a general DA natureare in order to supplementthe notes included in most of the following chapters.In practicethe maintenanceengineerrelieson his training and TAE experienceand also regulations,schedulesand procedureslaid down or approvedby national bodies responsiblefor aviationin generaland safetyin GS particular. The aircraft maintenanceenginber,of whatever specialism,is responsiblefor regularinspectionsof equipmentas laid down in the aircraft schedule.For the radio engineeran inspection will consist of a POS thorough examination of all equipment comprising WPT the radio installationfor cracks,dents,chafing,dirt, oil, grease,moisture,buming, arcing,brittleness, breakage,corrosion,mechanicalbonding, freedomof DIS movement,springtension,etc" as applicable. In XTK carrying out specific tasks the engineershould look for damageto parts of the airframeor its equipment which are not directly his or her responsibility. Vigilance is the key to flight safety. ETA Carrying out functional tests when called for in th0 schedule,or when a fault hasbeen reported,should be done in accordancewith the procedurelaid down Adjacent channel interference occurs when a in the aircraft maintenancemanual. A word of receiver'sbandwidth is not sufficiently narrow to waming should be givenhere,sinceproceduresarenot attenuateunwantedsigralscloseto the required alwayswhat they shouldbe: often testingof certain sigtal. Secondor imagechannelinterferencemay aspectsof a system'sfunction are omitted and, occurin superhetreceiverswhen an unwantedsigral rarely, there m.aybe errors in the procedure. separated froin the required sigrral by the twice A thorough knbwledge of the system is the best { intermediatefrequencyand lies on the oppositeside guardagainstmistakesor omissionswhich, if noticed, of the local oscillatorfrequency. A high intermediate strouldbe amendedthrough the proper channels. frequency will help reduce secondchannel Modern equipmentusuallyhassufficient built-in interferencesince the image will be outside the r.f. test equipment(BITE) and.monitoringcircuits to bandwidth,the separationbeinggreater. carry out a comprehensive check of the system. With Unfortunatelyfor a given Q factor, the bandwidth of radio systems,however,specialportable test the intermediate frequency amplifiers will be wide for equipmentmust be usedin addition to BITE in order this solutionto the secondchannelproblem. to be in a position to certify the system asserviceable. Increasingchannelseparationis not really acceptable Test setsshouldbe capableof testingby radiationand sincedemand for more channelsis forever rising. of simulating the appropriate sigrals to test all Somereceiversemploy two intermediatefrequencies functionsnot coveredby BITE. producedby two mixer stagesand two local The r.f. circuits,includingantennasand feeders, oscillators;this can give good adjacent and second are often neglectedwhen functional tests are carried drannel rejection. out. In particulartest set antennasshouldbe Magrreticfields associatedwith electronic and correctly positioned if a false impressionof the DTK

receiversensitivityor power output is to be avoided. Whenfault-firtdhg a completefunctional test shouldbe carriedout as far as posible in order to obtain a full list of symptoms. Naturally symptoms suchasa smell of burning or no supply must call a halt to the procedure. Fault-findingchartsin maintenancemanualsare usefulbut there is no substitutefor knowledgeof the system. One shouldnot forget the possibleeffects of non-radiosystemsand equipmentwhen investigating reporteddefects.Poorbonding,brokenstatic dischargers, open circuit suppressioncapacitors,low or inadequatelyfiltered d.c. supplies,low voltageor lincorrectfrequencya.c. supplies,etc. will all give rise \o symptomswhich will be reportedby the pilot as radiodefects. Sometimessymptomsare only presentwhen the aircraft is airborneand the systemis subjectto vibration,pressure and temperaturechanges, etc. A functional test during engineruns will gc part way to reproducingthe conditionsof flight. One should mention the obvioushazardof loose articles;so obviousthat many aircraft accidentshave beencausedin the past by carelessness. Tools and test equipment,including leads,must all be accounted for when a job is finished. A well-run store with signing-inand signing-outof equipmentis an added safeguardto personalresponsibility. Installationof equipmentshouldbe in accordance with the manufacturer'sinstructionswhich will cover the following, l. Weight of units: centre of gravity may be affected. 2. Current drawn: loadingof suppliesshould be carefully consideredand the correct choiceof circuit-breakermade. 3. Cooling: more than adequateclearanceshould be left and forced air-coolingemployedif appropriate. Overheatingis a major causeof failure. 4. Mounting: anti-vibrationmounts may be necessarywhich, if non-metallic,give rise to a need for bonding straps. 5. Cables:length and type specified. Usually maximum length must be observedbut in some casesparticularlengthsarenecessairy.Types of .' cableusedmust provideprotection against interferenceand be ablerohandle current drawn or supplied. Current capabiltiesare reduced for cablesin bunches. 6. Antenna: approvedpositionsfor particular types of antenna or particular types of aircraft are laid down by aviation authorities. t8

Strengtheningof the structurearound the antennain the form of a doublerplatewill probablybe necessary.A groundplaneis essentialand must not be forgotten if the antennais to be mountedon a non-conducting surface. If the antennais movable,adequate clearanceshould be left. Any alignment requirements must be met. 7. Interface: qompatibility with other systems/ units must be ensured. Both impedance matching(including allowing for capacitiveand inductive effects) and signalcharacteristics should be considered. Loading of outputs should be within limits. Particularcareshould be taken in decidingwhere synchro devices obtain their referencesupplies. Programming pins for choiceof outputs and/or inputs must be correctlyconnected. 8. Compass:safedistance. 9. Radiationhazards. The last item in the abovenon-exhaustivelist raises the topic of safety. Electric shock is an obvious hazardw'ren working on aircraft and it shouldbe rememberedthat one is liable to receivea shockfrom radiatingantennas,particularly h.f. antennas. A radiation hazardexistswith all transmitting antennas,thus the operator should ensureno-oneis working, particularly doping or painting, near an antennawhen the associatedtransmitter is on. The particularhazardsof microwaveradiation are consideredin Chapter9. It is up to all personnel working on aircraft to becomeawareof the dangers of harmful substances, the use(and position) of fire extinguishers,the dangersof mixing oil or greasewith oxygen,elementaryfirst aid, warning symbols,etc.

Regulatingand Advisory Bodaes All countriesset up bodieswhich are responsiblefor mattersconcernedwith aviatione.g. CAA (UK), FAA (USA), BureauVeritas (France),etc. These bodiesdraft air law and issueregulationsconcerned with the hcensingof engineersand aircrew,aircraft operations,aircraft and equipment manufacture, minimum equipment fits (including radio), air traffic control, etc. They are also the bodieschargedwith seeing,by meansof examinationsand inspections, that the law is obeyed. Aviation is an international activity and co-operationbetweencountriesis essential.This co-operationis achievedmainly through the ICAO, an agencyaffiliated to the United Nations. All

nationswhich are signatoriesto the Chicago Conventionon Civil Aviation 1944 aremember-srates of the ICAO which was an outgrowth of that convention. Table 1.7 Organizations,ordersand conference concernedwith aircraft radio systems Abbreviation

Orsanization

ARINC ATA AEEC CAA CAP FAA

AeronauticalRadio lnc. Air TransportAssociation AirlinesElectronicEngineeringCommittee Civil Aviation Authority Civil Aviation Publication FederalAviation Agency InternationalCivil Aviation Organization InternationalFrequencyRegistrationBoard InternationalRadioConsultativeCommittee InternationalTelecommunications Union TechnicalStandardOrder World AdministrativeRadioConference

lcAo IFRB CCIR ITU TSO WARC

The ICAO issuesannexesto the convention. Annex l0 beingof particularsignificanceto aircraft radio engineerssinceit is concernedwith aeronautical telecommunications and, amongother things,lays down a minimum specificationfor airborne radio systems.The materialpublishedby the ICAO does not automaticallybecomethe law or regulationsin all member-states; ratification is necessaryand may not take placewithout considerableamendment,if at all. In particular,systemspecificationsemergdin forms considerablydifferent from Annex 10, although similar in content. In the USA specificationsare issuedin the form of TSOswhile in the UK there is CAP 208. Volume I with its companionVolume 2 listing approvedequipmentunder variousclassifications. Licensingof engineersis one areain which, as yet, there is little internationalstandardizationin accordancewith Annex l. The licensedaircraft radio maintenanceengipeeris unknown in the USA but, of course,organizationsoperatingwith the approval of the FAA do so only if they employ suitably qualifiedpersonnel. In the UK the [censed engineer reignssupreme,except in the certification of wide-bodiedjets and supersonictransportswhere a systemof companyapprovalof personnelexists,the companyitself being approvedby the CAA for the operationand maintenanceof such aircraft. France hasno systemof statelicensing,it being left to the operatorsto assess the compettncy of its maintenance personnelunder the watchful eye of officials. Of further interestto those concernedwith aircraft

radio, the Chicagoconvration provides that aircraft registeredin contractingstatesmay carry radio transmitting apparatusonly if a licence to install and operatesuch apparatushasbeenissuedby the appropriateauthoritiesof the statein which the aircraft is registered.Furthermore,radio transmitting apparatusmay only be usedover the territory of contractingstates,other than the one in which the aircraft is registered,by suitably licensedflight crew. Various non-regulatory bodies exist with a view to extendingco-operationacrossnational boundariesin respectto aircraft equipmentand maintenance. ARINC is one such organization. It is a corporation the stockholders of which are drawn from airlines and manufacturers,mostly from the USA. As well as operatinga systemof aeronauticalland radio stations ARINC sponsorsthe AEEC, which formulates standardsfor electronicequipmentand systems designedfor usein airlinersasopposedto general aviation. Characteristics and specificationspublished by ARINC do not havethe force of law but nevertheless are,in the main, adheredto by manufacturerswho wish to sell their equipmentto the airlines. A specificationrelatingto the presentationof maintenanceinformation is the ATA 100. A standard Iayout for technicalpublicationsrelatingto aircraft hasbeen promulgatedand widely adopted. Of particularinterestto readersare Chapters23 and34 of the maintenancemanualwhich cover communicationsand navigationrespectively.In addition to prescribinglayout, a set ofstandard symbolsfor electricalwiring diagramshas beenissued. So far, bodiesconcernedwith aircraft and their equipmenthavebeenconsidered;in addition organizationsconcernedwith telecommunications strouldbe mentioned. The ITU is an agencyof the United Nations which existsto encourage internationalco-operationin the useand development of telecommunications.The CCIR is a committeeset up by the ITU to deal with radio communications. Among topics of interestto the CCIR arespectrum utilization and aeronauticalmobile services.The IFRB has alsobeen set up.by the ITU for the assignmentand registrationof radio frequenciesin a masterfrequencylist. In November 1979 an internationalconference(WARC '79), with representatives from 154 countries,met in Genevato considerradio regulationsand re-allocate frequencies.The resultsof WARC '79 will not be publishedwhile this book is beingwritten but it is unlikely that the frequenciesallocated to aeronautical mobile serviceswill suffer significant amendment the cost would be too great.

19

h

2 CommunicationsYstems

on v.h.f. lrequenciesis often found; unfortunately aeronauticaliommunicationssatellitesare not to be found (1979). There is a fundamental need for communication The audio integrating system (AIS) complexity beiweenaircrewand ground controllers,amongthe dependson the type of aircraft. A light aircraft aircrew and between aircrew and passengers.External ,yrt.* may provide two transmit/receivechannelsfor communicationis achievedby meansof dual u.h.f.iomms and receiveonly for dual v'h'f' (R/T) link while internal radio-telephone nav.,ADF, DME and marker. Each receivechannel communication'(intercomor audio integratingsystem) hasa speakeroff'phoneswitch while the microphone is by wire as opposedto wireless.Although intercom' canbe switchedbitween v'h.f. comms I and v'h'f' is not a radio system,it is includedin this chapter comms 2. A multi-crew large airliner has very many becauseof its intimate relationship with the aircraft more facilities,as describedlater. radio systems.Voice recordersand in-flight entertainmentsystemsarealso consideredsincethey areusually the responsibilityofthe aircraft radio V.H.F. Gommunications technician/engineerThe first items of radio equipmentto appearon Basic Principles aircraftwerelow-frequency(l'f.) communications An aircraft u.h.f. comrnstransceiveris comprisedof gap transmitters. setsin the World War I daysof spark either a singleor double conversionsuperhetreceiver lntercom was by meansof a Gosport(speaking)tube' and an a.m. transmitter' A modern set provides720 By the 1930sthe early keyed continuouswave(c'w') channelsat 25 kHrzspacingbetween I l8 MHz and was beginningto be replacedby (radio-telegraphy) 'key-bashing'hadits placeaslong as 135'975MHz; until recently the spacingwas 50 kHz R/T although givingonly 360 channels.The mode of operationis aircraft carriedradio operators. Early R/T was within iinglJ.itutntl simplex(s,c.s.),i.e. one frequencyand the l.f. and h.f. bands,the setsoperatingon only one both receiverand transmitter' If or very few frequencies.With airfieldswidely spaced onJ antennafor provision for satellite communication is included in and low-poweredtransmission,there was little iccordancewith ARINC 566 then in addition to interferenceand so the need for many channelsdid a.m. s.c.s.we will have f.m' double channelsimplex not arise. (d.c.s.), i.e. different frequenciesfor transmit and The situation has drastically changedsinceWorld ceive.. re the with War II; air traffic and facilities have increased 'line of Communicationby v.h.f. is essentially consequentdemandfor extra channelswhich cannot sight'by direct (space)wave. The rangeavailablecan be providedin the Lf., m.f. or h.f. bands. '23 (\/.\ + y'ft1)nm where ftt is be approximated by I Fortunatelyv.h.f. equipmenthasbeensuccessfully *re trilght, in feet, abovesealevel of the receiver developedfrom early beginningsin World Wu II while ftl.is the samefor the transmitter' Thus, with figtrtercontrol. the ground station at sealevel, the approximate The current situation is the v.h.f. is used for ,nr*I*urn range for aircraft at l0 000 and 1000 ft *tort-range communication while h'f. is used for (30 000 and 3000 m) would be 123 and 40 nm long-range. A large airliner, such as aBoeing74T , respectivelY. in such carriesthree v.h.f.sand dual h'f. In addition, are (Selcal) facilities aircraft, selectivecalling ' trstallation provided by a dual installation such that a ground A singlev.h.f. installationconsistsof three parts' station can call aircraft either singly or in groups namJy control unit, transceiverand antenna' In without the need for constant monitoring by the are connected to the v'h'f' via crew. Provision for satellite communication (Satcom) addition crew phones

lntrodoction

m

Frg.2.1 KY 196 v.h.f. comm.transceiver (courtesyKing Radio Corp.)

Fi& a2 CN-201Iv.h.f.comm./nav. equipment (courtesy BendixAvionicsDivision) selectionswitchesin the AIS. Light aircraft v.h.f.s usuallyhavea panel-mountedcombinedtransceiver and control unit, an examplebeingthe King KY 196 illustratedin Fig. 2.1. The current trend is for combinedCOM/NAV/RNAV; Fig. 2.2 illustratesthe BendixCN-2011,a generalaviationpanel-mounted unit comprisingtwo commstransceivers, two nav. receivers, glidepathreceiver,marker receiver,

frequency control for internal circuits and d.m.e. and last but not least,audio selectionswitches.Such equipmentwill be consideredin Chapter12. Figure2.3 showsone of a triple v.h.f. comms installationas might be fitted to a largepassenger transportaircraft: VHF2 and VHF3 are similarto VHFI but aresuppliedfrom a different 28 V d.c. bus bar and feed different selectionswitchesin the AIS.

21

ATE v.h.f.COMM

liitool

cO

o

rcool

Freq.

@

FWD MTR PWR REF oFF-\ \ t t . / fPwa

r\

Mic

l \

P.t.t.

Ars

Rcv Audio

I

\_./ DISABLE SOUELCH (o)

fo Aerial

Sidetone

PHONO E v.h.f. No. 2

v.h.f No.

Rcv Audio To Selcal

O MIC. 28 V d.c. Stby Bus

Fig.2.3 Typicalv.h.f.l installation

The transceiver.which is rack-mounted,contains all the electroniccircuitry and hasprovisionfor the maintenancetechnicianto connectmic. and tels direct, disablethe squelch,and measureVSWR. Theseprovisionsfor testingareby no meansuniversal but if the systemconformsto ARINC 566 a plug is providedto which automatictest equipment(ATE) canbe connected.A protectivecoverfor the ATE plug is fitted when the unit is not in the workshop. The antennacan take variousforms: whip, blade or suppressed.In a triple vl.f. commsinstallation thesemay be two top-mountedbladeantennasand onebottom-mounted:an altemativewould be two within the fincap dielectric. bladeand one suppressed The whip antennais to be found on smalleraircraft. All antennasaremountedso asto receiveand transmitverticallypolarizedwaves. The blade antennamay be quite cornplex. It will nearthe centreofthe band with be self-resonant bandwidthimprovementprovidedby a short'circuited stub acrossthe feed terminal or a more complicated reactivenetwork built in which will permit height and hencedrag reduction. Controls and Operation It is common to have in-useand standby frequencies available,the former controlling the transceiver frequency. This is the situationin Fig.2.3 wherewe have two setsof frequency controls and two displays, the in-useone being selectedby the transfer switch and annunciatedby a lamp abovethe display.

z2

Frequencycontrol is achievedby concentricknobs, the outer one of which variesthe tens and units while the inner pne variesthe tenths and hundredths. An alternativeis shownin Fig. 2.1 wherethere is one frequencycontrol and two displays' On rotating the frequencyknobs clockwiseor anticlockwise,the standbyfrequencyonly will incrementor decrement respectively.Standbymay then becomein-useby operationof the transferswitch. Thereare many controllersin servicewith only in-useselection. may Someor all of the following switches/controls be providedby manufacturerson request. Volume Control A potentiometer,which allows variableattenuationof audio,prior to feedingthe AIS may be fitted asa separatecontrol or as a concentric knob on the frequencyselector(s).Sucha volume control may havesidetonecoupledthrough it on transmit. $uelch Control A squelchcircuit disablesthe receiveroutput when no sigralsare beingreceivedso preventingnoisebeingfed to the crew headsets betweenground transmissions.The squelchcontrol is a potentiometerwhich allowsthe pilot to set the level at which the squelchopens,so allowingaudio output from the receiver.Whenthe control is set to minimum squelch(fully clockwise) the Hi and l,o leads,brought to the control unit squelch-disable from the transceiver,shouldbe shorted,so givinga definite squelchdisable.

Mode Selector Control Providesselectionof normal a.m.,extendedrangea.m.or Satcom.If the Satcom antennahas switchablelobessuchswitchingmay be includedin the mode switch,or couldbe separate.

ReceiverThe riceiver is a singleconversionsuperhet. The r.f. stageemploysvaractordiode tuning, utilizing the tuning voltagefrom the stabilizedmaster oscillator(s.m.o.). Both the r.f. amplifier and mixer are dual gatefield-effecttransistors(f.e.t.). The r.f. amplifier f.e.t. has the input signalappliedto gate I On-Off Switch Energizesmaster power relay in transceiver.The switch may be separate,incorporated while the a.g.c.voltageis appliedto gate2. The in mode selectorswitch as an extra switch position, or mixer connectionsare: gate l, signal;gate2, s.m.o. gangedwith the volume or squelchcontrol. The differencefrequencyfrom the mixer, I l'4 MHz, is passedby a crystalfilter, providingthe desired narrow bandpass,to the i.f. amplifiers. Two stagesof ReceiverSelectivity Switch Normal or sharp i.f. amplificationareused;the first of selectivity. WhenSatcomis selectedsharpselectivity a.g.c.-controlled which is a linear integratedcircuit. automaticallyapplies. The detectorand squelchgateutilize transistorson an integratedcircuit transistorarray. A further array Block Diagram Operation (KY 196) is used for the squelch-controlcircuitry. Noiseat Ftgure2.4 is a simplifiedblock diagramof the King 8 kHz the detectoroutput is sampledand used from v.h.f.comm.transceiver. KY 196 panel-mounted to closethe squelchgateif its amplitudeis as general for the aviation equipment, intended This expected from the receiveroperating at full gain. since market, is not typical of in-servicetransceivers When a sigral is received,the noise output from the frequencyand displaycontrol is achievedwith the aid detector decreases due to the a.g.c.action;asa within the lifetime of of a microprocessor;however the squelchgate opensallowing the consequence this book suchimplementationwill become audio signalto pass. The squelchcap be disabledby commonplace.

Frequencydisplay 118.70 Use

1

121.90 Standby

t

t;: I

F

F F

F

'__--__--.1

kHz Codc

MHz Code

P.t.t.

lncremenV Decrement

Fig. 2.4 King KY 196 simplified block diagram

a t F A( F F,

R.F. Input

Tuning Volts {s.m.o.}

Fig.2.5 KingKY 196simplifiedreceiver blockdiagram

meansof a switch incorporated in the volume control. When the receivedsigral has excessivenoise on the carrier,the noise-operated squelchwould keep the squelchgate closedwere it not for carrier-operated or backupsquelch. As the carrier levelincreases, a point is reachedwhere the squelch gate is opened regardlessof the noise level. The meandetectoroutput voltageis usedto determinethe i.f. a.g.c.voltage. As the i.f. a.g.c. voltageexceedsa set referencethe r.f. a.g.c.voltage decreases. The detectedaudio is fed via the squelchgate, low-passlilter, volume control and audio amplifier to the rearpanelconnector. A minimum of 100 mW -audio power into a 500 O load is provided.

transmitter chain comprisesa pre-driver, driver and final stageall broad band tuned, operated in ClassC and with modulatedcollectors. The a.m. r.f. is fedvia a low-passfilter, which attenuatesharmonics,to the antenna. On receivethe t.r. diode is forward biased to feed the receivedsignalfrom the antenna through the low-passfilter to the receiver.f. amplifier. The modulator chain comprisesmicrophone pre-amplifier,diode limiting, an f.e.t. switchingstage, integratedcircuit modulator driver and two modulator transistorsconnectedin parallel. The pre-ampoutput is sufficient to subsequentlygive at least 85 per cent modulation,the limiter preventingthe depth of modulation exceeding100 per cent. The mic. audio line is broken by Jhe f.e.t. switch during receive.

Tiansmitter The transmitter(Fig. 2.6) feeds l6 W of a.m. r.f. to the antenna. Modulationis achievedbv superimposingthe amplified mic. audio on the transmitter chain supply. The carrier frequency corresponds to the in-usedisplay. Radio frequency is fed from the s.m;o. to an r.f. amplifier. This input drive is switched by the transmit receiveswitching circuits, the drive being effectively shorted to earth when the pressto transmit(p.t.t.) button is not depressed.The

Stabilized Master Oscillator The s.m.o. is a conventional phaselocked loop with the codesfor the programmabledividerbeinggeneratedby a microprocessor.Discretecomponentsare usedfor the voltagecontrolledoscillator(v.c.o.)and buffers while integratedcircuits(i.c.) areusedelsewhere. The referencesigrralof 25 kHz is provided by an oscillatordivideri.c. which utilizesa 3'2 MHz crystal to give the necessarystability. Only sevenstagesof a fourteen-stageripple-carry binary counter are used to

24

R.F. {s.m.o.}

P.t.t.

Fig. 2.6 King KY 196 simplified transmitter block diagram

Phase detector

v.c.o.

MHz cont. lrom pp

MHz cont. lrom gp

Fig. 2.7 King Ky 196 simplifiedprogrammabte divider bkrck diagram gtve the necessarydivision of 21 = l2g. This reference, is made;i.e. zerosafter the displayeddecimalpoint.

with the outpur of the programmable l9e9lner divider,is fed to the phasedetecior-whichis part of an i.c., the rest of which is unused. The pulsating d.c. on the output of the phasedetectortrasa a.O. componentwhich after filtering is used to control the frequencyof the v.c.o. by varactortuning. If there is a synthesizermalfunction, an out-ofloclisignal from "s.m.o. the phasedetectoris usedto switch off the feedto the transmitter. The programmabledivider consistsbasicallyof . thrce setsof countersas shownin Fig. 2.7. Tie v.c.o.output is first dividedUy "ittrei+O o, !r1f!rea41, the former being so when a discreti MHz selection

The prescalerwhich perlbrmsthis division is a u.h.f. programmable divider(+ l0/l l) followedby a divide-by-four i.c. The wholeMHz diviclerusesa 7 4 L S l 6 2 b . c . d .d e c a d ec o u n t e ra n d a 7 4 L S l 6 3b i n a r y counterwhich togethercan be programmedto divide by an integerbetweenI l8 and 145,hencethe prescalerand whole MHz divider give a total division o,t^llSO(40 X I l8) to 5800 (40 X 145)in stepsof 40. Thus a requiredv.c.o.output of, say, t IO.OOUHz would be achievedwith a divisionof 5200 (40 X 130) since130 MHz + 5200 = 25 kHz = reference frequency. The 25 kHz stepsareobtainedby forcing the

25

prescalerto divide by 41, the requirednumber of timesin the count sequence.Each time the division ratio is 41, one extra cycleof the v.c.o.frequencyis neededto achievean output of 25 kHz from the programmabledivider. To seethat this is so, consider the previousexamplewherewe had a division ratio of 5200 to give 130'00MHz, i.e. 5200 cyclesat 130'00MHz occupies40 ps = periodof 25 kHz. Now a prescalerdivisionratio of 4l once during 40 ps means5201 cyclesof the v.c.o.output occupy40 ps so the frequencyis 5201/(40X 10-6) = 130.025MHz asrequired. The prescalerratio is controlled by the fractionalMHz divider, againemploying a 7 4LS| 62 and 74LS163. The numberof divide-by-41eventsin 40 ps is determinedby the kHz control code from the microprocessorand can be anywherefrom 0 to 39 times. Thereforeeachwhole megacyclecan have N X 25 kHz addedwhereN rangesfrom 0 to 39. This produces25 kHz stepsfrom 0 kHz to 975 kHz.

Microprocessorand Display The microprocessor used,an 8048, containssufficientmemory for the programand data requiredin this applicationto be storedon the chip. In addition to this memory and, of course,an eight-bit c.p.u.,we havean eight-bit timer/counterand a clock on board. Through twenty-sevenI/O linesthe 8048 interfaceswith the programmabledivider, displaydrive circuits and non-volatilememory.

1024 words program memory

Clock

8-bir CPU

64 words data memory

__JI___J 'l ( 8-bit Timer/ evont counter

27 I/g lines

Fig. 2.8 8048 eieht-bit microcornputcr (courtery King Radio Corp.)

216

The 8048 has been programmed to generatea 'use' and 'standby' frequencies. binary code for the The code,aswell asbeing storedin the 8048, is also storedin a l40o-bit electricallyalterablereadonly memory (EAROM). This external memory is effectively a non-volatileRAM, the data and address being communicated in serial form via a one-pin mode being bidirectionalbus, the read/write/erase controlledby a three-bit code. When power is appliedthe microprocessorreadsthe last frequencies stored in the EAROM which are then utilized as the 'use'and 'standby' frequencies.In the eventof initial failure of the EAROM the microprocessorwill display 120'00 MHz as its initial frequencies.The EAROM will store data for an indefinite period without power. The 'standby'frequency is changedby clockwise or counterclockwisedetent rotation of the frequency selectknobs. I MHz, 50 kHz and 25 kHz changescan be made with two knobs, one of which incorporatesa push-pull switch for 50125kHz step changes.The microprocessoris programmedto incrementor 'standby'frequency by the appropriate decrementthe whenever it sensesthe operation of one of the step knobs. frequency-select The code for the frequency in use is fed to the programmabledividersfrom the microprocessor. 'standby' 'Use' frequenciesare exchangedon and operationof the momentary transferswitch. When the transceiveris in the receivemode the 'us€' frcquency microprocessoradds I l'4 MHz to the code since the local oscillator signal fed to the receivermixer should be this amount higher than the desiredreceivedcarrier in order to give a difference frequency equal to the i.f. 'standby'codesare fed to the Both luse'and display drivers. The'use'code representsthe transmit frequency and is not increasedby I l'4 MHz in the receive mode. Each digit is fed in tum to the cathode decoder/driver, an i.c. containing a sevensegmentdecoder, decimal point and comma drives and programmable current sinks. The decimal point and comma outputs (i and h) are used to drive the '.'and 'T' (seeFig. 2.10). segments displaying'l', 'T'is The illumfrrated when in the transmit mode. The display is a gasdischargetype with its intensity controlled by a photocell located in the display window. As the light reaching the photocell decreasesthe current being supplied to the programming pin of the cathode decoder/driverfrom the display dimmer circuit decreases,so dimming the display. Time multiplexing of the display dri'res is achieved by a clock sipal being fed from the microprocessorto

T C

n s

llrito?y array IOO x 14

t d d

q C2

f

e s s LSB

C3

1O-bits Fig. 2.9 Electrically alterableread only memory, e.a.r.o.m. (courtesy King Radio Corp.) Display

A1

A2

A3

A4

A6

&'AA

Anode' driver

A7

",, lf"

A5

A8

f l h l

tl'lo

.Ll. d

Cathode decoder/driver

1f

o h T h

A B C D

I l T I T B.C.D. code

Multiplexer

Dimming current

Clocft

-

Anode drive

A1

B.C.D. code

I l

r_-l

A7

l-l l-1.

l-ll-ll-t

ll

t_-t ll U. U

t-l t:l

l/1lo sec. Fig.2.l0 King KY 195 simplifieddisplaydrive block diagram

n

arulofJurrsr/rrruuP

(Al to A8) are switched scquentially. As the anode drivesare switchedthe appropriateb.c.d. information from the microprocessoris beingdecodedby the cathodedecodery'driver,the result being that the necessary segmentsof eachdigit are lighted one digit at a time at approximatelyI l0 times per second. A synchronizationpulseis sent to the multiplexer from the microprocessor every 8 cyclesto maintain display synchronization.

dcsiredsignal,ttrc resultantaudiooutput shallnot exceed-10 dB with referenceto the output prcduced by a desiredsigralonly whenmodulated30 per cent (underspecified conditions). sigtallevel/offresonance UndesiredResponses in band108-135MHzshallbe All spuriousresponses includingimage, downat least100dB otherwise, at least80 dB down. Audio Output

Characteristics The selectedcharacteristicswhich follow are drawn from ARINC Characteristic566 coveringairbome v.h.f. communicationsand SatcomMark l. Details of Satcomand extendedrangea.m. are not included. System Units l. V.h.f. transceiver; 2. modulation adaptor/modem- f.m. provision for Satcom; 3. power'amplifier- Satcomand extendedrange; 4. pre-amplifier- Satcomand extendedrange; 5. control panel; 6. remote frequencyreadoutindicator - optional; 7. antennas- separateSatcomantenna. Note: I and 2 may be incorporatedin one line replaceable unit (l.r.u.). Frequenry Selection 720 channelsfrom I l8 through 135.975MHz, 25 kHz spacing. Receivermuting and p.t.t. de-energization during channelling. 2i 5 channelselection. Channellingtime: ( 60ms. Recciver Sensitivity 3 pV, 30 per cent modulation at 1000 Hz to give S+N/N>6d8. Selectivity Minimum 6 dB points at I l5 kHz (t 8 kHz sharp). Maximum60 dB pointsat I 31.5 kHz (t l5 kHz sharp). Maximum 100 dB points at i 40 kHz (t l8'5 kHz -sharp). Qoss Modulation With simultaneousreceiverinput of 30 per cent

Gain A 3 pV a.m.sigralwith 30 per centmodulationat 1000Hz will produce100mWin a 200-500Q load. Frequmcy Response Audio poweroutput levelshallnot vary morethan 6 dB overfrequencyrange300-2500Hz. by at > 5750Hz mustbe attenuated Frequencies least20 dB. HarmonicDistortion [.essthan 7'5 percentwith 30 per centmodulation. Irss than2Opercentwith 90 per centmodulation. AGC No morethan 3 dB variationwith input signalsfrom 5 gV to 100mV. Transmitter Stability Carrierfrequencywithin t 0'005 per cent under prescribed conditions. PowerOutput 25-40W into a 52 O loadat theendof a 5 ft transmission line. Sidetone With90 per centa.m.at 1000Hz the sidetoneoutput strallbe at least100mWinto eithera 200 or 500O load. Mic. Input of Mic.audioinput circuitto havean impedance 150O for usewith a carbonmic.or a transistormic. from the (approx.)20 V d.c.carbonmic. operating supply. Antenna Verticallypolarizedandomnidirectional.

2A

,J

'i6

,$

$

To match 52 O with VSWR ( l'5 : l. Ramp Testing After checking for condition and assemblyand making availablethe appropriate power suppliesthe following (typical)'checksshouldbe madeat each stationusingeachv.h.f.

-

l. Disablesquelch,checkbackgroundnoiseand operationof volume control. 2. On an unusedchannelrotate squelchcontrol until squelchjust closes(no noise). Pressp.t.t. button, speakinto mic. and checksidetone. 3. Establishtwo-way communicationwith a remotestation usingboth setsof frequency control knobs,in conjunctionwith transfer switch,if appropriate.Checkstrengthand quality of signal.

The current and future norm is to use single sideband(s.s.b.)mode of operationfor h.f. communications,although setsin servicemay have provisionfor compatibleor normal a.m.,i.e. carrier and one or two sidebandsbeing transmitted respectively.This s.s.b.transmissionand reception hasbeen describedbriefly in ChapterI and extensivelyin many textbooks. A featureofaircraft h.f. systemsis that coverageof a wide band of r.f. and useof a resonantantennarequiresefficient antenna tuning arrangementswhich must operate automatically on changingchannelin order to reduce the VSWR to an acceptablelevel.

Installation A typical large aircraft h.f. installation consistsof two systems,eachof which comprisesa transceiver, controller, antennatuning unit and antenna. Eachof NB . Do not transmiton I 2l '5 MHz (Emergency). the transceivers are connectedto the AIS for mic.. tel. Do not transmitif refuellingin progress. and p.t.t. provision. In addition outputs to Selcal. Do not interrupt ATC-aircraftcommunications. decodersare provided. Suchan installationis shown i n F i g .2 . 1l . The transceivers contain the receiver,transmitter, H.F. Communicataons power amplifier and power supply circuitry. They are mounted on the radio rack and providedwith a flow BasicPrinciples of cooling air, possibly augmentedby a fan. A The useof h.f. (2-30 MHz) carriersfor communication transceiverrated at 200 W p.e.p.needsto dissipate purposesgreatly extends the rangeat which aircrew 300 W when operatedon s.s.b.while on a.m. this canestablishcontact with AeronauticalMobile figure risesto 500 W. Telephoneand microphone Servicestations. This beingso, we find that h.f. jacks may be providedon the front panel,asmight a comm.systemsare fitted to aircraft flying routes meter and associatedswitch which will provide a which are,for somepart of the flight, out of rangeof meansof monitoring variousvoltagesand currents. v}t.f. service.Such aircraft obviouslyinclude public Coupling to the antennais achievedvia the transportaircraft flying intercontinentalroutes,but antennatuning unit (ATU). Somesystemsmay thereis alsoa market for generalaviationaircraft. employ an antennacouplerand a separateantenna The long rangeis achievedby useof sky waves couplercontrol unit. The ATU provides, which arerefractedby the ionosphereto suchan automatically,a match from the antennato the 50 Q extent that they arebent sufficiently to return to transmissionline. Closed-loopcontrol of matching earth. The h.f. ground wavesuffersquite rapid elementsreducesthe standingwaveratio to l'3 : I attenuationwith distancefrom the transmitter. or less(ARINC 559A). Ionosphericattenuationalsotakesplace,being Since the match must be achievedbetween line and greatestat the lower h.f. frequencies.A significant antennathe ATU is invariablymountedadjacentto featureof long-rangeh.f. transmissionis that it is part of the the antennalead-in,in an unpressurized zubjectto selectivefadingovernarrow bandwidths airframe. For high-flyinga'ircraft(most jets) the ATU (tensof cycles). is pressurized,possiblywith nitrogen. Someunits The type of modulation used,and associated may contain a pressureswitch which will be closed detailssuchas channelspacingand frequency wheneverthe pressurizationwithin the tuner is channellingincrements,havebeenthe subjectof adequate. The pressureswitch may be used for many papersand ordersfrom users,both civil and ohmmeter checksor, providingswitch reliability is military, and regulatingbodies. ARINC Characteristic adequate,may be connectedin serieswith the key No. 559A makesinterestingreading,in that it reveals line thus preventing transmissionin the event of a how conflictingproposalsfrom variousauthorities leak. Altematively an attenuatormay be swit;hed in (in both the legaland expert opinion sense)can exist to reducepower. at the sametime. Light aircraft h.f. systemsin serviceare likely, for

A

Mic.

28V---2-

Tcl.

No. 1 Xmit

No. I

t.r.

No. I p.t.t. No. 2 interlock No. I interlock

28V

2av

No. 2 p.t.t.

Mic. -

Tel.

l

28vl ruoz

+

Fig 2.1I Typicaldualh.f. installatbn financial reasons,to have a fixed antennacoupler. Sucha systemoperateson a restrictednumber of channels(say twenty). As a particular channelis selected,appropriate switching takesplace in the coupler to ensurethe r.f. feed to the antennais via previously adjusted,reactivecomponents,which make the effective antennalength equal to a quarter of a wavelength,thus presentingan impedanceof approximately 50 O. The required final manual adjustmentmust be carried out by maintenance personnelon the aircraft. The antennaused variesgreatly, dependingon the type of aircraft. For low-speedaircraft a long wire antennais popular although whip antennasmay be found on somelight aircraft employing low-powered h.f. systems. The aerodynamicproblemsof wire

c,

antennason aircraft which fly faster than, say, 400 knots, haveled to the useofnotch and probe antennaswhich effectively excite the airframe so that it becomesa radiatingelement. Modernwire antennasare constructedof copper-cladsteelor phosphorbronze,givinga reduced comparedwith earlierstainless.steel r.f. resistance wires. A coveringofpolythene reducesthe effectsof precipitationstatic. Positioningis normally a single spanbetween forward fuselageand vertical stabilizer. I:rger aircraft will have twin antennaswhile a single 'V' configuration,is more installation,possiblyin a aircraft. The r.f. feed is usually for smaller common at the forward attachment via an antennamast. The rear tetheringis by meansof a tensioningunit. The aniennamaqtis subjectto pitting and erosion

of the leadingedge;a neoprpnecoveringwill provide someprotection,nevertheless regularinspectionsare called for. Protection againstcondensationwithin the mastmay be providedby containersof silicagel which shouldbe periodicallyinspectedfor a changein colour from blue to pink, indicatingsaturation. Hollow mastsare usuallyprovidedwith a water-drain path which shouldbe kept free from obstruction. The two most important featuresof the rear tetheringpoint are that the wire is kept under tension and that a weak link is providedso asto ensurethat any break occursat the rear,so preventingthe wire wrappingitself around the verticalstabilizerand rudder. On light aircraft a very simplearrangementof a spring,or rubber bungee,and hook may be used. The springmaintainsthe tensionbut if this becomes excessivethe hook will open and the wire will be free at the rearend. On largeraircraft a spring-tensioning unit will be usedto copewith the more severe conditionsencountereddue to higherspeedsand fuselageflexing. The unit loads the wire by meansof a metal spring,usuallyenclosedin a barrelhousing. A serratedtail rod is attachedto the tetheringpoint on the aircraft and insertedinto the barrelwhereit is securedby a springcollet, the grip of which increases with tension. The wire is attachedto a chuck unit which incorporatesa copperpin servingas a weaklink desigredto shearwhen the tensionexceedsabout 180 lbf. Someunits incorporatetwo-stageprotection againstoverload. Two pins of different strengthsare used;shouldthe first shear,a smallextension(3/ 16 in.) of overalllength results,thus reducingtensionand exposinga yellow warningband on the unit. Notch antennasconsistof a slot cut into the aircraft structure.often at the baseof the vertical o stabilizer. The inductanceof the notch is series-resonated by a high-voltagevariablecapacitor driven by a phase-sensing servo. Signalinjection is via matching circuitry driven by a SWR sensingservo. 'Q' Since the notch is high the input is transformed to a voltageacrossthe notch which is ofthe order of thousandsofvolts. This largevoltageprovides the driving force for current flow in the airframe which servesas the radiator. A probe antenna,which is aerodynamically acceptable,may be fitted at either of the wing-tipsor on top of the verticalstabilizer. Againseriestuning providesthe necessary driving force for radiation. The probe antenna,aswell as the wire antenna,is liable to suffer lightningstrikes,so protection in the form of a lightning arrester(sparkgap)is fitted. Any voltagein excessof approximatelyl6 kV on the antennawill causean arc acrossthe electrodesof the hydrogen-iilledsparkgap,thus preventingdischarge

through the h.f. equipment. Build-up of precipitation static on antennas,particularlyprobes,is dealt with by providinga high resistance static drain (about 6 MSl) path to earth connectedbetweenthe antenna feed point and the ATU. It is important in dual installationsthat only one h.f. systemcan transmit at any one time;this is achievedby meansof an interlock circuit. This basic requirementis illustratedin Fig. 2.11 whereit canbe seenthat the No. I p.t.t. line is routed via a contact of the No. 2 interlock relay, similarlywith No. 2 p.t.t. The interlock relayswill be externalto the transceivers often fitted in an h.f. accessory box. While one of the h.f. systemsis transmittingthe other systemmust be protectedagainstinducedvoltages from the keyedsystem.In addition,with some installations,we may havea probe usedas a transmitting antennafor both systemsand as a receivingantennafor, say,No. I system. The No. 2 receivingantennamight be a notch. It follows that on keying either systemwe will havea sequenceof eventswhich might proceedas follows. HF I keyed: l. HF 2 keyline broken by a contactof HF I interlock relay; 2. HF 2 antennagrounded; 3. HF 2ATU input and output feedsgounded and feed to receiverbroken. HF 2 keyed: l. HF I keyline broken by a contactof HF 2 interlock relay; 2. HF I probe antennatransferredfrom HF l; ATU to HF 2 ATU; 3. HF 2 notch antennafeed grounded; 4. HF I ATU input and output feedsgrounded and feed to receiverbroken. C,ontrols and Operation Separatecontrollers are employed in dual installations, eachhaving'in-use'frequencyselectiononly. Older systemsand some light aircraft systemshave limited channelselectionwhere dialling a particular channel number tunesthe system,includingATU, to a pre-assigned frequency,a channel/frequency chart is required in such cases.With modern sets,indication ofthe frequency selectedis given directly on the controller. The controlsshownin Fig. 2.1 I arethosereferred to in ARINC 559A; variationsare common and will be listed below. Mode Selector Switch. OFF-AM-SSB Thc'turn off' function may be a separateswitch or indeed may not

31

be crnployed at all; snritchingon and offbeing achievedwith the masterradio switch. The 'AM' positionmay be designated'AME'(AM equivalentor compatible)and is selectedwhenevertransmission and receptionis requiredusinga.m. or s.s.b.plus full carrier(a.m.e.). The 'SSB'position providesfor transmissionand reception of upper sidebandonly. Although useof the upper sidebandis the norm for aeronauticalh.f. communicationssomecontrollers 'USB' 'LSB'positions. have and In addition 'DATA' 'CW'modes and may be available.The former is for possiblefuture use of data links by h.f. using the uppersideband- the receiveris operatedat maximumgain. The latter is for c.w. transmissionand reception,morsecode,by 'key bashing',being the information-carryingmedium. Flequency SelectorsFrequency selecton consist of, typically, four controls which allow selectionof frequenciesbetween 2.8 and 24MHz in I kHz steps (ARINC 559A). Military requirementsare for a frequencycoverageof 2 to 30 MHz in 0.1 kHz steps, consequentlyone will find systemsoffering 280 000 'channels'meeting theserequirementsin full or 28 000 channelsmeetingthe extendedrangebut not the 0'l kHz steprequirement. When a new frequency is selectedthe ATU must adjustitselfsincethe antennacharacteristics will change.For this purposethe transmitteris keyed momentarily in order that SWR and phasecan be measuredand usedto drive the ATU servos.

Indicator A meter mounted on the front panel of the controller may be providedin order to give an indication of radiatedpower. Block Dhgram Operation Tlansceiver Figure 2.12 is a simplified block diagram of an a.m./s.s.b.transceiver.The operationwill be describedby function. Amplitude Mo dulated Transm issio n The frequency selectedon the controller determinesthe output from the frequencysynthesizerto the r.f. translatorwhich shifts the frequency up and provides sufficient drive for the power amplifier(p.a.). The mic. input, after amptfication, feedsthe modulator which produces high-levelamplitudemodulation of the r.f. amplified by the p.a. The r.f. signalis fed to the ATU via the antennatransferrelay contact. The PA output signalis sampledby the sidetone detectorwhich feedssidetoneaudio via the contact of the deenergizedsidetonerelay and the sidetone adjustpotentiometerto the audio output amplifier.

Single Sideband Transmission Low-level modulation is necessarysincethere is no carrierto modulateat the p.a. stage,hencethe mic. input, /n., is fed to a balancedmodulator togetherwith a fixed carrier frequency,/., from the frequencysynthesizer.The balancedmodulator output consistsof both sidebands f" + f^ andf" - f^, the carrierbeingsuppressed. The requiredsidebandis passedby a filter to the r.f. SquelchControl Normel control of squelch translatorafter further amplification. thresholdmay be provided. As an alternativean r.f. Ifwe consideran audio responsefrom 300 to sensitivitycontrol may be used,but where Selcalis 3000 Hz we seethat the separationbetweenthe utilizedit is important that the receiveroperatesat lowestEs.b. frequencyand the highestl.s.b. full sensitivityat all timeswith a squelchcircuit being frequencyis only 600 Hz. It follows that the filter employedonly for aural monitoring and not affecting usedmust havevery steepskirts and a flat bandpass. the output to the Selcaldecoder. A mechanicalfilter can be usedin which an input transducerconvertsthe electricalsignalinto Audio Volume Control Providesfor adjustment of mechanicalvibrations,theseare transmittedby audiolevel. Sucha control may be locatedelsewhere, mechanicallyresonantmetal discsand coupling rods suchas on an audio selectorpanel, part of the AIS. and finally convertedback to an electricalsignalby an output transducer. C:lanfter This control is to be found on some h.f. Frequencytranslationis by a mixing process controllers. With s.s.b.signalswhile the phaseof thc rather than a multiplicativeprocesssinceif the re-insertedcarrier is of little consequenceits u.s.b./, + /n' were multiplied by try'we would frequencyshouldbe accurate.Should the frequency radiatea frequencyof//(/c + /n,') rather than be inconect by, say,in excessof t 20 Hz ft + f " + /.. The amount by which the u.s.b.is deterioration of the quality of speechwill result. translated,fi, is determinedby the frequencyselected A clarifier allows for manual adjustment of the on the controller. Final amplification takes place in re-insertedcarrier frequency. Use of highly accurate the p.a. prior to feedingthe r.f. to the ATU. md stablefrequency synthesizersmake the provision To obtain sidetonefrom the p.a. stagea carrier of such a control unnecessary. would needto be re-inserted.A simplermethod, 32

Sidetonc relay

To r.f./i.f. stages

f"+ff"-

f.

Fq.2.l2 Typicalh.f. a.m./s.s.b. trmsceiverblockdiagram

which neverthelessconfirms that a sigral has reached the p.a.,is to usethe rectified r.f. to operatea sidetonerelay. When energizedthe contact of this relay connectsthe amplified mic. audio to the output audio amplifier. Amplitude Modulated Reception The receivedsignal passesfrom the ATU via the de+nergized antenna transferrelay contact to an r.f. amplifier and thence to the r.f. translator. After the translatornormal.a.m. detection takes place, the audio so obtained being fed to the output stage. A variety ofa.g.c. and squelch circuitsmay be employed.

t

Single Sideband Reception The circrit action on s.s.b.is similarto that on a.m. until after the translatorwhen the translated r.f. is fed t6 the product detector along with the re-inserted'carrier' /". The output ofthe product detector is the required audio

signal,which is dealt with in the sameway as before. Antenna Tuning Unit Figure 2.13 illustrates an automatic ATU simplified block diagram. On selectinga new frequency a retune sigrralis sent to the ATU control circuits which then: l. keys the transmitter; 2. insertsan attenuatorin transceiveroutput line (Fig.2.t2); 3. switcheson the tuning tone sigral generator (Fig.2.l2) and drivesa tune warninglamp (optional); 4. switcheson referencephasesfor servo motors. The r.f. signalon the input feed is monitored by a loading servosystem and a phasingservo system. If the load impedanceis high then the line current, /L, is low and the line voltage ZL is high. This is dctected by the loading s€rvodiscriminator which

El

I

Aru

Tune Tx Rctunc tone keY

Transceiver

p;g.2.13 Typicalh.f. a.t.u.blockdiagrarn

appL:: the appropriate amplitude and polarity d.c. sigral to a chopper/amplifier which in turn provides the control phasefor the loading servomotor. The auto transformertap is drivenuntil the load impedrnceis 50 O. Should Iy and Vynot be in phasethis is detected by the phasingservodiscriminator which appliesthe approp:iateamplitudeand polarity d.c. signalto a chopper/amplifier which in turn provides the control phasefor the phasingservomotor. The reactive elemenis,inductanceand capacitance, are adjusted untri.Il and Vy are in phase. As a result of the action of the two servo systemsa resistiveload of 50 O is presentedto the co-axial feed from the transceiver. When both servosreach their null positions the control circuits removethe signals listedpreviously. Ctaracteristics The following brief list of characteristicsare those of a systemwhich conformswith ARINC 559A. frequency Selection An r.f. nnge of 2'8-24 MHz coveredin I kHz increments. Method: reentrant frequency selectionsystem. Orannelling time lessthan I s. Mode of Operation Singlechannel simplex, upper singlesideband. g

Tlansmitter Poweroutput: 400 W p.e.p.(200 W p.e.p. operatiohal). Absolutemaximum power output: 650 W p.e.p. Mic. input circuit frequencyresponse:not more than I 6 dB variation from 1000 Hz levelthrough therange 350 Hz to 2500 Hz. Spectrumcontrol: componentsat or below /" -100 Hz and at or abovef" +29O0Hz shouldbe attenuatedby at least30 dB. Frequencystability: ! 2OHz. Shop adjustmentno more often than vearly. Pilot control (e.g.clarifier) not acceptable. lnterlock: only one transmitter in a dual system 'first-come, strouldoperateat a time on a first-served' basis,this includestransmittingfor tuning purposes. Receiver Sensitivity:4 pV max.; 30 per cent modulation a.m. (l pV s.s.b.)for l0 dB signaland noiseto noiseratio. A.g.c.: audio output increasenot more than 6 dB for input signalincreasefrom 5 to I 000 000 pV and no more than an additional 2 dB up to I V input signal level. Selectivity: s.s.b.,6d-Bpoints atf"+ 300 Hz and /. + 3100 Hz, t 35 dB pointsat f"andf" + 3500 Hz. A.m.: toensureproper receiveroperation(no adjacentch4nnelinterference)assumingoperationson 6 kHz spaceda.m. channels.

Overall response:compatible with selectivity but in addition no more than 3 dB variation between anv two frequenciesin the range300-1500 Hz (for satisfactorySelcaloperation). Audio output: two-wire circuit isolated from ground, 300 O (or less)output impedancesupplying 100 mW (0'5 Selcal)into a 600 O load.

w h e r e N= 1 2 ,1 3 . . . 2 7 ,

giving a total of sixteen tones betwecn 312'6 and 1479.1Hz. The tonesare desigratedby lettes A to S omitting I, N and O so a typical code might b,: AK-DM. The re ue 297Ocodesavailablefor assigrmentusing the first twelve tones, the addition of tonesP, Q, R and S (1976) bring the total to 10920. Codesor blocks ofcodes are assignedon Ramp Testing ard Maintenance Whilst regularinspection of all aircraft antennasis requestto air carrier organizationswho in turn assigt called for, it is particularly important in the caseof codesto their aircraft'either on a flight number or h.f. antennasand associatedcomponents. Any aircraft registration-related basis. maintenancescheduleshould require frequent Figure 2.14 illustrates a singleSelcalsystem inspection ofantenna tensioning units and tethering large passengertransport aircraft would norma,ly' points in the caseof wire antennas,while for both carry two identical systems. The decoder will probe and wire antennasthe spark gap should be recognizea receivedcombination of tones on rny of in3pectedfor signsof lightning strikes (cracking five channelswhich correspondsto that combrnation . and/or discolouring). selectedon the code selectand annunciator pa:;el. A functional test is similar to that for vh.f. in that When the correct code is recogrized the chime switch two-way communication should be establishedwith a and appropriate lamp switch is made. The lamp .witch remote station: all controls should be checked for supply is by way of an interrupter circuit so that the lamp will flash. A constant supply to the chime satisfactoryoperation and meter indications, if any, svitch causesthe chimes to sound once. Each lanrp strouldbe within limits. Safety precautions are particularly important sincevery high voltagesare holder, designatedHF I , HF I I etc. incorporatesa reset presenton the antenna systemwith the resulting switch which when depressedwill releasethe latched lamp switch and chime switch. The tone filters in the dangerofelectric shock or arcing. No personnel decoderwill typically be mechanicallyresonant should be in the vicinity of the antenna when devices. transmitting, nor should fuelling operations be in Variations in the arrangementshown and progress.Rememberwith many h.f. systemsa change describedare possible. Mechanicallythe control and of frequency could result in transmissionto allow annunciator panel may be separateunits. Should the automaticantennatuning. operatorrequireaircraft registration-related codes there will be no need for code selectswitches.the appropriatecodebeingselectedby jumper leadson Selcal the rear connectorofthe decoder. Although five resetleadswill be provided they The selectivecalling (Selcal.) system allows a ground may be connectedindividually, all in parallel to a group of aircraft using station to call an aircraft or singleresetswitch or to the p.t.t. circuit of the h.f. or vir.f. commswithout the flight crew having transmitter. In this latter caseisolation associated continuously to monitor the station frequency. (within the decoder)prevent'sneak'circuits, diodes A coded sigral is transmitted from the ground and keying one transmittercausingone or more i.e. to the tuned the v.h.f. or h.f. receiver rcceivedby othersto be keyed. appropriate frequency. The output code is fed to a The lamp and chime srrppliesshown can be Selcaldecoderwhich activatesaural and visual alerts at the operator'soption. Possibilitiesare to changed if and only if the receivedcode'correspondsto the reverse the situation and havesteady lights and the aircraft. code selectedin multi-stroke chimes,or havesteady lights and Each transmitted code is made up of two r.f. single-strokechime, in which casethe interrgpt bursts(pulses)eachof-l t 0'25 s separatedby a circuit is not used. period of 0.2 t 0'l s. During eachpulsethe The Selcalsystemswhich do not comply with per with two 90 modulated transmitted carrier is cent ARINC 596 may not providefacilitiesfor decodingof tones, thus there are a total of four tones per call; . five channelssimultaneously. A switch is provided on the frequenciesof the tones determine the code. the control panel with which the singledesired The tones available are given by the formula channelcan be selected;in this caseonly Selcalcodes receivedon the correspondingreceiverwill be fed to = antilog (0'054(/V- t) + 2O), [y

35

b L*-ntgs

Sslf test

[amp drive (5 wiresl

v.H.F.1

v.H.F.2

V.H.F.3

H.F.2

Fig. 2.f4 Typical Selcalblock diagram

the decodcr. Only one annunciator lamp is required. Codeselectionin an ARINC 596 systemis achieved by meansof a 'b.c.d.' format. Eachof the four tone selectorshas four wires associatedwith it; for any particular tone an appropriate combination of the wires will be open circuit, the rest grounded. If the

3G

tones A to S arc numbered I to 16 (0) the open wires will be as given by the correspondingbinary number; e.g.tone M-12-l l(X), so with the wires designated 8,4,2 and I we see8 and 4 will be open. Note this is termed so. not really b.c.d. but is nevertheless Testing of Selcalis quite straightforward. If

possiblea test rig,consistingofa tone generatorin conjunctionwith a v.h.f. and h.f. transmittershould he used,otherwisepermissionto utilize a Selcalequippedground station shouldbe sought. G

Audio IntegratingSystems(AlS) - lntercom Introduction All the systemsin this book exhibit a variety of characteristics but none more so than AIS. In a light. aircraft the function of the audio systemis to provide an interfacebetweenthe pilot's mic. and tel. and the selectedreceiverand transmitter;sucha 'system' might be little more than a locally manufactured junction box with a built-in audio panel-mounted amplifier and appropriateswitching. ln contraSta largemulti-crewpassenger aircrafthasseveral

sub-systems making up the total audio system. The remainderof this chapterwill be concernedwith the AIS on aBoeing747. It is unusual to considerall the systemsand sub-systems which follow as part of AIS, a term which should perhapsbe restricted to the system which provides for the selectionof radio system audio outputs and inpuis and crew intercommunications. Howevera brief descriptionof all systemswhich generate,processor recordaudio signalswill be given. The following servicescomprisethe complete audio system: l. flight interphone: allows flight deck crew to communicatewith eachother or with ground stations; 2. cabin interphone: allows flight deck and cabin crew to communicate:

Attendant's chime call

PA bverride VOR/ILSNAV systom Markerbsacon systom Low range radioaltimeter systom

Visual Pass.ent. audio (motionpic.) system

ATC system DMEsystom ADFsystem HF comnr un ication system

lSatcom I sysrem

I I

Headsets and microph,

tjgg'"q"rlj Fig. 2.15 Boeing747: typical communicationsfit (courtesyBoeingCommdrcialAeroplaneCo.)

37

3. serviceinterphone: allows ground staff to communicatewith each other and also with the flight crew; 4. passengeraddress(PA): allows announcements to be made by the crew to the passengers; 5. passengerentertainment system: allows the showing of movies and the piping of music; 6. gound crew call system: allows flight and ground crew to attract each other's attention; 7. cockpit voice recorder: meets regulatory requirementsfor the recording of flight crew audio for subsequentaccident investigation if necessary. It *rould be noted that the aboveare not completely separatesysremsasillustratedin Fig. 2.15 and describedbelow. The dividing lines between zub-systemsof the total audio system are somewhat arbitrary, and terminologl is varied; however the facilities describedare commonplace. Flight Interphone This is really the basic and most esential part of the audio system. All radio equipments having mic. inputs or tel. outputs, aswell asvirtually all other audio systems,interface with the flight interphone which may, in itself, be termed the AIS. A large number of units and componentsmake up the totd systemas in Table 2.1 with abbreviated termsaslisted in Table 2.2. Figuie 2.16 showsthe flight interphone block diagram,simplified to the extent that only one audio selectionpanel(ASP), jack panel etc. is shown. An ASP is shown in F i g .2 . 1 7 . A crew member selectsthe tel. and mic. signals required by useof the appropriate controls/switches on an ASP. The various audio signalsentering an ASp a.ie'selected by twelve combined push selectand volume controls. Each ASP has an audio bus feeding a built-in isolationamplifier. The v.h.f. and h.f. comm. ADF, interphone and marker audio signalsare fed to the bus via the appropriate selectbuttons and -volume controls. The vh.f. nav. and DME audio is fed to the bus when voice and rangeare selectedwith the Voice pushbutton; with voice only selectedthe DME audio is disconnectedwhile the vJr.f. nav. audio is pased through a sharp 1020 Hz bandstop filter (FLl) before feeding the bus. With the flii-normal switch in the fail position only one audio channel can be selected(bypassingthe amplificr) and the pA audio is fed direct to the audioout lines. Radio altimeter audio is fed direct tb the audio-out lincs. The above audio switching arrangementsare illustrated h Fig. 2.18. Note the sericsresistorsin the input

38

Table 2.1

Flight interphone facilities UPT

FlO

FIE

ASP

x

x

x

x

x

Jack panel

x

x

i

x

x

Int - R-T p.Lt.

x

Handheld mic.

x

X

llcrdrct

Jack

Jack

Boom mic. headset

x

Oxygen mask mic.

Jack

lnterphone speaker

X

OBSI

OBS2

M.E.

x

x

Jack Jack

feck

Jack

Jrct

hck

Jac*

Iack

x trck

lack Jrck

J.ct

A'X'indicates the particular unit or component is fitted at that station (column), 'Jack' indicatesa jack plug b fitted to enableuseof the appropriatemic. and/or tel.

Table2.2 Abbreviations CAPT F/O OBS m.e. -

Captain First Officer Observer Main Equipment Centre mic. - Microphone

a.s.p.int. rlt p.t.t. tel.

Audio SelectorPanel Interphone Radiotelephone hes to Transmit

- Telephone

a\dio lineswhich,togetherwith loadingresistors in the interphoneaccessory box, form an anti-cross talk network;if onecrewmemberhas,say,h.f.l selected on his ASPthen the resistivenetworkwill greatly attenuatesayh.f.2 whidr would otherwisebe audible shouldanothercrewmemberhaveselectedh.f.l and h.f.2. Six mic. selectbuttonsareprovidedon an ASP; threevJr.f. @mm.,two h.f. cornm.andPA. Additiond sritchesasociatedwith mic. selectand transmission aretheboom-mask andr.t.-int.p.t.t. on eachASPand alsop.t.t. buttonson the hand-heldmicrophones. jack panelsandthe captain'scontrolwheel (R/T-int.). To speakorrerinterphonea crewmembershould selectinterphoneusingthe r.t..int. switchon the a.s.p.which will connectmic. high(boom or mask)

c.E@{r-8..|& r^.&rEr.@*ccrd 'Eg-B

E] lr. s lucu O'El

or',a..,.r

D+-i

I

i I

ll ir r t

aaaaa

- - - - l

l l l

:

l!lri4

t t u g rrr,r l:JJ

n ti !r

!

+= :l---+i

F.--di

#--i il'F.'Li"ri-i il--r---l

t

|I

iI + Il s # - . "

|i 1,. i,..*. L-re'g!'e$!sret-l :i

i---r'"* T------1 ij-#S-.i i I

i

i

r

i Fi& 2.lt Ardb signelselectbn(courtcsyBoeiru Aeroplane Commercial Co.)

Fig. 2.16 Boeirg ?4?: night intqphonc (courtcry Boeirg CommercialAeroplaneCo.)

tr trtr trtrtr

i_p-

l l t l l

EOOM

ooooo. l-oo'l l-"i"_.] s o o--fio,, o CI

rE

,r'i

El

I t

-Gql#

rNr

rrr

o

**"

atr@

xrt

.3

r' l l -

F .

Fig.l.l7 Audioselection panel(c€urtesy Boeing Commercial Aeroplane Co.) to the interphone mic. high output feeding the flight interphoneamplifier in the interphoneaccessorybox. Alternativelythe captaincan seiectinterphoneon his control wheel p.t.t. switch which will energizerelay K2 thus making the mic. higft connectionas before. Note that the ASP r.t.-int.p.t.t. switchdoesnot rely on power reachingthe ASP for relay operation (see

f'

rrcw

rs

s*

*rcx

r|rrrftsa ro rLrcir

E r.Mro

IC'Off

{rcaet{d rrtat,da

hIGN

rg

E

Fi& 2.19 Microphone signalselection(courtesy Bocing CommercialAeroplaneCo.)

39

Fig. 2.19). Interphone mic. signalsfrom all ASh are fed to the flight interphone amplifier which combines them and feedsthe amplified interphone audio to all ASPsfor selectionas required. Pressing a mic. selectbutton on the ASP will connectthe correspondingsystemmic. input lines to relayK2 and to contactson the ASP r.t.-int. p.t.t. switch. Thus when a p.t.t. switch is pressed,the mic. lineswill be madeby either the contactsof K2 or by the ASPp.t.t. switchin the r.t. position. In Fig. 2.19 the h.f.2 selectswitch is shown as typical of all comni. selectswitches.Whenthe PA selectswitch is pressed the flight interphonemic. circuit is interruptedand PA audio is applied to the fail-normal switch; in additionthe mic. linesto the PA systemare made. Operationof any p.t.t. switch mutesboth interphone speakers to preventacousticfeedback. Cabin Interphonc The cabin interphoneis a miniatureautomatic telephoneexchangeservicingseveralsubscribers: the cabin attendantsand the captain. In addition the systeminterfaceswith the PA to allow to be made. announcements Numbersaredialledby pushbuttonson the telephonetype handsetsor on the pilot's control unit. Eleventwo-figurenumbersare allocatedto the plus additionalnumbersfor PA in subscribers, 'all-attendants' variousor all compartments,an call 'all-call'. and an Two dialling codesconsistof letters: P-Pis usedby an attendantto alert the pilot (call light flasheson control unit and chime soundsonce) while PA-PAis usedby the pilot to gain absolute priority over all other usersof the PA system. The directory is listed on the push-to-talkswitch incorporatedin eachhandsetto minimizeambient noise. All diallingcode decodingand the necessarytrunk switchingis carriedout in the centralswitchingunit, CSU(automaticexchange).The CSU also contains three amplifiers,one of which is permanently allocatedto the pilot on what is effectivelya private trunk. Of the five other availabletrunks, two are allocatedto the attendants,two to the PA systemand -onefor dialling. (Note a trunk is simply a circuit which can connecttwo subscribers.) The cabin interphoneand serviceinterphone qystemsmay be combinedinto a common network by appropriateselectionon the flight engineer's interphoneswitch panel,captain'sASP and cabin interphonecontrol unit. Any handsetmay then be lifted and connectedinto the network (dial'all-call'). In a similarway the flight interphonecircuits may be usedto make specificcallsover.thecabin interphone system.

n

Crll light

Attondrnt's .t tion3 (typrcrl)

c.:to!:S"hin! Itntcrphonc I lrudio acccrl lbox I Ft. 2.20 Boeing 7rt7: cabin interphonc (courtcsy Boeitg Commercial Aeroplane Co.) .** --o;*

The systemis more complex than has been suggestedabovebut a basic description has been given, zupportedby Fi1.2.20. ServiceInterphone A total of twenty-two handsetjacks arelocatedin variousparts of the airframein order that ground crew can communicatewith one anotherusingthe serviceinterphonesystem. The systemis rather simplerthan thoseconsideredabove. Mic. audio from 'pressto talk' depressed, are all handsets,with combinedin and amplifiedby the serviceinterphone amplifier in the interphoneaudio accessorybox. The amplified signalis fed to all handsettels. Volume control adjustmentis providedby a preset potentiometer. With the flight engineer'sinterphoneswitch selectedto ON the input summingnetworks for both serviceand flight interphonesystemsare combined. All mic. inputs from either systemare amplified and fed to both systems. Address' PassengQr The systemcomprisesthree PA amplifiers,tape deck, annunciatorpanel,attendant'spanel,PA accessory speakerswitch paneland box, control assemblies, fifty-three loudspeakers.The variousPA messages havean order of priority assigredto them: pilot's ts, attendant'sannouncements, announcemen prerecordedannouncementsand finally boarding music. All PA audio is broadcastover the speaker systemand also,except for boardingmusic,overrides

L--------

r----- -T----1

,:.*Q I

*.*A i

L-

l-*.,^f-J, .*,,"."

L__------.J fr

aLaclioir6

cricull

r!r6xr ,i[ma

I ,".",*,.1-liii',T:f..

' i''' *llF^*.; ^*' l;n:'6--'"'"" ; *,,*,* ,"*

f

euo'o

?il6

Fig,2.2l Boeing?47: serviceinterphone(courtesyBoeing CommercialAeroPlaneCo.)

stethoscope entertainmentaudio fed to the passenger emergencyannouncement headsets.A prerecorded may be initiated by the pilot or an attendant,or automaticallyin the event of cabin decompression' 'fasten A chimeis generatedwhen the pilot tums on 'no smoking' siglts. seat-belt'or addressamplifiersare fed via the The passenger flight or cabininterphone systemsfor pilot oratiendantannouncementsrespectively.Distribution of audio from the amplifiersto the speakersin various zonesdependson the classconfiguration,sincesome *noun.itntnts may be intended for only a certain classof passengers. The necessaiydistribution is achievedby meansof switcheson the speakerswitchingpanel. Audio is also fed to the flight interphonesystemfor sidetone purposes. Number 2 and number 3 ampliliers ere slavedto number I for all'classannouncements.Should be requiredthe parallel separateclassannouncements control relay is energized,so separatingthe number I audio from that of number 2 and 3. The control assembliesin the PA accessorybox contain potentiometersused to set the gain of the PA

lf,b__^-rc,

L____r (courtesy Boeing address F1g.2.22Boeing?4?:passenger Co') AeroPlane Commercial

amplifiers. When the aircraft is on the ground with ;;;i;g gearlocked down and ground powgl applied the lev-efofspeakeraudio is reducedby 6 dB' The tape deck containsup to five tape cartridges apart from the necessarytape'{rive mechanism' piaybackhead and a pre-amplifier' Boarding musicis Ltectea at an attendint'spanelwhile prerecorded imnouncementsare selectedby meansof twelve pushbuttonson the annunciatorpanel' PassengerEntertainment SYstem entertainmentsystemof the Boeing The pa-ssenger 747 andan! other modern largeairliner is perhaps also the mmt complex of 3ll airbome systems'lt is and, the systemliklly to caus€most trouble the fortunatelv, teait litcelyto affect the safety of or a fire to leads aircraft unlessbad servicing ^ loose-articlehazard. Evenon the sametype ol aircraft a variety of serviceswill be availablesince different op.r"iott will offer different entertainment the in a bid to capturemore customers' In view of is description following abovecomments, the particularlybrief and doesnot do justice to the complexitY involved.

41

Movie audio

P.A. override

Other submultiplexers Seats 1 2 3 Channel select Other seat demultiolexers

Other seat colurnns

1

2 3 Seats

Fig.2.23 Boeing747: simplified passengerentertainment system

'system', Both moviesand music are provided,the movie as can be seenfrom the schematicdiagram audio being fed to individual seatsvia the music n Fig.2.?4. The horn and flight-deckcall button are portion of the system.Ten tape-deckchannels,four locatedin the nosewheel bay while tl'reground-crew movieaudio channelsand one p.a.channel(total call(with illumination)and auralwarningbox areon fifteen) are provided usingtime multiplexing. A time the flight deck. Operation is self-explanatoryfrom interval,ternreda liame, is divided into fifteen the diagram. Should horn or chinte sound, the ground channeltimesduringwhich the signalamplitudeof crew, or flight crew respectively,will contact each eachchannelis sampled.The audiosigrralamplitudes other usingone of the interphonesystems. arebinary coded(twelve bits) and transmitted, togetherwith channelidentification, clock and sync. pulses,over a co-axialcablerunning throughout the aircraft. The music channels(five stereo,ten monauralor a mixture)aremultiplexedin the main multiplexer,the resultingdigitalsignalbeingfed to six submultiplexers CFO CiCW CALL +. in series,the final one being terminatedwith a suitable load resistor. Movie and PA audio are multiplexed with the musicchannelsin the zonesubmuliiplexers, Fig.2.24 Boeing747:groundcrewcall(courtesy Boeing eachof which feedsthree or four columnsof seat Aeroplane Conrmercial Co.) demultiplexers.Channelselectionis madeby the passenger who hearsthe appropriateaudio over his Cockpit Voice Recorder - stethoscopeheadsetafter digital to analogue conversionin the demultiplexer. Alternate zone An endlesstape provides30 niin recordingtime for submultiplexersare usedasback-upin the event of audio signalsinput on four separatechannels.The prime submultiplexerfailure (classpriorities exist if channelinputs are captain's,first officer's and flight failuresmean somepassengers must havethe engineer'stransmitted and receivedaudio and cockpit entertainmentservicediscontinued). areaconversation.Passenger addressaudio may be The controls necessaryfor activationof the substitutedfor the flight engineer'saudio in an entertainments systemarelocatedon attendants' aircraft certified to fly with two crew members. control panels. The microphoneinputs should be from so-called 'hot mics', i.e. microphoneswhich are permanently Ground Crew Call Syrtem live regardless of the setting of ASP or control Ground crew call is hardly worthy of the title column switches. The areamicrophone(which may

42

Flt. eng. hot mic. tel. Record head

lst. off. hot mic. tel.

Area Mic.

lo Q Playback I head

Pre-amp-

Erase

Test Jack

4, Landing

parking

P"::;

ffif

Essontirl flt. inst. bus bar

Ft1.2.25 Typicalcockpitvoicerccorderblockdiagram

be s.eparate from the control panel) is strategically situatedso that it can pick up night crew speechand generalcockpit sounds. While the control panelis situatedin the cockpit, ., the recorderunit (CVR) is locatedat .:heother end of the aircraft where it is leastlikely to suffer damagein the event of an accident. The CVR is constructedso asto withstand shock and fire damage,and additionally is paintedin a fire-resistantorangepaint to assistin recoveryfrom a wreck. The recorded audio may be erasedproviding the landinggearand parking brake interloik relav' contactsare closed. As a further safeguardaiainst accidentalerasurea delay is incorporited in the bulk erasecircuit which requiresthp operator to depress the 'erase'switch for two secondibefore "r"a*" commences. Test facilities are provided for all four channels,

separatelyor all together. A playbackhead and monitor amplifier allowsa satisfactorytest to be observedon metersor heardover a headsetviajack plug sockets. Pressingthe test button on the control panel or the all-testbutton on the CVR causesthe channelsto be monitored sequentially. The power supply for the system should be from a sourcewhich provideqmaximum reliabilitv. Sincethe tape is subjectto wear and thus has a limiied life, the CVR should be switchedoff when nqt in use. A suitable method would be to remove power to the CVR wheneverexternalground power is connected.

Testingand Trouble Shootingthe Audio Systems Variousself-test facilitiesmaybeprovided by which tl:l

tonesmay be generatedand heardover headsets. However,to testproperlyall switchesshouldbe operatedand all mic. and tel.jacks,aswell as speakers,shouldbe checkedfor the requiredaudio. This shouldbe sufficiently loud, clearand noise-free. Amplifier gainpresetsin accessoryboxesmay needto be adjusted. A full functional test is best done by two men, althoughit is not impossiblefor one man with two headsetsand an extensionlead to establish two-way contact betweenvariousstations. Faults can be quite difficult to find owing to the complicatedswitchingarrangements.Howeverthe wide rangeof switchingcan be usedto advantagein order to isolatesuspectunits or interconnections. Disconnectingunits providesa good method of

4

finding short circuits or howls due to coffee-induced tel.-mic.feedback(i.e. spilt liquid providinga conductingpath betweentel. and mic. circuits). Whereone has a number of units in series,e.g. demultiplexersin an entertainmentsystem, disconnectingcan be a particularly rapid method of fault-finding;it is usually.best to split the run in half, then in half again,and so on until the faulty unit or connectionis found. Continuity checkson very long cablescan be achievedby shorting to earth at one end and then measuringthe resistanceto earth at the other. The resistanceto earth should also be measuredwith the short removedin casea natural short exists.

3 Automaticdirectionfinding

Introduction Most readerswill havecome acrossthe principle on which ADF is basedwhen listeningto a transistor radio. As the radio is rotated the signalbecomes weakeror stronger,dependingon its orientation with respectto the distant transmitter. Of courseit is the antennawhich is directionaland this fact has been known sincethe early days of radio. In the 1920sa simpleloop antennawas usedwhich could be rotated by hand. The pilot would position the loop so that there was a null in the signal from the station to which he was tuned. The bearingof the stationcould then be readoff a scaleon the loop. Tuning into anotherstation gaverise to another bearingand consequentlya fix. Apart from position-fixingthe direction-findingloop could be usedfor homing on to a particularstation. This primitive equipmentrepresentedthe first use of radio for navigationpurposesand came to be known as the radio compass. The systemhas been much developedsince those early daysand in particularits operationhasbeen simplified. Within the band 100-2000kHz (I.f./m.f.) thereare many broadcaststationsand non-directional beacons(NDB). An aircraft today would have twin

Athwartships loop

receiverswhich, when tuned to two distinct stations or beacons,would automaticallydrive two pointerson an instrumentcalleda radio magneticindicator (RMI) so that eachpointer gavethe bearingof the correspondingstation. The aircraft position is where the two directionsintersect. Sincesucha system requiresthe minimum of pilot involvementthe name radio compasshascome to be replacedby automatic direction finder (ADF).

BasicPrinciples TheLoop Antenna

'l

he first requirementof any ADF is a directional antenna. Early loop antennaswere able to be rotated first by hand and subsequentlyby motor, automatically. The obviousadvantageof havingno moving partsin the aircraft skin-mountedantennahas led to the universaluseof a fixed loop and goniometer in modern equipments,althoughsomeolder types are still in service. The loop antennaconsistsof an orthogonalpair of coils wound on a single flat ferrite core which concentratesthe magnetic(H) field componentof the e.m. waveradiatedfrom a distant station. The plane

Rotot (sctrch coil)

Forc and aft loop Fb.3.l

Loop entcnnaand goniometcr

45

ol one coil is alignedwith the aircraftlongitudinal axiswhile the other is alignedwith the lateralaxis. The currentinducedin eachcoil will dependon the directidnof the nragneticfield. Whenthe plane of the loop is perpendicular to the directionof propagation, no voltageis inducedin the loop since the linesof flux do not link with it. lt canbe seen that if one loop doesnot link with the magneticfield the other will havemaximumlinkage. Figure3.1 showsthat the loop currentsflow through the stator (resolver)where.providing windingof a gonionreter the characteristics o1'eachcircuitareidentical,the magneticfield detectedby the loop will be recreated in so far asdirectionis concerned.We now effectivelyhavea rotating loop antennain the form of the eoniometerrotor or searchcoil. As the rotor turnstf,rough360otherewill be two peaksand two nullsof the voltageinducedin it. The output of the rotor is the input to the ADF receiverwhich thus sees is the rotor asthe antenna.Suchan arrangement known asa Bellini-Tosisystem. Sincewe areeffectivelybackwith a rotatingloop situationwe shouldconsiderthe polardiagramof suchan antennaaswe areinterestedin its directional properties. In Fig. 3.2 we havea verticallypolarizedt.e.m. wavefrom the direction shown. Tl.ratcomponentof the H field linking with the loop will be H sin 0, so a plot of the loop current againstI producesa sine curveasshown. The polar diagramof such an antennawill be asin Fig. 3.3. It canbe seenthat natureof the plot the nulls of the sinusoidal because arefar more sharplydefinedthan the peaks. The abovehasassumeda verticallypolarizedwave which is in fact the casewith NDBs and most

Fig,3.3 Loopaerialpolardiagram broadcaststations.Howevera verticallypolarized earth and signaltravellingovernon-homogeneous strikingreflectingobjects,includingthe ionosphere, can arriveat the loop with an appreciable horizontallypolarizedcomponent.The currentin the loop will then be due to two sources,the vertical and horizontal cornponents,which will in generalgive in the a non-zeroresultarrtnull, not necessarily direction of the plane of the antenna. This polarizationerrordictatesthat ADF ihould only be usedwith groundwavesignalswhich in the l.f./m.f. bandsare usefulfor severalhundredmiles. However. polarizedsky they arecontaminated by non-vertically wavesbeyond,say,200m at 200 kHz and 50 m at 1600 kHz, the effect beingmuch worseat night (night effect) $*o read , The SenseAntenna The polar diagramof the loop (Fig. 3.3) showsthat the bearingof the NDB will be givenas one of twtr

Plane of looP

l

tl

Direction of propagatron -------------+

/

l,/

(l \

i

E fieldO

|

.-0,,

F8. 3.2 To illustrate degrccof coupling of loop acrbl

tt6

H field

figures,l80o apart, sincethere are two nulls. In althoughnot as clearlydefinedas the nulls for the order to determinethe correctbearingfurther figure-of-eight(Fig. 3.4). information is neededand this is providedby an omnidirectionalsenseantenna. In a verticaliv polarizedfield an antennawhich is omnidireitional in Simplified Block Diagram Operation the horizontal plarreshouldbe of a type which is excited by the electric(E) field of the t.e.m. wave Automatic direction finding (ADF) is achievedby i.e. a capacitanceantenna. The output of suchan meansof a servoloop. The searchcoil is driven ro a antennawill vary with the instantaneousfield stablenull position,a secondnull beingunstable. strengthwhile the output of a loop antennavariesas the instantaneousrate of changeof field strength - The searchcoil o-utput,after amplification,is phase-shifted by 90" so as to be either in phaseor out (Faraday'slaw of inducede.m.f.). As a of phasewith the senseantennaoutput, dipending on consequence, regardless of the direction of the t.e.m. the direction of the NDB. prior to addingto the wave,the senseantennar.f. output will be in phase sensesignalthe phase-shifted loop signalis switched quadraturewith respectto the searchcoil r.f.butput. in phasein a balancedmodulator at a rate determined In order to sensethe direction of the NDB the two by a switchingoscillator,usuallysomewherebetween antennaoutputs must be combinedin such a way as a 50 Hz and 250 Hz rate. Whenthe compositesignal either to cancelor reinforce,and so either the sense is formed in a summingamplifier it will be or the loop signalmust be phaseshifted by 90.. amplitude-modulatedat the switchingfrequencysince A compositesignalmadeup of the searchcoil for one half period the two input signalswill be in output phaseshifted by 90' and the senseantenna phasewhile for the next half period they will be in output would appearas if it camefrom an antenna antiphase(seeFigure3.6). the polar diagramof which was the sum of thosefor The amplitudemodulationis,detected in the last the individual antennas.Now the figure-of-eightpolar stageof a superhetreceiver.The detectedoutput will diagramfor the loop can be thought of asbeing be either in phase,or in antiphase,with the switching generatedaswe considerthe output of a fixed search oscillatoroutput and so a further 90" phase-shiftis coil for variousn.d.b.bearings or the output of a requiredin orderto providea suitablecontrolphase rotatingsearchcoil for a fixed n.d.b.beaiing,either for the servomotor. The motor will drive either separatehalvesof the figure-of+ightwill be clockwiseor anticlockwisetowardsthe stablenull. 1v%the rdu out ot phase.As a consequence the sense When the null is reachedtherewill be no searchcoil antennapolar diagramwill add to the loop polar output henceno amplitudemodulationof the diagramfor somebearings,and subtractfoiothers. compositesignalso the referencephasedrive will be The resultantdiagramis a cardiodwith only one null, zero and the motor will stop. Should the servomotor Le in sucha position that the searchcoil is at the unstablenull the sliehtest -a--\. disturbancewill causethe motor to drive aiay fiom / t / \ \ \ \ , / l \ \ this position towardsthe stablenull. The senieof the / t l \ connectionsthroughoutthe systemmust be correct / t l \ for the stablenull to give the bearing. / \ t . A synchrotorquetransmitter(STTx),mountedon the searchcoil shaft, transmitsthe bearineto a remote indicator. '.

r \

'. \-

'l

\

l /

/ \

-\-__-i--

./

z'

Fig. 3.4 Compositepolar diagram

Block Diagram Detail Tuning Modern ADFs employ so-calleddigital tuning wherebyspot frequenciesare selected,as opposedto older setswherecontinuoustuning *.s usurl. A conventionalfrequencysynthesizeris usedto generatethe local oscillator(first l.o. if double superhet)frequency. The tuning voltagefed to the v.c.o.in the phaselock loop is alsousedfor varicap

47

Loop antenne Synchro.torque Tx o l P

S u m m i n ga m P .

Fig.3.5 An ADF simplifiedblockdiagram

tuning in the r.f. stages.Remoteselectionis by b.c'd. (ARINC 570) or someother codesuchas 215. BalancedModulator Figure3.7 showsthe balancedmodulator usedin the King KR 85. DiodesCR 1 l3 and CR I l4 areturned on and off by the switchingoscillator(Q 3l I and Q 312) so alternatelyswitchingthe loop signalto one of two sidesof the balancedtransformerT I 16. The output of Tl l6 is thus the loop sigral with its phase swiichedbetween0o and 180" at the oscillatorrate. Receiver A conventionalsuperhet receiveris usedwith an i.f. frequencyof 14l kHz in the caseof the KR 85 ; i.f. andr.f. gain may be manuallycontrolledbut in any casea.g.c.is used. An audio amp, with normal gain control, amplifiesthe detectsdsignaland feedsthe AIS for identificationpurposes.A beat frequency oscillator (b.f.o.) can be switched in to facilitate the identificationof NDBs transmittingkeyed c.w- The /t8

b.f.o. output is mixed with the i.f. so asto produce an audio differencefrequency. Good sensitivityis requiredsincethe effectiveheight of modern low-dragantennasgivesa low levelof signalpick-up' Good selectivityis requiredto avoid adjacentchannel interferencein the crowdedI'f./m.f. band. Indication of Bearing In all indicatorsthe pointer is alignedin the direction of the NDB. The angleof rotation clockwisefrom a lubber line at the top of the indicator givesthe relative bearingof the NDB. If the instrumenthas a fixed scaleii is known as a relativebearingindicator (RBD' More common is a radio magneticindicator (RMI) which hasa rotating scaleslavedto the compass heading. An RMI will give the magretic bearingof the NDB on the scaleaswell as the relativebearingby the amount of rotation of the pointer from the lubber line. Figure3.8 illustratesthe readingson RBI and RMI for a givenNDB relativebearingand aircraft heading. An RMI normally providesfor indication of

A

A

NDB 1

NDB 2

two magneticheadingsfrom a combinationof two ADF receiversand two VOR receivers.Figure 3.9 showsa typical RMI while Fig.3.l0 showsthe RMI which may circuit and typical switchingarrangements be internal or external to the RMIs.

Assumesearchcoil aligned with zero bearing

Sourcesof SystemError

NDB 2 to right

NDB 1 to left

Loop

r.t. Switching voltage

I

Automatic direction finding is subjectto a number of sourcesof error, asbriefly outlined below.

\A/\A I

ra_

hdg.

I

Balanced mod. O/P Sense r.f.

Composite signal

M,|AA

N.D.B.

A

I

AAA -iAA I

Detected Rx out Reference phase

I

l-.-1

r i r -

N.8. Waveshapes and relative time scalesare not exactly as shown. Fig. 3.6 DiagramshowingADF phaserelationships

R.M.l.

R.B.I

Fig. 3.8 Diagramof RMI and RBI readings

T116 Astable multivibrator o311-312

Fig. 3.7 King KR 85 balancedmodulator - simplified

49

Night Effect This is the polarizationerror mentio,ned previouslyunder the headingof the loop antenna. The effect is most noticeableat sunriseclr sunset when the ionosphereis changingmost rapidly. Bearingerrorsand instabilityareleastwhen tunedto an NDB at the low end of the frequencyrangeof the ADF. CoastalRefraction The differing propertiesof land and waterwith regardto e.m.groundwaveabsorption leadsto refractionof the NDB transmission.The effectis to changethe directionof traveland so give riseto an indicatedbearingdifferent from the actual bearingof the transmitter. Mountain Effect If the wave is reflected by mountains,hills or largestructures,the ADF may measurethe direction of arrivalof the reflectedwave. The nearerthe reflectingobject is to the aircraft the greaterthe error by the geometryof the situation. Stotic Interference Static build-up on the airframe

Fig.3.9 KNI 581 RMI (courtesyKing RadioCorp.) ADF No. 1

VOR No.l

ADF No

No. 2.

No.1.

cto

cto

No. 2. R.M.l

26V 4OO Hz Ref.

t ---/ / Red

No'1 R'M'l' ,/tr\ (w E) \9/ A_-A

Fig. 3.10 Radio magneticindicator: simplified circuit

50

VOR No. 2.

and the consequentdischargereducesthe effective rangeand accuracyof an ADF. Thunderstormsare alsoa sourceof static interferencewhich may give rise to largebearingerrors. The ability of ADF to pick up thunderstormshasbeenusedby one manufacturerto give directionalwarning of storm activity (Ryan Stormscope). Vertical or Antenna Effect The vertical limbs of the crossedloopshavevoltagesinducedin them by the electriccomponentof the e.m.wave. If the planeof a loop is perpendicularto the direction of arrivalof the signalthere will be no H field coupling and the E field will induce equalvoltagesin both vertical limbs so we will havea null as required. Should, however, the two halvesof the loop be unbalanced,the current inducedby the E field will not sum to zero and so the direction of arrival to give a null will not be perpendicularto the plane of the loop. An imbalancemay be due to unequalstray capacitanceto earth either sideof the loop; howeverin a well-designed Bellini-Tosisystem,where eachloop is balancedby a centretap to earth, this is not a severe problem. Station Interference When a number of NDBs and broadcaststationsare operatingin a given areaat closelyspacedfrequenciesstation interferencemay

result. As previouslymentionedhigh selectivityis requiredfor adequateadjacentchannelrejection. Quadrantal Enor (QE) It is obvious that the two fixed loops must be identicalin electrical characteristics, asmust the stator coils of the goniometer. If the signalarrivesat an angle0 to the planeof loop A in Fig. 3.1I the voltageinducedin loop A will be proportionalto cos0 and in loop B to cos(90-d)= sin0. If now the searchcoil makesan angled with the stator P then the voltageinducedin the searchcoil will be proportionalto (cosOX cos@)- (sinOX sin@)providedthere is no mutual couplingbetweenthe interconnectingleads. So when the searchcoil voltageis zero: cosOXcos@=sinOXsin@ or: cot0 = tan0 and: 0=6+ 90+ifX 180 wherey'y'is 0 or any integer. This is simply a mathematicalmodel of the situationpreviously ddscribedunder the headingof the loop antenna. Now considerthe two loopsnot electrically identicalso that the ratio of the maximum voltages inducpd in the two loops by a given signalis r. The condition for zero voltagein the searchcoil is now:

Direction of arrival

Fig. 3. I I Diagram showingsearchcoil signalas a function of direction of arrival

51

cotO=rXtan@' when 0=O

cotp=o

circuits and the loop connectionswill lead to errorsin the searchcoil Position.

lnstallation

therefore A typical transport aircraft ADF installationis shown i n F i g . 3 . l 2 ; N o . 1 s y s t e mo n l y i s s h o w n ,N o ' 2 b e i n g sirnilir except that different power bus barswill be when used. Main power is 28 V d.c', the 26 V, 400 Hz cot0=0 0=90 being usedto supply the synchros' lt is vital that the 26 V 400 Hz fed to the ADF receiveris from the therefore samesourceas that fed to the RMI' tan6'=0 so 0'=0+NXl80 The loop antennaand its connectingcableform g = part of the input circuit of the receiverand so must 180 or 270) we (alsowhen In thesetwo cases p = n o ' s t t iravea fixed known capacitance(C) and inductance 0 h a v et h e s a m es i t u a t i o na sb c l o r el . e . (L). This being so the length and type of loop cableis error. the so error' an ipecified by the manufacturerof the loop. The will be there angles At intermediate not be exceeded,but it can be bearingindicatedby the searchcoil will be incorrect' length specifiedmust compensatingC and L are provided in tlnee shorter value made Sincethis type of errorhasa maximum the circuit. in placed correctly error' quadrantal eachquadrantit is called equalizercontainsthe loop corrector r'f' The will cause QE NDB the from wave t.e.m. Now the to compensatefor a components reactive currentsto flow in the metalstructureof the aircraft' necessary provide to and QE correction' A cable loop the from short Eachof the loops will receivesignalsdirect 3.13. Cl,C2,Ll,L2 Fig. given in is ciicuit typical airframe' the trom signals NDB and alsore-radiated (loop una C:, C4,L3, L4 providecompensation Sincethe aspectratio of the aircraft fuselageand correction provide L6,L7 L5, QE while equalization) energy wingsis not I : I the effect of the re-radiated stator of appropriate the in current the attenuating by equivalent is on th. t*o loopswill be different:this is equalizer loop the goniometer.The QE corrector to makingtwo physicallyidenticalloopselectrically to the IooP. dissimilar.The resultingquadrantalerror could be up mounted close Similar considerationsapply to the senseantenna to 20" maximum. to can be nradeby usinga which is required to presenta specifiedcapacitance Fortunately,compensation of cable given length a we have Again receiver. the possibly and QE QE correctorloop equalizer the combined which must not be exceededbut can be madeshorter correctionbuilt into the loop. Nt>rrnaliy an iqualizeris fitted. Often both an r.f. field producesa gleatervoltagein the longitudinal proviclecl are usedto achieve and a suscepti-former equalizer identical' loop than in the lateralloop if the loopsare receiver'The the to capacitance input statetl the more have antennas This beingthe casesomeloop devicewhich matching passive susceptlformeris a turnson the lateralloop than the longitudinallocp, the effective to increase transformer auto utilizesan typical correctionUelngt Z|' in the middle of the antenna.Typicalunitsare sense the of capacitance quadrants. shbwnin Fig. 3.14. As an alternativethe necessary may be achievedin a single Loop Alignment Error If the longitudinalloop plane matchingand equalization matching/equalizing The coupler. senseantenna is not parallelto the aircraft longttudinalaxis then a the antenna' to close mounted are unit(s) constantloop alignmenterrorwill exist. Tire loop antennawill consistof the crossedcoils wound on a ferrite slab and encapsulatedin a Field Alignment Error If the loop antennais offset low-draghousing. On high-speedaircraft the loop will from the aircraft centreline the maxima of the zeros' be flush with the skin but on slower aircraft the will the as quadrintal error will be shifted, housingmay protrude slightly, givingbetter signal Consequentlythe situation wherethe NDB is at a pick-up. relativebearingof 0, 90, 180pr 2?0o will not give The senseantenna can take many forms' On large zeroerror. is c-apacitiveplate Ft transport aircraft a suppressed 'towel rail' a aircraft comrnon,whereason slower Loop Connector Stmy Coupling Reactive coupling type of antennamay be used. Generalaviation external between or connections the loop between t a n @ ' = es o

52

Q'=90+NXl80

N o .1 2 8 V d . c .

4OOHz

Panel lights supply

I

Corrector box

No. I VOR

From No. 2 ADF or No. 2 VOR

Compass hdg

Sense aerial Fig.3.l2 TypicalADF installation

o i -cc

Fig. 3.13 Quadrantalerror corector/loop equalizer (straight-through connectionsnot shown)

aircraft might usea wire antennaor, asan alternative, a whip antenna. Somemanufacturersnow producea combinedloop and senseantennafor the general aviationmarket. The position of both antennasis important. The loop shouldbe mounted on, and parallelto, thecenire line of the aircraft with nomore than 0'25" alignmenterror. While the loop may be on top or

Sense ae. cable equalizer lnsulated sense ae. terminal

lnner screen Fig; 3.t4 Senseaerial matchin!

53

bottom of the fuselage it shouldnot be mountednear the nose,tail, largeor movableprotuberances or near othersystemantennae.Similarconsiderations apply to the senseantenna,althoughbeingomnidirectional alignmentis not a problem. Ideallythe senseantenna will be mountedat the electricalcentreof the aircraft in orderto giveaccurateover-station turn-aroundof the bearingpointer. The interconnectionsin the systemmust take into Fig. 3.15 ARINC 570 control panel (typical) accountthat the phasingof voltagesproducedby senseand loop antennaswill be different for top and bottom mounting. The methodusedwill dependon the manufacturer but if the systemconformsto Functiort Switch. OFF-ANT-ADF In the antenna ARINC 570 the synchrorepeaterconnections will be position(ANT) the receiveroperatesfronr the sense asin Table3.1. If, asin somelight aircraft a n t e n n ao n l y , t h e b e a L i n p g o i n t e rb e i n gp a r k e da t 9 0 " r e l a t i v e p o s i r i o nm a y b e u s e dl b r b e a r i n g . T h i : Table3.1 Synchroconnectionsfor alternateaerial t u n i n g N D B , l s t l t i o n a n d i d e n t i f i c a t i o nI.n t h e A D F locations.Indicatorsynchroreceivercorrections positionsignalsfrorn both loop and senseantenna p r o v i d en o l n r a lA D F o p e r a t i o nt,h e R M I i n d i c a t i n g Aerial position Bottom Bottorn Top Top t h e b e a r i n go l ' t h e s t a t i o n . loop, loop, loop,

ottO'oo'

SI Synchro 52 transmitter S3 corrections Rl R2

bottorn sense

top scnse

top sense

sl s2 s3

sl s2

S3 S2

S3 R2 RI

SI RI R2

RI R2

loop, bottom sense S3

s2 sl R2 RI

Fretluenq,St,/ecl(rrobs Threeknobsareused;one is nrorrntedco-ariallywith the functionswitch,to s e l e c ft r e q u e n c yi n, 0 . 5 , l 0 a n d 1 0 0k H z i n c r e m e n t s . D i g i t r l t v p e t ' r e q u e ny ed i s p l a ys e g m e n ti sn d i c a t et h e selectedflr'qrrencv.The informationis passedto the r e c e i r eur sp l r l l k ' l b . c . t i .

Ilcat F-requt'rtct'Oscillatr'r Sx,itch Selectsthe BFO installations,the goniometeris in the indicator and lirr useu'henthe NDB selec:ted is identifiedby the bearingis presented directlyratherthan by synchro t ' r n - o lk- ie-r i n g o l ' t h e ea r ri e r . feed then the following correctionsare necessary: A nurrrberof other su,itches nraybe found on v a r i o u sc o r r t r o l l e r as s. b L i e l l yd c s c r i b ebde l o w . l. loop from top to bottom: longitudinalcoil connections to goniometerstatorreversed: Ftur<'tit,rrSrt,irclr. OFI-'-ANT-ADI:-l.OOPAn extra 2. sensefrom top to bottom: searchcoil p o s i t i o nr r l ' t l r ef u n c t i o t rs w i t c hn t a l b e p r o v i d e dt o conneCtionS reverSed. ()pr'rilethe receiverfronr the loop ;rc.rial only. This Obviouslyone must check fcrrwhich positior.r.t()p (rr p o s i t i o nL . O O P .w o u l db e u s e di n c o n i u n c t i o n with a bottom, the connections arernadt-in tht supplicd I o o pc o n t r o l . unit. Protectionfrom interferenceis of vital intportance. Ittop Corttrol Springloadedto ot'f. Whenop!'rated and to this end adequatescreening of cablesshouldbe clockwiseor anticlockwise the searchcoil rotatesin employed.ARINC 570 callsfor four individually t h e s e l e c t e d i r e . c t i o nT. h i s c o n t r o lc l n b r .u s e df o r shieldedco-axialcablesinsulatedand twisted.then n r a n u adl i r e c t i o n - l i n d i n tgh. e s- e a L ccho i l b e i n gr o t a t e d jacketed.The senseantennaconnectorshoulduse until an audionull is achit'vedor. il'pr,rvidcd.I visual doubleshielding(tri-axial)cable. Thc.cableruns t u n i n gi n d i c a t o ri n d i c a t eus n u l l . A l t h o t r g hr t o t u s e d shouldbe clearof any high-leveltrlnsrrrittingcables in most rnodernequipmentsthis doeshuvethe or a.c.powercables. advantage overADF that the nulls arr.sharper;ADF operationwould lraveto be usedto scnsethe correct null. Controls and Operation Gain Cotttntl An auiliogaincontrol is usually A standardARINC 570 control prrrcirsillustratedirr providedand rnayhe annolatedvolume. On at least F i g .3 . 1 5 . one svstr'nrthe gain0f'tht' R.F. ampsis manually 54

adjustablewherrANT or LOOP is selected,whereas audio gain is controlled on ADF. Beat Frequency Oscillator Tone A rotary switch, givingb.f.o. on-off,and a potentiometermay be mounted on the sameshaft turned by the b.f.o. control. Whenswitchetlon the frequencyof the b.f.o. canbe adjusted,so varyingthe tone in the headset. heselect Frequency Capability Provision can be madefor in useand standby frequenciessele'cted by meansof a transferswitch. Whenfrequencyselection is madeonly the standbyfrequencychanges. Switchingthe transferswitch (TFR) will now reverse the rolesof in-useand standby frequencies.Both frequenciesare displayedand clearannunciationof which is in useis required.

Characteristicrs The following characteristicsare selectedand summarizedfrom the ADF SystemMark 3 ARINC 570. Frequency Selection Range:190-1750kHz; spacing:0.5 kHz; channelling time lessthan 4 s; parallelb.c.d.frequencyselection with provisionfor serialb.c.d. ADF Accuracv + 2o excludingq.e. for any field strengthfrom 50 !V/m to 100000 pV/m, assuminga senseaerial quality factor of 1.0. (Senseaerialquality factor = effectiveheight X squareroot of capacitance, i.e.ii-root-cap). 13- excludingq.e.for a field strengthaslow as 25 pYlm. + 3" after q.e.correction. ADF Hunting k s s t h a nI 1 " . Table 3.2 Station interferenceconditions.with referenceto desiredfrequency Undesiredfrequency

Unde sired signal stre ngth

t2kHz t3kHz t6kHz t7 kHz

-4 dB -10 dB -55 dB -70 dB

Sensilivity Signal+ noiseto noiseratio 6 dB or better with 35 pV/m field strengthmoduiated30 per cent at = l'0. 1000Hz and l.ri-root-cap Station Interference An undesiredsignalfrom a source90o to that ofthe. desiredsignalat the frequenciesand relativcsignal levelslisted in Table 3.2 shallnot causea changein indicatedbearingof more than 3". Receiver Selectivity Passband at leastl'9 kHz at -6 dB pointsnot more than 7 kHz at -60 dB points. Resonantfrequency within 1 175 Hz of selectedfrequencv.

Calibrationand Testingon the Ramp Loop Swing The procedurefor determiningthe signand sizeof errorsin an ADF installationis known as a loop swing. On initial installationa swingshouldbe ;arriedout at l5o headingintervals.Checkswings should be carriedout whenevercalledfor in the maintenanceschedule,after a lightning strike, when an airframemodification closeto the ADF antennais completedor when a new avionicsystemis installed. The checkswingis carriedout at 45o intervals. A sving should not be carriedout within + 2 h of sunset or sunriseto avoid night effect. The loop swingmay be carriedout in the air dr on the ground. The advantageof an air swingis that the aircraft is operatingin its nclrmalenvironmentaway from external disturbancesbut, in someways, a ground swingis to be preferred,sincereadingsmay be taken more accurately. If the loop is mounted on the bottom of the t'uselage the swingmay be affectedby the closeproximity of the ground,in which casean air swing should be carriedout. An installation should be checkedby air test after q.e.shavebeen corrected. Ground Swing A groun{ loop swing must be carried out at a site known not to introduce bearingerrors. A basesuitablefor compassswingswill not necessarily be suitable for loop swings. A survey using portable direction finding (D/F) equipmentmust be carried out if the site is doubtful. The loop may be swung with referenceto true or magneticnorth. Using true north hasthe advantage that the loop swingingbasemay be permanently marked out. If the swing is with referenceto magretic north the loop should be calibrated using a

55

datum compass,suchas the medium landingcompass' whic,hshouldbe alignedwith the longitudinalaxis of the aircraft and positionedabout 100 ft from the aircraft. To sight the longitudinalais, sightingrods or clearlyvisibleplumb linesmay be fixed to the aircraft centreline. Use of an upright nose'mounted propellerand the verticalstabilizermay suffice providingsightingis carriedout carefullyfrom a suitabledistance. For a checkswingthe aircraft gyro magneticcompassmay be usedprovidedthis hasbeen recentlyswung,correctedand a calibrationchart madeout. The aircraft must contain its full complementof equipment. Doors and panelsin the vicirfity of the ADF antennasmust be closed. Internal power suppliesshouldbe usedwheneverpossiblesincethe externalgeneratorand lead may causeerrorsin the readings. The ADF is tuned to a station or NDB within rangeand of a known magneticbearingfrom the site. With the aircraft on the requirednumber of headings the ADF readingand the aircraft magneticheading

are recordedon a loop swingrecord chart, a specimen of which is shownasTable3.3. The correction(D) is the sigred anglewhich must be addedto the indicatedmagneticbearingof the station (B + C) in order to give the true ma^gnetic bearingie). So, for example,adding-5'5o to 41" + 354' gives389'5' = 29'5oasrequired. When completedthe valuesobtainedin the final column shouldbe plotted as a q.e- correctioncurve, of the absolute asshownin Fig. 3.16. The average of correction amount gives the peaks the of values required.the polarity being givenby the sign of the correctionin the first quadrant. So in the example givenwe have: 12.5+ 12.5+ 16 + l7'5 = +14'625" 4

asthe requiredcorrection. The correctionis made by suitablechoiceof componentsin the QE corrector loop equalizeras instructedby the manufacturer. The correctionshouldbe more or lessthe samefor identicalinstallationsin a particularaircraft type. Once the prototype hashad a calibrationswingand the componentvaluesare chosen,subsequentswings Table 3.3 Loop swing recordchart on seriesaircraft shouldshow the error boundedby I 3" asrequired. A/C Type............-.. A/CTailNo ............. Loop alignmenterror is givenby the averageof the Time10.00 Base'..'...........-......'. Date....................,.... (A)029'5 peaks. So in the examPlewe have: Station Droitwich Freq.200kHz Mag'Brg

Magneticheading datum compass

(B) 028 041 054.5 073 089 105'5 r22 135 153 t72 184 198 213 230 242 257 271.5 286 302 317 332 34s 358

s6

Autornattc direction linding relative bearing

Correction (D)

001.5 354 346 333 318 298 272 247 224 206 197 187 179 169 160 r48 134 115 088 063

0 -5.5 -l1.0 - l6'5 - 17.5 -14.0 -4.5 7-5 12.5 I1.5 8.5 4.5 -2.5 -9.5 -12.5 - 15.5 - 16.0 -11.5 -0.5 9'5 12.5 t2'5 ' 10.5

(c)

u5 032 02l

l2'5 - 16+ l2'5 - l7'5 = -2'125 4 Sincethis is in excessof t 0'25o this error should be taken out by re-alignmentof the loop. The line correction= -2'125 has beendrawn on Fiq. 3.16. The correctioncurveshouldcrossthis line atb, gO, 180 urd 270' if thereis no field alignment error. Within the limits of the accuracyof the plot and the scalewe can seethat this is the casein our example.If therewerea field alignmenterror it would be measuredalongthe horizontal axis. Air Swing An *ir swing should be carried out in smooth air conditionsin order to eliminatedrift errors. There are variousmethodswhich may be employedbut all involveflying a particular pattern over a clearly definedpoint or points sgmedistance from the transmitterwhich is to be usedfor the swing' Magneticheadingand ADF relativebearingare noted forl number of headings,every 10" or I 5o,depending on the pattern flown. The aircraft should be inland of the transmitterwhen readingsare taken to avoid coastalrefractionproblems' Recordingand plotting is as for the ground swing.

various headinp when overhead of the referenoe point. SenseAntenna CapacitanceCheck The total capacitanceof the senseantennaand feeder shouldbe checkedwhen calledfor in the maintenance schedule.wheneverthe senseantennaor feederis changed,upon initial installationor if a possiblefault condition is suspected.A capacitancebridgeor Q meter operatingat 650 kHz shouldbe usedfor the measurement. Functional Test Ideally there will be at leastone station or NDB within rangein each quadrant; in busy regionsthis will ccrtainly be so, for example,at London Heathrow more than twenty beaconscan be receivedunder good conditions. The true magneticbearingof those beaconsto be usedmust be known. An accuracy checkis performedby tuning into a beaconin each quadrantand ensuringthat for eachthe pointer Fig.3.16 Quadrantal errorcorrection curvefor Table3.3 indicatesthe bearingto within the limits laid down in the procedure,say t 5o. The figure givenwill only be achievableif the aircraft is well away from largemetal Two possiblemethodsare position-lineswinging objects,and if the test is not carriedout within 2 h of and single-pointswinging. With the first method a give ground seriesof landmarkswhich a referenceline sunriseor sunset. Useof externalpower may also giveerroneousreadings. alignedwith a distant transmitterare chosen. A zig-zagpattern is flown, both toward and away from All controls shouldbe operatedto ensurecorrect the transmitter,the readingsbeing taken'asthe functioning. In particularif a loop control is^ aircraft crossesthe line at variousheadings.With the providedthe pointer shouldbe displaced170" first secondmethod a clearly defined point, some distance clockwiseand then anticlockwisefrom the correct from the transmitter,is chosen. A cloverleaf pattern reading,to which the pointer shouldreturn without hunting. is flown centredon this point, readingsbeing takenon excessive

6'

4 V.h.f.omnidirectionalrange

(voR) lntroduction

Prior to World War II it was realizedthat the propagationanomalies with low- and experienced medium-frequency navigationaidslimited their usefulnes3 asstandardsystemsfor a sky which was becomingevermore crowded. A systemcalled four-course low-frequencyrangewaswidely in the United Statesduringthe 1930s; implenrented this gavefour coursesto or from eachgroundstation and fltted in quite nicelywith a systemof fixed airways.A problenrwith the four-coursesystemis that eachstationonly providesfor two intersecting airways;a more complexjunction requiresrnore courses.The above,coupledwith increasedaltitude of flying rnakinglineof-sightfrequencies usefulat longerranges,and the developtnent of v.h.f. comms, led to the adoptionof v.h.f.omnidirectionalrange ( V O R ) a ss t a n d a r d i n t h e U n i t e dS t a t e si n 1 9 4 6a n d internationallyin 1949. The competitionftrr an internationalstandardsystemwas fierce,the leading contenderafter VOR beingDeccaNavigator.lt is debatablewhetherthe technicallysuperiorsvstem waschosen,but certainlyVOR wascheaper.had the advantage of a largehome market,and hasdone the job adequatelyeversince. The VOR systemoperatesin the 108-l !8 MHz band with channelsspiced at sO kHiffiT[ sharedwith ILS localizertt. ffiing allocatedto 160of the 200 availablechannels.Of these160 cfrannels12Oareallocatedto VOR stationsintended for en route navigationwhile the other forty are for tClrn!ryLYQR statim!_(TVORL The output porver o f a n e n r o u t es t a t i o nw i l l b e a b o u t2 0 0 W p r o v i d i n s a serviceup tL49-laulieal n,iles.itTfleouency wilf b e w i t h i n t l r eb a n d I l 2 - l l 8 M H z . A T V O R w i l l h a v e an output power of Ebgnl5)-t{-providing a serviceof up to about ]5_!sg.l[salmiles,its frequencywill be

to, or departurefrom, a stationon a particular bearing,steeringinformationcanbe derivedfrom the receivedVOR signals.It is this latter facility which makesVOR so usefulin airwaysflying, stationscan Victor airways be placedstrategicallyalongso-called and the pilot can then, by selectionof the appropriate radials,fly from stationto stationeitherby obeying steeringcommandsor by feedingthe sameto the autopilot. To obtain a position fix liom VOR one needs bearingsto two separatestationstwhen usedin this way VOR can be considereda theta-tl-reta system.If a VOR stationis colocated with a DME stationan \ aircraftcan obtain a fix usingthe pair asa rho-thetai system.The VOR/DME systemircurrentliTil internationalshort-range navigationstandard.ln recentyearsthis systenrhasbecomeevenmore versatileu'ith the adventof airborneequipmentwhich caneffectivelyrepositionan existingVOR/DME s t a t i o nt o g i v ea ' p h a n t o mb e a c o n ' c o m p l e twei t h radialswhich can be flown usingVOR-derived steeringinforrnation. This developmentis considered i n C h a p t e r1 2 .

Basic Principles

A sinrpleanalogyto VOR is givenby imagininga which enritsan omnidirectionalpulseof lig;hthouse light everytime the beam is pointingdue north. If the speedof rotation of the beamis known, a distant observercould recordthe time intervalbetweerr seeingthe onrnidirectionalflashand seeingthe beam, and l'rencecalculatethe bearingof the lighthouse. In realitya VOR stationradiatesv.h.f. energy modulatedwith a retbrencephasesignal- the omnidirectionallight - and a variablephasesignalthe rotatingbeam. The bearingof the aircraft within the bundI9&I2_I4I&, thisbelngthe part of dependson the phasedifferencebetweenreference thetotalbandshared with ILS localizer. and variablephases-- time differencebetweenlight The crew of an approp.iatelyequippedaircraftcan a n db e a m . tune into a VOR stationwithin rangeand readthe The radiation frorn a qgnventionalVOR (CVOR) bearingto the stationand the relativebearingof the stationis a horizontallypolarizedv.h.f.wave station. Should the flight plan call for an approach modulatedas follows:

58

$el{*,!

Sin pt coc

\ a/

Cosnt Cos qrt

rcos I

I

I

I I

2r.3OHz 2n.1"MHz

Fig.4.l Groundstationblockdiagram, v.o.r,

t . 30 Hz a.m.: the variablephasesignal. at Xo magneticbearing/roz the station the variable 2 . 9 9 6 0 H 2 a . m . : t h i s i s a z u b c a r r i e r f r e q u e n c y , p h a s e w i l l / a g t h e r e f e r " e n c e p n u t " u y i ;r.i g u r e s + . t ,

modulatedat 30 Hz with a deviation of 1480 Hz. The 30 Hz signalis the reference phase' 3 . l02Q Fz a'm': identification signalkeyed to providemorse code identification at least threegjnes each39-s. Wherea voR and5ffitreco-locatedthe identificationtransmissions are synchronized (associated identity,see Chapter7). 4. Voice a.m.: the VOR systemcan be usedas a ground-to-aircommunicationchannelas long as this doesnot interfere with its basicnavisational function. The frequency rangeof the vo]ce modulation is limited to SQOXZ-

4.2 and4.3 illustrate th. b.ii" priniiples. The airborneequipmentreceivesthe composite signalradiatedby ihe station to which the receiveris tuned. After deiection the variousmodulatingsignals are separatedby filters. The 30 Hz referenceiigrul is Variabled 3O Hz a.m.

The 30 Hz variablephaseis spacemodulated in that the necessaryamplitude variation in the received signalat the aircraft is achievedby radiatinga cardioid patternrotatingat 1800 r.p.m. The frequency modulated9960 Hz sub-carrieramplitude modulates the r.f. at sourcebefore radiation. It is arransedthat an aircraft due north of the beaconrryillreceiie variableand referencesignalsin phase,for an aircraft Flg 4.2 Frequency spectrum:CVOR spacesignals

59

:l

1 ,s I PHASE SIGNAL (FM) ALL RAOIALS

REFER€'{CE

V A R I A E L €P H A S E S I G N A L( A M )

ib

sEcoNo

FESULI ilI PATTERil ROTATTNG

I O oR A D i A L I

UNIO0ULATE0

996otsr SU8CAnRIER FR€O MOO AT 3oxt

ROfAT ING

oIPOLE PATT€RI

I

+sEcoro -

stcono

-"2to'

-r-

r-f-F-1o

r tttono

a

I

R E f E R E T C EP H A S € V O L T A G E I A F T E Rf X O E T E C T I O N )

= ' . c

<

Z

l

VOLTAGESAT AIRCRAFT ON 24O' RADIAL

a

240"

.l vaRrAEL€ IHASE VOLTIGE o TAFTERAT OETECTIOtr)

. o r R € c T l O NO F P O Sr r V E L O S E ATTENNA OF ROTATING

CVOR (courtesyKing Radio Fig 4.3 Phaserelationships, Corp.)

be presentedto the pilot' Figure 4'4 variablesignal,the difference station ian the with phase compared stationis the 'ln itfurt*t.t thatihe relativebearingto the the station ohaseeivinethe bearingfrom the station' The Aiii.r.n.. betweenthe magneticbearing-to to actualreiding presentedto the pilot is the bearing to.display used is RMI An .na tn. aircraftheading. in in the station ralher than from, so if the difference the information. Suchinstrumentsare considered 135' is signal ohasebet*een variableand reference drivenby is card the application this In 3. 'to' bearingwould be 135 + 180 = 315", asshown air;;i.r ift" readingat th. .n*putt, asnormal,so that the card in -- Fig.4.4. the same At heading' lubber line is the aircraft information (heading)is combinedwith ttre liio.p*s by determined position a to titnt u pointtr is driven the the VORderived bearingthe relativebearingof 60

fly-left or fly-right signalsare derived and presentedto the pilot. A complicationis that radial information depends 135"(Froml only on the phasedifferencebetweenmodulating signalsand is independentofheading; hencethe fly-right or fly-left information may sendthe aircraft 'long way the round'. Further, when an aircraft is on course,i.e. the steeringcommandis nulled, the aircraft may be headingeither toward or away from the station on the selectedradial. A TO/FROM indication removesthe ambiguity. With the aircraft heading,roughly, towards(away from) the station and the TO/FROM indicator indicatingTO (FROM), the steeringinformation givesthe most direct path in order to intercept the selectedradial. If the referencephase(R) is phaseshiftedby the selected course(C) and then comparedwith the Fig.4.4 To/frommagnetic bearings andrelativebearing variablephase,a fly-right indicationwill be givenif R + C lagsV, while if R + C leadsV, the command will be fly-left. If we now add 180 to the the differenceb€tweenthe bearingto the station and phase-shifted relbrencephasewe haveR + C + 180 the heading. A differentialsynchroor resolveris used will, addition, either cancelV, partially or which on to give the requiredangulardifference. Figure4.5 completely,in whict casea TO indicationwill be strowsthe RMI presentationcorrespondingto the situationdiagramshownin Fig.4.4. Only one pointer given,or reinforceV, partially or completely,in which casea FROM indication will be given. is shown,for clarity. 'automatic' Figure4.6 showstwo possiblesituations. ln both The previoustwo paragraphsrefer to casesthe selectedcourseis 042,i.e. the pilot wishes VOR, so calledsincethe pilot needdo no more than to fly towardsthe station on the 222 radial or away svitch on and tune in to an in-rangestation in order 'Manual'VOR from the station on the 042 radial. With aircraft A to obtainbearinginformation. we havea fly-left and a TO indication;with aircraft B requiresthe pilot to selecta particularradial on which we havea fly-right and a FROM indication. Note that he wants to positionhis aircraft. The actual radial on if the headingsof the aircraft were reversed,the which the aircraft is flying is comparedwith the indicationswould be the satne,so sendingthem the desiredradial. If the two are different the appropriate 'long way round'. Figure4.7 showsan electronic deviationindicator correspondingto aircraft B. The indication at top right showsthe aircraft to be on the 022 radialfrom a secondVOR station.

DopplerVOR (DVOR)

Fig. 4.5 RMI presentation

The useof CVOR leadsto considerablesite errors where the station is installedin the vicinity of obstructionsor where aircraft are requiredto fly over mountainousterrain while usingthe station. The error is causedby multi-path receptiondue to reflectionsfrom the obstructions,and givesrise to coursescalloping,roughnessand/or bendswhen the aircraft is flown to follow steeringcommands.The terms useddescribingthe courseunder these conditionsrefer to the nature of the departurefrom a straightline course. DVOR is relativelyinsensitiveto siting effectswhich would renderCVOR unusable. Although the method of modulationis completely different DVOR is compatiblewith CVOR in that

61

Flv right O42 (Froml

Fly left

From

R+420

v.From fly right (R + 42' lags V)

R+1800t42'

R+42'

il, :;ii ,'i'

To fly left (R + 42' leads V)

O42 (To)

R+1800+42" Fig' 4.6 Fly'left/flv-right and'to/from' situation diagram

,i*: at 30 Hz anticlockwisearoundthe ring of antennas. airbome equipment will give the correct indications when usedwith stationsof either type. In the DVOR To a receiver,remote from the site,it appearsasif the signalsourcesareapproachingand receding,and the referencesigrralis 30 Hz a.m. while the variable hencethe receivedsigral suffers a Doppler shift sigrralis 30 Hz f.m. on a 9960 Hz sub-carrier.Since (see Chapter l0). -With a diameterof l3'5 m and the rolesof the a.m. and f.m. are reversedwith respect speedof 30 r.p.s.the tangentialspeedat the rotation the to CVOR the variablephaseis arrangedto lead is n X l3'5 X 30 = 1272m.p.s. At the periphery X" magtetic at an aircraft phase X" for by reference centrefrequencyof the v.h.f. band, I l3 MHz, one bearingfrom the station (cf. CVOR). rycle occupiesapproximately2'65 m, thus the In a double sidebandDVOR (DSB'DVOR) the maximum Dopplershift is 127212'65= 480 Hz. carrier,/", with 30 Hz (and identification)a.m. is ln the airbornereceiverthe sidebandsmix with the radiatedfrom an omnidirectionalantenna. Two unmodulatedr.f. sidebandsigrals,one 9960 Hz above carrierat /. to produce9960 t 480 Hz. Single sidebandand altematesidebandDVOR are possible, /c, the other 9960 Hz below /", are radiatedfrom but sincethey compromisethe performanceof the of about in a ring antennasdiametricallyopposite systemthey will not be discussed. fifty antennas. Theselatter sigtals are commutated 62

Fig.4.7 IN-2014electroniccoursedeviationindicator (courtesyBendixAvionicsDivision) Referencery' 30 Hz a.m.

instrumentson which VOR information is displayed aremulti-function hencequite complexswitching arrangements are involved. Figure4.9 showsone VOR/ILS systemof a typical dual instatlation;only thoseoutputs from VOR are shown. The antennamay serveILS aswell asVOR but someaircraft haveseparateantennas,particularlyif all-weatherlandingis a requirementwhen the. optimum position for the localizerantennamay not suit VOR. If separateantennasare usedwith a common r.f. feed to the receiverthe switchinglogic will be derivedfrom the channelselectionmadeat the control unit. The VOR antennaemploys horizontal polarizationwith an omnidirectional radiationpattern. A horizontal dipole is often used with the dipole elementsforming a 'V' shapeto givea more nearly omnidirectionalpattern. Sincethe dipole is a balancedload and the co-ax.feederis unbalanced(with respectto earth) a balun (balanced to unbalancedline transformer)is used. The dipole may be mounted on the verticalstabilizeror on a stand-offmast, top-mountedon the fuselage. The VOR/ILS receivercontainsa conventional superhet,a filter for separationof signiilsand a converterto providethe requiredoutputs which aro l. 2. 3. 4. 5.

audioto AIS; bearinginformation to two RMIs; deviationfrom selectedradial; TO/FROMsignal; flag or,warningsignal.

The RMI feed is the result of the automaticVOR operation. Sincethe pointer on the RMI movesto a position givingrelativebearingwith respectto the lubber line the magneticbearing(omnibearing)must be combinedwith headinginformation aspreviously 9960 t +8oHz described.The necessary differentialsynchroor resolverwill be in the receiveror, in the caseof Fig.4.8 Frequencyspectrum- DVOR spacesignals equipmentconformingwith ARINC 579,in the RMl. ln the former casemagneticbearing(mag.)is required by the receiver,this beingobtainedfrom the compass Aircraft lnstallation systemvia the RMI. The necessary switchingfor displayingVOR as opposedto ADF information, on SinceVOR and ILS localizers occupythe sameband either or both of the pointers,is on the RMI. of frequenciesthey invariablysharethe samereceiver The deviationand TO/FROM signalsare the result which will alsocontain the necessary circuits to of manualVOR operation. Thesesteeringcommands extractthe requiredinformation. It is not aredisplayedon a coursedeviationindicator(CDI) an uncommonfor v.h.f. comm. and v.h.f. nav. to share electronicversionof which is shownin Fig.4.7. The the samereceiver,particularlywith generalaviation CDI, however,may not be a stand-alone unit; it is equipment.It is expectedthat the 'all v.h.f.in one likely to be part of a multi-function indicator known box' trendwill continueby a variety of namessuchashorizontal situation A largeairliner,and indeedmost aircraft from indicator(HSl) or pictorial navigationindicator PNL twins up, hasa dual v.h.f. nav.installation. The In the installationshownin Fis. 4.9 an HSI is used

63

Captain'scontrol panel

DME1

Norm

l1lo

Comprss

S/byt ) Test DME RF

Power Captain RMI

-:-|

N o .1 V O R / I L S R x

F----------,-----

FlO RMI

To From

DEV

Fl

ight interohone

Flag

I J

oBs

Audio ,.

-----f

transfer relay

Captain'sVOR/ILS transfer relay lentral nstrumenl rarning panel

rNs

Captain'sRAD/INS relay

Captain'sHSI

Fig 4.9 TypicalVOR installation

Fig.4.l0 KPI 552 pictorialnavigationindicator (courtesyKing RadioCorp.)

with remote courseselection(o.b.s.- omnibearing selection)fitted, say,on an autopilot/flightdirector mode selectpanel. Figure4.10 illustratesthe King KPI 552 PNI wherethe built-ino.b:s., iniolporated(courseknob),hasbeensetto 335", a fly-right commandis beinggivenby the deviationbar and we havea TO indication(largearrow'headabove aircraft symbol). arepossible;those Variousswitchingarrangements shownillustratethe captain'schoiceof VOR/lLS I ' VOR/lLS2 or inertialnavigationsystem(lNS) informationbeingdisplayedon his HSI. Switching by meansof the betweenVOR/lLS I or 2 is achieved transferrelay(TFR/RLY) while VOR/lLS or INS switchingis by meansof the radio/lNS(R.AD/lNS) relay. The first officer (F/O) hasa similar arrangementat his disposal.Deviationsignalsfrom numberI systemto F/O'sHSI and from number2 systemto captain'sHSI may be via isolation amplifiers. The flag signalis of vital importance since it gives

il

.ir*

warningof unreliabledata from the VOR/ILS receiver.It will be fed to all instrumentsselected to displayVOR/ILS information and often to a central instrumentwarningsystem(CIWS). Shouldthe deviationsignalbe fed to the automatic flight control system(AFCS),then obviouslyso must the flag or warningsignal.

Controls and Operation The controlleris not particularlycomplicated. Frequencyselectionis achievetlby roiation of two knobs,mountedco-axiallyor separately, so determiningthe appropriate2/5 code fed to the receiver.It is normal for DME frequencyselection to be madefrom the sarnecontroller with DMg standby,normal and test switchingalsoprovided (s-eeChapter7). Self-testswitchingfaciities for the YOR/ILSareprovided. Additiona]controls,other than thoseshown in Fig. 4.9, may be provided, namelyVOR/lLS on-off and audio vo-lume. The locationof displayswitchesand the course . selector hasbeentnentionedpreviously,ashas the interpretationof indicationsgiven to itre pilot.

Simplified Block Diagram Operation Receivedsignalsare selected,amplified and detected by a conventionalsingleor doubie superhetreceiver. The detectedoutput is a compositesignalwhich must be separated into its cornponentpartsby meansof appropriate filtering circuits. The audiosignal,1020Hz identification,is routed . via an amplifier and possiblya volume contiol on the v.h.f. nav.controller to the flight interphone sub-system of the AIS. The associatedaudio filter may be switchableto give a passbandof 300-3000Hz when the VOR systemis being usedas a ground-to-air communicationlink. ^^ lh9_referencephasechannel(CVOR) consistsof a 9960 Hz filter, a discriminatorto deteci the 30 Hz l.m. and,-not shown, amplifier circuits. Limiting of the signaltakesplace before the discriminatorto remove unwantedamplitude variatipns. The 30 Hz referencesignal(R) then undergoesvariousphase shifts. For manualVOR operation,as previously mentioned,we need to shift R by the selectld course.This is achievedby the phase-striftresolver, the rotor of which is coupledtqthe courseor OBS knob. A digital readoutof the selectedcourseis provided. The phase-shiftedR is now comparedwith the variablephasesigrral. If they are in phaseor lg0"

out of phasethere is no lateralmovementof the deviationbar. Sinceit is simplerto determinewhen two signalsare in phasequadrature(at 90.) a 90" phaseshifter may be inclutledin the referencechannel prior to feedingthe phasecomparatorwhere detectionof phasequadraturewill giveno movement ofthe deviationbar. In the absenciofeither or both of the signalsthe flag will be in view. 'To'or 'from' informationis derivedby comparing the variablephasewith the referencephaseshifted bi the OBSsettingplus l80o It follows-tharif the referencephasehas beenshifted by 90o before feedingthe deviationphasecomparatorwe only reqrrirea further 90" phaseshift before feedingthe 'to/from'phase comparatorratherthan a lgOdphase shift asillustratedin Fig. 4.1l. If the inputs to the 'to/from' phasecomparatorarewithin, say,+ g0o of beingin phasethen a TO indicationis given;if within t 80'of being in anri-phasea FROM intication is given;otherwiseneither a TO nor a FROM indication is given. For automaticVOR operationthe reference channelis phase-shiftedand comparedwith the variablephase.If the two inputsarein phase quadraturethere is no drive to the motor, otherwise the motor will turn, changingthe amount by which the referencephaseis shifted until phasequadratureis achieved.The motor connectionsare arrangedso that the stablenull of the loop givesthe requirei shaft position which representsthe magneticbearingto the station. Compassinformationis fed to a differential synchro,the rotor of which is turned by the motor following station magneticbearing. ThL difference signalrepresentsrelativebearingwhich positionsthe appropriateRMI pointer via.asynchrorepeater. The RMI card is positionedby compassinformation.

Characteristics The following characteristics are selectedand summarizedfrom ARINC characteristic579-1. li shouldbe noted that there are radicaldifferencesin outputs,betweenARII{C 579-l and the older ARINC 547 with which many in-servicesystems conform. Frequency Selection 160 channels,50 kHz spacing,range108-117.95MHz. Standard2/5 selectionsystem. Channellingtime lessthan 60 ms. Receiver Satisfactoryoperationwith 1.5 pV sigral.

65

To AIS

Ref. channel

Deviation bar

Flag

cDt/oBS To/From -------l

Compass

VOR blockdiagram' Fig.4.lI Simplified

natf "t 31'5kHz' 60dBattenuatio reast

"

i"pr1""l'it'-i:"t:mmfll fri?:n|;Tfii:"' stationis requtrt

Nomorethan6dBattanuationat/.i|7kiz;atanaloguesigralprooortionaltoperpendicularlinear

outputs . ^^n .nn , of 200-500Q Audio:at least100mWinto a load ..nt'lt"' 30 per t:: -:I modulated from a 3 pV input ,ignuirnoaututed it 1000Hz. blli:'fo'*

infractional ouJput digital omni-bearing:

of the of whichis ananalogue 'Tt;digh-leutl o*'' the spaiing shouldgive2 V across output ffiil; within adjustable i ziio ii r*a.fo'?"ou'* deviation corresponding rhe miles' ;;;; t s to t lb nautical

io*i'u'r ":f:;.'lfi1.Ti;:'%tti\;b"o*;l*"

:J"'i,*:::i1!";;;;;:;r'.'^'n3-n1 flightcontrolsystem #:'fi:iJffift,:ft1,;Tlsts":lT{:: i;;; ;t usedby the automatic ""'"' bearing, the of -ih. eventof lossof the In proportiori?,rr."rt. one voltages, moi.iocprt' Gat-l 19 .nulogu.ou,puti, DMEthe deviation theotherto its cosine. iistanctinfotmationfrom the t*o ;i;l''"'*irnu*'of OBS revertto an angular to feed"t desiened 'r^6-*-'- o;tp"t should.automatically uigruute lnt"r.t not friiJn'.;. v for 10" off nr'" purau.lconnected er-vi1g^z atui'tion *"d;;'f;;;;'n outputs 547' ARINC deviation 579 with . used ARINC to Wts *i*, "out"' (Note:iti"i voltage d'c' ani'f-.*-i**f Deviation:a trigh-levef 66

Fig.4.l2 Ttc T-308VOR/ILStestset(courtesy Tel-lnstrument Electronics Corp.)

represented angulardisplacenrent.) TO/FROM:groundreferenced providing2 mA for eachof two 200 Q loadsin parallel. In additiona lowlevel output of 200 gA may be provided to feed olderinstruments. Warning:high level,28 V d.c. valid, absentinvalid. low level,between300 and 900 rnV valid, lessthan 100 mV invalid. The low level signalshould be capableof driving frorn one to five 1000 Sl parallel loads. The VOR digital output should also include warningbits. Ramp Testing Testingof VOR shouldalwaysbe carriedout with a ramp test set capableof being tuned to any VOR frequency,radiatingsufficient energyto allow satisfactoryoperationof the VOR and providing a meansof simulatingvariousVOR radials. Most test setsincludeprovisionfor testingILS aswell as VOR. Among thoseavailableare the CossorCRM 555. IFR NAV4OIL.

The CRM 555 operateson any of the 160 VOR channelswith a frequencyaccuracyof 10.0035 per cent(--10'C to +30oC).Modulationof the carrieris suchthat the simulatedbearingmay be set to any readingbetween0 and 360o with a calibrationaccuracvof t l" or mav be switchedin 45o stepswith an u..uru.y of t 0.3". Carrierpower canbe attenuatedin I dB stepsbetween0 dBm and -120 dBm (0 dBm corresponds to an output of I mW). A self-testfacility is provided. The NAV-401L offers similarfacilitiesbut is more 'state of the art' and so offersStighttymore in the way of performance. The TIC T-278 is part of the T-308 test set illustratedin Fig.4.12. The facilitiesarenot as 'extensiveas either of the previouslymentionedtest setsbut it has the advantageofeaseof operationand lesscost. It is FCCtype accepted.Operationis on 108'00MHz radiatedfrom a telescopicantenna. Bearings of 0,90, 180 and 270" canbesimulated both TO and FROM, alternatelyvariation,90-l l0' 'to' or270-290o'from', is available.A J lo switch

6'

*:;

givesa usefulsticky needlecheck. Actual testingshouldbe carriedout in accordance with the procedureslaid down, but briefly it would involvecorrectly positioningthe test set antennaand radiatingon sufficient frequenciesto test frequency selectionof the VOR. Sensitivitymay be checkedby reducingthe r.f. levelreceivedeither by use of the test

68

set attenuatoror moving the test set antennafurther away. Various bearingsshouldbe simulated(check 'to'or 'from' whether they are station), the appropriatereadingshouldbe checkedon the RMI and the OBS operatedso as to check the manual mode of VOR.

5 lnstrumentlandingsystem

Introduction In order to be able to land the aircraft safelyunder visualflight rules(VFR), i.e. without any indication from instrumentsasto the aircraft'sposition relative to the desiredapproachpath, the pilot must haveat least3 mileshorizontal visibility with a ceilingnot lessthan 1000 ft. Although most landingsare carried out under theseconditionsa significantnumber are not; consequently, wereit not for instrumentaidsto landinga considerableamount of revenuewould be lost due to flight cancellationsand diversions. One method of aidingthe pilot in the approachto an airport is to usea precisionapproachradar(PAR) systemwhereby the air traffic controller,havingthe aircraft 'on radar',can giveguidanceover the v.h.f.-r.t. The alternativemethod is to provide instrumentation in the cockpitgivingsteering information to the pilot which, if obeyed,will cause the aircraft to make an accurateand safedescentand touchdown. The latter, which may be complemented/monitored by PAR, is the method which concernsus here. Early ILS date back to beforeWorld War II; the GermanLorentz beingan example. During the war the currentILS wasdeveloped and standardized in the United States. The basicsystemhasremained unchangedeversincebut increasedaccuracyand reliability haveresultedin landing-minimum visibilityconditionsbeingreduced. The ICAO havedefinedthree catesoriesof visibility.the third of which is subdivided . All categoriesare definedin termsof runway visualrange (RVR) (seeICAO Annex 14) and,exceptCategory III, decisionheight(DH), belowwhich the pilot must havevisualcontactwith the runwayor aboit the landing(seeICAO PANS-OPS).The various categoriesare definedin Table 5.1 where the standardsaregivenin metreswith approximate equivalents in feet (in parentheses). Sometimes categories IIIA and B arecalled'seeto land'and 'see to taxi'. The ILS equipmentis categorizedusingthe same Romanirumeralsand lettersaccordingto its

Table5.1 ICAOvisibilitycategories Category

d.h.

r.v.r.

I II IIIA IIIB IIIC

60 m (200ft) 30 m (100ft)

8 0 0 m 2( 600ft) 400 m l 200ft) 200m 700 ft) 3 0 m l 50 ft) Zerc

operationalcapabilities.Thus if the ILS facility is categoryII, the pilot would be able to land the aircraft in conditionswhich correspondedto those quotedin Table5.1. An obviousextensionof the idea of a pilot manuallyguidingthe aircraft with no extemal visualreferenceis to havean autopilot which 'flies' the aircraft in accordancewith signalsfronr the ILS (and other sensors includingradioaltimeter)i.e. automaticlanding.

BasicPrinciples Drectional radio beams,modulatedso€s to enable airborneequipmentto identify the beamcentres, define the correct approachpath to a particular runway. In addition verticaldirectionalbeams providespot checksofdistance to go on the approach. The total systemcomprisesthree parts,eachwith a transmitteron the ground and receiverand signal processor in the aircraft. Lateralsteeringis provided by the localizerfor both front-courseand back-course approaches; provides the glideslope vertical steeringfor the front courseonly while markerbeaconsgivethe distancechecks. [,ocalizer Forty channels areallocatedat 50 kHz spacingin the band I 08' 10-l I I .95 MHz usingoniy thoseliequencies wherethe tenthsof a megacycle count is odd; so,for example108'I0 and 108.I 5 MHz arelocalizer channelswhile 108.20and 108'25MHz arenot. Thosechannelsin the band not usedfor localizerare

69

allocatedto VOR. The coverageof the beaconwill normally be asshownby the hatchedparts of Fig. 5.1,but topographical featuresmay dictatea wherebythe ! 10" sectormay be restrictedcoverage reducedto l8 nauticalmilesrange.

(extendedcentreline) and limited by the lineson which thereis a d.d.m.of 0'155. The changein d.d.m.is linearfor I 105 m alongthe line perpendicular to the courseline and passingthrough the IIS datum point on the runway threshold;these points 105 m fronr the courseline lie on the 0'155 d.d.m.lines,asshownin Fig. 5.2. The beaconis Azimuth situatedsuch that the abovecriterion is met and the coursesectoris lessthan 6'. Outsidethe course s e c t o rt h e d . d . m . ' i sn o t l e s st h a n0 ' 1 5 5 . The ICAO Annex 10 specification for the pattern is more complicatedthan localizer-radiated the descriptionaboveindicates,in particularin the varioustolerances for categoryI, II and iII facilities; howeverwe havecoveredthe essentialpoints for our purposes. The airborneequipmentdetectsthe 90 and 150 Hz tonesandhencecauses a deviationindicatorto show a fly-left or fly-right command. Full-scaledeflection is achievedwhen the d.d.m.is 0'155, i.e. the aircraft is 2-3ooff course.Figure5.3 showsa mechanicaland electronicdeviationindicator both showingslightly overhalf-scaledeflectionof a fly-right command. Providedthe pilot flies to keep the commandbar at zero,or the autopilotfliesto keepthe d.d.m.zero, Fig.5.l Localizer frontbeamcoverage the aircraft will approachthe runway thresholdalong the courseline. The horizontally polarizedradiatedcarrieris In addition to the 90 and 150 Hz tonesthe modulatedby tonesof 90 and 150 Hz suchthat an localizercarrieris modulatedwith an identification aircraft to the left of the extendedcentreline will be tone of 1020 Hz and possibly(exceptionallycategory in a regionwherethe 90 Hz modulation predominates. III) voicemodulationfor ground-to-air Along the centreline an airbornelocalizerreceiver communication.The identificationof a beacon will receivethe carriermodulatedto a depth of 20 consistsof two or threeletterstransmittedby keying per centby both 90 and 150 Hz tones. Deviation the 1020 Hz tone so as to give a Morsecode from the centreline is givenin d.d.m.(differencein representation.The identification is transmittednot depthof modulation),i.e. the percentage modulation lessthan six times per minute when the localizeris of the largersignalminus the percentagemodulation operational. of the smallersignaldividedby 100. The localizercoursesectoris definedas that sector Glideslope in the horizontalplanecontainingthe courseline Glideslopechannelsarein the u.h.f. band, 15OHz > 90 Hz

0.155DDM Bercon Course sector < 6o

0 ' 1 5 5D D M

ILS datum point

150 Hz < 9O Hz Fig. 52

70

Localizer ooursc,lelector

Fig. 5.3 Electromechanical and electronic course deviatron indicators (courtesy Bendix Avionics Division)

s p e c i f i c a l l3y2 8 . 6 - 3 3 5 .M 4 H z a t 1 5 0k H z s p a c i n g . Eachof the forty frequencies allocatedto ihe glideslope systemis pairedwith a localizerfrequency, the arrangement beingthat localizerand glidesiope beaconsservingthe sarnerunway.will haveliequencies takenfrom Table5.2. pilot selectionol'the required localizerfrequencyon the controllerwill cause'both localizerand glideslope receivers to tune to the appropriatepairedfrequencies. Table5.2 Localizer/glideslope frequencypriring (MHz) Lot'alizer

()li
108.10 1 0 8 .51 t08.30 r08.35 r0 8 ' s 0

334.70 -r,r4.55 3 3 4 .0r 33.r.95 329.90 329.75 330.50 330.35 329.30 329.15 33r .40 3 3 1. 2 5 332.00 3 3 1. 8 5

r0u.55 r0 8 . 7 0 108.75 108.90 108.95 109'10 109.15 109'30 109.35

Localizer

Glidepath

l0e.-s0

332.60 332.3s 333.20 333.05 333.80 333.65 334.40 334.25 335.00 334.85 329.60 329.45 330.20 330.05 330.80 330.65 3 3 1. 7 0 331.55 332.30 332'15 332'90 332.75 333.s0 3 33 . 35 3 3l . 1 0 330.95

1 0 9 .55 l0e 70 109.75 109.90 109.95 I10.10 l 10.15 110.30 I10.35 I l0'50 I 10.55 l10.70 110.7s I10.90 l10.95 ll l.l0 lll'15 I I 1.30 l l 1.35 I I 1.50 l I 1.55 I I1.70 I I1.75 I I 1.90 I l l.9s

-

71

The principle of glideslopeoperation is similar to that of localizerin that the carrieris modulatedwith 90 and 150 Hz tones. Above the correctglidepath the 90 Hz modulationpredominateswhile on the correctglidepaththe d.d.m. is zero,both tonesgiving a 40 per cent depth ofmodulation. The coverageand shown in Figs 5.4 and 5.5 are beamcharacteristics givenin termsof the glidepathangle,typically 2|-3'. CategoryI facilitiesmay haveasymmetrical upperand lower sectors,the figure of 0'0875 d.d-mto an angulardisplacementof betweert corresponding 0'070 and 0'140 0. By contrasta categoryIII facility is asshownin Fig. 5.5 with a toleranceof + 0'02 0 on the 0'12 0 lines. Althoughd.d.m.= 0lines occur al2 0,30 and4 0 thev are not stablein the sensethat if the pilot obeys

t o n 'n

I

I

Course line Azimuth

Fi;. 5.4 Glideslopecoverage

DDM : 0.0875

Fig. 55

72

Glideslopebeamcharacteristics

the steeringcommandshe will not maintain the correspondinganglesof descent.The first stablenull o".un at S 0 wtrich for a glidepathof 3" is at l5o. This is sufficiently different from the desireddescent angleto createfew problems;howeverto avoid 'captured' confusionthe glideslopebeamshouldbe from below. Oncein the correctbeam fly'up and fly-down signalsare indicatedto the pilot in much the same *-uVur with the localizer. Figure5.3 illustratesa fly-up commandof just over half-scaledeflection. The glideslopeoutput is more sensitivethan localizer in that typically a |' off the glidepathwill give deflection(about0'175 d'd.m.) compared full-scale with about 2t off the courseline for full'scale deflection. Marker Beacons A marker beaconradiatesdirectly upwardsusinga carrierfrequencyof 75 MHz. The modulatingsignal dependson the function of the marker. An airways,fan or'Z'marker is a position aid for en-routenavigationlocatedon airwaysor at holding points. As suchit is not part of ILS. The carrieris modulatedwith a 3000 Hz signalwhich causesa white lamp to flashin the aircraftwhile station identificationin Morsecode is fed to the AIS. The outer marker is normally located4f miles from the runway threshold. The carrieris by 400 Hz keyed to give two amplitude-modulated which can be heardvia the AIS and per second dashes causesa blue (or purple) lamp to flash. The middle marker is located3500 ft from the runway threshold. The carrieris amplitude'modulated by 1300Hz keyedto givea dot-dashpair 95 timesper minute which can be heardvia the AIS and causesan amberlamp to flash. The ILS markerbeamwidths are sufficiently wide in the planeperpendicularto the courseline to cover the coursesector.

SimplifiedBlock DiagramOperation

Sincethe localizerand VOR frequenciesoccupy the sameband it is normal to havea v.h.f. navigation and detectssignals receiverwhich selects,.amplifies the frequencyselected. on depending either aid, from Figure 5.6 illustratesthe basicblock diagramof a localizerreceiver.A conventionalsingleor double o.0875 superhetis employed.A.g.c.is importantsincean increasein the 90 and 150 Hz output signalsby the samefactor would increasethe magnitudeof the difference,so givingmore deflectionof the deviation

3OO-3OOOHz

I Deviation indicator

r__ _ ___:=___

__J

Fig. 5.5 Localizer simplified block diagram

Fig.5.7 KingKN 72 bandpassfilter,simplified

indicator for the samed.d.m. Sigral separationis achieved by threefilters:audio,90 and 150 Hz. The audio signal,identification and possiblyvoice,is passedvia audio amplifiers(incorporatinga noise limiter)to the AIS. The 90 and 150 Hz iignalsare full waverectified, the differencebetween-the rectifieroutputsdrivingthe deviationindication while the sum drivesthe flag out of view. The 90 and 150 Hz filters,togetherwith the rectifienand any associated circuitry.are often part of the so-calied VORi LOC converterwhich may be within the v.h.f.navigationreceiveror a separaieunit. A combinedconverterwill usuallyenrployactive filters which serveaseither 30 Hz bandpissfilters for VOR operationor 90/ I 50 Hz bandpassfiltersfor

localizeroperation. Figure5.7 showsthe circuit used in the King KN 72 VOR/LOC converter. Whena VOR frequencyis selectedthe ILS Hi line is low, so turning off Ql which effectivelydisconnectsR2 from the circuit, the centrefrequencyof 30 Hz is set by R3. Selectionofa localizerfrequencycauses the ILS Hi line to go high,so turningon Ql and placingR2 in parallelwith R3. R2 is setto givea centrefrequency of 90 or 150 Hz asappropriate. The glidepathreceiverconverterblock diaeramis similarto that of the localizerexceptthat the-audio channelis not required.A separate receivermay be usedor all navigationcircuitry may be within the sameunit. In any eventseparate antennas areused for localizerand glidepath.

3

5 :t

White

Blue

Amber

To AIS

blockdiagram Fig.5.8 Markersimplified The marker is fixed tuned to 7 5 MHzand may employ a t.r.f. (tuned radio frequency)or superhet .r..iu.r. The detectedaudio is t-edto three filters for tone separationand alsoamplifiedand fed to the AIS' The filtir which givesen output causesthe appropriatelamp-switchingcircuit to give an - . lamp' interruptedd.c. output to drivethe associated receiver the of sensitivity the Hi to When sq'itched is such that it respondsto airwaysmarker beacons eventhough the iircraft is at a relativelyhigh altitude. Wittr trigtrsensitivitythere is a dangerthat when at lower altitudes,for examplewhen flying over the outer and middle markerson approach,the lamps is may be lit for longerthan the maximumof 10 s' It lamps marker middle and outer evenpossiblefor the to be lit simultaneously.To avoid this, low sensitivity wherebyan attenuator(i0 dB) is piacedin is selected, line with the receiverinput. Switchingmay take placeat 10000 ft. Fig.5.9 "l*':

Typical attitude director indicator

J'

the and scaleconventionallyon the left-handsideof rising a drives localizer the ADI the tn instrument. runway laterally tc displaydeviation(vertical. in Chapter4 an installationincorporatinga movementrepresentingradio altitude) while the VOR/IiS receiverwas discussedand illustrated glideslopedrivesa pointer over a scale,againon the ILS we areinterestedin (Fig. a.9). In considering left-handside of the instrument' itro"seouiputs derivedfrom the localizer'glidepath Localizer,glideslopeand marker signalsarealso glideslope and marker receivers.Localizerand fed to an autolandsystemwhen fitted' The localizer deviation(fly-leftifly-right, fly-up/fly-down deviationwill be usedto supply the appropriate respectively)will be fed to a conventionalor (rudder) HSI an ciemandsignalto the roll (aileron) and yaw Lt.it.onic ieviation indicator (Fig' 5 '3) and/or respond will channels.ihe pitch (elevator)channel (Fig.a.l0) and an attitudedirectorindicator'ADI touchdown approaches aircraft to glideslope.As the localizerdrivesa iateral ieii. s.sl. In the HSI the thJresponie of the pitch channelto glideslope while arrow course the of left and aril.tion bar right reduced;this deviatiln signalsis progressively glideslopedeviationis givenby a deviationpointer Installation

74

reductionis triggeredby the outer marker and thence deviationoutput circuit hasan output impedanceof controlledin accordancewith the radio altimeter 200 O and suppliesthe required current to five output. A modern ILS will provide dual parallel indicators in parallel then when lessthan five outputsfor both localizerand glidepathdeviationin indicatorsare usedthe deflectionwill not properly orderthat the AFCS.may acceptinformation only correspondto the d.d.m. Considera d.d.m.of 0.155, when the samesignalappearson eachfeed of a then 750 pA must be suppliedfor five loadsin parallelpair. parallelfrom a generateddeviationvoltageof A generalaviationinstallationis illustratedin 300 mV. Now considerfour loadsfed from a 300mV, Fig. 5.10 incorporatingKing equipment. The 200 Cl source,the total current will be KX 175Bis panel-mountedand containsa 300 x 103(200 + 250) = 666.7ptA divided equally 720-channelv.h.f. comms receiver,a 2O0-channel amongthe four loadsso that eachload has v.h.f. nav. receiverand all necessarycontrols with 666'7l4 = 166'7 pA, i.e. the indicatorswill over-read digitalreadoutof comm. and nav. frequencieson the by about I I per cent. Unlessthe receiveroutput is a front panel. The KX 175B alsoprovidestuning constantvoltagefor a variety of loads(ARINC hformation for the DME and glideslopereceiver. 578-3)we must comp€nsatefor loadingvariations. Various methodshavebeenusedfor loading compensationin the past. One possibilityis to choosedifferent receiveroutput impedances dependingon the numberof loads;inthis casethe receiverand the mounting rack shouldbe suitably labelled. Another possibilityis to fit a shunt resistor in an aircraftjunction box throughwhich the deviation signalis fed. With two indicatorsa 330 O shunt would be neededgivinga 330 O and two 1000 Q loadsin parallel,i.e. total load of 200 (-). Finally, but not exhaustingthe possibilities,five separate buffered outputs may be provided,eachindicator beingfed from one of the buffer amplifiers. Similar considerations apply to flag circuitswhere using four 1000Q loadsin parallelis standardprocedure. F8. 5.10 Kirg general aviationcomm./nav. system vary betweendifferent Antenna arrangements typesof aircraft. Mentionof combinedVOR/ The KN 72 andKN 75 are remote-mounted VOR/LOC converterand glideslopereceiver respectively. The KN 72 gles localizerdeviationand flagsignals (aswell asVOR deviation,TO/FROM and flag),while the KN 75 givesglideslopedeviationand flag. The KMA 20 is an audio control console providingspeaker/phoneselectionfor sevenreceive channelsand mic. selectionfor two transmit channels aswell as containinga marker receiverplus its controls Phillip3 hcad scrcw andlamps.The indicator,Kl 206, showslocalizer (16 pt.ccs) deviation(verticalbar) and glideslopedeviation (horizontalbar) aswell as showingVOR deviation and TO/FROM indication if a VOR frequencyis selected,the deviationrelatingto the OBS setting also on the KI 206. lf a Kl 204 is usedinsteadof the Lo[r l@rlirct antennt KI 206 then the KN 72 may be omitted sincea VOR/LOCconverteris built in. Typically a deviationindicator movementwill be of 1000S) impedanceand require150 pA for Fig. 5.t I Boeing747 localizer aerials (note Bendix weather full-scaledeflection(f.s.d.), thereforethe voltage radar scanner with spoiler grid on parabolic reflector for acrossthe deviationoutput of the receivershould be mapping purposes - see Chapter 9). (Courtesy Boeing 1 5 0m V f o r a d . d . m .o f 0 . 1 5 5 . I f t h e r e c e i v e r C o m m e r c i a l A e r o p l a n eC o . )

75

localizer antennashasbeenmadein Chapter4 but the glideslopeand marker antennaswill alwaysbe aircraft, separate.As an exampleof a largepassenger considerthe Boeing747. ThreeVOR/ILS receivers areinstalledfed by one V-type VOR antennaat the top of the verticalstabilizer,two dual localizbr antennasin the noseand a total of six glideslope antennasin the nose-wheeldoors. One marker beaconreceiveris instalied,fed by a flush-mounted antennaon the bottom centrelineof the aircraft. The localizerantennasare mounted aboveand below the weatherradarscanner.The lower antenna feedsreceiversI and 2 while receiver3 is fed from the upper antenna. Antenna switchingbetweenVOR and localizeraerialsis achievedby either solid stateor switchesmounted behind the electromechanical VOR/ILS receivers. The six glideslopeantennasare split into two groupsof three,one group in eachnose-wheeldoor. A non-tunableslot (track antenna)dual unit is

installed in each door leading edgewhile two tunable arrays(captureantennas)are mounted on the sidesof eachdoor. A total of four hybrid antennacouplers combinethe r.f. outputs of the glideslopeantennas providingsuitableimpedancematching.

Controlsand Operation Normallya combinedVOR/ILS/DME controlleris employed(Fig. a.9). Such a controller is briefly describedin Chapter4. The marker receiver switchingis likely to be remote from the combined controllerand its action hasbeendescribedabove. In usethe glidepathshouldbe capturedfrom below, approachingfrom a direction determinedby the approachproceduresfor the particular airfield. The marker sensitivityshouldbe on low for the approach.The appropriateselectionshould be made on the audio control panel.

VOR/ILS receiver No. 3 Glide slope antenna hybrids 8554 and 8555 and att€nuators 8582 and 8583 Trrck antenna coaxial cable

Capture antcnna coaxial cable { Coaxial recbiver No. 2 cable 9uard Antcnna coaxial connectors D8558

G/S track antenna B 561 Antenna coaxial connector D8561

See@r

G/S capturc a n t e n n a8 5 5 8

G/S capture a n t e n n a8 5 5 8 G/S capture antenna hyg49-{-\ -8560 c/S;;t';; \ .antennaarray . \\ t \

Tuning"trg, .

t -]\'*. -.\\ . .\\.

Attachins screws(5 places)

Right aft nose gear door

S rwo

Aft door shown in retraciedposition (6t \J

Fig.5.l2 Boeing?47 glideslopeaerials(courtcsyBoeing CommercialAeroplaneCo.)

76

Track antennas

Capture antonnas

Right door

Track aerials

Fig.5.l3 Boeing747 simplified glideslopeaerial coupling alrangements

Characteristics The basisfor the following is ARINC Characteristic 578-3 althoughmuch of the detail hasbeenomitted and not all sectionscovered. Units Tlte receivershouldcontain all the electronic circuitry necessaryto providedeviationand flag signalsfor both localizerand glideslope.The control unit should provide for frequencyselectionof ILS, VOR and DME using2/5 coding.

carrierwithin I l2 kHz of tuned frequency. Sensitivityis such that the flag shouldclearwith a 5 pV 'hard' input sigral ('hard' pV: the output of a signalgeneratorcalibratedin terms of open circuit load). The receivershouldbe protectedagainst undesiredlocalizersignals,VOR sigralsand v.h.f. comm. signals.The a.g.c.shouldbe such that the receiveroutput shouldnot vary by more than 3 dB with an input signallevel rangeof l5-100 mV.

Gli
77

shouldnot vary for AFCS. Output characteristics loadSbetween200 Q and no load. When90 Hz 'hot' sideof all deviationoutputs predominatesthe 'common' side; shouldbe positivewith respectto the 'fly-left'is given. in this case Glideslope:similarto localizerbut high- and lowlevel for outputsare2Y and 150 mV respectively 0 ' 1 7 5d . d . m . Flag Outpu* Two highJevelwarningsignals(super flag) and one lowlevel waming signalshould be providedby both localizerand glidepathreceiversThe high-levelflag characteristicis 28 V d.c. for valid statuswith current capabilities;25 mA for AFCS waming;250 mA for instrumentwamings.The low-levelflag shouldprovidea voltageof between 300 and 900 mV into up to five parallel1000 Q loads. Monitoring Warningsigtalswhen: no r.f., either90 or 150 Hz missing,total depthof modulationo1'composite 90/150 Hz signalis lessthan 28 per cent,etc.

RampTesting A radiatingtestsetmustbe usedwith a basic capabilityof simulatingoff-glidepathsignals.In additionthe testsetshouldoperateon one or more and providefaciliticsfor accuratespot frequettcies deletingeitherof the modulatingfrequencies. TIC T-308 This test set wasmentionedin Chapter4 in connectionwith VOR testing.In additionto the VOR testset modulewe havethe T-268,T-288 and T-298 for testingthe marker,localizerand glideslope receivers respectively.The T-268 providesat least 7Cper centmodulationfor the 400, 1300and 3000Hz tones. The T-28Boperateson 108'I MHz a n dc a ns i m u l a t e0 d . d . m . , 0 ' 1 5 5d . d . m . l e f ta n dr i g , h t (switched)or 0 to t 0'199 d.d.m.(variable).The T-298 ooerateson 334'7 MHz and cansimulate b a.a.m.,0.175 d . d . m .u p a n dd o w n( s w i t c h e do) r 0 to t 0'280 d.d.m.(variable).Eitherthe 90 or the 150Hz tonesmay be deletedwith both the T-28B and the T-298.

beingmarkedin decibels,e.g.: 6'6 dB fly-right(+ 0'1549 d.d.m.)' 4 ' 0 d B f l y - l e f t ( - 0 ' 0 9 2 ed . d . m . ) ' 3 ' 7 6d B f l y - u p ( + 0 ' 1 7 5d ' d . m ' ) e' t c . Further switch positionson the d.d.m. switch allow for deletingone or other of the tones. ln additiona variable0 to 1 150gA deviationis available.Stepped attenuatorsprovideoutput levelsvariablebetween 0 dBm and - 120 dBm in I dBrn steps,in orderthat receiversensitivitymay be checked(testsetaerial positioningwill affectthis check). Modulatingtones fbr marker of 400. 1300and 3000 Hz are available for checks.Finally, l0l0 l-lzmodulationis available 4 the in Chapter audiochecks.As menticlned CRM 555 canalsobe usedto checkVOR. IFR NAV402 AP Conttinsa modtrlatedsignal and generatorfor marker,VOR, localizer,glideslope t e s t s e ti s < r l ' t h e o u t p u t c o m m u n i c a t i o nt es s t i n g .T h e frecluencies all on I l0 dBnr 7 and variablebetween ad t s e tb y a v a r i a b l et ) e q u e n c cy o n t r o l( p h a s e - l o c k e 25 kHt.on eacltblnd exceptfor glidepatltwhcre deviatitlncanbe irrtervalis 50 kt-lz). The localiz-er d . d . n rw . hile 0 ' 1 0 0 o r 0 ' 1 5 5 s w i t c h e dt o 0 ' 0 9 - 1 . g l i d e s l o pde. d . t no. f l e r s0 ' 0 9 1 , 0 ' 1 7 5a n d0 ' 4 0 0 ' . l l t l r r e cr t t a r k ctro n c s T o n ed e i e t i o nc a nb e s e l e c t e dA a r ea v l i l a b l ea. si s 1 0 2 0t l z .f o r l u d i o c h c c k '

WM,n*#]"

F r c r 6 +. -,' ; - , ,

-EFf,Gr,t

Fig. 5.14 NAV402 AI' tcst sct (cotrrlcsyllrR l'llcctrontcs lnc.)

htrcedure The prdredure for a lunctional checkis if the operationof ILS is understood straightforward and full detailsof the testset areknown. In practice, the procedurewill be listedin the aircrllt nlaintcnance manual. Carefulattentionmust be paid to testsct with low sensitivity CossorCRM 555 Forty localizer and forty glideslope antennapositioningif receivers Self-testfacilities passed as serviceable. to be not are channelsmay be selected,all crystalcontrolled. Thereare sevend.d.m. settingsfor localizer-simulated on both the testset and the aircraftinstallationshould be usedif available. deviationand five for glidepath,the d.d.m. switch

78

6 Hyperbolicnavigationsystems

GeneralPrinciples The needfor a co-ordinatesystemfor navigation purposesis self-evident,the most important being the geat circlelinesof longitudeand the linesof latitude parallelto the equator,itself a greatcircle. Figure6. I illustratestwo alternativesvstemssuitablefor usein ndio navieation.

we usethe terms circularl.o.p. (lines of be discussed position)and hyperbolicl.o.p.The patternsconsideredare not suitablefor position fixing sincetwo circularLo.p. intersectat two placeswhilst knowing the differencein rangeto two points simply placesone anywhereon one of two hvperbolic l.o.p. Knowing the startingposition and subsequentlythe track and ground speed(or heading and true airspeed)will make it possibleto usethe rho-rho system,sincea position caiculatedby dead reckoningwill identify at which of the intersections the aircraftis. To usethe hyperbolicl.o.p.we must generateanotherfamily of linesby taking a third fi;red point, we then havethe co-ordinatesystem shown in Fig.6.2. A fix is givenby the unique point wheretwo hyperbolicl.o.p.cross.Of coursethe use of three fixed points givesthe possibilityof a rho-rho-rhosystemwhere three rangecircles intersectat a uniquepoint.

Fig.6.l Circular andhyperbolic co-ordinate systems of If two fixed pointson earthhavea sequence concentriccirclesdrawn around them, eachcircle representinga particularrangefrom the fixed centre, then points of intersectionare definedbut ambiguous excepton the line joining the two points (baseline) where they areuniquely defined. Sucha systernis calledrho-rhosincetwo distance(rho) measurements areinvolved. We can usethe concentriccirclesto define hyperboliclines. Whereany two circlesintersectwe will havea differencein rangedefined;for example, the rangeto point A lessthe rangeto point B. I,he locusof points which havethe samedifferencein rangewill describeahyperbola. Thusin Fig. 6.1 the hyperbolicline hh' is the locusof the point X such that AX - BX = constant.By plottingthe linesfor severaldifferent constantswe obtain a family of hyperboliclines. In the radio navigationsystemsto

positionfix (courtesy Litton Fig.6.2 Hyperbolic navigation Division) Inc.,AeroProducts Systems International The co-ordinatepatternsdescribedaboveare currently usedin three radio navigationequipments, namelyLoranC, DeccaNavigatorand Omega. Predecessors of thesesystemsinclude GEE, a British World War II hyperbolicsystemdevelopedto navigate bomberson missionsto Germany.

79

It shouldbe clearby now that the requirementof a hyperbolicsystemis that it can measuredifference in rangewhile a rho-rho-rhosystemmust nleasure absoluterange. Two methodsare in use: time for Loran C and phase differencemeasurements for Deccaand Omega. Thesemethods measurements dictatethat Loran C is a pulsedsystemwhile the other two arecontinuouswave(c.w.). A basicproblem with phasemeasuringsystemsis that rangecan only be determinedif the whole numberof cyclesof e.m. radiationbetweenthe aircraft and the transmittingstation are know. This is illustratedin Fig. 6.3. An aircraft at X measuresthe phaseof the signalfrom station,4 which is

'r" r"l

,fi

'Raceivor

l0^-06l./

LOPS

Tg

navigation uiavehyperbolic Fig.6.4 Continuous Inc.,AeroProducts (courtesrInternational LittonSystelns Division)

( | / l00th of a lane) subdividedinto say.centilanes and so determiningon which l.o.p.the aircraftis llying is sirnplya matterof laneand centilanecounting liorrrsonreknown point. A lix requiresa separate between count to be rnadeof the lanesand centilanes onc transmitternlay be anotherpair of transnlitters. to the two pairs. The two l.o.p' will conlrnor.r intersectai the aircraft'sposition. The possibilityof laneslipexists;i.e. missingI lane Fig. 6.3 Received signal phase measurement in the count. lf this happensthe correctlanetttttstbe beingterrnedlaning. this prt'rcess established, Obviouslylaningis easierwlten liuresarewidcr. transmittingat a frequencyof l0 kllz. The Supposethe frequencyof transmitterA is l0 kHz wavelength, L, is givenby C/ I 0 000 where C is the while that of B is l5 kHz. tltenwe havea dit'ference to a lanewidth speedof light; thustr = l 6 nauticalnriles.lf the frequencyot'5 kt{z which corresponds for l0 kHz and l5 000 m opposed to phasenreasured as is. sayt)0", the distancc,4Xis 30 000 nr of ( l6// + 4) nauticalrnileswhereN is the nurnberof l0 000 m for l5 kHz. In this way lanewidth canbe wlrolecyclesoccupyinglhe spacebetwccnA andX. madewide without havingto transmitimpossiblylow Wc-sav that thc lanewidth is l6 nauticalrnilesand fiequencies.While the useof wide lanesis of + (N thc lircr:rl'tis inrportancefor tlte purposesof laning.narrowlanes i-) l:rnesl'ronrA. givegreaterresolutionand hencegreaterpotential ContinuousWaveHyperbolic Principles accuracy. in phasebetweensignals W i t l rl h y p c L t r o l iscv s t c nw the dift-erence r c a r cc o n c e r n ew dith Measuring will only be nreaningfulif the in nrngcnrthcr than ubsoluterurge: dil'lr'rcrrcc from two transntitters tlrt' uirlrornc' crluiirrnentrnustnleasure transnrissions ltavea knowtrand t'ixedphase consctyrrcrrtly exist: in phlsc bt'twcenrrdio wavesfronr two relationship.Tyo possibilities thc'rlil'li'rcrrcc r o t r n tsl t a t i o r r sl." ' i g u r6e. 4 s h o w st l t a t l n r n s r r r i t l i rgr g the canbe tlesignated tlrcrcrvill br.'zcrrrplrlsedil'lert'nccbctwcen l. one of'tl'retransntitters svrrchltlrt izctl t nrnsrrtissions r'vcry lrll [' l wavelength. master,the other the slavewhich. clnreceivittg frotrttltc'ntaster.will ensulc An lircrll'i nrcusuring a phlsc dil'ti'rcnccolthe transnrission is synchronized: c o r r l d b c o n u n y o l ' t h c d r s l r r d own transtnissiott l . o . p . . i . e . i n its d l | 0A to sotrre aresynchr-oniz-ed a n y o l ' t l r c I l n c sl r c t w e e rl r a r r s r r r i l t cAr sa n d l l . e a c h 2. both transntittcrs h i d c a t t l r eb l s c l i n e . standardtinrescalesuchasprovidedby an o l ' w h i c hi s h l l l ' a w u v c l c n g tw a t o n t i cc l o c k . S i n c c ' c v c rlya n ci s i t l c r r t i c at o l t l r r 'r e c c i v eor n t h e luircrll'ta lunt'corrntnruslbc cstlblishcdcitlrcr ll'orn PrrlsedHyperbolic PrinciPles l h c l i r c r u l ' t ' s t l r t i r r gp o i n l o r , t l u r i n gl l i g h t .l ' r o n ra n laningis.rrota problclttsitrcc' positionlix. lilch lanerrraybe lrr suchsystertts indcpcndt'rrt 80

d
T = 2t+d

Extended base line

Extended - - - - - t : obase line

-Slave

Master

L.O.P.= line of constant time ' difference.T

Fig.6.5 Pulsedhyperbolicnavigation

unambiguousl.o.p.sare obtained. Considerthe two transmittersat A and B in Fig. 6.5. One is designated the master;this transmitspulsesof energyat a fixed publishedp.r.f. On receiptof the masterpulsethe slavewill transmit, usually after somefixed delay,say d ps. If the propagationtime from masterto slaveis I gs then we c.ul seethat an airCraftat the master stationposition, or anywhereon the extendedbase line outward from the master,will measurea time differenceof (2t + d) gs when comparingthe tinre of arrivalof the masterand slavetransmissions.An aircrafton the extendedbaseline outward from the slavewould record a time differenceof d ps. Should an aircraftbe anywhereother than on the extended baseline the time differencewill be someunique readingbetweend and (2t + d) ps. Disadvantages of hyperbolic systemsare that lane width varieswith distancefrom baseline and that signalgeometryis important. With a hyperbolic co-ordinatesystemthe angleof cut betweentwo l.o.p. canbe suchthat the tangentsto the lines at the aircraftposition are almostparallel;for other aircraft positionsthe hyperbolicI.o.p.may cut almostat right angles.Of courseif more than two l.o.p. are availablethe geometryproblem is of little consequence sincea most probableposition can be computed. Figure6.6 illustratesthe variousl.o.p. geometries. Continuous Wave Rho-Rho and Rho-Rho-Rho Systems To measurethe phaseof a receivedsigral a suitable referencemust be available,generatedwithin the receiver.Let the phaseofthe referencesignalbe @r, the phaseof the receivedsignal be @"when the aircraft is at point A and fu when the aircraft is at

L.O.P.3

).P.2 L.O.P. Best geometry very accurate L . O . PI.

Worst geometry poor accuracy Fig. 6.6

Multiple L.O.P. good accuracy due to redundancY

Various geometrics for hyperbolic systenrs

point B. The airborneequipmentwill tncasurctlre differencein phasebetweenthe receivcdsignaland signal,i.e.: the reference Q^=Qr-Qr when the aircraft is at point A, while: Qm=h-Q, when the aircraft is at point B. The changein phasc asthe aircraft moveswill providea measureof the changein range,we have:

@.:(0" -Q)=Qa-Qb

( 0 b- 0 ' ) (6.1)

Theseideasare illustratedin Fig. 6.7. The aboveworking hasassumedthat the reference sigrraldoesnot drift in the time it takesfor the aircraft to travel from A to B. If the referencephase is @r.and @16when the aircraft is at point A and B respectively,then equation(6.1) must be modified

81

AtB

+ 0 ^ : Q a- 6 u Fig. 6.7 Changein measuredphasewith afucraftmoveinnenl

to accountfor this drift, it becomes:

Q^=(0^-@u)-(0'"-0'u)

(6.2)

Thus an equipmentcontinuouslymonitoring the changein measuredphasein order to calculatechange in rangewill be in error by an amount dependingon the referenceoscillatordrift. At the moment of switch-onthe referencesignal on board the aircraft is not phaseJockedto the gound transmitter'sfrequency. Further, sincethere is a signalphaseshift due to the transmissionpath, therewill be a phaseor clock offset (@o)between receivedsignaland the local reference.If at switch-onboth the transmitterand receiverpositions areknown @ocan be calculated,and if at subsequent aircraft positions this phaseoffset remainsthe same then by measuringphasedifferenceas described earlierthe changein rangefrom the known starting point may be computed. Referenceoscillatordrift can be consideredas a changein phaseoffset' Errors arisingdue to a changein phaseoffset can be minimized in three ways: l. usc difference in phasebetween synchronized sigrralsfrom two remote transmitters,in which caseany changein referencephasecancelsout (this is the hyperbolic approach); 2. a precisionreferenceoscillatorof atomic clock standardcan be carried on the aircraft, in which cas€drift is negligible over the duration of the flight (this is the rho-rho approach); 82

3. estimate the phaseoffset throughout flight by utilizing signalsfrom more than two transmitters(this is the rho-rho-rhoapproach)' The operation of a rho-rho system is illustrated in Fig. 6.8. As the aircraft flies from I to 2 the phase changesin the signalsreceivedfrom transmittersA andB are continuously measured;this allowsthe airborneequipment to count the number of range lanesand centilanestraversedwith respectto both transmitters.Equation(6.1) applies,sincereference oscillatordrift is negligible. Thus if the aircraft position at point I is known it can be computedat 2.

\dIA-(6A2-aLO2)

- l c l 1 1- d s 6 1 )

l 6 M O ' l d 3 2 - O ! O 2 l- t o g l - C 1 g 1 ;

.t.'12-6A1t-{6192-cLg1) rclE

LGAL mnLUrOn

-tc62-ca1l-16192-{1611

OrrFt ' t6LO2 - 6LOtl

Fig. 6.8 Rho-rho navigation (courtesy Litton Systems International Inc., Aero ProductsDivisio')

With a rho-rho.rho s),stenltwo range(circular) l.o.p. givea position fix while a third can be usedto eliminateerror in phaseoffset,@". ln Fig.6.9 it can be seenthat a non-zero@,givesus the situation where threel.o.p.do not intersectat one point but-form a trianglewithin which the aircraft is positioned.

hyperbolic and rho-rho-rho methods require three ground transmittersfor a fix but the geonretryof the aircraft and transmitterswill degradethe accuracyof the hyperbolicsystemmoreso than eitherrho-rho-rho or rho-rho. A hyperbolicsystenrhasthe most complexcomputerprogram,but is nevertheless

o Transmitter 2

x

Transmitter

,

,'\1 \

True position

True position

Ptc-

- -

P. P^ P.

Kncwn points

--- CalculatcdL.O.P. - T r u eL . O . P . Fig.6.9 Rho-rho-rho navigation of this positiontrianglcshou.sthat the probably the leastcostly sinceits local oscillator Consideration p e r p e n d i e u ld a irs t a n c elsi o r r rt l r c t' r u e p o s i t i o nt o t h e stabilityrequirenrents arelessstringentthan eventhe c a l c u l a t eld. o . p .u r ee q u a lt o e l c h o t h r . -l rn d g i v ea rho-rho-rhosystem. n l c e s u roc f 'f " . S u l ' l i c i c nitr r t i r r n r a t i oi rsra v a i l a b l e l i o n r t h e t h r e el . c l . p t. o e v a l u a t p e " . a s s u n t i nteh a t rr-fercnce oscillatordrift is the orrlv sorrrceof t'rror. Omega Navigation System (ONS) S h o u l do t h e re r r o r sc o n t r i b u t et o t h e c a l c u l a t e d l . o . p .t h e p e r p e n d i c r r ldairs t a n c els' r o mt h e t r u e Omegais a very low-frequency. c.w.,long-range p , o s i t i otno t h e c a l c u l a tdel . o . p .w i l l n o l n c c e s s a r i b he , navigationsystem.Threetime-multiplexed signalsof L-qual and an approxinratesolutionnrustbe sought. l 0 ' 2 , I l ' 3 3 a n d l 3 ' 6 k H z a r et r a n s n - r i t t e d omnidirectionallyfrom eachof eight stations Cdmparisonof Systems strategically locatedaroundthe world. Althoughthe Thereis no clear-cutbestsystemto enrploy,and in conceptwaspatentedin 1923it wasnot until the fact all are in useas follows: mid 1960sthat the US Navy establishedthe first experimentalstations.By 1968it wasestablished pulsedhyperbolic I-oranC that ONS was t'easible and the setting-upof a c.w. hyperbolic DeccaNavigator,Omega worldwidenetwork commenced.The USA is c.w. rho-rho Omega responsible for the stationsin North Dakota,Hawaii, c.w. rho-rho-rho Omega Liberiaand a temporarystationin Trinidad,while It is interestingto observethat manufacturersof stationsin Norway,Japan,Argentina,La Reunion Omeganavigationsystemshaveopted for different and,by 1980,Australiaarethe responsibilityof methodsof calculatingposition,illustratingthat nationswhich haveestablishedbilateral agreements thereis no universallyacceptedbestmethod. with the USA. Although the responsibilityfor The rho-rhomethodis certainlythe simplestof co-ordinationwasoriginallyallocatedto the US Navy the three,needingonly two qroundtransmittersand it hasnow beentaken overby the US CoastGuard. employinga relativelysimplecornputerprogram. 'The Omega Stations and Broadcast Patterns It does,however,havethe costlydisadvantage of requiringa very stablereferenceoscillator. Both Eachstationhasa transmitterDowerof l0 kW with

83

Table5.1 Sigralformat,o.n.s.

{_-Ios 0 ' 9s <---'

Stations

Norway Liberia Hawaii North Dakota [: Reunion fugentina Trinidad/Australia Japan

A B C D E F G H

r0.2

1 ' 0s <|*.'

r3 ' 6 t0.2

l'l s H

I1.33 13.6 to.2

1.2s H

I 1.33 13.6 10.2

I'I s <----}

I l-33 13.6

ro.2 I 1.33 r3.6

l 1.33

0 ' 9s e

I1.33 13.6 to.2

1 - 2s H

I 1.33 13.6 10.2

I'0 s <'_}

I I '33 13.6 10.2

the exceptionof the temporarystation in Trinklad which hasa I kW transmitter. Radiationis from an omnidirectionalantennawhich takesthe form either 16 NM 9 CYCLES of a verticaltower, approximately450 m high, supportingan umbrellaof transmittingelements,or a valleyspantypically 3500 m in length. Equipment redundancyensuresreliableoperation99 per cent of the time. As mentionedabdveeachstation transmitsthree 1 4{ N M lo cYcrts frequenciesin a time-multiplexedpattern which is uniqueand providesidentification. The transmission formatis shownin Table6.1. It canbe seenthat at any one time only three stationswill be transmitting, eachon a different frequency. There are short intervals(0'2 s) betweentransmissionbursts. The pattern is repeatedevery l0 s. Fig. 6.10 Omegafrequencyrelationships(courtesyLitton to a nearly All transmittersare phase-locked SystemsInternationalInc., Aero ProductsDivision) absolutetime standardprovidedby the useof atomic clocksat eachof the station locations. The result is that the three frequenciesin all transmitters simultaneouslycrosszero with positiveslopeat precisetimes every l5/ l Tths of a millisecond. This phaserelationshipis illustratedin Fig. 6.10. The net resultis that the timing error betweenstationsis at most I ps, leadingto a maximum error in position fix of 300m. (courtesy Litton waveguide Fig.5.1I Earth-.Ionosphere Division) Inc.,AeroProducts hopagation Interpational Systems The band of frequenciesl0-14 kHz is an appropriate since predictabilityof the changesin phase. choicefor a phase-measuring navigations1)'stem e.m. radiationat thesefrequenciescan travel A requirementof the systemis that four or more thousandsof mileswith predictablephase-change stationscanbe receivedeverywher€.Account must characteristics.A natural waveguideis formed by the be taken of the attenuation of the signalwhich varies earth'ssurfaceand the D layer ofthe ionosphere,the with direction due to the rotation of the earth. dimensionsof which are suitablefor.propagationof Signalstravellingin an easterlydirection suffer approximately2 dB/1000 km attenuation,while the ONS frequencies.This mode of propagation thoseon a westerlypath suffei approximately accountsfor the rangeof the signalsand the 84

4 dB/1000km. North and southattenuationis the sameat 3 dB/1000km. A further consideration is that signalscannot be usedcloseto the sourcesince the phasevariationsare unpredictablein this region. The implementationof ONS with eightstations,which arenot equi-spaced around the world, leadsto a situationwhere,undernormalconditions,between four and sevenstationsare usabledependingon the receiverlocation.

meansthat a conductivity map can be stored in the computer,so enablinga propagation-correction factor to be calculatedfor the path betweenthe receiverand the known stationlocation. A complicationarisesin that it is possibleto receivea direct sigral and one which hasgonethe 'long way round', in which casewe havea mutual interferenceproblem. Automatic deselectionof stationsat rangesin excessofsay 8000 nauticalmiles is usedto minimize this effect. 3. Geomagnetic FieA The earth'smagnetic(H) field altersthe motion of ions and electronsin the lower regionof the ionosphere,thus affectingv.l.f. propagation. Again the equipmentsoftwaremay be usedto apply corrections. 4. Norspheroidal Effects The computationof aircraft position must take into accountthat the signalpath from transmittingstation to aircraft receiveris not on the surfaceof a sphere. Further, pressuredifferencesat variouslatitudes effect the height of the ionosphereso compensation must be made for the effect on phasevelocity.

5. Modal Interference There are variousmodesof propagationin the waveguide.If one mode is earth.ionosphere dominant the phasegrid producedwill be regular; howeverin practicea competingmode can be almost Fig,6.12 Typicalusablecoverage equal to the dominant mode in which case irregularitiesappearin the phasepattern. The most seriouscaseoccurswhen one mode is dominant at Factors Affecting Propagation night and a secondduring the day. It follows that during sunriseand sunsetthe two modeswill be l. Diurnal Effect equal. SomeOmegareceiversautomaticallydeselect The height of the ionospherevariesby approximately station B (Liberia) at critical times sincesignalsfrom 20 km from day to night, beinghighestat night. The this station are particularlysusceptibleto modal phasevelocity of the propagatedwavewill be greatest interferenceat night. during the day when the dimensionsof the 'waveguide' areleast;this leadsto phasevariationswhich 6. Solar Effects fortunatelyarepredictableand cyclic. Correctionsto A solar flare givesriseto a largeemissionof X-rays compensate for diurnai effect may be implemented which causesa short-termdisturbancein a limited by meansof a softwareroutine. The entry of GMT part of the ionosphere.Suih an eventis calleda anddateat switch-onis requiredbv the routine. suddenionosphericdisturbance(SID) or a sudden phaseanomaly(SPA) and may last for I h or more; 2. Ground Conductivity l.o.p. in the affectedregionsmay be shifted by up to The different attenuatingeffectsof the oceansand say 5 nauticalmiles. TheseSIDsoccur about 7 to l0 varioustypes of landmasschangesthe phasevelocity times per month, but during the peak of the I l-year of the v.l.f. signal.The greatestlossof signalstrength sunspotcycle a major solarflare may product a shift occursin the ice-capregionswherethe changein in l.o.p.by up to 15 nauticalmiles. This latter event phasevelocityis significant.Waterhasleasteffect. is predictable,and warningsmay be issued. The effect of ground conductivitybeingwell known Infrequently largequantitiesof protons are 85

releasedfrom the sun,producinga so-calledpolar cap (p.c.d.).The effectof a p.c.d.,which is disturbance to shift l.o.p. from say 6 to 8 nauticalnriles,may last for severaldays. Only thosetransmissionpaths passing overthe polesareaffected.Sincethe p.c.d.is may be of long duration navigationwarningmessages broadcast. Position Fixing ONS may usehyperbolic, As previouslydiscussed rho-rhoor rho-rho-rhomethods,a root meansquare accuracyof l-2 nauticalmilesbeingobtainablewith all methodsprovidingthe computersoftwarecorrects for predictableerrors. Whatevermethod is usedthe lane in which the aircraft is flying must be established.Lane widths for the basicfrequenciesand differencefrequenciesare givenin Table6.2. It canbe seenthat the broadest lane for the direct rangingmethodsis 144 nautical mileswhile that for the hyperbolicmethod is 72 nauticalmiles. If it is known which broad lane the aircraftis in thenit is possibleto resolvelane as ambiguityfor the narrowerlanesautomatically, shownin Fig.6.l3. In this exampleit is known in and lanewidths,o.n.s. Table6.2 Frequencies (lanewidthsin nm)

Basic frequ encies

13.6-10= .2 l 3 ' 6 - 1I 3 = I 1 . 3 - 1 0 .=2

1 0 . 2k H z 1 1 . 3k H z 1 3 . 6k l ' l z 3.4 kHz 2.3 kHz l.l ktlz

Dircct ronging

ltltperbolic

16 14.4 12 4ti '12

8 7.2 6 24 36 i2

114

'13 6 KHZ lanes

13.6KHZ LOP

Rate Aiding patternextendsovera period The ONS transmission of l0 s. If the phasesof all usablesignalsare overthis periodand then l.o.p. are measured generated for positionfixing.an error will result. sincesomeof the phaseinformationwill be up to l0 s old. Aircraft directionand speedinformation niay be usedto updatethe phaseinforrnationlor this processbeingknown as rate l.o.p.calculations, l.o.p.at lessthan aiding. In practicewe cangenerate l0 s intervals,sayeveryI s, thusONS can be asa deadreckoningsystemwith considered position-fixingupdateseverysecondor so. Directionand speedinformationcan comefrom a headingand for examplecompass numberof sources, true air speedfrom an Air DataConrputeror track and groundspeedtiom Doppleror INS. SomeOmega trackand groundspeed equipmentsgenerate internallyfrom computedpositionchanges. If for any reasonthereis a lossof signaldead reckoning,dataon directionand speedinputsot track and ground last-knowninternallygenerated speedcan be usedto continuouslycalculatethe aircraft'sposition.so that on receiptof sufficient laneambiguityis easilyresolved. usablesignals, last-knowntrack Obviouslyif the internallygenerated and groundspeedareusedduringdeadreckoning cause duringtlrisphase.may then aircraftmanoeuvre laningproblemswhen signalslre receivetiaglrin.

Broad lane

10.2KHZ lanes

10-2 KHZ LOP Unique LOP

Fig: 6.13 Resolvinglane ambiguity (courtesyLitton Systems lnternational Inc., Aero ProductsDivision) 86

which 3.4 kHz lanethe aircraft is flying. Phase measurementof the l0'2 kHz signalgivesthree possiblel.o.p.while the l3'6 kHz signalgives tbur possibleLo.p. Only one of the possibleLo.p. from eachgroupis coincident,this beingthe uniquel.o.p. on which the aircraftis positioned.

In sucha caseaircraft approximatepositionwould haveto be enteredby the pilot.

Fig. 6.14. The ONS consistsof a receiverprocessor unit (RPU), control displayunit (CDU) and antenna . couplerunit (ACU). Sucha break-downof 'black boxes' conformsto ARINC Characteristic599 but Most Probable Position somemanufacturerschooseto separatethe receiver Thereis a redundancyin the Omegasystemin that and computerand alsothe antennaand couplingunit. normally more signalswill be receivedthan are The RPU is fitted in a convenientlocation. the necessary to computethe two l.o.p. neededfor a fix. most important considerationbeingcooling ln this case,data from all receivablestations,and as a forced downdraughtor integal many frequenciesaspossible,may be usedto generate arrangements, a numberof l.o.p. If all frequenciesarereceivedfrom blower being typical. The CDU must of coursebe mounted in view, and in reach,of the pilot; all stationstherewould be 3 X 8 = 24 phase normally specialcoolingarrangements are not giving every l0 up to measurements s, twenty-four required. l.o.p.for a singlefix. The multiplel.o.p.will not The antennausedmay be of H field or E field crossat a point but will definea smallpolygon type, the latter possiblyemployinga separatecoupler within which the aircraft is positioned. The computer unit with a suppliedinterconnectingcable. An E will calculatethe aircraft'smost probableposition field systemis sensitiveto precipitationstatic within this polygon. discharge, thus good bondingand sufficient In practicetherewill be far fewer than twenty-four strategically spacedstaticwicks are essential.An H phasemeasurements available.Automatic deselection field system is sensitiveto magnetic(a.c.)noise place poor for will take reasonsof signalto noise sourcesand a skin mappingshouldbe carriedout on ratio, poor geometry,susceptibilityto modal initial installationto determinethe optimum location interferenceor outsideusablerange(too closeor too for the antennawhich may be on the top or bottom far). Manualdeselectionwill be accomplishedas a of the fuselage. resultof pre-flightor in-flight information received concerningstationstatusor unusualionospheric Skin Mapping Detailed procedureis given in activity. manufacturer'sliterature,but basicallythe aircraft shouldbe parkedaway from all powerlines,both Communication Stations, v.l.f. aboveand below ground,and away from all A worldwide high-powermilitary communications obstructions. Ambient signalplus noiseis then network operatingin the band l5-25 kHz is initially approximately100 ft from the recorded maintainedby the US Navy. As a secondarypurpose aircraft with analysersetat l0'2, I l'3 and a spectrum provide to of the network is worldwide aremadewith an synchronizationof time standards,the carriersignals l3'6 kHz. Similarmeasurements arepreciselytimed, and so may be usedfornavigation ACU securedby tape at variousairframelocations. Comparisonof ambientand airframemeasurements purposes.Sincecontrolof the stationsis out of the handsof thosebodies,eithernationalor international, will identify severalpossiblepositions. The optimum position(s)can then be found by repeatingthe responsiblefor civil aircraft navigation,use of the measurements undervariouson and off conditions network for navigationcan only be consideredas ofengines, electronicsand fans. The final lighting, supplementaryto other forms of navigation. position shouldbe checkedout for signalto noise Hyperbolicnavigationis not suitablefor usewith v.l.f. commsstationssinceabsolutephasedifferences ratio with enginesrunningat 90 per cent minimum. betweentwo receivedsignalscannotbe determined Brief Description of Units due to eachstation operatingon an unrelated The descriptionswhich lollow arebasedon the Litton frequency. A further disadvantage is that the diurnal LTN-2 I l; other systemshavesimilarunits which vary phaseshifts are not aspredictablefor v.l.f. signalsas in detail. they are for ONS sigrals. Severalmanufacturersoffer equipmentwith v.l.f. and Omegacapability;in somecases v.l.f. is optional: Receiver hocessor Unit The RPU is the major part In suchequipmentOmegasignalsprovidethe primary of any ONS. Omegabroadcastsignalsfrom the ACU navigationinformation while v.1.f.signalsprovide are processedtogetherwith inputs from other sensors back-upshouldinsufficientOmegasignalsbe usable. to give presentposition and guidanceparametersas required. The major partsof a RPU will typically be: Installation A typical simplifiedinstallatibndiagramis shownin r.f. circuitry;

cl

Autopilot

H.S.t. Installation Program

Atc

F.D.t. Aircraft Data Bus

Speed Source Aircraft Instrumentation

Fig. 6.14 Litton LTN-21I ONS installation(courtesyLitton SystemsInternational Inc., Aero ProductsDivision)

88

central processorfor computing function; scratch-padRAM for temporary data storage; specialRAM in which pilot-entered data is saved during power interrupt; ROM to store program which will incorporate corrections; power supply assembly; analogueinterface; digital interface;

BITE; antennaswitching; .chassis. bnnol Display Unit T\e CDU provides the interface between the flight crew and the ONS. Data transmissionbetween CDU and RPU is via two one-wayserial digital data buses. The RPU transmits four 32-bit words to the CDU while the CDU transmitsone 32-bit word to the RPU. The d.c. voltagesfor the CDU are provided by the RPU. The CDU annunciatorsare driven by signalsfrom the RPU. Antenna Coupler Unit Two H field bidirectional loop antennasare wound on ferrite rods arrangedat right anglesto eachother. Pre-amplificationof the signaltakes place in the ACU. Provision is made for the injection of a test signalto eachloop. ONS Interface Operator Inputs l. Presentposition latitude and longitude: entered duringinitialization, i.e. during preparationof the systemprior to take off; 2. waypoint latitude and longitude: rrp to nine enteredas requiredduring initialization; editing facility availablefor in-flight entry; 3. GreenwichMean Time/date: enteredduring initialization.

2. 26V a.c. 400 Hz reference:from external equipment acceptingsynchro feedsfrom ONS; 3. aircraft data bus: interfacewith digital air data syst€m(DADS), inertial referencesystem(lRS), flight muragementcomputersystem,etc. In stalla tio n Programming Various receiverprocessorunit connectorpins, termed 'programdiscretepins' are groundedby meansof a link to earth in order to select: l. Speedinput format; 2. frequencystandard; 3.magnetic/trueheading input/output; 4. oleo strut logic; 5. synchrooutput; 6. gid mode: local, Greenwich or t$ro alternatives; 7. antennamount: top/bottom. System Outputs Analogue/Discrete l. Track angle; 2. crosstrack deviation; 3. track angleerror; 4. drift angle; 5. track angleerror plus drift angle; 6. true heading: 7. desiredtrack angle; 8. track changealert; 9. track leg change; 10. steeringsigral (roll command); I l. To/from. Digital 12. Presentposition (lat./long.); 13. heading(mag./true); 14. track angle; 1 5 . g r o u n ds p e e d ; 16. distanceto waypoint; 17. time to GO; 18. wind angle; 19. wind speed; 20. crosstrack distance: 21. track angleerror; 22. drift angle; 23. desiredtrack.

Extemal SensorInputs l. Speed:from air data computer(ADC) or Doppler radarin a variety of sigral formats; 2. heading:from compasssystem; 3. drift angle:from Doppler radar,optional; 4. speedvalid sigral; 5. headingvalid signal; 6. compassfree/slavedinput; 7. oleo strut switch input; 8. drift anglevalid signal.

Warning 24. Cros track deviationfailure; 25. true headingwarning; 26. steeringsignalwarning.

Other Inputs l. Frequencystandard:rho-rho opti6n;

Figure l.l3 definespictorially thoseoutputs relating to angleand distance.

89

Right numerical display

Dim control

Alcrt ann (amber) Dead reckoning ann (amber)

From/to and waypoint display

Sync ann (amber) Waypoint soloctor switch

Ambiguity ann (amber)

Track change pushbutton (green)

Warn ann (red) M a n u a la n n (amber)

Mode switch Entcr pushbutton

roi

I

64r;'l][\s11

Hold pushbutton (greenl

..orsrrME

Clcar pushbutton (green) rAS

Data keyboard pushbuttons Fig. 5.15 Litton LTN-21I CDU (courtesyLitton Systerns lnternationalInc., Aero ProductsDivision)

C-ontrols and Operation The following information may be displayedby Figure6.15 showsthe Litton LTN-21I CDU which is appropriatepositioningof the displayselectorswitch: similarto thoseof other manufacturers.A very rangeof controlsand displayeddata is GMT/DAT GreenwichMeanTime and date comprehensive Track angleand ground speed TK'GS available.Brief detailsonly are givenhere. HDG/DA Heading and drift angle The pilot is able to enter his presentposition and up to nine waypointsdefininga great-circlenavigation XTK/TKE Crosstrack distanceand track angleerror Presentposition POS flight plan which can be updatedduring flight' (selected) fly Waypoint pilot to WPf helps him to the Information displayed DIS.TIME Distanceand time (to'go') the specifiedroute from waypoint to waypoint or fly Windditection and velocity WIND parallel offsets from the flight plan. If the autopilot DTK/STS Desiredtrack and status(malfunction) is engagedsteeringinformation from ONS causesthe Magneticheadingand true airspeed MH/TAS aircraft to automatically follow the flight plan, in Station status STA which casethe display is used for monitoring FROM/TO From and to waypoints for current leg. purposes. The system also demandsentry of GMT and date. System Softward A keyboardis usedfor all dataentry which is The major tasks for the software employed in a a coded which case error, in checkedfor operator rho-rho-rhoOmegasystemare describedbriefly warning is given. System failure waping is given by below. the WRN annunciatorwhen malfunction and action codesmay be displayed. Severalother annunciators The transmissioripattern must be Synchronization givewaming of track leg changeimminent (ALR), 'identified in order that the ONS will know when each systemin deadreckoningmode (DR), synchronization of systemwith transmittedsignalformat taking place stationis broadcasting.Sincethe Omegatransmission patiern repeatsevery l0 s synchronizationis attempted (SYN),lane ambiguity(AMB) and manuallyent;red offset track by ampling l0 s of data in order to try and find the cross true airspeed,magneticheadingor stait time (station A transmitting lO'2 kHz burst). (MAN). displayed being 90

If synchronizationis not successfula further l0 s periodis sampledand so on. Sincebefore synchronizationis completesignal directionis not known, the antennais set to an omnidirectionalmode. The idea of the synchronization routine is to look from sample for correlationin phasemeasurements to samplefor the three broadcastfrequencies.Noise alonewill of courseappearwith random phase,not correlatedbetweensamples. Phaseand Signal-to-NoiseMeasuremenrs The phase differencebetweenthe receivedsignaland a local referenceis sampledat regularintervalsthroughout the burst. Forming sineand cosinesumsof the sampledphaseangleswill allow a burst phase measurement(averageof samples)and signal-to-noise to be made. If no signalis being measurement receivedduring the samplethen no contribution will be made to either the sine or cosine sums. We have:

l. eliminatemanuallydeselectedstations; 2. eliminatestationsfor which the aircraft is not within areacovered(seeFig.6.l2); 3. eliminatestationswith known modal interferenceproblem at night in certain areas (Liberia); 4. eliminatestationson the basisof poors.n.r.; 5. eliminatefrequenciesfrom particularstations whosephasedifferencebetweencomputedand measuredexceedsa certainfigure. All qualifying frequenciesare used for position determination. If lessthan minimurn number of stationsare available,the deadreckoningmode is entered. hopagation Conection The computer must calculate a propagationcorrection0p, the valueof which will dependon the path from station to aircraft, the timg of day and the date. Factorsaffectingpropagation earlierand while complete havebeendiscussed

. t / \\ =tan-r phase shiftHllT::lx'L: f bursr ilir:",,Ti::TlT;:ii'"illfo*' /r t'urg ) )- antenna (6'J) cosd

// \lJ l R burst= (rrin o\' * /> .o, p\' / \-N \-T-/

(6.4)

wherethe summationsare of the samplesover the burst, @is the phaseangleand R, which lies between 0 and l, givesa measureof sigral to noise ratio (s.n.r.). The valuesof @burst and R burst are fed to a trackingfilter in order to give smooth values@andR. Thereare a total of twenty-four tracking filters (threefrequenciesX eight stations). Rate aidingis appliedto @to compensatefor known aircraft motion. Each tracking filter is updatedafter the appropriateburst, i.e. every 10 s, rate aiding values arecalculatedevery 0'l s. Antenna Selection Every l0 s the bearing to the eight stationsis computedand stored. Every second the differencebetweenbearingand headingis computedand usedto selectthe longitudinal loop, lateralloop or combinationof loops to make the antennadirectional,the main lobe being in the direction of the station to be received.. Station Selection For Omegarho-rho-rho navigation threestationsmust be receivedto calculatethe three unknowns(latitude, longitudeand clock or phase offset). Various criteria areusedin the selectionof stationsto be employed:

andcomputation ti,nitin.a in orderto savestorage include: tim;' simplifications integationstepsizealongpathto say l. increasing 2. usingcoarsememory map, i.e. subdividingearth into, say,4" x 4" block and assigtinga conductivity index to eachcorrespondingto averageconductivity in that area; 3. simplifyingsub-routinewhich computesbearing of signalpath to earth'smagrreticfield.

Computercomparisonsof simplihedand more accuratemodelshavebeencarriedout and show excellentagreement. Cunent Least-SquaresEnor Cabulttton The measuredphase@is correctedfor propagationshifts, 0p, and estimatedclock offset, f., and then compared with the phase,@r,derivedfrom the current calculated rangebetween aircraft.and station to give A 0, we have LQ=(Q-Op-QJ-Qr

(6.5)

There is one A@for eachstation frequency so there will be at most twenty-four. If thereis no error, that is 0p, 0. and @,are all correct,then A0 will be zero. In practiceeqrorswill exist,so the purposeof the error estimationroutine is to find least-squares correctionsto @.,and the computedposition to in use. minimizeAd for the stations/frequencies

It

Sincethe mostreliableinformationcomesfrom the strongest signaleachA@is weightedby its s.n.r. (smoothed R from equation(6.4)). Thesquares of theA@arecomputedto preventcancellation in the sum.Wehave:

is wind (north and east),latitude and longitude. The wind is not computed when the aircraft is on the ground,asindicatedby the oleo strut switch. Summary The above notes on system softwaveare by

(6.6) no meanscomplete;somefunctions of the softwave

minimize>R(AO)2

havenot been mentionedalthoughhavebeen implied elsewherein the chapter. The major navigationtasks and their implementationare best summarizedby a flowchart (seeFig. 6.16).

where the sum is taken over all the stations and frequencies in use. The phasedifference,LQ, canbe expressedin termso(A@., AN, AE and B where the first three termsare the correctionsin clock offset. north position and eastposition respectivelywhile B is the bearingto the station. If we considerthe signal receivedfrom station I on frequencyJ, we have: A0(l,J) = Adc - AN . cosB(l) - AE . sin B(l) (6.7) Thus equation(6.7) is usedin (6.6), the minimization of the weighted sum of squares,giving a least-squares estimateof the current error which enablescorrectionof clock offset and position. The correctionvectorX = (A0., AN, AE)T is smoothedby clock and position filters, rate aidingof speed,resolvednorth and eastabout aircraft heading, beingappliedto thesefilters. The output of the filters

The Program The actualprogramusedin any ONS is proprietary and will vary greatly dependingon the type of microprocessorused,the method of navigationand the ingenuity ofthe author. In generalthere will be a main loop which checks for power interrupts, computespropagation correction,carriesout self-testing,etc. The main loop will be interruptedwhen 10.2, I l'3 or 13.6kHz information is availablefor processingand alsowhen the CDU is ready to input or output data. In the LTN-21I the phasedata interrupts for the three frequenciesoccur regularlyat 6.25 ms intervals. The I I '3 and I 3'6 kHz interrupt loops simply serve to read the appropriatephasedata while the

Clock

Phase | detector

Omegr signal

0

SinI

-lSin/Cosine look up

C"t C

Clock offset estimator

Qc

t(r, J)

Tracking filter

-

+

Position f ilter

Propagation prediction

+

Lat./Long. Wnd

Rrtc rid north

Ratc aid oast

Fig.6.16 Softwareflow chart

gi2

Least

?

6o

-t

LOc AN AE

R (r,Jl

Rate aid

C o u'sr(1,J) R - "il' J)

Burst processing

estrmator

6, t

GMT Lat./Long.

Rangeto station

-l

Expected phase

L

l0'2 kHz interruptloop processes all phasedataand drecksto seeif it is time to startloopswhich occur regularlyandperformvariouscomputationsand drecks.Brieflyu,ehave: 'update lfl) msloop: clock

amplified and limited sigrals are comparedin phase with referencesigrals derived from a 4.896 MHz clock. A real-time interrupt is generatedon completion of a phasemeasurementfor each of the Omegafrequencies.The l0'2, I l'3 and 13'6 kHz interruptsareeachgenerated160 timesper secondto rate aid computation inform the computer that phasedata is availablefor check synchronization 368 gs, during which time the appropriate interrupt antennaselection routine is entered and the data read. The sensor burst processing phasedata forms word I in a four-word, l6-bit serveCDU digital multiplexer. I s loop: horizontal steeringcommands Heading and speedinputs enter the system in the l0 s loop: estimator least-squares form of three-wire synchro feeds. Scott-T savekev data transformersresolvethis input into the sine and cosinecomponentswhich are then demodulated and Hardware filtered to provide d.c. signalsto an analogue multiplexer controlled by the computer. After C.omputo Inputs Figure 6.17 showsa simplified systemblock diagramof the LTN.2I l. Omegasignals analogueto digital conversionthe headingand speed from the ACU are fed via an anteniraswitching matrix sine and cosine componentsare multiplexed asword 2 in the digital multiplexer. Words 3 and 4 contain to three narrow-band receivers.one for each of the data relatingto variousdiscretes,programpins, three Omegafrequencies. Antenna switching is validities and sourceselectors. derived from the computed relative bearingof the ARINC 575 data from the CDU is converted to station being receivedat that particular time. The

l?u?*^ss'-s I l ; -

I sTtiltNG rNTtttAct

[**r**' )

POWTISUPPTY

Fq. 6.17 Litton LTN-21I block diagram(courtesy Lftton SystemsInternationalInc,, Aero ProductsDivision)

sl

logic) levelsand shifted into TTL (transistor-transistor a serialto parallelshift register.When the input data is ready an interrupt is generatedand the contentsof the registerare readby the computer. A digital interfacecard,part of the steering interface.is a link betweenthe ONS and other systems.Thereare four ARINC 575 receiversfor DADS TAS (digital air data systemtrue air speed), flight management,IRS (inertial referencesystem) and an inter-systemb.c.d. or binary bus for interface with anotherONS or possiblyanothertype system. The addressbus from the computeridentifiesthe requiredreceiverwhich storesthe particularinput an interrupt. The word in a registerand generates this interrupt, so requestinga computeracknowledges transferofdata onto the l6-bit paralleldata bus via a tri-stateregister. Sincethe ARINC 575 serial-parallel word is 32-bitslong, transfertakesplacein two sections. Memory The navigation computer program is stored in a 20K X l6-bit word (K = 1024) UVEROM (ultravioleterasableROM) which can be programmed from cassettetapeusinga programmingadapter. The datais retainedin the WEROM for an estimated100 yearsunlesserasedby exposingall twenty chips to ultravioletlight. Additional memory is providedon the card in the form of a 2K word computer/processor scratchpad RAM for temporary storageand a 128-worddata savememory usedto store present position,time/date,waypointsand additional data requiredto resumenormal operationsafter a temporarypower failure. Back-uppower for data saveis provided by a 4500 pF capacitor for at least 7 min. The Computer The computer usesthe input and storeddata referredto in the aboveparagraphsto carry out the necessarynavigationproblem computations and to output the resulting employedis a information. The microprocessor TMS-9900,a l6-bit CPU (centralprocessingunit) capableof addressing32K words of length l6 bits. Input/output (l/O) functionsare treatedin the sameway as memory for addressingpurposes,the twelve addressmap beingsplit into sixteensectio,ns, of which are assigredto WEROM and two each to RAM, savedata memory, steeringinterfaceI/O and other I/O. As an example F A 0 0 hexadecimal= 1 5x 1 6 3+ l 0 x 1 6 2+ 0 x 1 6 r + 0 x 1 6 ' = 6 4 0 0 0 1 e , is the addres of the digitd multiplexer phasesensor word which will contain phasemeasurementdata correspondingto l0'2, I I '3 or l3'6 kHz depending 94

on which interrupt is beingserviced.Not all the are assigned in the LTN-21l. availableaddresses The computercan recognizesixteeninterrupt levelswith the highestpriority level0 and the lowest priority level 15. An interrupt maskis containedin a statusregisterand continuouslycomparedwith the interruptcode. Whenthe levelof system-generated the pendinginterrupt is lessthan or equal to the current-enablinginterrupt masklevel(higher or equal the interrupt. On recognizes priority) the processor recognitionthe current instructionis completed, detailsof the position in current programstoreo, the appropriateinterrupt serviceroutine started,and the interrupt mask forced to a levelthat is one less than the level of the interrupt beingserviced.When the interrupt routine is completethe interrupted programcontinueswhereit left off. In the LTN-21I seveninterrupt levelsare impiemented: 0 I 2 3 4 5 6 7

reset(power on)l pendingpower fail or programcycle fail; lO'2 kHz sensordata input; l3'6 kHz sensordatainput; 1 l'3 kHz sensordatainput; reservedfor a sensordata input; CDU ARINC data ready; ARINC receiverdataready.

Cttmputer Outputs A l6-bit word is transmittedto the receivermodule for control of the matrix and the antenna-calibrate antenna-switching functions. Various output functions respondonly to the addressbus stateand do not requirespecificdata to be placedon the output bus. Thesefunctions,with are: hexadecimaladdresses resetprogramcycle fail (F100), selectanalogue multiplexer address(F2X0), start analogueto digital converter(F300), acknowledgec.d.u. ARINC Rx (F600) and ARINC interrupt acknowledgement(CFAO). 'X' in addressF2X0 can be any number from 0 The to 7 dependingonthe funption, e.g.headingsine, headingcosine,speedsine,etc. Sigralsassociatedwith other systemsand instrumentsare output from the computer via the steeringinterfacewhich is dividedinto three main sections:(l) analoguefunctions;(2) digital communications;(3) discreteflag drivers. The analogueinterface card provides four three-wire synchrooutputs and both high- and lowlevel two-wire d.c. cross-trackdeviation. Digital data is fed to the analoguecard on the data bus and convertedto d.c:

in a digital to analogueconverter. The analogued.c. of an ONS is suchthat comprehensive monitoring and sipal is fed in parallelto sampleand hold circuits self-testroutinesmay be incorporatedin the system addressed by the computervia a decoder. Each of software. Monitoring of systemperformancetakes the synchrochannelshasa modulator for the sine and placevirtually continuouslyduring flight. In cosined.c. inputs followed by a Scott-T transformer addition, operatorerror detectionaidssmooth which providesthe three-wiresynchrooutput. The operationand a reductionin reporteddefectsdue to 'finger particularoutputs are determinedby the stateof two trouble'. With a malfunction code readoutand synchrooutput selectprogrampins, ground or open. self-testdisplay,turn-rounddelaysfor ONS. In this way synchronumber I will giveheadingor installationdefect investigationand repair are track, synchronumber 2 will give 1 drift angleor minimized. track angleerror, synchronumber 3 will give track angleerror + drift angle,track angleerror or desired track anglewhile synchronumber 4 will give aircraft The Decca Navigator steeringor track angleerror. The digital interfacecard hasbeenreferredto above Introduction in connectionwith its input function. In addition Deccanavigatorwasinventedin Americaby there are ARINC 575 and ARINC 561 serialdigital W. J. O'Brienbut first usedby the Britishin the outputs,two-wireb.c.d./binaryfor the 575 and closingstagesof World War II. Sincethen a number six-wire(clock, sync and data) for the 561. The of marks of the equipmenthaveemergedfrom the ARINC transmittersare selectedwhen the continuousdevelopment of this, the most accurateof appropriatedata is on the data bus by suitable all the radio navigationaids. The systemcamesecond addressingfrom the computer. The output word is in the two-horseracefor adoption by the ICAO as 32 bits long, and so must be enteredinto a registerin the standardshort-rangenavigationsystem. That it two parts under the control of the addressbus. survivedis a credit to the DeccaNavigatorCompany Parallelto serialconversiontakesplacewhen the whoseconfidencein the basicmerits of the system word is assembledin the register. were such that it continuedits airbornedevelopment Flag signalsat TTL levelsare output from the programdespitethe setbackin 1949. computerand latchedvia drivers. The signalsare then Deccais a low-frequencyc.w. hyperbolic buffered,scaledand/or levelshifted befoie output. navigationsystem. The serviceis providedfor suitably equippedaircraft, shipsand land vehiclesby Characteristics . chainsof transmittingstations. Each chain comprises Much of this sectionon ONS hasbeenbasedon the a masterstation and normally three slavestations,all Litton-21I which is an ARINC 599 system.'This at known geographicallocations(typically 70 miles being so, what follows is a particularlybrief apart), radiatingphase-locked signals.The choiceof summaryof the ARINC Mark 2 ONS sincedetails frequencycould givea ground wavecoverageof zuchasinput and output havealreadybeencovered. l0O0 nauticalmilesbut c.w. operationpreventsthe The systemcomprisesthree units: a receiver separationof ground and sky wavesignalsso the processor,control/displayand antenna/couplerunit wable rangeis limited to about 240 nauticalmiles which, togetherare capableof receivingand by night and about twice that by day. There are processingOmegaground station signals(v.l.f. not chainsin variousparts of the world, in particular precluded)so as to provideminimum functional north-westEuropeand the northeast seaboardof capabilitiesofpresept position readoutand North America. horizontal track navigation.The systemshould Phasedifferencesbetweenthe masterstation and operateworldwide with a presentposition error of eachof its slavesare displayqdto the pilot on three lessthan 7 nauticalmiles. phasemetersor Decometers.The observedphase The power supply for the systemis I 15 V 400 Hz differencesidentify hyperboliclinesmarked on singlephasefed via a circuit breaker. In addition a speciallypreparedcharts. By noting at leasttwo 26 V zt00Hz referencein accordancewith ARINC phasereadingsthe pilot can plot his position as the 4134 will needto be suppliedvia a circuit breaker intersectionof the correspondinglines. For easeof from the appropriateinstrumentationbus for use the charts are printed with the three different excitation of synchros. familiesof hyperboliclinesin purple, red and green, hencethe red slavestation and the greenDecometer, Rmp Testing etc. Decometersare still usedbut for airborne Uttle needsto be said here since the computing po*er systems,automaticand computer-based methods

s

are uzually found, with aircraft position being shown on a roller map (flight log display)' The Radiated Signals Each chain is assigneda fundamental frequenry / in the range 14-14'33kHz. The stationseachradiatea harmonicof /, namely6/from the masterznd 5f,8f and 9/from the purple, red and greenslaves respeitively. Thus with f = 14kHz we haveradiated = frequenciesof 6f = 84 kHz, 5f = 70 kHz, 8/ l.l2'ktlz for frequencies different Using kHz. 126 andg/= the stationsin a chain allowsseparationin the airbornereceiver. Deccachains are designedby an alphanumeric code. The basiccodesare08, lB, . . ., l0B' the correspondingfundamentalfrequenciesbeing by a nominal 30 Hz (separationis 29'17, separated 30 or 30'83 Hz). A subdivisionof this basic 'half allocation is provided by the so'called frequencies'0E, lE, . . ',98 which havea nomind spacingof l5 Hz from the B fundamentalfrequencies. Additional subdivisionis achievedby the use of frequencies516Hz aboveand below the B and E Co' (courtesy theDeccaNavigator Fig.6.l8 Zonepatterns tb givegroupsof six frequencies frEquencies Ltd) by the appropriatenumber and the letters designated A, B, C, D, E, F. An examPleof a grouPof frequenciesis givenin Table 6.3 showingcode, fundamental and master (6/) frequencies;the slave fundamental frequency of the chain. frequenciesarepro-rata. The zonesare subdividedin two ways' depending on the method of phasecomparisonusedin the aircraft. The transmittedsignalscannotbe compared Table 5.3 Deccafrequencies numerical group 5 in phasedirectly sincethey are of different or division may Master frequerrcies;frequency multiplication Fundamental Chain frequency' to a common signals the 6f (Hz) be usedto bring lf (Hz) code Sincethe transmittedfrequenciesare relatedto the 84 995 fundamentalby multipliers5,6,8 and 9 (purple' 1 41 6 5 . 8 3 5A 85 000 master,red and greenrespectively)comparisoncan t4166.67 5B 85 005 take placeat the l.c.m. of any two of the multipliers l4 167.50 5C 8s08s which include 6, Thus purple and master l4 18c.83 5D canbe'phasecomparedat 30/, i'e' at 85 090 transmissions 1 41 8 1 . 6 7 5E and 429'9 kHz. Similarly the red and 420 between 85 095 l4 182.50 5F greencomparisonfrequenciesare24f and l8f respectively.A lane is definedasthe regionbetween lines with zero phasedifference,at the hyperbolic PositionFixing comperisonfrequency,i.e. everyhalf'wavelength Aswith anyhyperbolicsystemtwo hyperboliclines (pig. O.+). Thus there are 30 purple,24 ted and of positionmustbe identified,the fix beinggivenby i8 gtt.n lanesper zonewith baselinewidths of wherethey intersect.With Deccathe hyperbolic m,440 m and 587 m respectively' are app-roximately-352 patternsaredividedinto zonesandlanes.Zones startsfrom the masterand runs numbering end Lane at the master J starting A to by letters designated of letters 0-23 for red,3O'47for greenand 50-79for purple' baseline. Thesequence of the master/slave Decometerscan be readto one or two hundredthsof to coverthe wholepattern repeatsasnecessary a lane. Figure6.19 illustratesa position fix in terms (Fig.6.18). Alongthebaseline,zoneshavea km, oflanes: the red Decometerreadszone I (bottom l0'71 width of between10'47and constant lane l6 (outer scale),lane fraction 0'30 window), at the a wavelength to haff corresponding

96

at'; z InEiaEcTtoIlot ndt totmox LtxEs ts./ . at oF Potntox / - osl

DEccA co ORDTNAT€ RtD I ro i0\

DECQACO-OnDttrATC dar:l O rs.O

FB 6.19 Plotting a position fix from Decometerreadings (courtesythe DeccaNavigatorCo. Ltd)

(inner scale)while the green Decometer reads zone D, lane35,lane fraction0.80. Thus the Decca co-ordinatesare I 16.30 and D 35.80, intersectingas $town. The accuracyobtained by using frcquency rnultiplicationis often not required for air navigation; furthermorea better s.n.r.can be achievedby dividing the receivedsigrals down to the fundamental. Sincephasecomparisonis at / the zonesare the 'lanes' for a dividing type receiver. Fractionsof a zonearemeasuredto a resolutionof I llO24, i.e.just over l0 m.

Although the dividing type receiverdoes not measurelanesthere is still ambiguitywithin a zone causedby the divisionprocess.For exampledividing the mastersignalby six givesriseto an output which can start on any of six cycles,only one of which is correct. The ambiguouscyclesare known as notches. The resultingambiguity is the sameasdescribedin the previousparagraphsince,for example,an error of*l notch in the masterdivider output givesan error in the zone fraction readingof 1/6 while an error of -l notch in the red divider output givesan error in the zone fraction readingofl/8. The net error in the red zone fraction readingwould be ll6 - l18 = ll24 zone Resolvinglane Ambiguities or I lane. Sinceeach lane appearsto be the same,in so far as To resolvelane ambiguitiesmost Decca chains phasedifferencemeasurementis concerned,the pilot operatein the MP (multipulse)mode (an older V $rould know where he is, to within half a lane, in mode will not be discussedhere). Eachstation in order to initially set the appropriateDecometerby tum, startingwith the master,transmitsall four hand. Thereafter,sincethrough gearingthe lane (5f,6f,8f ,9f) simultaneously.A frequencies fraction pointer drivesthe lane pointer and zone read- s€quenceof transmissions lasts20 s during which out, the Decometerwill recordthe correctco-ordinate time eachstation transmitsthe MP signalsfor 0'45 s. by an integratipnprocess.Any interruption in ln the receiverthe four frequenciesare summed, receptionwould require a resettingof the Decometers. producinga compositewaveformwhich has a

97

5f

6t

- summatibn(courtesy Fig. 6.20 Multipulsetransmission the DeccaNavigatorCo. Ltd)

Fig. 6.21 Mk 19 DeccaNavigationSystem(courtesythe DeccaNavigatorCo. Ltd)

98

predominant spike or pulse occurring at the fundamentalrate (Fig. 6.20). In a receiveremploying the multiplying method, with readout on Decometers,lane ambiguity is resolvedby feeding a lane identification meter such that one arm of a six-armedvernierpointer, identified by a rotating sector,indicatesthe correct lane. The sector is driven in accordancewith the phase differencebetween l/6 of the mastertransmission (rememberedduring the MP transmissionby a phase-locked oscillator)and the fundamentalderived from an MP transmission.The vernierpointer is drivenin accordancewith the phasedifference betweenthe mastertransmissionand six times tne fundamentalderivedfrom an MP transmission,the drive being through I : 6 gearing. During the master MP transmissionthe lane identification meter should readzero and may be adjustedto do so if in error.

As eachMP trafismissionoccursthe appropriate Decometerlane readingshould correspondto the lane identification meter readingand may be adjusted if necessary.The current lane identification may be held to assistcheckingbut will only be valid for a few secondsdue to aircraft movement. ln a receiveremploying the dividing method the MP transmissions provide a referencefrequency/ with which the phaseof eachdivider output can be compared. The phaseconrparisonand subsequent correctiontakesplace automaticallywithin the divider circuits. This process,known as notching, removesambiguity from the zone fraction data. ResolvingZone Ambiguity We haveseenhow the particularlane (within a zone) in which the aircraftis flying can be identifiedbut the porsibilityof an incorrectzonereadingstill exists sincezones,like lanes,areindistinguishable by the normalphasenreasurements. A zoneidentification meterresolves the zoneambiguity,not completely but to *'itlrin a group of five zones,i.e. a distanceof over50 km on the baseline. An 8'2/signal is transmittedwith the MP signals from eachstationin turn. A beat note betweenthe 8f and8.2f componentsof an MP transmissionis producedhavinga frequencyof fl5. The masterbeat frequencyis 'remembered' and comparedwith each slavebeat tiequencyin turn. The resultinghyperbolic pattern haszero phasedifferenceson lines five zones apart,givingthe required resolution. hstallation Different options are availabletwo of which are strownin Figs6.2 I and 6.22. The Mk 19 receiveris capableof drivingDecometers (or digitalieadout) and/ora l1ightlog; the multiplyingmethod is usedto drive the Deconreterswhile the dividing method is usedto drive the flight log through a computer unit. The Mk l5 receiverusesthe dividingmethod only with readouton a flight log. Wherespaceis at a premium,a Dectracposition fixing unit (PFU) may be installedin conjunction with eithera Mk 15 or Nlk l9 receiver.The Dectrac PFU containsone indicatorwith four scalesand a singlepointer, effectively replacingthe Decometers but not allowingthe samedegreeof accuracyin reading.althoughthis may be recoveredby the additionof anothersmallunit. A zoneidentity readingcan be taken from the singleindicator. A capacitivetype antennais used,comprisinga coppermeshwithin a fibreglassplate mounted flush with the aircraft skin. The meshis at least2 ft2 in area. A pre-amplifier/matching unit allowsa long

Fig.6,22 DeccaMk l5/Danacnavigation (courtesy system theDeccaNavigator Co.Ltd) feederrun. The antennashould be mounted as near to the centre of the aircraft aspossible,either above or below the fuselage.If below a fixed I 80" phase shift is appliedin the pre-amplifier. Mk lS/Danac Block DiagramOperation Figure6.23 showsa sirnplifiedblock diagramof a Mk l5i Danacinstallation.A significantfeatureof the systemis the degreeof automaticcontrol achieved. The receiveroutput is fed to the computer as four pulsetrainsrepresenting the receivedmaster,red, greenand purple signalsdivideddown to the fundamentalfrequency,f. .Themaster/slavephase differencesat f aredigitallymeasuredin the computer, thus givingthreehyperboliclinesof positioneach derivedasa l0-bit binarynumberrepresentinglll}2a of a zone. The computer convertsthe Deccaco-ordinatesinto X and Y demandsignalsfor the servosdriving the laterallymoving stylus and the verticallymoving chart respectively.The major computingeffort is carried out oft'-lineon a more powerful computer which calculates constantsto be usedin the hyperbolicto X-lz conversion.Theseconstantsarewritten on a 99

zln"i"J",

Fast/normal lock

Low signal Flightlog warning

P/E cclls Warning lamp Decca Navigator Mark 15 Receiver

#r; ffil I controlI r|*'r L- l |Irl \

\v_-/

./

a

Danac controller F i g . 6 . 2 3 D e c c a M k 1 S / D a n a cn a v i g a t i o ns y s t e m b l o c k diagram (courtesy the Decca Navigator Co. Ltd)

part of the chart,amongother data,in the non-visible form of a black and white ten-trackdigital Gray code readby a line of ten photoelectriccells. The )/ servoposition feedbacksignalis in the form of a 9-bit word derivedfrom nine of the digital tracks referredto above. The X servoposition feedback sigrralis a 9-bit word derivedfrom printed circuit tracksreadby wiper contactsmountedon the stylus carriage.The servodrivesare,of course,the differencesbetweenthe demandand feedbacksisnals. Selectionof the correctDeccachainis automaticallyachievedby includingthe code represenlingthe frequencyamongthe constantsread by the photoelectriccells. Otherdata amongthe constantsare the zone valuesfor one or two checkpointson the chart (to which the stylus goes initially) and the chart scale. Settingup is largelyautomatic. If the required chart is in view the following sequencetakesplace when the systemis switchedon: l. pushbutton lampslight for checkingpurposes, and the chart constantsareread into the computerl 2. the stylus movesto a checkpoint and receiver locks on to requiredsignals; 100

3. zone fraction computation takesplaceusing MP transmissionsfor notching; 4. stylus takesup a position within a zone on the to the aircraftposition chartcorresponding within the true zone. If the zone is correctthe OP button is pressedand thereafterthe stylus and chart shouldmove so as to foilow the aircraft'smovements.lf the stylusis in the wrongzoneit may be manuallysetby pressing SET and operatingthe pressure-sensitive slewingcontrol. The correctzoneis known from the zone identification indicator and the pilot's knowledgeof his positionto within five zonewidths. Whenthe 'flies off'-the current chart and on to the next aircraft (on the samechart roll) the stylus goesstraightto the aircraft position on the new chart except under certainconditions,e.g.chainchange,in which case the aboveinitial procedurerelatingto zone identificationand slewingis cariied out. The pilot may bring anotherchart into view by pressing LOOK AHEAD and operatingthe slewing the LOOK AHEAD button a second control. Pressing the new chart to remainin view,otherwise time causes the stylusis returned io the aircraft'spresentposition

(still calculatedduring LOOK AHEAD) on the previouschart. the systemto go Pressing the INT button causes where.the MP and only mode into the integration zoneidentificationfacilitiesareswitchedoff. It may be desirableto selectINT when flying in a fringe area sincespuriousor imperfect notching signalsmay causethe warninglamp to comeon, indicatinga betweendisplayedpositionand receiver discrepancy output. On somechartscoveringfringe areasor INT is selected chainswithout MP transmissions, automaticallyby a suitablechart constant. The LOCK button hasseveralfunctions,one of oscillator which is to put the receiverphase-locked hto a fast lock condition, providingthe warning lamp is on. It may alsobe usedto initiatethe automatic setting-uproutine when a new chart, brought into view by LOOK AHEAD, doesnot havethe same coloursasthe previouschart. The stylus may be preventedfrom marking the chart by selectingWRITE off, otherwisethe track of the aircraft will be tracedout on the ghart. The TEST-DIM-BRILswitchis the only control not previouslymentioned,it may be usedfor lamp test or to selectthe brightnesslevel of the lamps. Theabovedescriptionis sketchyto saythe least,but I hopeit will givethe readeran ideaof how one Decca Navigatorsystemconfigurationperformsits function.

Loran C

with it, up to four slavetransmittersdesignatedW, X, Y and Z. The masteroccupiesa centralposition surroundedby the slavesso far as the geography allows. Baselinesareof the orderof 500-1000 nauticalmilesoverseabut arereducedoverland. The rangeof the systemis about 1000 nautical miles(from master)usinggroundwavesand up to about 2000 nauticalmilesusingskywaves.The accuracydependson the geometryof the chainbut may be in the orderof about400 ft at 350 nautical miles rangeto 1700 ft at 1000 nauticalmiles range are used. With skywavesthe providedgroundwaves in the orderof l0 nauticalmiles would be accuracy at 1500nauticalmilesrange. The Radiated Signals Pulsesof 100 kHz r.f. are transmittedfrom all are synchronized stations. The slavetransmissions with thoseof the mastereither directly (triggeredby or by useof atomicclocks. The mastergroundwave) delaybetweenthe time of transmissionof the master and eachslave(codingdelay) is fixed so that whereverthe aircraft receiveris locatedin the area covered,the slavesignalswill alwaysarrivein the same order after the master. Sinceall chainstransmit the samer.f., mutual must be avoidedby useof differentpulse interference repetitionperiods(p.r.p.)for eachchain. Therearea basicrates,eachof which have total of six so-called eightspecificratesasgivenin Table6.4. The chains areidentifiedby their p.r.p.,thus chainSS7(Eastern of North America)hasa basicrateperiodof seaboard ps 100000 (SS)which is reducedby 700 ps sincethe specificrateis 7, hencethe periodbetween from the master(and from eachslave) transmissions per i.e.fractionallyover l0 transmissions is 99 300 1,rs,

lntroduction l.oran A was proposedin the USA in 1940, hdd trials n 1942andwasimplementedovermuch of the north has and westAtlantic in 1943. Sincethen coverage beenextendedto many of the oceanicair routesof the world, but sometime in 1980the last Loran A Table 6.4 Basicand specificratesfor transmittershouldbe switchedoff. Sincethe Loran C implementationof Loran A the family hasbeen extendedto B, C and D. Loran B wasfound to be Specific Basic impracticaland Loran D is a short-range, periods repetition low-altitudesystemintendedfor usewhereline-of-sight ( subtract) peiod systemcoverageis inadequate (tts) ( t t s ) pulsedhyperbolic LoranC is a long-range" navigationaid with accuracyapproaching that of 0 0 30 000 H It was Deccaunderfavourablecircumstances. 100 I 40 000 L introducedin 1960and now providesa valuable 200 2 s0 000 S servicein rpanypartsof the world, in particularthe 300 3 000 60 SH north andeastPacificand Atlantic. The systemis. 400 4 80 000 SL usedby many shipsand aircraftandwould appearto 500 5 100000 SS havean indefinite future. (hain Layout A transmitter,designated the master,hasassociated

6 7 8

600 700 800

101

second. Not all the basicratesare in useand indeed somemay neverbe usedsince6 X 8 = 48 chainsare unlikely to be needed. Groupsof eight pulsesof r.f. are transmittedfrom eachstation once during a repetition period. With synchronousdetectionin the receiverthe eight pulses are combinedto give a much better s.n.r.than one would obtain with a singlepulse. The spacing betweenpulseswithin a group is I ms. The master transmitsa ninth pulsein its group, 2 ms after the eighth, for identification. Sometypes of interference(e.g.skywave contamination)can be discriminatedagainstby use of phasecoding. The r.f. of certainpulseswithin a group hasits phasereversed;unlessthis is properly decoded in the receiversynchronousdetectionwill give a loss of sigral power. Additionally, sincemasterand slave phasecoding is different for a particularchain, decodingcan be usedto separatethe receivedmaster signalsfrom the slavesignals.

To measurethe time differencebetweenmasterand slavetransmissions corresponding'events' must be identified in each. Obviouslyfrom Fig. 6.24 it is impracticalto measurefrom leadingedgeto leading edgeor evento usethe laggingedges,consequently one of the cyclesmust be chosenin masterand slave transmissions and the time betweenthem measured. Such a processis known as cycle matchingor indexing. From the point of view of s.n.r.the eighth cycle is the obviousone for indexing;howeverit may be subjectto skywavecontaminationand therefore difficult to identify. The minimum differencein propagationtimesbetweenskywaveand groundwave is 30 prs,so up to and includingthe third cycle the pulseis clear. For this reasonthe third cycle is usuallychosenfor indexing,particularlyin fully automaticequipment. An automaticreceiverwould selectthe third cycle by looking for the unique changeof amplitude betweenthe secondand fourth cycle; in this way the indedng circuitsare able to lock on. The transmission of the first eight pulsesmust be accurateand consistentsincean error in indexing of one cycle would give a l0 ps time differenceerror. If indexingis carriedout manuallyusinga c.r.t. to display the pulses,on time-bases of decreasing durationasthe processproceeds, useofup to the eighth cycle may.bepossiblewith a skilled operator.

Installation A Loran C systemmay consistof up to five units, namely antenna,antennacoupler,receiver,c.r.t. indicatorand controlunit. A c.r.t. displayis used where the indexingprocedureis manualor where, if automatic,it is thought necessaryto provide the operatorwith monitoring of the procedure. On some systems indexingis manualbut thereafterthe third Fig 6.24 Loran C pulseand pulse format cycle is trackedautomatically. Figure6.25 showsthe DeccaADL-S1 Loran C/D The pulseduration is approximately270 ps, i.e. a receiverand control indicator; an aerialand coupler total of about 27 cyclesof r.f. in eachpulse. To would be neededto completethe installation. The radiatea pulseof short rise-timeleadsto problemsin ADL-8 I is fully automaticprovidingdigital time frequencyspectrumspreadingand transmittingantennadifferencereadoutswith a resolutionof 100 ns on the designat the low carrierfrequencyinvolved. In fact control indicator and 50 ns via a computer interface. -indexing 99 per cent of the radiatedenergymust be in the band Synchronizationprovidesthird cycle in 90'l l0 kHz, hencethe slow rise and dbcaytime goundwave covdrand optimum cycle indexiig during illustrated\n Fig. 6.24 (in which the signalformat, iky*au. working. Three time-difierencesare m:Nterand three slaves,is alsoshown). The maximum computed,two of which may be displayed. Tunable amplitudeoccursby the eighth cycle. automaticnotch filters providerejeition of the strongestinterferingsignals.Overallsystem Principlesof Operation performimcechecksmay be performedusingbuilt in . The basicprinciplesof a pulsedhyperbolic iest equipment(BITE). navigationaid havebeengivenearlierin the chapter. The antennais usuallya capacitivetype, sometimes 102

Fig.6.25 ADL-81LoranC/D (courtesy theDeccaNavigator Co.Ltd) servingboth ADF senseand Loran, in which casean antennacouplerwould providethe necessary impedancematchingand isolationlor the two receiversserved.A pre-amplifiermay be included for the Loran feed. Block Diagram Operation The receivedsigralsare separatedinto n:asterand slavegroupsby the phasedecodecircuits,the groups beingfed to the appropriatemasterand slavephase lock loops(p.l.l.). In Fig. 6.26 only one slavep.l.l. is shownbqt in practicethere will be a minimum of two to providethe two hyperbolicl.o.p. requiredfor a fix. Threeslavep.l.l.swould enablean automaticsystem to selectthe two which gavethe bestangleof cut, althoughthe calculationsinvolvedwould probably be

carriedout by a computerto which the three time-differencereadingswould be fed. Gatepulseformersfeedthe p.l.l.swith a seriesof eight pulsesspacedby I ms and of, say,5 ps duration. The object of the p.l.i. is to providea signalto the appropriateoscillator,.drivingthe correspondinggate pulseformer, so that the phaseof the oscillator,and hencetiming of the gatepulse,is alteredsuchthat eachgatepulseis coincidentwith somespecificpoint on the third cycle in any receivedpulse. The rate at which the gatepulsegroupsaregeneratedis set to equalthe rateof the requiredchain. The basicideaof indexing,as carriedout by the p.l.l.s,is asfollows. ln Fig. 6.27 line I is a of the leadingedgeof a receivedpulse. representation Line 2 is a representationof the leadingedgeof a

103

,

Fig.6.26 LoranC simplifiedblock diagram

O

10

20

30

40

5O rrs

Fig.6.27 A methodof indexing

receivedpulseafter it hasbeendelayedby l0 ps and amplifiedby a factor of 1.5. It can be seenthat lines I and 2 crossat a time 30 ps after the pulseleading 104 ,

edge. Line 3 is a representationof the result of subtractingthe delayedand amplifiedreceivedpulse from the receivedpulse. Sincethe crossoverpoint of this differencesignalis at 30 ps it can be usedasa servosignalto set the gatepulsetiming for coincidencewith the tlfrd cycle. The futl bandu,idth of 20 kHz is requiredto preserve pulseshapeduring the indexingprocess,althoughinitial signal acquisitionmay take placewith a restricted bandwidthin orderto improvethe s.n.r. Oncephaselock is established, time-difference rcadoutis easilyachieved b1istartingand stoppinga counterwith the masterand 4ppropriateslavegate pulsegrouprespectively. Acquisitionof the receivedfrasterand slave pulses.i.e. the initial aligrmentto a point wherethe p.l.l. can takeover,may be carriedout by the operatoror automatically.With manualacquisition both receivedpulsesand gatepulsesaredisplayedon a c.r.t.;a slewingcontrolallowsthe operatorto align the gatepulseswith the third cycleof the received pulsesby useof diff'erenttime-baseselectionsfor the A-typedisplay(time-base horizontaldeflection,signal verticaldeflection).With automaticacquisitionthe gatepulsegroupsmustbe slewedor swept autolnaticallyuntil the p.l.l. cantrack successfully.

7 Distancemeasuringequipment

VOR in fact providethe standardICAO short-range navigationsystem. A DME beaconmay also be (DME) is a secondary locatedon an airfield equippedwith ILS, thus giving Distancemeasuringequipment continuousslantrangereadoutwhile on an ILS radarpulsedrangingsystemoperatingin the band approach,suchuseof DME is limitedat present. 978-1213MHz. The originsof this equipmentdate I'ACAN is a military systemwhich givesboth systemdevelopedin back to the Rebecca-Eureka range and bearingwith respectto a fixed beacon. War II. International agreement BritainduringWorld The rangingpart of TACAN has the same of the current systemwas not on the characteristics as civil DME. Thereare,however,more reacheduntil 1959but sincethen implementationhas characteristics channelsavailablewith TACAN sinceit utilizes an beenrapid. extendedfrequencyrangeof 962-1213MHz. Thusa The systemprovidesslantrangeto a beaconat a civil aircraft equippedwith DME can obtain range fixed point on the ground. The dift-erencebetween measurementfrom a TACAN beaconprovidedthe slantrangeand ground range,which is neededfor navigationpurposes,is srnallunle5sthe aircraft is very DME canbe tunedto the operatingfrequencyof the TACAN concerned.Many civil aircraftcarry a DME high or closeto the beacon. Figure7.1 showsthe which coversthe full frequencyrange. relationshipbetweenslant range,ground rangeand heightto be:

Introduction

s2 = G2+ (r46oso)2

(7.1)

Transponder

lnterrogator

igroring the curvatureof the earth. To seethe effect of this consideran error in rangeof I per cent, i.e. S = l'01G. Substitutingfor G, rearrangingand evaluatingwe have:

s +/t8s3 for a I per cent error. Thus at 30 000 ft if the DME readoutis greaterthan about 35 nauticalmiles the error is lessthan I per cent,while at 5000 ft greater than about 6 nauticalmiles readoutwill similarlv givean error lessthan I per cent. Givingrange,DME alonecan only be used for position fixing in a rho-rho scheme,three readings beingneededto removeambiguity. With the additionof bearinginformation, suchas that derived from VOR, we havea rho-thetascherne;DME and

Gnm Fig. 7.1 Slantrange/ground r?ngetriangle

Frg.7.2 The d.m.e.sYstem

BasicPrinciples The airborneinterrogatorradiatescoded r.f. pulse pairsat a frequencywithin the band978-1213MHz antenna.A ground from an omnidirectional transponder(the beacon),within rangeof the aircraft and operatingon the channelto which the interrogator is selected,receivesthe interrogationand automaticallytriggersthe beacontransmitterafter a fixed delayof 50 irs. The omnidirectionalradiation from the beaconis codedr.f. pulsepairsat a frequency 63 MHz below or abovethe interrogationfrequency. This reply is receivedby the suitablytuned is fed to the intenogator receiverand after processing

105

rangecircuits where the round trip travel time is ccmputed. Rangeis givenby:

beaconif there are reflectingobjectsinconveniently placedwith respectto the aircraft and the beacon. possibilityarisessincethe antennasat both ends This (7.2) R = (r - so)lt2'3s9 of the link areomnidirectional.Shouldsucha 'dog where:R is the slant rangedistancein nautical leg'path occur.the round-triptraveltime Z in milesto or from the beacon;I is the time in equation(.7.2\ may be that for the long way round microseconds of the furs)betweentransmission and thus lead to a readoutin excessof true short interrogationand receptionof the reply. The range. constantsin the equation are 50 prscorresponding To describethe way in which the systemdesign to the fixed beacondelay,l2'359 prsbeingthe with this it is necessary copes to introduceseveral time taken for r.f. energyto travel I nautical mile new termswhich aredefinedand explainedbelow. and return.

Both beaconand transponderusea single omnidirectionalantennasharedbetweentransmitter and receiverin eachcase. This is possiblesincethe systemis pulsed,and diplexingis simplesincethe transmit and receivefrequenciesare different. Onceevery30 s the beacontransmitsits identity which is detectedby the pilot asa Morsecodeburst of threelettersat an audiotone of I 350 Hz. It should be noted that the r.f. radiatedfrom the beacondurine identificationis of the samelorm aswhen transmittingrepiies,i.e.pulsepairs. The difference is that when replyingthereare randomintervals betweentransmissions whereasduringidentification the intervalsareconstantat l/l350th of a second.

Jitter Dellberaterandom variationof the time intervalbetweensuccessive interrogations.Each i n t e r r o g a t oprr t r d l l g sa5J i t t e r i n gp u l s e( p a i r ) (p.r.f.)'which,overa periodof repetitionl-requency severalinterrogations, describes a uniquepattern sincethc variationsare random. With an interrogationrate of, say, 100,the average interval betweeninterrogations will be l0 ms,with any particularintervalbeingbetweensay9 and I I ms. The uniqueiuterrogationpatternenablesthe DME to recognizerepliesto its own interrogationby stroboscopictechniques.

Automatic Stattdby Often rei'erredto as signal-activated search.Whenthe aircraftis out of rangeof the beaconto rvhichthe airborneDNIEis Further Principles and Terminology tuned,no signalswill be received.This stateinhibits By now the readermay haveidentifiedseveral interrogations until suchtime as the aircraftis within problemswith the principlesof systemoperationas range and signals are received. described.With DME, many aircraftwill be asking The implementationof this featuredetermines the beacon'what is my range?',the beaconwill reply whetherinterrogations commenceasa resultof mean to all of them,the problembeinghow eachis to signallevelexceedingsomepredetermined levelor the identify its own reply. Anotherproblemis how to rateof signalsbeingreceivedexceedssome preventthe airborneDME interrogatingan predetermined rate. The two alternatives areequivalent out-of-rangebeaconsincethis would be wastefulof asthe aircraft approachesthe beaconfrom beyond equipmentlife. It is obviousthat the DME operationmust be in at maximum range,and typically interrogations commencewhen the receivedsignalcount is in excess leasttwo phasessincewe cannot expect an of 300-400per second.They arenot equivalentwhen instantaneous readoutof the correctrangethe the aircraft is closeto the beaconsincethe mean momentwe selecta beacon.Theremust be some periodwhen the DME is acquiringthe rangefollowbd signallevelwill be raiseddue to signalstrength; by a period, hopefully much longer,during which the consequentlythe requiredrate is much reducedfor the former altei'native.This is of littie consequence indicatorcontinuoushdisplaysthe correctreading. In this latter period rvemust considerthe eventuality when the aircraft is well within range;one would not of a temporarylossof reply suchasoccursduringthe expect the DME to be on auto standby. When transmission of the identiflcation(ident)signalby the gound testing,however,an auto standbycircuit which monitors mean signallevel-cangive unexpected, all beacon,or perhapsduring'manoeuvre'when but not unexplainable, resultssincethe test set signalsmight be iost. (beaconsirnulator)will normallyoutput We haveassumedthat the r.f. anergywill travelin signaisregardless of rangesimulated. a straightline from aircraft to beaconand back. This constant-strength of coursewill be the caseunlessthere are any Squitter The auto standbycircuit will not allow obstructionsintervening;however,it is possiblethat interrogations to commenceuntil it detectssignals theremay be more than one path to or from the

106

from the beacon. When a sufficient number of Should more than 2700 interrogationsper secondbe intenogatingaircraft are within rangeof the beacon receivedthe sensitivityis reducedstill further, thus thereis no problem, sinceanother aircraft coming maintainingthe servicefor thoseaircraft closestro within rangewill receiveall the replies and thus bigin the beacon. to interrogate. If, however,we considerthe beacon In fact the nominal madmum of 100 aircraft is havingjust come on line or the first flight, after a exceededsinceinterrogationrateson track (see quiet period, approachingthe beacon,we havea below) are considerablylessthan twenty-sevenfor -chicken-and-egg situation: the beaconwill not reply modernequipment,and further the interrogatordoes unlessinterrogated;the interrogatorwill not not need 100 per cent repliesin order to maintarn interrogateunlessit receivessignals. readoutofrange. The beaconcapabilityof 100 From the explanation thus far there are in fact aircraft may be reducedif peak traffic is much less sigralsavailable,namely ident, but this meansan than this figure. aircraft may haveto wait 30 s, perhapsmore in weak signalareas,before coming out of auto standby. This Search During searchthe range-measuring circuits of is unacceptable;consequentlythe beaconis made to the interrogatorhavenot recognizedthose pulses transmit pulse pairs even in the absenceof arnongstthe total receivedwhich have the same interrogations.Such transmissions jittering pattern as the interrogation. The from the ground beaconare known collectivelyas lsquitter' to interrogationrate is high so as to decreasesearchtime, distinguishthem from replies. When the random the maximum rate allowedbeing 150 s-r. The search squitterpulsepairs are receivedthe airborne time in a modern equipmentis typically lessthan I s. equipmentstartsto interrogate. A p.r.f. of 135 is avoidedsinceit may cause A beaconmust transmit randomly distributed interferencewith the bearingmeasurementfunction pulsepairsat a repetition rate of at least700; this of TACAN. The readoutwill be obscuredby a .flag' minimum rate includesdistancerepliesas well as if of the mechanicaltype, or will be blankedif squitter. Beaconswhich supply a full TACAN service, electronic.The counterdrumsof an electroi.e. rangeand bearing,must maintain a rate of 21.00 mechanicalindicator can be seento be rotating when pulsepairsper second. In order to achievethis during the interrogatoris searching;an electronicindicator ident an equalizingpair of pulsesis transmitted may havea lamp or Le.d.which illuminatesduring 100ps aftereachidentity pair. A range-onlyDME search. beaconat a constantduty cycle of 27OOpuisepairs It is an ICAO recommendationthat if after l5 000 per secondis not ruled out. pairsof pulseshavebeentransmittedwithout Ifwe considerthe caseof a beaconwith a constant acquiringindication of distancethen the p.r.f. should duty cyclein a quiet period all transmittedpulsepairs not exceed60 until a changein operatingchannelis aresquitter,apart from during the dots and dashesof madeor a successfulsearchis completed. In practice the ident signaltransmission.With one aircraft using useof automaticstandbycircuitsand searchp.r.f.s as the beaconinterrogatingat a rate of, say, 27 then the low as,say,40 in modernequipmentsmakesthis numberof squitter pulsepairswill be recommendation redundant. 2700 - 21 = 2673 s-r while the reply pulse pairswill number27 s-t. Two aircraftwould Gad to isquitter Track Dunng track the range-measuring circuits, rate of 2646 s-t and a reply rate of 54 s-r andio on havingacquiredthe reply pulses,follow their early or until we arriveat a condition of beaconsaturation late arrivalas the aircraft movestowardsor away from with a nominalmaximumof 100 aircraft the beacon.Continuousrangereadoutis givenwith interrogating.We can seethat all the squitter pulse. the'flag'out of view. The p.r.f. is low. In order to pairshavebecomesynchronizedwith received optimize beaconcapabilitya maximum averagep.r.f. interrogations.From the interrogator'spoint of view of 30 is laid down. This assumes that 95 per -ent of all receivedpulsepairs appearto be squiiter except the time is occupiedby tracking,thus: thoseidentified by the rangecircuits as being ,rp-lim 9sr+ss < 3000 to its own interrogations. Q3) where:7is the track p.r.f. andS the searchp.r.f. Maintaininga constantduty cycle for the beaconis achievedby varying the receiversensitivity. When no In practicemodernequipmentsmay havetrack p.r.f.s interrogationsare receivedsensitivityis sufficiently o f l e s st h a n 1 0 . high for noiseto trigger the beaconmodulator 2700 In someequipmentsthe transitionfrom searchto timesper second. As interrogationsare receivedthe track, during which the rangemeasuringcircuits check sensitivitydecreases so maintainingthe duty cycle. they havein fact acquiredthe correctsignals,is

107

known asacquisition.lt is convenientttl identity terntsinceit takesa llnite. this eventby a separate thoupilrshort.time and the equipntcntis neither s e a r c l t i rnr go r t r a c k i n g .

a rangeof zero,or nearzero,nauticalmiles. simulates 'lhus after self-testthe outbound searchcommences fiorn at or nearzero.

PcrcentageRepll' rNecan seefrom the abovethat not will giveriseto repliesevenif the all interrogations Mennry If repliesarelost an interrogatorwill not immediatelyrevertto searchor auto standbybut will aircraftconcernedis well within range.lt may happenthat an interrogationarrivesduringthe enterits memorycondition;this rrtaybe one of two of lossof groundreceiverdeadtirne. Other causes W i t h m e n l o r y static t y p e se , i t h e rs t a t i co r v e l o c i t y . and the beacon from transmission ident are replies with whereas steady, is maintained the readout of the interrogatorrecbiverby other suppression velocitymemorythe readoutcontinttesto changeat airborneL band equipment.Everytime an L band its lastknown rate. Mentorytime will norrnallylie i.e. ATC transponderor DME equipment, between4 and 12 s. pulseis senton interrogator,transmits,a suppression the If, duringmemory,repliesare re-acquired, a commonline to all other L band equipment.This equipmentwill continuetracking:thus the pilot will may well be when a reply would otherwisehavebeen havebeenspareda falsewarning. At the end of received. nremory,if tltereareho signalsat all beingreceived, Ignoring,for the moment, ident transmissionfrom the equipmentwill autostandbywill ensue;otherwise the ground and suppressiondue to ATC transponder commencesearching. replieswe can calculatea worst-casepercentagereply figure. Assuminga beacondeadtime of 60 ps and Echo Protection The possibilityof the interrogator capability operatingconditionsof 21OO maximum reflection must suffered which have trackingreplies interrogationsper second,we havea total deadtime be guardedagainst,both on the ground,for the = ps d e a dt i m e interrogationpath,and in the air, for the reply path. o f 6 0 X 2 7 0 O 1 6 20 0 0 s - r ; i . e . The time. of total per cent 16'2 constitutes On the ground,dependingon the geographyof the is 30 with 2 DME (average) of No. p.r.f. maximum will arrive terrain,the reflectedor echointerrogation greaterthan 60 ps; of not pulse width a suppression Thus interrogation' line-of'sight after the a short time for at most thus No. I DME will be suppressed for long enough if the ground receiveris suppressed = p e r p s c e n to f t h e t i m e . 0 ' 1 8 s r ; i . e . l 8 0 O X 6 0 3 0 a{'terreceptionof an interrogationthe echowill not - l6'38 = 83'62 per cent 100 with left we are Thus period, or trigge.r a reply. Normallya suppression deacltime, of up to 60 gs is sufficient;exceptionally asthe reply rate exPectation. The ident transmissionoccursonce every 30 s up to 150ps may be necessarY. the total key-downtime will be lessthan 4 s. when A similarsituationexistsin the air but a different group transrnittedconsistsof dots and code The and solutionis normallyemployed.The line-of-sight 'dashes t i m e d u r a t i o n0 ' l - 0 ' 1 2 5s a n d0 ' 3 - 0 ' 3 7 5s o f jittering the same exhibit will both the echo replies respectively.The time betweendots and dashesis p.r.f. asthe interrogator;however,the line-of-sight uuullubl.for replies. We havethe situation where echo' To reply arrivesbeforethe corresponding three replieswill be lost during a dot, and about arranged is achieveecho protection the interrogator a track p.r.f. of ten duringa dash,assuming about zero at commences the search If to searchoutbound. with a lower equipment a modern about2l . For nauticalmilesand movesout, then the first set of these Under less. even be will losses reply p.r.f., repliessatisfyingthe rangecircuit's searchfor the the to calculate not sensible it is circumstances true to the jitter patternwill be thosecorresponding of the effect the since reply percentage exp'ected protection on changing guarantee echo To iung.. ident is possibiyto make the interrogatorgo into channelor before commencingsearchafter memory memory, particularlywhen a dashis transmitted' or auto standby,the rangecircuitsshouldbe Sincethe memory time is at leastaslong as the is This miles condition. returnedto the zero nautical total key-downtime the momentaryswitch between done in someequipmentswherethe reverse will not be movementtowards zero may be known as a reciprocal track and memory and back to track the bY noticed place. In Pilot. search,althoughno interrogationtakes It shouldbe noted by the maintenanceengineer other equipments,wheresearchis outbound from the that in simulatingident durlrg a ramp test the ident last reading,echoprotection is likely but not guaranteed.In this latter situationuseof the self-test sigpalwill be continuous,ratherthan keyed,aslong asthe appropriateswitch on the test set is held on. svitch or button will givefull echo protection since if ident is simulatedfor longerthan the memory which Thus facility a self-test virtually all interrogatorshave 108

circularpath centredon the beaconwould registera time the interrogatorwill start to search,This is ground speedof zero on the DME indicator! test set the on switch one usefulsinceoperationof If the airborneequipmenthas calculatedgound associated with its tone ident of checking allowsthe speed,it is a simplematter to give time to station volume control, memory time and searchp.r.f. = DST/KTS whereTTS is time to The ATC transponderproducesreplies,and hence (beacon)sinceTTS station.DST is slant rangeand KTS (knots) is the pulses,only when interrogated. If an suppression Again this is only a usefulindication aiiciaft is wilhin rangeof one interrogatorit will only gound speed. is on courseto/from the beaconand aircraft ttrJ wtren be interrogatedabout thirty timesper sweep' With a it. from distance some say sweeprateof say 12 r.p.m.and a beamwidth of of the ground speedmeasuring time constant The time a during occur will 5o thesethirty interrogations circuitis longbut cancopewith aircraftacceleration' interval givenby the product of 5/360 and 60112, ln groundtesting.however,one mustwait sometime the p.r.f' i.e. aboui 0'07 s. For thirty interrogations foithe groundspeedreadingto take up the simulated the would needto be 30/0'07 430 which is closeto on the ramp testset,slnce of velocityselected to value situation similar a maximum p.r.f. of 450. We have one is simulatingan infinite velocity a switching-in in the i.e' transmission, ident loss during the reply acceleration. o..uti.n". is relativelyinfrequent;for example0'07 in 5 s. If the aircraftis within rangeof more than one interrogatorthe total interrogationtime in, say, 5 s is Interrogation increased. Consideringthe effect on DME only during the The full TACAN interrogationfrequencyrangeis time the ATC transponderis replyingwe have, a 450 and of rate 1 0 2 5 - l1 5 0M H z w i t h 1 N { H zs p a c i n gT. } r u st h e an ATC interrogation assuming ps, percentage will be one of 116 possiblefrequencit's of 30 interrogation pulse duration suppression tlependingon the channelselected.The r'f is keyed iime = 450 X 30 X 100/I 000 000 = l'35 suppression 'rn per cent. If we alsotakeinto accountthe worst'case by pulsepairs. The timing,which is dcpendenl 7'-1' Fig' in illustrated per Y' is or X 83'62 of selection, cirannel reply for the DME system percentage ..nt *. haveduring this short time 83'62 1'35, 3'5trs 82 per cent replies.In fact the DME interrogator - 1 l l x stroutO.op. *ith this and remainon track' J The aboveis not quite the whole story. The intention is to allay the fearsof studentswho, on I I l2irs findingout how manywaysrepliescanbe lost, wonder how on earthDME worksat all. The few simplecalculationsgivenshowthat the situationis in fact It can,however,be worsethan sugquitesatisfactory. only requiresthat gestedsincethe ICAO specification iit. OVf beaconhavea 70 per centreplyefficiency; pulsespacing Fig.7.3 Interrogation not previouslymentioned,being one of the reasons, Even that time mustbe allorvedfor self-monitoring' The p.r.f. is dependenton the nlodeof operationof percentage with will cope interrogators DME so most theDME: repliesasiow as.orlower than 50 per cent' Search 40-I 50 Average l0-30 Track Ground Speedand Time To Station The interrogator -Average continuouslymonitorsthe slantrangeto the beacon actualp.r.f.dependson tl-reequipmentdesign The which,of course,will changeasthe aircraftflies and may be lower than minimutrlliguresgiven. There of awayfrom the beacon.Measurement p.r.f. due to ' towardsor will be a smallvariationin the average the speedof the rateof changeof slantrangegives. that 95 per centof p.r.f.,assuming jitter. The average approachor departureto the beacon.Such the time is spenton track' must be lessthan 30' The is carriedout by most airborneDMEs measurement with verticalpolarization' radiationis omnidirectional groundspeed.tt is asso-called and presenteci that the readoutcan importantthat the pilot realizes only be consideredasground speedwhen the aircraft R e p l y is flying directly towardsor away from the beacon between962 and The r.f. at one of 252 frequencies and is somedistancefrom it. An aircraft flying a

L=--------*l

r09

l2l3 MHz isteyed by pulse pairs the timing of which is similar to that given in f ig. I .5, the differince being e1l f channelspacingis 30 prs,not 36 pr. ffrc radiationis omnidirectionalwith verticalpoiarization.

conjunction with VOR and, largely as a future requirement,ILS. To achievettris, OUe Uru.on, "r. co-locatedwirh VOR or ILS beaconr, in1r" i.j"g prescribedmaximum separation fi.its 1a"nr"'i to the conventionon InternationalCivil A'v",j"rl. Wherewe havecoJocationconstituting a sinele X and Y Channel Arrangements fapility the two systemsshould ""'"ii""o"ra ;;;il frequencypairing(Table7.1).r,i rr."r1nii r""'' and 252 replyfrequenciesassociatedidentity signal. Jh...rr1.-..lZ!^i1t-elryCation

in the full IA!{I-fr.guency range.The-repty_.._. trequency is 63 MHzaboveor belowtheintiriogating Table7.1 Frequencypairing asshownin Fig.7.4. fne chann.ispacing fre.quency, is I MHzfor bothinterrogation andreply. The' v.h.f. nav.freq. v.h.f. allocation TACANchannels arenumbered lX,ly,'. . . liOX, r26Y. r08.00 VOR UsingFig.7.4we seethat channel20X say, , 108.05 VOR corresponds to aninterrogation at 1044M;Hz';nda r08.10 ILS replyat 981MHz,whilechannel I16I,, say, 1 0 85 .1 ILS corresponds to aninterrogation at I140 MHzanda replyat 1077MHz. Eeacon reply

63Y 1 1 5 0 I Y 1c|aR r26Y 1087 64Y

1025

Aircraft interrogation

I r50

--'--'t'-

Beacon reply X

1 0 8 7 -*__\l\_ 1 0 2 5 -_|.__\a

Fig,7.4 .Y/y channel arrangements

to24 63X 962 t x

l7x t7Y l8x t8Y

ii1lo

19t

l I 1.95 I12.00 l12.05

.'?{

n2.ro

ILS VOR VOR VOR

56Y 57X 57Y 58x

r12.30

VOR VOR

59Y 70x

t17'95

VOR

t26y

1213 126X 1't51 d4x 112.25

t088 -'f-'-r-

TACAN channel

With standardfrequehcypairingthe need for separate DME and v.h.f.nav.contiol unitsis eliminated.It is normalpracticefor u .o.Uin"O For civil DME beaconsthe 52 channels l-16, X controllerto be used,the selectedfrequency and nd 60-69,X andy,are avoidedi"it*. indication beinggivenin termsof tne v.h.f.nav. reasons.Firstly DME is meant to be used in frequency.Thusa selectionof 10g.05Uffr-woufO coniunctionwith VOR and ILS, which occupy 200 channels ratherthan 252. Secondly,havingh'fty_two tune the v.h.f. nav. receiverto that frequencyund the DME to the pairedchannell7y. redundantchannels,the gapsare chosen to-overlap Someequipmentshavea hold facility whereby, fre[uencies"f l0i0;;;' . lle lTC transponder when engaged. a changein the selectea 1090MHz to avoid any p-ossible ".fr.f."u". interference, lrequencywill not causethe DME channel althoughdifferent codesand ,nr,u.i ,"iprJrJion to change. "r" W.hen.using hold, rangeand bearinginfo.rn"iion i, also usedfor this purpose. givenbut not to a common point. The useof_thefifty+wo missingchannels This could lead to is, howpilot navigationerror, to .uoid ttris ever,not precludedby the ICAO;they a warninf tr*nr r, may be alloc- illuminat ed when hold is selected; n.*rtf,.i."rr,'ror. areoon a nationalbasis.The fact that civii aircraft national authoritiesfrown upon the availabilit rnay wish to useTACAN beaconsmeans of thai many sucha faciliry. DME interrogatorshavethe full ZSi.f,.""rfr.' In Table 7.1 the frequencypairingarrangements lt1:Lo*n. The frequenciesshown.i Uring"uiio.ut.O to-ILS are,ofcourse,localizerfrrqu.n.irrit. The Link With v.h.f. Navigation tigtrrt of which is I I I .95 MHz. The gfi.iepatfr/ioJirel frequencypairingis not affectel tV IUE p.iri"g. As statedpreviously DMEis meantto beusedin ThoseTACAN channelsnot pairedwith v.h.f. nav.

110

dnnnels may nevertheless still be required. In this v.h.f. nav. controllers. The RNAV facility (see casethe pairingsfor channelslX to I 6Y are Chapter l2) nray not be available,in which caseslant 134.40-135.95 MHz and for channels60X to 69Y are rangewould be fed direct to the HSI or often, a 133'30-134'25MHz solely lor the purposeof separateDME indicator in which speedand time is selectionon combinedcontrollers.Selectionof one computed. wirh a DME indicator fitted the HSI may ol thesechinnelswould only give rangeinformation still act asa repeaterfor slant range. ro an aircraftnot equippedwith full TACAN. Associatedidentity is the term givenfor qynchronization ofthe ident signalsfrom co-located beacons.Each30 s intervalis dividedinto lbur or rnoreequalpartswith the DME beaconident transn'ritted duringone periodonly and the associated v.h.f.facility ident duringthe renrainingperiods. Associated identity would alsobe usedwith a Vortac beaconwhich providesbearingand rangeinformation to both civil and military aircraft. A TACAN (or DME) beaconnot co-located with VOR would use ndependentidentity. .

Instattation The DME interrogatorcomesin rrranyforms; airline standardequipmentis rack-mounted whereasgeneral aviationinterrogators rriaybe panel-rnounted with integralcontrolsand digrtalreadout. King havegone one betterwith their KNS 80 integratednav.system sinceone panel-rnounted box containsthe DME interrogator: v.h.f.nav.recejverand converter, glideslopereceiver,RNAV computerplus integral controlsand readoutof range,groundspeed,and Fig.7.6 KPI 533pictorialnavigation indicator(courtesy time to station(seeFig. I .10). KingRadioCorp.) Figure7.5 showsa singleDME installationwith a combinedv.h.f. nav./DMEcontroller,an output to an Co-axialcablesareusedfor antennafeederand suppression.With a dual ATC transponderand dual DME installationall four setswill be connectedin parallelfor suppression purposes,so that when one transmitsthey are all suppressed.The antennais mounted on the undersideof the fuselagein an approvedposition. Sufficientspacingbetweenall Lband equipmentantennasmust be allowedto help prevent mutual interference,althoughsuppression, different frequencies,p.r.f.s and pulsespacingall contribute to this. Tuning information to both DME and the v.h.f. nav. receiveris likely to be 215,althoughb.c.d. and Fig.7.5 DMEinstallation with RNAVtie-in slip codesmay be found. Screenedcables,preferably twisted and screened,areusedfor transferof RNAV computer/controllerand with slant rangeand analogueor digital data and alsofor audio groundspeedor time to station displayedon an HSI identificationto the audio integratingsystem. The (Fig. 7.6). All largeraircraft would havea dual audio may be routed through the controllerif a installation,possiblywith changeover relaysfor HSI volume control is irrcorporatedin the system. Other feeds. A combinedcontrolleris usuallyfound, but it controller/DMEinterconnectionsare for self-test,off, is possibie(not advised)to haveseparateDME and standbyand on.

111

Controls and Operation A drawingof a combinedcontrolleris shown in Chaptera (Fig.4.9). Controlsfor DME areminimal. Frequencyselectionis usuallyby rotary click stop knobs,the digitat readoutof frequencyon the controllerbeingthe v.h.f.nav.frequency,e'g. may 108'00 MHz. The DME on/off switching 'standby' incorporatea standbyposition. Usually indicatesthat VOR/ILS is on, while DME is on standbyi.e. transmitterdisabled.Sucha switchis 'off'-v.h.f. nav, and DME off; often marked 'receive'- v.h.f.nav.on, DME standby; 'transmit' - both v.h.f. nav.and DME on. A self-test switch will be providedon the controlleror, rarely, be panel-mounted.Further switchingtakesplaceon the indicatorfor groundspeed(KTS or SPD)or (TTS or MIN). A hold switchmay time-to-station (see previousnote on'hold'). alsobe found Operationis simple;just switchon, tune to requiredbeaconand ensurelock-on after a brief search.lf the indicator employsa mechanicallydriven digital readouta flag will obscurethe readingduring whereaswith an electronicdigitalreadoutthe search, displaywill be blanked. Whentuning to a different beaconthe ident signalshouldbe checkedto ensure the correctchannelhasbeenselected.Also if the out from its DME is of a type which searches last-knownreadingthe self-testmust be operatedto return the dialsto near zeroso that an outbound searchwill resultin lock'on to a line-of-sightreply andnot an echo. Evenwith DMEswhich automaticallysearchout from zero af'tera channel changethe self-testshouldbe operatedoccasionally' Simplified Block Diagram Operation The bl
The pulsests are fed to the modulator and thus decidethe time of transmission.The modulator producespulsepairsof the appropriatespacingwhich in turn key the transmitterpower amplifiers. The r.f. is generatedby a frequencysynthesizerthe output of which servesas receiverlocal oscillatoraswell as transmittermasteroscillator. The amplifiedr.f. is fed to and radiatedfrom an omnidirectionalantenna. The peak power output of a modem airline standard DME will be about 700-800W nominal. Receivedpulsesare fed to the receivermixer via a tuned preselectorwhich givesimagerejectionand someprotection from the transmittedsignal. In addition duplexingaction will normally be employed to ensurereceivermixer protection during transmission.Sincethe transmit and received frequenciesare always63 MHz apart,the frequency synthesizercan be usedasdescribedaboveand the i.f. amplifieris tunedto 63 MHz. A dual superhetmay be employed. The receiveroutput will be the detectedvideo signal. The decodergivesan output pulsefor each correctlyspacedpair of pulses.The decoderoutput consistsof repliesto all interrogatingaircraft plus squitteror pulsesat the identificationp.r.f. of 1350Hz, in which casea bandpassfilter givesa 1350Hz tone output to the audiointegratingsystem. The auto standbycircuitcountsthe pulsescoming a from the decoderand if the rateexceeds predeternrinedfigure(say400 per second)euablesthe jitter generator.If the rateis low therewill be no modulatortriggerandhenceno interrogation.A third decoderoutput is fed to the rangegate. The zero time pulses/o are effectivelydelayedand stretchedin the variabledelay which is controlled eitherby the searchor track circuits' The output of the variabledelay,often termedthe rangeSate waveform,opensthe rangegate?'ps after every interrogation.If a reply or squitterpulseis received at a time when the rangegateis open.a pulseis fed to counter' Assumethe DME is the cr.rirrcidence with an interrogationrateof 100,and searching the rangegatewavelormgatingpulses further,assume duringa period are20 ps in'duration,then on average = gate will be openfor prs range the 10000 of l/100 o n l y 2 0 p s ,i . e . l / 5 0 0 t h o r 0 ' 2 p e r c e n to f t h e t i m e ' Now squitterand unwantedrepliesoccurrandomly so the chanceof full coincidenceat the rangegateis roughly I in 500 for eachof the decoderoutput pulsepairsper puises.Sincethere are2700 received 2700/500,i'e. secondwe will have,on average' 5-6 pulsesper secondfrom the rangegate. During searchthe variabledelay is continuously increasedat a rate correspondingto anythingfrom

Suppression pulse gen. PRF change

t-

*-oirt"-nJl

I I

to ind.

r l

l l _ll I I

Range. measunng circuits

I I I I I

replios

Rx supprcssion Frg.7 .7 Interrogator block diagram

liming gen.

o/P

Jittcr gcn.

otP

i li l i l l i l i l i l l l l l ll ll ll ll ll l l l l l l l ll ll l l l l l l l l l l l l l l l l

r

t

l

Rengo gate YVtVCtOrm

Dodcr

otP

l|nec g!r. otP

r---1

H

F3 7.8 Stroboscopicprinciple

It3

20 to 400 nautical miles per second,dependingon the vintageof the design. While in searchthe rangegate, output rate, as detectedby the coincidencecounter, is low. When the delay lps is equalto the round trip travel time plus the 50 ps beacondelay the range by a sigrificant amount. Bateoutput rate increases Assumingas abovea searchp.r.f. of 100, and also a 50 per cent reply rate, then the output.of the range gatewill jump from say 5 pulsesper secondto 50 pulsesper second. This is the situation shown in Fig. 7.8. Whenthis easilydetectedincreasein rate occursthe mode control circuit will: (a) enablethe tracking circuit; (b) inhibit the searchcircuit; (c) senda p.r.f. changesigral to the jitter generator; and (d) lift the indicator blanking or flag as appropriate. During track the variabledelay is controlled by the trackingcircuitsso as to keepeachwanted reply in the centreof the correspondingrangegatewaveform pulse. Should the aircraft be flying towards the beacon,successive replieswill appearearly within the gatepulse,so causingthe delay to be reduced. The opposite occurswhen the aircraft is flying away from the beacon. The variabledelay representsthe slant rangeand so a signalproportionalto or representing this delayis fed to the indicator and/or RNAV computer. Ifwanted repliesare lost, the coincidencecounter output registersa zero tate and hencethe mode control switchesto memory. With static memory the trackingcircuits are 'frozen', whereaswith velocity memory the trackingcircuitscontinueto changethe variabledelay at the last known rate.

zero crossingsof the delayedsine wave turn on (Q = l)a bistablewhich is turned otr (Q = 0) by the zero time pulsesfrom the jitter generator. The bistableoutput is connectedto the positive-going triggerinput of a monostable;in this way the resulting30 ps pulsesoccur at a time determinedby those delayed timing pulseswhich occur I ps after transmission.The elapsedtime I representsthe rangereadout which will be obscured'by a flag during search. In the logic employed in Fig. 7.9 a low rate output from the rangegate will give a logic zero output from the coincidencecounter, so enabling the search circuit but disablingthe early and late gates.When Zps correspondsto the actualslant rangeofthe beaconthe rangegateoutput rate is high, hencethe searchcircuit is disabledand a logic one is fed to the early and late gates. The other inputs to the earlyand late gatesare the decodedpulsesand a ramp waveformsymnietricalabout zero volts and coincident with a 30 ps rangegatewaveformpulse. The ramp input to the late gateis invertedso that the late gate is open for almost all of the latter half of the 30 ps period,while the early gateis open for almostall of the first half. The slopeof the ramp waveformis chosenso as there is a period (equalin duration to the decoderoutput pulsewidth) when neither early nor late gateis open. Thus when on track the wanted repliesare steeredto the decreaseor increaserange circuits,dependingon whether the repliesarriveearly or late within the rangegatewaveformpulse respectively. The motor drive circuits supply the motor so that when in searchthe readoutand delay progressively increases.While in track the motor will turn in a RangeMeasuringand ModeGontrol direction dependenton which of the decreaseand increasecircuits givesan output. It can be seenthat Analogue in track we havea servosystemwhich maintainsthe Typically in an older analogueDME the variable delay wantedrepliesin the centreof the rangegate takesthe form ofa phaseshifter resolver,the rotor of waveformpulses. which is fed from the timing oscillator and is The memory circuit is enabledwith the early and mechanicallycoupledto a distancemeasuringshaft. late gateswhen it clearsthe flag. Subsequently,should The trackingcircuitsin suchequipmentoften employ therebe a lossof replies,searchwill be inhibited and a ramp generatoi. Figure7.9 illustratesa block the motor held (staticmemory)or madeto continue diagramand waveformswhich may be used to explain rotating with the samesenseand speed(velocity the operationof sucha DME but is not meant to memory) for the memory time. representany particularequipment. The timing generatoroutput is sinusoidaland so Digital must be fed to a pulseformer (zero crossingdetector) What follows is an explanationof the principlesof a beforethe jitter generator.The timing signalis also first-generationdigital DME basedon, but not fed to a phaseshift resolverwhereit is phase-shifted accuratelyrepresenting,the RCA AVQ 85. Currently (delayed) by an angledependingon the position of the trend is to usea special-purpose l.s.i. chip to the distance-measuring shaft which also drives the perform the rangemeasurementand mode control readout. Pulsescoincidentwith the positive-going tasks. 114

----l I

\

l

-

l

roln ming en.

l

Delayed timing pulses

I t \ ; i____

I I I I I I I I i I I

-'l t

-J---..i

I l

M

o T o

Ranoe oat""

Enable

r

ro Delayed tlmrng pulses

R D R I V E

r

l

'

+iTe

I

Bistable

o

t

Mono^stabre o

h l

'

l

j*

so r,s

h

Monostable

o

Ramp gen. Earlv Decoder o/P

n

Late

Fig. 7.9 Analoguerangemeasuringand mode control block diasram

The AVQ 85 hasa searchp.r.f. of 40, a track p.r.f. of l2 and a maximumrangeof 400 nauticalmiles, which correspondsto a two-way travel time of 5000ps. Duringthis time the nuntberof pulsepairs receivedfrom a beaconwill be, on average: 5000x10-6x2700=13.5. Of the thirteen or fourteenpulsepairsreceivedone will, hopefully,be a wantedrepiy. During the searchmode the elapsedtime between f6 and the time arrival of a particulardecodedpulse

after the is measured.If rn is the time measured nth interrogationthen /1111is the time to the first decodedpulseto arrivesuchthat tn+1) tr, where r,l= 0, L . ., and /o = 0. Whenwe haveequality,i.e. tn+L = /r, then /r, is, subjectto further checking,the roundtrip traveltime to the beacon.It can be seen that if the aircraftis at rnaximunlrangewe shallneed, to complete interrogations l3-14 successive on average, the searchtime- At a searchrate of 40 this will take sayl3'5/40 s,i.e.about one-thirdof a second.The acquisitiontime of the AVQ 85 is quotedaslessthan I s. 115

Int.rrogatiorl lst mcesurornont

2nd measuremart 3rd measurement 4ttl melsurc|tranl

Sth mcaermont 6th measuremetrt 7th- moasurotnctrt

Valid reply

- search Fig.7.l0 Digital rangemeasurement

With the aboveoperationonly one time measurement needsto be storedin a register. With a modestamount of memory the wanted reply could be identified within two successive interrogations, providedthat sucha reply was receivedafter eachof the interrogations.If we assumea 50 per cent reply rate then four interrogationswould be needed. During a 5000 ps interval lessthan eighty-fourpulse pairswill be received,assuminga minimum spacingof 60 ps betweenbeaconsquittertransmissions and allowingfor a 60 ps deadtime. Eachtime measurement would need l2 bits if a resolutionof one-tenthof a nauticalmile is required. Thus'a faster searchtime could be achievedif a RAM of 12 X 84 X 4 = 4032 bits were provided. A practical circuit would consistof 4 X lK bit (lK = 1024\ RAMs, the pulsearrivaltimes,expressedas distances, dfter eachof four interrogationsbeing recorded successively in eachRAM chip. The first chip would thus record the arrival times after the first, fifth, ninth, etc. interrogations,similarly for the second, third and fourth chips. Of courseonly one 4K bit drip is needed,providedit can be organizedinto four linear arraysof l2-bit words. With a searchp.r.f. of 40 thereis a periodof 30000 ss (= l/40) less 5000 ps in which to checkfor equal arrivaltimes 116

which, when detected,signalthe end of search. per In Fig. 7.1I we return to the one measurement interrogationsituation. Initially the distance measuringcircuit countersand registersare cleared. Time measurementfrom /6 is carriedout by the distancecounter which counts809 kHz clock pulses, thus givinga rangeresolutionof one-tenthof a mile. The sequenceof eventsfollowing the (r + I )th interrogationof a searchcycle is as follows: l. te+20ps Distancecountgrciears. Blankingcounter loadedwith contentsof distancestorageregister= rn. 2. ts + 47 tts Blankingbounterstartsto count down. 3. ls + 50 r/s Distancecounter startsto count up towards maximum range. 4. to+tn Blankingcounter reacheszero and hence enablesblanking gateand triggersrangegate waveformgenerator. 5. ts + tral through A decodedpulsearrivesand passes enabledblanking gateto stop distancecounter

tlccodcd pulsos Distance to lnd 8O9kHz

s

PRF chang€

T

o

Ind enable

P

809 kHz F!. ?.ll diagram

Range gate wavaform gen.

Decoded pulsss

Digital rarge measuringand mode control block

and trigger transfer of data to distancestorage register;n becomesn + I and circuit waits for next to. The abovesequenceis repeatedafter each interrogation. Within, on average,fourteen interrogationsthe time to a wanted reply will be counted and the distancestorageregisterwill contain the number of tenthsof a nauticalmile actualrange. After the next interrogationthe blanking counter will enable the blanking gate 3 prsbefore the arrival ofthe wanted reply, sincethe blankingcounterstart is 47 ps after re while the distancecounter start is 50 ps after fe. It thereforefollows that the distance subjectto counterwill recordthe samed'.stance, aircraft movement.thereafter. The pulse from the rangegate waveform generator is of 6 ps duration,its leadingedgebeingX ps after te whereX is the time of arrivalof the previously measureddecodedpulseless47 ps. This gatingpulse is fed to the rangegatetogetherwith the decoder output. Coincidenceindicatesthat the two latest pulsesto be measuredhavearrivedwith the sametime delay t 3 prswith respectto /s and are thus probably wanted replies. The percentagereply checkingcircuit then checksthat two of the next eight interrogations giverise to a decodedpulsewithin the track gateand if so the mode switchesto track. On track the p.r.f. is reducedand the indicator givesa readoutofthe range asmeasuredby the distancecounter. After switching to track, at leastfour of any sixteensuccessive

interrogations rrtust give rise to a rangeSateoutput; failure initiates a switch to memory. Five seconds after memory is entered the mode will revert to search,subject to auto standby, unlessthe four-from' sixteen check indicatessuccess,in which casetrack resumes.

Characteristics is drawnfromtheARINC Thefollowingsummary Characteristic568-5for the Mk 3 airborneDME, it is not completeand doesnot detail all the conditions under which the following should be met. Channels 252 channelsselectedby 215switching. hise Spacing Interrogationl2 r 0'5 ps modeX;36 x 0'5 prsmode X Decoderoutput if lZ ! 0'5 1rsmodeX; 30 i 0'5 tts mode Y. Decoder: no output ifspaeing of receivedpulse pain more than I 5 ps from that required. Range 0-200 nauticalmileswith overrideto extend to 300 nauticalmiles. TmckW Sped 0-2000 knots.

717

AcquisitionTime I s or less. Memory 4-12s velocitymemory. r.f. PowerOutput > 25 dBWinto 50 O load.

Fig.7.l2 TIC T-24A (courtesyTel-InstrumentElcctronics Corp.)

118

:r i : i: Intenogation Rote Overalllessthan 30, assumingon track 95 per cent of time, searching5 per cent of time. Auto Standby At least650 pulsepairsper secondreceivedbefore interrogationsallowed.

Tx Frequency Stability Better than t 0.007 per cent. Rx Sensitivity -90 dBm lock-on sensitivity. Suppressiort PulseDurat ion Blanket: l9 prsmodeX;43 ttsmodeL Pulsefor pulse: 7 gs. Antenna v,s.w.r. l'5: I over 962-1213MHz referredto 50 O. Antenna Isolotion > 40 dB betweenL-bandantennas. Outputs l. Digital: 32-bit serialb.c.d.word at leastfive times per second,resolution0.01 nauticalmiles. Buffers in utilization equipment. 2. Analogue:pulsephirs5-30 timesper secondwith spacing,in ps, 50 + l2'359d (d beingslant range). Eachload l2K in parallelwith lessthan I 00 pF.

3. Rangerate pulsetransmittedfor each0.01 nautical mile changein range. 4. Audio ) 75 mW into 200-500Q load. 5. Output impedance< 200 st. 6. Warningflag( I V d.c. for warning,2i.S y d.c. satisfactoryoperation. RangeOutput Accuracy From t 0.1 to t 0.3 nauticalmiles,dependingon sigral strengthand time sinceacquisition.

Ramp Testing A DME installationshouldbe testedusinga ramp test set which will test by radiation,simulatevarious rangesand velocities.operateon at leastone spot frequencyfor mode X and mode I/, and provide for simulationof identification. Two suchtest setsare the TIC T-24A (Fig. 7.12) andthe IFR ATC"600A ( F i g .8 . 2 3 ) . TIC T.24A A battery-operated,one-mantest set operatedfrom

*

.*tF

-trf:l;l: a{{dtir-ril /! *,

'k

"r

|

-,'a.

in$

d*

@

*rt

tlt

^

{i *. * { r t f * @ r f

nl'

a

-EE-I : 4*.

..

. a

:a.r/:{

-.** e

*$

,. ..:;,..

?. d

T

Et t

,

ciil, .,") ;

. t. *.r& tlq nfr _ft i*+ffi 4,frwa.:r .s,:,ai: d; :i {

e a

fr :

^t

ld

Fig. 7.13 TIC T-50A (courtesyTel-lnstrument Elcctronrcs Corp.) '!19

the cockpit and testingby radiation. ChannelslTX and 17Y are available(108.00 and 108.05MHz VOR frequencies)with rangesimulationftom 0 to 399.9 nauticalmilesin 0.1 nauticalmile increments. The velocity, inbound or outbound, can be selectedin lO-knot incrementsfrom 0 to 9990 knots. Squitter is selectableat 700 or 2700 pulsepairsper second. Identity is availableas 1350 or equalized1350 pulse pairsper second. An additionalpulsepair l0 nauticalmilesafter the reply pulsepair can be selected,to enablea checkof echo protection. The p.r.f. meterhastwo ranges0-30 and 0-150. Finally the percentagereply may be selectedin l0 per cent incrementsfrom l0 to 100 per cent. ATC 600A This test set doesnot have all the facilities ofthe T-24A but doesoffer comprehensive testingability for the ATC transponder(Chapter8). Like the T-24Athe ATC 600A operateson channelsl7X and l7Y. The rangecan be set from 0 to 399 nautical milesin I nauticalmile steps. Twelvedifferent rrelocitiesmay be simulatedin the range

120

50-2400knots inbound or outbound. The identity is equalized1350 pulsepairsper second. The percentage reply is either50 or 100 per cent by selection. Featuresof the ATC 600A not availablewith the T-24A arean interrogatorpeak r.f. power readout, accuracyt 3 dB (t 50 per cent) and interrogation liequency check.

Bench Testing Various test setsexist for the benchtestingof DME, one of theseis the TIC T-50A (Fig. 7.13) which also providesfacilitiesfor ATC transponderbench testing. This is not the placeto detail all the featuresof sucha complex test set; sufficeit to say that the test set is madeup of optional modulesso that the customer can choosethe most suitablepackage.One feature which must be mentionedis the ability to measure the pulsedr.f. from the DME interrogatorwith a resolutionof l0 kHz. TIC havefound that many units changetheir output frequency,sometimes beyond allowablelimits, when a changein pulse spacingoccurs;i.e. X to I/ mode or vice versa.

I ATC transponder

in particular Recognitionof thesedisadvantages, No. 3, led to the developmentof a military secondary surveillanceradar (SSR)known as identification With the rapid build-up of international and domestic friend or foe (lFF). With this systemonly specially civil air transportsinceWorld War II, control of air targetsgive a return to the ground. This equipped traffic by meansof primary surveillanceradar (PSR) has sincebeenfurther developedand extended system and proceduresis not adequateto ensuresafetyin the aswell asmilitary air traffic; the special to cover civil air. equipmentcarriedon the aircraft is the air traffic control (ATC) transponder. lntroduction

?---=__

m

BasicPrinciples

Secondarysurveillanceradar forms part of the ATC radarsurveillancesystem:the other part being PSR. Two antennas,one for PSR,the other for SSR,are Fig E.l Primarysurveillance radar mounted co-axiallyand rotate together,radiating directionally.The SSRitselfis capableof giving A PSRdoesnot rely on the activeco-operationof rangeand bearinginformation and would thus appear the target. Electromagnetic(e.m.) radiation is pulsed to make PSR redundant:howeverwe must allow for from a directionalantennaon the ground. Provided aircraft without ATC transponderfitted or a possible they arenot transparentto the wavelengthused, failure. targetsin tine with the radiation will reflect energy We can briefly explain SSRin terms of Fig. 8.2. back to the PSR. By measuringthe time taken, and The SSR transmitterradiatespulsesof energyfrom a noting the direction of radiation, the rangeand directionalantenna.The directionand timing of the bearingof the target are found. Display is by means SSRtransmissionis synchronizedwith that of the of a plan position indicator (seeChapter9). Such a PSR. An aircraft equippedwith a transponderin the qystemhasthe following disadvantages: path of the radiatedenergywill reply with specially the codedpulsedr.f. providedit recognizes l. Sufficient energymust be radiatedto ensurethe interrogationasbeing valid. The aircraft antennais minimum detectablelevel of energyis received omnidirectional. b! the p.s.r.after a round trip to a wanted The coded reply receivedby the ground is decoded, targetat the maximum range. Rangeis and an appropriateindication givento the air traffic ,' proportional to the 4th root of the radiated controlleron a p.p.i..display.The reply will give energy. information relatingto identity, altitude or one of messages.Figure8.3 showsa 2. Targetsother than aircraft will be displayed severalemerElency (clutter). This can be much reducedby using As can be seen,a variety of presentation. typical data Doppler effect (seeChapter l0) to detect only symbolsand labelsare usedto,easethe task of the controller. moving targets. 3. Individual aircraft cannot be identified except by requestedmanoeuvre. Interrogation 4. An aircraft's altitude is unknown unlessa One interrogationconsistsof a pair of pulsesof r.f' energy,the spacingbetweenthe pulsesbeing one of separateheight-findingradaris used. four time intervals. Different modesof interrogation 5. No information link is set up.

721

Fig 8.2 Secondarysurveillanceradar

The maximum interrogationrate is 450 although, in order to avoid fruiting (seebelow - FalseTargets), the rate is as low as possibleconsistentwith each targetbeing interrogatedtwenty to forty times per sweep. The pulsesof r.f. are 0.8 ps wide and at a frequencyof 1030 MHz (L band) this being the same for all interrogations.

Fig. 8.3 Typical data presentation

lre codedby the different time intervals,eachmode correspondingto +different ground-to-air.question'. For examplemode A - 'what is your identity'? Figure 8.4 illustratesthe modesof interrogation. Many transpondershaveonly mode A ind C capability;this is sufficient to respondto an interrogatoroperatingon mode interlacewhereby mode A and C interrogationsare transmittedin sequence,thus demandingidentification and altitude information. Mode D hasyet to be uti[zed. 122

Reply A transponderwill reply to a valid interrogation,the form of the reply dependingon the mode of interrogation. A valid interrogationis one received from the interrogator mainlobe(seebelow,- Side I-obe Suppression),the time interval betweenpulses beingequal to the mode spacingselectedby the pilot. In every reply two pulsesof r.f. 1090 MHz, spaced ^20.3 ps apart are transmitted,theseare the frame or bracketpulses,Fl and F2. BetweenFl and F2 there are up to twelve code pulsesdesignatedand spacedas shown in Fig. 8.5; a thirteenth pulse,the X pulse, may be utilized in a future expandedsystem. The presenceof a code pulsein a reply is determinedby the settingof code selectorswitcheson the pilot's controller when the reply is in responseto a mode A (orts) interrogation.If the interrosationis mode C the codc pulsestransmittedare autoriaticallv determinedby an encodingaltimeter. A pulse4.35 gs after F2 may be transmitted. This is the specialposition indicator (Spl) pulse.otherwise known as the indicateposition (I/p) or simply ident pulse. lf the reply is in responseto a mode A interrogationthe SPI pulseis selectedby a spring-loadedswitch or button on the pilot's controller. A brief depressionof the switch will causethe Spl pulseto be radiatedwith every reply to a mode A interrogationreceivedwithin l5-30 s. Someolder transponderswill transmit a SpI pulsein reply to a mode C interrogationwhen the reply code containsa D4 pulse;this corresponds to an altitudein excessof 30 700 ft.

'i

:i

j

'i

-3

3

.: I

ldontity

ldentity o !

o

=

Altitude

Unassigncd

Fig 8.4 Interrogation pulse spacing

Fr cl

A1 c2 A2 C4 A4 'X'

8r _Dr 82 d2 84 D4 F2

l*z.sr,t-l

Fig 8.5 Replytrainformat

Coding: Identification The codepulsestransmitteddependon four code selectorswitches,each of which controls a group of threepulsesin the reply and may be set to one of eight position,0-7. The code groupsare designated A, B, C and D, the pulseswithin eachgroup having suffixes4,2 and I (seeFig- 8.5). The resultingcode is binary codedoctal, the most significantoctal digit beingdetermined.bythe group A pulses,the least significantby the group D. Selectionof the A pulsesgivesthe familiar binary code,asshown in Table 8.1. Similarly with selection of the B, C and D pulses. The number of possiblecode combinationsis easilyarrivedat sincewe havefour octal digits giving 8a = 4096. Someof the code combinationsare siven specialsignificance;we have:

Table 8.1 Group A codeselection(similarly for groupsB, C and D)

A4

A2

0 0 0 0

0 0 I I 0 0 I I

AI

0 I 0 I 0 I : 0 I

Selection

0 I 2 3 4 5 6

il ,o,o

'7600 Radio failure 77OO Emergency Thereis also a specialcode for hijack. Coding: Altitude The flight level of the aircraft referencedto a pressure

o'o' Fig. 8.6 Examples of pulse trains lbr particular selected codes

123

of 1013.25mbar(29.92 inHg) is encoded Table8.2 100-Footincrementcoding automaticallyin incrementsof 100 ft, the codeused beinglaid down by the ICAO. The maximumencodedMod,o(A) cr c2 c4 rangeis from -1000 to 126700 ft inclusive.With 100 ft incrementsthis requires1278differentcode 8 0 I combinations, with 4096 availablewe seethereis 9 0 I considerable redundancy.It is impracticalto usemore 0 0 0 of the availablecodessincethe aciuracy of I I 0 barometricaltimetersis suchthat it is not sensibleto 2 I 0 have,say,50 ft increments; in any casethe objective Reflection is to indicateflight levelswhich arein hundredsof I 0 feet. 4 I 0 To accommodatethe redundancythe Dl pulseis ) 0 0 not usedand, further,at leastone C pulseis 6 0 I transmittedbut neverCl and C4 togetherin a single 1 0 I reply. Thus for eacheightpossibleA group combinationsof pulseswe haveeight B group,five C goup and four D group,giving8 X 8 X 5 X 4 = 1280 The A, B and D pulsesform a Gray code givinga possiblecodecombinations; two more than necessary.total of 256 incrementsof 500 ft each,i.e. The extra two. if assigned. would correspondto 128000 ft, commencingat -1000 ft. In orderof I 1 0 0a n d - 1 2 0 0 f r . frequencyof bit changewe have84,82, Bl, 44, A2, The C pulsesform a unit distancereflectedbinary Al , D4, D2; thus 84 changes every 1000 ft whereas codegivingthe 100 ft increments.As shownin D4 doesnot enterthe codeuntil 30 800 ft and D2 Table8.2 the reflectedpattern,startingat until 62 800 ft. Needless to sayaircraftin the general C l = C 2 = 0 , C 4 = l , b e g i n sr v h e nM o d l e ( A )= 8 ; aviationcategorywill not needto employ encoding i.e.when the renrainder on dividingthe altitude(in altimetersgivingD4 and D2 selection. hundredsof feet)by 10 is 8. Thus to find the C To find the A, B and D pulseswe can use pulsesin the codefor. say,25400 ft we have Table8.3. Sincethe entriesin the tablecommence A = 2 5 4 ,M o d 1 6 ( A = ) 4 , s o C l a n d C 2 a r ei n t h e r e p l y . with zero,whereasthe altitudecommences at Table8.3

0 0 0 0 - 0

0 0 0 0

0 0

o 0 0 I I

I I I t

124

0 0 0 0 I I 0 0 0 0

0 0

i

, I

:

{

500-Footincrementcodins B4 B2 B1 A4

D2 D4 AI

:

rl

0 0 0 0

1 0 0 0

1 1 0 0

0 1 0 0

1 30 33 62 55 94 97 126 129 158 161 190 193 222 225 2s4

2 29 34 61 66 93 98 125 130 r57 162 189 t94 22r 226 253

3 28 35 60 6't 92 99 t24 131 156 163 188 195 220 227 252

0 1 I.

1 1 r

0

1 0 t

0

0 0 1

0

1 0

0 0 1 1

r 0 I r

1 t r t

0 r 0 I

0 1 0

1

t

I

I

I

I

l0 2t 42 53 74 85 106 tt7 138 t49 170 r8l 202 2t3 234 245

lt t2 20 19 43 44 52 5l 7S 76 84 83 107 108 116 l15 139,140 148 t47 l7l 172 180 t79 203 204 212 2tr 23s ?36 244 243

13 l8 45 50 77 82 109 114 l4l 146 r'13 178 205 210 23't 242

14 t7 46 49 78 8t ll0 l13 142 145 174 t77 206 209 n8 241

15 16 47 48 79 80 l1l tr2 r43 t44 t' t5 t76 207 208 239 240

r 0

0

0 0

0

A2

0 I I 0 0 I I

0 0 I I 0 0 I I 0

0 31 32 63 64 9s 96 t27 t28 159 160 l9l t92 221 224 255

4 27 36 s9 58 9l 100 t23 r32 155 t64 187 196 219 228 25t

s 5 7 26 2s 24 3 7 3 8 39 58 57 56 69 70 7 t 90 89 88 101 t02 I 03 122 t2t I 2 0 r33 134 I 3 5 154 153 Is 2 165 166 I 67 186 185 I84 r9't 198 I 99 218 217 21 6 229 230 23 1 250 249 24 8

8 9 23 22 40 4 1 55 54 7 2 . 7f 87 86 104 1 0 5 ll9 ll8 135 137 1 5 l 150 168 169 183 182 200 20t 2 t 5 2r4 2 3 2 233 2 4 7 246

-1000, we must add 1000 to the altitude before enteringthe table. The tablerecordsthe altitudein incrementsof 500 ft. The followingalgorithmwill give the requiredcode: (i) add 1000 to the altitude; (ii) enter table with the integerpart of the result of (i) dividedby 500; (iii) readthe code:row, column. As an examplewe will find the completecode for I l0 200 ft: ( i ) 1 1 0 2 0 0 +1 0 0 0 =l l l 2 0 0 ; ( i i ) I n t . ( l l l 2 0 0 / s 0 0 )= 2 2 2 ; ( i i i ) 1 0 1 1 0 0 0 1= D 2 D 4 A l A 2 A 4 B l 8 2 F . 4 . To find the C pulses: Modls (l102) = 2, therefore100 = Cl C2 C4. C o m p l e t ec o d ei s l 0 l 1 0 0 0 1 1 0 0 . False Targets There are severalcausesof unwanted returnsbeing displayedon the air traffic controller'sp.p.i.,one of which is interrogationby sidelobes. This is discussed in somedetail below under the headingside lobe suppression. Sincethe transponderantennais omnidirectional the reply pulsesnreantfor one interrogatormay also be receivedby another,providingit is within range and its antennais pointingin the directionof the aircraftconcerned.Suchunwantedreturnswill not of the with the transmission be synchronized interrogatc'r sufferingthe interferenceand would appearas randombright dots on the p.p.i. This type of interi-erence, known as fruiting.carrbe dealtwith by makinguseof the tact that different interrogatorswork on difl-erentinterrogationrates,

repliesmay thus be sortedon this basis. A reply from a transponderlastsfor a period of 20'3 ps, thus the transmittedpulsetrain will occupya distanceof 163000 X 20.3 X l0-6 = 3.3 nautical milesin space(speedof propagationbeing 163000 nauticalmilesper second).As a consequence any two aircraft in line with the interrogator,and with a differencein slantrangeof lessthan 1.65 nautical miles,will transmit replieswhich overlapin spaceand consequently mutuallyinterfereat the interrogatorreceiver. Such repliesare said to be garbled. Secondarysurveillanceradaris most useful when traffic densitiesare greatest.These garbling. circumstances, of course,giveincreased Reflectionsof the transmittedenergy,either interrogationor reply,from mountains,hills or large structureswill givean indicatedreply at an incorrect range.Sincethe directpath is shorterthan the reflectedpath echoprotectionmav be used;i.e. the receivermay be suppressed or desensitized for a limited periodon receiptof a pulse. Side Lobe Suppression(SLS) When a reply is receivedits angularposition on the controller'sp.p.i.is determinedby the directionof the main lobe radiationfrom the interrogator.If the reply is due to an interrogationtiom a sidelobe then the indicatedbearingwill be incorrect.Two systems havebeen designedto suppressrepliesto sidelobe interrogation:they are the ICAO 2 pulseand the FAA 3 pulseSLS systems.Someoldertransponders havecircuitry applicableto eithersystem;however three-pulseSLS hasadvantages over its virtually obsoleterival and is the only systemconsidered, The polar diagramfor the interrogatorantenna systemis shownin Fig. 8.7. Pl and P3 are the i n t e r r o g a t i opnu l s e s p a c e d a t 8 , 1 7, 2 l o r 2 5 p s radiatedfrom the directionalantenna.P2 is the SLS control oulseradiatedfrom an omnidirectional

P 1 ,P 3

r *'J I2rs fFig. 8.7 Three-pulse s.l.s.

125

antenna. The field strength for P2 is such that an aircraft within the Pl/P3 main lobe will receiveP2 at a lower amplitudethan Pl/P3 whereaselsewhereP2 will be greater. The condition for a reply/no reply are:

lnstallation

Figure 8.9 showsa typical transport category aircraft dual installation. Two transpondersare mounted side by sidein the radio rack mating with back plate connectorsin the mounting tray junction box. Two n >Pl no reply encoding altimetersand the control unit havetheir P2
:i

Fig. 8.8 Probability ofa reply

j

l J

Supply: 115V, 4OO Hz and/or 28 V d.c.

{

lllumination

S0ppressioninlout

EE ,rrl/{o *-(/

Fig. 8.9 Dual transponderinstallation

126

EE

@@

.s ..{

E

,i 3 ! I

:

other threeto be suppressed for a perioddepending on the_source; a transponder suppression puir. will be about30 ps. controlunit is, of course,panel-mounted in the cockpitandprovidesthe piloiwith.o*fi.t" controloverthe transponders. Only onetransponder at a timeis in usewhilethe otheriJon standbv. Controls and Operation FunctionSwitch Off-Standby-A-B-C_D. ModeC operationwill dependonly on thepositionof the altitudereporting(a.r.)switch;henceif the function svitch hasa'C' positionsucha selection wouldserve to switchmodesA, B andD off but haveno effect on the selection of modeC. Manytransponders have only A andC capabilityin which.ur. th. function switchmighthavepositionsOff-standby-A. Rotaryswitches mountedco-axiaily ?!:.S*:!rhes m palrs;thumbwheel switches area common

alternative. The code setectedappearsin a window abovethe switchesand will determinethe code pulsespresentin the reply in responseto a modeA (or.B) in_terrogation.Sometranspona.r, t .u" " facility for remote automaticteying fo, moOes e ana B; this may be selectedby settingtlie code ,rit.h., to 8888. In fact the necessaryextra equipment has neverbeenintroduced. I/P.Syvit-chSpring-loadedto_off pushbuttonor toggle s'uritchfor selectionof the SpI puise. May be labelled SPI or ldent. Lo SenseSwitch Whenselectedto .on'reduces the transponderreceiversensitivityby l2 dB. This featurewas introducedas an inteiim measurefor reducingsidelobe response.Subsequent developmentshavemade this facility unnecessary.

A.R. Switch On-off selectionfor altitude reporting. Test (T) Switch Spring-loadedto off pushbutton or

Spike eliminator pulse width limiter

Visual monitor Fig. 8.10 Transponderbtock diagram

727

.,i

i

I

j j

toggleswitch which energizesthe self'testcircuitry. Indication of a successfulself-testis givenby a green visualmonitor lamp which alsoilluminateswhen a reply is sent in responseto a valid interrogation. The self-testswitch and visualmonitor may be repeated on the transponderunit for the use of the engineer. maintenance TlansferSwitch Selectseither transponderNo' I or No. 2 in a dual installation. Pilot work load with transponderis minimal, there beingno indication other than the monitor lamp while control switch changesareinitiated on ATCs instructionseitherby r.t. or throughstandard procedures.

Simplified Block Diagram Operation The interrogatingpulsesofr.f. energyare fed tcl the receiverby way of a 1030 MHz band passfilter' The pulsesare amplified,detectedand passedas a video iignal to the spikeeliminator and pulse-wihthlimiter circuits;theseonly passpulsesgreaterthan 0'3 gs in duration and limit long pulsesto lessthan that duration which will causetriggering.The decoder examinesthe spacingof the interrogationpulsesPl and P3, if it is that for the mode selectedan output will be givento the encoder.If the Pl-P3 spacingis of the mode 2l prsan output will be givenregardless selected. The encoder.on being triggeredby the decoder. producesa train of pulsesappropriateto the required reply which is determinedby codeswitches(mode A) or encodingaltimeter(modeC). The encoderoutput triggersthe modulator which keys the 1090 MHz

transmitter. Radiationis by an omnidirectional antenna. circuitsare fed with the The sidelobe suppression receivervideo output. In the event of an interrogation for about by sidelobe, the receiverwill be suppressed 35 ps commencingat a time coincidentwith the P2 of P3. pulse,thus blocking the passage circuit is triggeredwheneverthere The suppression pulseso producedis used is a reply. The suppression to suppressthe receiverand is alsofed to other L-bandequipmentfor the samepurpose. The featurebetweenL-bandequipmentsis suppression mutual. Automaticoverloadcontrol(a.o.c.)otherwise reduces known asgroup countdown,progressively the receiversensitivityafter the reply rate exceeds typically 1200groupsper second.AssumingI 5 pulse replies,0'45ps pulsewidths and a maximumreply rate of 1480 groupsper secondwe havea typical airline requirementof a I per cent duty cycle to handlernultipleinterrogations. Self-testfacilitiesareprovided.On beingactivated, signalis injectedinto the front end of the test a self-testis indicatedby the receiver.A successful visualmonitor lamp which lights wheneverthere is an havean Sometransponders adequatetransmission. audiomonitor facilityin additionto the visual monitor.

i .{ ? { *

I1

:q ii n

{ n I :

Block Diagram Details Transmitter Receiver The r.f. sectionswill employnormalu.h.f. techniques suitablefor processingsignalsin the regionof internalco-axialleads 1000MHz. Interconnecting

! ':i! .t

$ i

l

.;

j l

l

I 'i

To vidco st6gos

Fig. 8.1I Logarithmici.f. amplifier

128

must be of a length laid down by the manufacturer as they play a part in the circuit action; in particularthe lead to the transmitterwill be of a leneth such that receivedsigralswill seethe transmitteifeed as a high impedance,thus assistingin the duplexing action. In flying from maximum rangetowards the interrogator a large dynamic rangeof signalstrength is received,at least50 dBs from minimum triggering level. Providingadequateamplification for the weak signalscan result in saturation of the last stage(s)of the i.f. amplifier when strongersignalsare received. Such saturation may give rise to the suppressionof valid interrogations by the SLS circuit since the strongerPl main lobe pulseswill be limited. Successive detectionof the video sigral can overcomethe saturationproblem. Figure 8.11 shows a logarithmicamplifier usingsuccessive detection. fusuming a gain of A for each stageand a maximum stageinput signalZ before saturationof that stage, an input to the i.f. amp. of VlAi wrll causestage4 to saturate.In this event the detectedoutput to the video stagewill be (VlA') + (VIA) + (V) + (AV) which is approximatelyequal to AV for'a reasonable gajrnA. Table 8.4 illustratesthe differencebetweena conventional and logarithmic amplifier. Decoder Many transpondersstill in serviceuse multi-tapped delaylinesin both decoderand encoder;howeverall moderntranspondersemploy monostablesand shift registers,in the form of integratedcircuits,which will be the only type of transponderconsideredhere. The detectedvideo signalsare appliedto the spike eliminator and pulsewidth limiter in series. The action of the 0'3 ps delay and AND gateis to prevent any pulsesor noisespikeslessthan 0'3 ps in duration

Table8.4 Comparison of conventionaland logarithmic amplifiers Input signal

Output signal Conventional

Logarithmic

V I3

AV

AV

V T2

AV

2AV

V

AV

3AV

v

AV

4AV

T

beingpassedto the decoder. Long pulseswhich might causea'reply are reducedin duration by the pulse-widthlimiter monostable,the output of which is a pulseabout 0'5 ps in duration regardless of the width of the input pulse. The decoderinput monostableensuresthat only the leadingedgeof Pl (positive-goingt) triggersthe mode A and C delay circuitseachof which consistsof a monostableand a differentiatingcircuit. The mode A and C AND gateswill givean output if the delayed Pl pulsesfrom the differentiatorsare coincidentwith the undelayedP3. An output from either AND gate will give a trigger('T') output from the OR gate. Encoder ln the exampleof an encodershown,two lO-bit shift registersare used(e.g.Signetics8274).The operating

O ' 5p s

Output O'3lrs Pulse width limiter

Odprrt Spike eliminator fig. E.f 2 Spike eliminator and pulse width limiter wavcforms

129

.{

r

,{

J

{ : : :. .!

Pulse width limiter Fig.8.l3 Decoder

n t t

30 trs mono output

8 lrs mono-output

n

| Ln-] P'I P2

Input

-

t l

P3

l

.

l

I lrs

21 1rsmono output

n t l

Differentiator I O/P

n

Differentiator 2 O/P

'A'

'C'

n l l

output

output

T'output

- modeA intertogation Fig,8.14 Decoderwaveforms mode is controlledby inputsS0 and Sl, hold, clear, load and shift being selectedby the input values $rown in the truth table(Fig.8.l5). Thesetwo control waveformsare producedby two monostables, one triggeredby the positive-going edgeof the T output from the decoder,the other by the negative-goingedge. This anangementgivesus the 130

'lhc s e q u e n o el o a d , s h i l t , c l c a r , h o k l . S 0 w a v e f o r mi s a . l s ou s e d t ( ) g a t c a c l o c k g c r r c r a t o r p , c r i o d 1 . 4 5g s . 'l Load l'ha lcading cdgc of triggcrs thc controlling m o n o s t a b l c ss o t l r a t S ( ) = l , S l . , ( . 1a n d t l r c s h i f t r e g i s t e re l e r n c n t sw i l l b c l o a d c r lw i t l r t l r c b i n a r v information present on tlrc input lirres. Since il an
sxo

$lBo,,o

10 bit shift re$ster

10 bit shift register

I

:

Pilot code switch Encoding altimeter switch

r-

ct

D4

Fig 8.15 Encoder F2 are requiredin all replies,the appropriateinputs arehigh (+5 V). The one time-slotbetween.A4and B1 and the two time-slotsbetweenF2 and SPI are their input lines,together neverused;consequently with the two spareafter SPI.arelow (earth). The dynamicinput linesCl to D4 will be high or low dependingon the code switch selection(mode A or the altitude(modeC interrogation). interrogation) Assuminga validmode A interrogationhasbeen 'A' line receivedtherewill be a pulsepresenton the from decoderto encodercoincidentin time with the 'T'line. After inversionthis A pulseis. puiseon the appliedin parallelto one of twelveNOR gates,the otherinputsof which areconnectedindividuailyto the twelvepilot code switches.Thosecode pulses selected resultin a groundon the appropriateNOR gateinput. Sincethe other input in eachcaseis also

low for the dur4tion of the load mode, the NOR gate to selectedcodesgo high and outputscorresponding thusset the appropriateshift registerelement. Similar are whenevermodeC interrogations action.occurs receivedalthoughDl selectionis not involved.Noise filteringis employed(r.c.networks)within the for eachof the altitudeinformation transponder input leads.SPIloadingoccurswhen the A pulseis coincidentwith the l5-30 s output pulsefrom the of the SPIswitch. SPItimer producedon depression Shift The trailing edgeof pul5eT triggersthe S1 monostablethus S0 = Sl = I and the shift registers arein the shift operatingmode. Shiftingoccurswith a high to low transitionof the clock pulses,thus in the l'45 1lsafter the first transitionthe shift register output is high (Fl); after the next transitionthe 131

:l

::

s1

I

l

:

so cLx c2 A2 C4 A4

D2 B4 D4 F2

S/R out

:

oooo

S/R out 7777 Differentiator

o|rt

AND out

oooo

AND out 7777 Fit. 8.16 Encoderwavefolms

output will be high or low dependingon hqw the Cl elementwas loaded,and so on.

monostable is to provide pulsesof the correct duration to the modulator.

Aear and Ho;ld S0 and Sl pulsesare each of approximately30 ps duration, thus S0 goeslow ltrst = = $ving S0 0, Sl I when all the elementsclear. ln fact during shift all elementswill haveclearedso this clearmode of operationis not vital. What is important is that S0 goeslow first, sincewe do not want to load againuntil just before the next transmission.At the end of SI we haveS0 = SI = 0, the hold mode which is maintained until the next valid interrogation. The output ofthe shift registercontainsthe coded information but not in the form required,i.e. a train of pulses. Differentiatedclock pulsesare fed to an AND gatewhich is enabledby the shift register output, thus positivepulsesappearat the output of the AND gatein the appropriatetime-slots. The final

Encoding Altimeter As the aircraft ascends,the decreasein static pressure causesexpansionof the capsuleand consequent device,an optical or movementof a position-sensing (Fig. 8.17). The resulting magnetictransducer drive, through appropriategearing, servo-assisted display and encodingdisc. The altitude drivesthe pressurereferencefor the indicatedaltitude can be lhangedby the barosetcontrol. It should be noted *rat this doesnot affect the encodingdisc position which is alwaysreferredto 1013'25 mbar (29'92 in'Hg) the standardmean sea-levelpressure'As a result all aircraft report altitude referencedto the samelevel; an essentialrequirementfor ATC purposes. Variationson the aboveareencodingaltimeters which give no indication of altitude, blind altimeters,

132

ffi

Barowr

i l

i'---@

I I

Tach. gen.

-{

no,o,

I T

fu--ffi

-l ffi' t--: I I I

-----

Posi sensrng

Lioht

Light sourcc

d e v i ce

\-^-N Capsule

I *1.-

Static connection

sonsitive devices Encoding disc

Porver

crr(urts

Supply

-l switch i n g c l r c u l ts

To transponder

Fi& 8.17 Servoenodingaltimetet minimizesthe error when the readingline is aligned with the junction of two segments. In the simplifiedencodercircuit shownin Fig. 8.20 with A/R switchON ZenerdiodeVR2 is shortedand emitterof Q2 returnedto earth. When Altitude Encoding A transparentdisc,usually glass,is divided into tracks light from LED VR1 falls on photo transistorQl, via part of the encodingdisc,currentis a transparent and segments.Eachconcentrictrack representsone drawn through R switchingon Q2. Thus collector of of the code pulses,the outer track beingC4, while a Q2 falls to a low value,i.e. input 2 to NOR gateis segmentrepresentsa oarticularaltitude. An opaque low. If input I is drivenlqw by an output from the pattern is formed on the disc so that on a particular decodera high output lrom the NOR gateis available will s€gmentthe areaof intersectionwith eachtrack for loadinginto the encodershift register. on the depending transparent either opaque or be If no light falls on Ql we haveno volts drop across code assignedto the altitude representedby that R, thus Q2 is off and input 2 to NOR gateis high. segment. Undertheseconditionsa'zero'will be loadedinto The disc rotatesbetweena light sourceand the appropriateshift registerelement.The circuitry photosensitive.devices, one for eachtrack, aligned 'reading describedis repeatedfor eachtrack of the disc. line'. As the disc is driven by the along the barometricaltimeterthe appropriatesegmentis read. Side Lobe Suppression Referringback to Tables8.2 and 8.3 we seethat by I bit for each100 ft increment. Of the severalpossibleways of desigringan SLS the codechanges circuitone is illustratedin Fig. 8.2l, with the This useof a l-bit changeor unit distancecode

drivesbut and thosewhich do not haveservo-assisted do employ a vibrator to givesmooth movementof pointer and encodingdisc.

133

ft---*

||-.+ lF-+ ll-> ll--

To transpondcr

tl-+

ll-*

t= \.,r n, sensitivc devices

Fi& E.lt

Encodingdisc

Tracks

Fig.8.l9 Segmentfor12300ft,code0l0ll0l0o. Encodingaltimcter range- l000-32 700 ft

Mode C load

f-

tl

U I

*-1 Encoding alttmeter

+v

A/R onloff

j

-L =

Fig. 8.20 Simplifiedaltitudeencodercircuit

134

Transponder

Segments

D2 R1

-v2

Ditch digger

t-__

circuit Fig.8.2l Sidelobe suppression

lnput

in, d

GI M2,O

M3, O

waveformsin Fig. 8.22. MonostableMl associated and AND gateGl separatethe Pl pulsefrom P2 and P3 so that M2 will be triggeredby Pl only and will not be triggereduntil the next interrogation. M2 and M3 provide a gating waveform about I ps wide in the P2 pulseposition. Th" input pulsesarealso appliedto a'ditch-digSer' circuit. Prior to Pl, Dl is forward biasedand the junction of Cl/R2 is low. Both inputs to G2 ate low. The leadingedgeof Pl causesDl to conduct chargingCl rapidly. The laggingedgeof Pl causes Dl to cut off, sincethe junction Cl/R2 falls by an amount equal to the amplitudeof Pl . Cl now through Rl and R2. WhenP2 arrivesDl discharges will conduct providingthe amplitudeof P2 is sufficient. The time constantClRlR2 and the bias voltagesVl and Y2 ue chosenso that if P2>Pl AND gate G2 will receivean input via D2 which will be coincidentwith the gatingwaveformfrom M3' Thus the SLS pulsegeneratorM4 will be triggeredif suppression and only if Y2> Pl, the subsequent pulsebeing usedto inhibit the receivervideo output to the spikeeliminator.

Characteristics Junction c1 R2

Fig. 8.22 Side lobe suppressionwaveforms

The followingsummaryis drawnfrom ARINC No.572.1for theMk 2 ATC Characteristic It is worthpointingout that several transponder. arenot requiredfor featuieson the Mk I transponder will find many engineer the the Mk 2, however whichhavesomeor all of still in service transponders the following: 135

l. two-pulseSLS; 2. SLS countdown - receiverdesensitized when the number of SLS pulsesexceedsa limiting figure; 3. low sensitivity selection; 4. receivervideo signaloutput socket; 5. remote automatickeying; 6. externaltransmittertriggeringposition; 7. audiomonitor: 8. transmissionof SPI pulsewheneverD4 is one bit ofthe altitudereportingcode. Receiver Minimum Ttiggering Level (MTL) -77 to -69 dBm at antennaor -80 to -72 dBm at transponder. Dynamic Range M T L t o 5 0 d B sa b o v em . t . l . Frequency and Bandwidth Centrefrequency1030MHz. - 3 d B p o i n t sa t 1 3 M H z . -60 dB pointsat ! 25 MHz. Decoding Facilities Decoderoutput lbr pulsesspaced8, I 7 and 2 I ps tolerancet 0.2 gs on spacing.Automaticmode C decodingregardless of modeselectionswitch. Spaceprovisionfor 25 ps decoding.

Reply Delay 3 t 0'5 ps. Reply RateCapabiliry 1200repliesper second. Reply Pulse Interval Tolerance t 0'l gs for spacingof any pulse,other than SPI.rvith respectto F I ; t 0'15 ps for spacingof any pulsc'with respectto any otherexceptFl: t 0.1 prsfor spacing of SPI with respectto F2. Mutual SuBpression P.ilse 25-331s duration. Manitor Lamp To light when five repliesaredctectedat a rategreater thar 150 repliesper second.To stayilluminatedfbr l 5 s a f t e rl a s tr e p l yd e t e c t e d . Antenna Polarization:vertical. v . s . w . r .b: e t t e rt h a n l . 4 l : I a t 1 0 3 0a n d 1 0 9 0M l t z .

Ramp Testing A transpondercan be testedril situ usingone of s e v e r apl o r t a b l et e s ts e t s .A s u i t a b l er a n t pt e s ts e t will testby radiatitrrr._bc. ceplbleof interrogatingon at leastmcldesA and ( . be capablerlf simulatinga sidelobe interroXptlon. displaythe transponder reply and providea ffeansof measuringthe transponder transmitterfrequency.

Side Lobe Suppression Facilities Pl > P2 + 6 dBs shouldgive90 per cent reply rate. 6 dBsratherthan ICAO 9 dBsensures ATC 5OOA adequate marginto allow for performancerundown in service. A popular test set is the IFR ATC 6004 illustratedin Figure8.23. A reasonfor its popularity is the fact SLSPulse Duration that it can testboth DME and ATC transponder with 25-45 ps. a comprehensive rangeof checks,making it suitable for functional testson the ramp or bench. Transmission Pl, P2 and P3 pulsesaregenefatedand usedto key a crystal-controlled 1030MHz oscillator.The interval Tlansmitter Frequency betweenPl and P3.isswitchedto simulatea mode 1 0 9 0 13 M H z . Ay'Cinterlace,two mode A interrogationsbeing transmittedfor eachmode C. The following Minimum Peak Power characteristics may be variedby front panelcontrols:

500w.

l. Pl-P3 intervd - to checkdecoder; Reply hIttlseCharacteristics 2. P2 amplitude- to checkSLS; Duration 0.45 1 0'l ps measuredbetween50 per cent 3. Transmitterpoweroutput - to checkMTL. amplitudepoints. 0'05-0'l ps risetime, l0-90 per cent. The reply is displayedby a bank of lamps,one for 0'054'2 ps delaytime,90-10per cent. eachcode pulseand oire for the SPI pulse. There is 136

#ffi "n[f*

x:

l

*

t

l

l

r

* * ) l

r* i I

# ' $'tl*$ i . ffi

t r

) poweroutput of transponder (1 50 per cent accuracy);

3 . frequencyof the'transpondertransmitter; 4 . percentage reply; 5 . invalidaltitudecode,i.e.no C pulsesor C I and C4 together;

t

';*'!,r,".t'-'.1;u

,:-:

t+

Fig.8.23 ATC600A(courtesy tFR Electronics lnc.)

6 . absenceof code pulsesin reply to nrode C interrogation. Supplyis by rechargeable batteryor a.c.,battery operationis limited by a timer. Further l'eatur.es are directconnectionto the transponder via an external 34 dB pad, self-testing of display,lampsand battery and direct connectionofencodingaltirneter.

alsoa numericalreadout which showseither the pilot codeor the altitude in thousandsof feet. In addition TIC T-33B and T-438 to this basicinformation the following can be checked: The TIC approachto ramp testingis to useseparate testsetsfor L bandequipments, the T-338 and l. F2 timing; T-43B beingthosefor ATC transponder.

Fig. 8.24 TIC T-438 (courtesyof Tel-InstrumentElectronics Corp.)

137

Specificationsfor the two test setsare identical exceptfor the addedfacility of direct connectionof an encoderwhich is availableon the T'438. of thesetest setsand the ATC The capabilities 6O0Aare similar in so far as ATC transponderramp testingis concerned,in that they both meet the FAA requirements.Differencesarelargelydue to the use of the ATC 6004 as a bench test, although it should be noted that a particularATC 600A is best usedas eithera benchtest setor a ramp test setbut not both. the TIC test setsdo not To detailthe differences, havefacilitiesfor continuouslyvaryingPl -P3spacing or strobingthe F2 pulse,and do not indicateinvalid 'no altitude'informationor transmitterpower. or

Featuresof the TIC test setsnot availableon the ATC 600A are provisionof all military and civil modesof interrogationand changeof scalefor percentage reply meter(0-10 per cent SLS on: 0-100per cent SLS off). Thereare other minor and one other major difference.in that differences, the TIC test setsare designedftlr use in the cockpit on the groundor in flight, the antennabeing mountedon the test set asopposedto the ATC 600A wherethe antennais mountedon a tripod nearthe l'or the aircraftantenna-The antennaarrangements the useol direci connection TIC test setsnecessitate to the tr;ulsponderfor receiversensitivitychecks.

:

.J

, t <<"

138

I Weatheravoidance

Introduction

can remainin a cloud without falling to earth is dependenton the speedofthe up-draughtof air. Weatherforecastingis by reputation and, until the In the appendixto this chapterit is shown that the introduction of satellites,in fact, notoriously signalin a weatherradaris proportional to the sixth unreliable. Evenwith modern techniquesrapidly power of the droplet diameter,so strongsignaisare changingconditions and lack of detailed associated with a rapid up-draught. If a small volume information on the exact location and severityof bad of the cloud resultsin strongsignalsfrom one part weatherresultsin diversionsor cancellation,of fligt t, and weak from an adjacentpart we havea steep 'rainfall where the forecastis the only availableinformation. gradient',most probably due to a down- and What is requiredis an airbornesystemcapableof up-draughtcloseto one another. The region around detectingthe weatherconditionsleadingto the this verticalwind shearis likely to be highly hazardsof turbulence,hail and lightning.. turbulent. Attention has been concentratedon developins The correspondingargumentfor associating systemswhich will 'detect'turbulence.tf an alrcr-aft electricalactivity with turbulencegoes as follows. passesthrough regionsofsevere turbulenceit is A wind shearwill resultin the separationof positive obviouslysubjectto rnechanicalstress,which may and negativeelectricalchargesin the air due to the causedamage,possiblyleadingto a crash. On friction of the moving air currents. An electrical commercialflights, passenger comfort is also dischargeoccursafter sufficientchargeof opposite important sincethe number of customerswould soon polarity has accumulatedin distinct parts of the declineif the discomfort and sicknesswhich cloud. Thesedischarges occur repetitively,most turbulencemay bring becamecommonplace. beinghidden from view but occasionallyseenas Unfortunatelythe phenomenonof rapidly and lightning. Each dischargeis accompaniedby a large randomlymoving air currentsis not amenableto burst of e.m. radiationwhich canbe receivedat some detectionby any currently realizabletechnique; distance.In most radioapplications the 'noise' however,someprogressmay be madeby utilizing a receiveddue to lightning is a nuisancebut its pulsedDopplersystem. associationwith turbulenceis put to good usein a As a consequence of our inability at presentto Ryan Stormscopein a way similarto that in which a detect the turbulencedirectly, systemshave been weatherradarusesithe'nuisance'signalsof weather developedwhich detect either water droplets or clutter. electricalactivity, both of which are associatedwith We can summarlzeand combinethe above convectiveturbulencein cumulonimbusclouds. arguments by saying:convective turbulenceoccurs Clearair turbulencehas no detectableassociated wherewe havelargeshearforceswhich imply: phenomenawhich can give a clue to its presence. (a) an up-draughtsupportinglargeraindropsformed To detectwater dropletsor raindropsa from the water vapour in the warm moist air rising conventionalprimary radaris usedwith frequency from ground level;(b) a nearbydown-draughtwhich and specialfeatureschosento optimize the cannotsupport largeraindrops;(c) frictional forces presentationof signalswhich would, in a normal search givingriseto chargeseparation; and (d) electrical radar,be unwanted. Weatherradar hasbeen usedfor discharge due to chargeseparation arida saturated many years,and is mandatory for largeaircrift. interveningmedium. Therehasbeena steadymove into the generalaviation Causeand effect are very much bound up iri this market by radar manufacturersbut here a relatively argument,the variousphenomenabeing recentinnovation is the Ryan Stormscope,a patented interdependent.Howeverreasonable the theory, the devicewhich detectselectricalactivity. ultimatejustificationfor the association between The maximum diameterof a water droplet which turbulence,steeprainfallgradientand electrical

139

activity is recordedcorrelationduring many flights. Weatherradar is certainly well provenwith many yearsin service,while the Stormscope,although only availablesince1976,hasbeenindependently evaluatedand shown to be a useful aid'

WeatherRadar Basic Principles Weatherradar operationdependson three facts: l. precipitationscattersr.f. energy; 2. the speedof propagationof an r.f. waveis known; 3. r.f. energycan be channelledinto a highly directionalbeam. Utilizing thesefacts is fairly straightforwardin principle. Pulsesof r.f. energyare generatedby a iransmitter and fed to a directionalantenna. The r.f. wave,confi4edto asnarrow a beamaspracticable, will be scatteredby precipitationin its path, someof the energyreturning to the aircraft as an echo. The and receptionis elapsedtime betweentransmission particular in to range R, directlyproportional = propagation of the speed is where c ct R 12 (= 162000 nautical miles per second);t is the elapsedtime; and the divisor2 is introducedsince travelis two-way. The direction of the target is simply givenby the direction in which the beam is radiated. Sincethe pilot needsto observethe weatherin a wide sectoraheadof the aircraftthe antennais made to sweepport and starboardrepetitively,hencewe use the term scannerfor a weatherradarantenna.Any stormcloud within the sectorof scanwill effectively of the be slicedby the beam so that a cross-section viewed. is cloud Displayof threequantitiesfor eachtargetis requirid: namelyrange,bearingand intensityof echo' .l ptanpositionindicator(p.p.i.)displayis invariably displayof the utid tinc. this allowsthe simultaneous ' threequantitiesand is easyto interpret. A cathoderay tube (c.r.t.)is usedin which the beamof electronsis velocitymodulatedin with the receivedsignalstrength' accordance Whereverthe beamstrikesthe'phosphorcoatingon the back of the viewingscreena glow occurs,the intensity of which is dependentof the velocity of the with a electrons.Thus a strongsignalis associated intensity term the hence screen; the on bright spot modulaiion. The beamis made to sweepacrossthe screenin synchronismwith both the time of and the antennaposition' transmission 140

In a conventional(rho'theta) display the beant strikesthe screenat bottom centre(origin) at the instantthe transmitterfires. Subsequentlythe beam will be deflectedacrossthe screenin a direction dependenton the scannerazimuthposition,e.g.if the scanneris pointing dead aheadthe beant is deflected vertically from the origin. In this way a time'baseline is tracedout on the screenand is made to rotate in synchronismwith the scanner.' The duration of the time-base,i.e. the length of time it takesto tiaversethe screen,dependson the rangeselectedby the pilot. Everymicrosecondof to a rangeof round trip traveltime corresponds rangeof a selected 0'081 nauticalmiles,thus for about will be time-base the 20 nautical miles 250 ps in duration. An echo received125 ps after transmissionwould causea bright spot to appear so indicatingto the half-wayup a 250 ps.time-base, is l0 nautical miles' target of the pilot that the range The net result of the aboveis that a cross-section of the targetswithin the selectedrangeand scanned sectorof the radarareviewedin plan. The position of the bright patcheson the screenrelative to the origin is representativeof the position of the targets relativeto the aircraft. Figure9.1 illustratesthe .situation.

and Features Choiceof Characteristics Frequency The higher the frequency(smallerthe wavelength) per unit' the largeris the backscattercross-section greater the hence (see A9.12) volumeof the target suffer frequencies high power. However, echo the more atmosphericabsorptionthan do low, and further cannotpenetratecloudsto the sarneextent' Thus the choiceof frequencyis a compromise'An additionalconsiderationis the beamwidth; for a givenscannerdiametera narrowerbeamis produced with a higherfrequencY' Practically,takinginto accountavailabilityof standardcompqnents,the choicecomesdown to eitherabout 3'2 cm (X'band) or 5'5 cm (C-band)' The majority of radarsin serviceand currently manufacturedare X-band. Pulse Width The volume of the target givingrise to an echo is directly relatedto the pulsewidth (seeA9.8) thus use of long pulseswill give improvedrange' There are two argumentsagainstlong pulses: l. Sincethere is only one antennaand a common

(-,\\

,\l

L_ \", r\-l c\', l

\

)

--^-\-1 ()

\

Time-base

a

{

r \

\__

Fig.9.1 Displayprinciples frequencyfor transmit and receive,the antenna Sincerangeresolutionand minimum rangeare not must be switchedto the transmitter for the criticalin a weatherradar,pulsestend to be longer duration of the pulse;thus the pulsewidth than in other radars,say2-5 ps. A shorterpulsewidth, determinesminimum range. For a 2 ps pulse say I ps, may be switchedin when a short displayed no return can appearfor the first 2 ps of the rangeis selected. givinga minimum range= time-base, A techniqueis availablewhich realizesthe 2c X tO-612t one-sixthof a nauticalmile. advantages of both long and short pulses. The with increasing 2. Rangeresolutiondeteriorates transmittedpulsecan be frequencymodulatedso that pulsewidth. A pulseof 2 ps durationoccupies the r.f. increasesover the duration of the constant about 2000 ft in space. If two targetsare on the amplitude pulse. The frequencymodulatedreturn is samebearingbut within 1000ft of one another passedthrough a filter designedso that the velocity the echofrom the nearesttargetis still being with frequency. Thus the of propagationincreases receivedwhen the leadingedgeof the echofrom higher frequenciesat the trailing edgeof the echo the furthest targetis received.The resultis that 'catchup'with the lower frequencies at the leading both targetsmergeon the p.p.i.display.The edge. In this way, the duration of the echo is rangeof the targetsdoesnot affect the resolution. compressed.It shouldbe noted that the bandwidth

141

TargetI Harfa wavelength = IOOO'

t-\-\l

| i

Target 2 | k+

I l---

--- t

+ 2 ------

time (7rs)

t + 3

+ 4 + 5

Rang€

[D K3

Incidentpulse (2 1rs)

Reflectedpulse

Fig.9.2 Range resolution requirementsareincreasedby usingfrequency modulationof the pulsedr.f., this being the penalty lbr obtainingbetterrangeresolution.This technique is known as pulsecompressionbut, so far as the author is aware,is not usedon existingairborne weatherradars. hrlse Repetition Frequency,p.r.f. Changingthe p.r.f. will affect the number of pulses striking agivenvolume of the targetin eachsweep and hencechangethe displayintegrationfactor (see A9.17). Howeverin order to maintain a constant duty cycle(pulsewidth X p.r.f.) an increasein p.r.f. in pulsewidth, so must be accompaniedby a decrease keepingaveragepower and henceheat dissipation in p.r.f. constant.Alternativelyan increase accompaniedby a reductionin peak power will also keepheatdissipationconstant.The net resultis that for constantaveragepower a changein p.r.f. doesnot, in theory, affect the maximum rangeof the radar. Limits areimposedon the choiceof p.r.f. sinceif it is too low the rate at which information is received 142

echoes Fig.9.3 Second.trace is low, while if it is too high the seriousproblem of secondtraceechoesmay arise. If the characteristics of the radarare such that the maximum rangefrom which echoescanbe detectedis, say,200 nautical miles then the round trip travel time for a targetat maximumrangewould be about,2500ps. In sucha systema pulserepetitionperiodp.r.p.(= l/p.r.f.) of 2000 gs would meanthat the time-basestart would occur 500 ps before the return of an echo of the previoustransmittedpulsefrom a target at 2OO nauticalmiles. This secondtraceecho would appear

to be at about 40 nautical miles range. It follows that in the aboveexamplethe maximum p.r.f. would be 400 and in generalp.r.f. ( c/2R whereR is the maximum range. A popular choicefor older radarswas a p.r.f. of 4O0 synchronizedto the supply frequency. With improved performanceleadingto increasedrange,a submultipleof the supply frequency,e.g.200, was used. In modernsystemsinternaltimingis independentof the supply frequencyand one finds p.r.f.sfrom about 100 to 250.

is that ground returnswill appearat closerrangesfor wider beamwidths,thus maskingthe cloud retums.

Tilt and Stabilization The reasonfor requiringstabilizationis relatedto the previousparagraph.A weatherradarmay scanup to 300 nauticalmiles aheadof the aircraft.within azimuth scananglesof typically t 90". Unlessthe beam is controlled to move only in or abovethe horizontal planepart or all of the weatherpicture may be maskedby ground returns. Imaginethe aircraft rolling with port wing down. If the sweptregionis in the sameplane as the aircraft'slateraland longitudinal Power Output axes,then when the scanneris to port the beamwill In older radarspeak power outputs of about 50 kW or evenhigher were common,while maximum range be pointing down towardsthe ground,while when to starboardthe beamwill be pointing up, possibly wasmodest. In modernradars,peakpoweris about previous abovethe weather. Figure9.4 illustratesthis with l0 kW with increasedrangecompared problem. systems.This apparentspectacularimprovementis In fact stabilizationholds the beamnot in the put in perspectiveby consideringthe rangeequation horizontal planebut at a constantelevationwith (see A9.7) where we seethat maximum rangeis power. peak respectto the horizontal. This constantelevationis of the root proportional to the fourth Thus reducingpower by four-fifths reducesmaximum determinedby the tilt controlassetby the pilot. later. rangeby about one-third. This shortfall of one-third Detailsof stabilizationand tilt arediscussed hasbeen more than madeup by improvementsin rerial design,receiverdesignand sophisticatedsignal Contour The pilot will be interestedin thoseregionswherethe processing. precipitationis greatest.In order to make the situation clearer,thosesignalswhich exceeda certain Beam Width predeterminedlevel are invertedso as to show the Although the largerthe beamwidth the greaterthe cellsof heavyprecipitationas dark holeswithin the echo this the volumeof the targetcontributingto 'paint' causedby the cloud surroundingthe bright the inverse to out due than cancelled is more effect 'paint' aroundthe cell is an cell. The width of the relationshipbetweenaerialgainand beamwidth indication of the rainfall gradient. The narrowerthe (G o ll02). A narrowbeamis alwayspreferred, width the steeperthe gradient,and hencethe greater improve range and to increase is sincethe net effect turbulence.This bearingresolution.Simplegeometricconsiderations the probabilityof encountering (equal echo) contour iso€cho show that with a 4" beamwidthtwo targetsseparated techniqueis known as pre sen tation. by about 3! nauticalmiles,at a range) 50 nautical miles,will appearas one on the p.p.i. Bearing Sensitivity Time Control, s.t.c. resolution,unlike rangeresolution,is dependenton the targetrange.An equallyimportantconsideration For correctcontouroperationthe signalstrength No stabilitv

B\

./ <______

__ Stability

\ No stabilitY

7///////////////////////////////ru

7//////27'////////////////////////ru

Fig. 9.4 Scannerstabilization

1/13

arrangementis to havea.g.c'noise'derived.During strould depend only on the characteristicsof the the output target,but of coursethe rangealso affectsthe received the time immediatelybefore transmission p.r.f. will the only, since is noise receiver the from inversion contour ifthe power. As a consequence echoes,i.e. second-trace to avoid chosen been have a certain cells at storm to indicate as so levelis set the time correspondingto maximum rangeexpires rangethen innocent targetscloserthan that range before the next pulse. The a.g.c.circuit is gated well may causeinversionwhile storm cellsbeyond that the receiveroutput is connectedto it only for that so rangemay not. In order to solvethe problem the time beforetransmission. a short being with range, receivergainis made to vary of havingsuch a.g.c.is to keep the result The thereafter increasing and range at zero minimum output constant,which under normal noise receiver (i.e. with time), hencesensitivitytime control or the gainis constant. If, for any means conditions sweptgain,asit is sometimescalled. or received, noisegenerated reasonthereis excessive the gainwill fall, so keepingthe backgroundnoise displayedat a constantlevel,althoughsignalswill Tx will not highlight all storm cells. ------+Time fade and contour An alternativearrangementis to keep the receiver Rx of how the receiveroutput gain constantregardless gain may change.This hasthe virtue of keepingthe (S.t.c.) conditionsfor inversionin the contour circuit unchanging.Such presetgain is found in modern a.g.c.is found digital systems,whereasnoise-derived in older analoguesystems. timecontrol Fig.9.5 Sensitivity The aim is to make the receiveroutput independent'udrange. Unfortunately the received as the squareof the rangefor targets power decreascs which fill the beam,but as the fourth power of the rangeotherwise(seeAppendix). To achievethe aim would requirea complex gain control waveform which would, even then, only be correct for a certain sizedtarget. Many systemshavebeendesigned assuminga 3 nauticalmile diametercloud as standard; s.t.c.hasoperatedto a range amongsuchsystems, wheresucha targetwould fill the beam;beyond that gainis constantwith time. It hasbeen observedthat it is uncommonfor water dropletretumsto come from a regionwhich fills the beamvertically except at closeranges. Following from the above,s.t.c.may be arranged to compensatefor the rangesquaredlaw out to about 30 nauticalmiles for a 6o beam. A changein scannersizewould lead to a changein maximum s.t.c. rangesincethe beamwidthwould alter. Alternativeiy, a modified law may be compensatedfor out to, say, 70 nauticalmiles,ignoringthe possibilityof beam filling.

Display A problem in displaydesignfor cockpit useis the largerangeof ambientiighting conditions. Some storageof the informationreceivedis inevitableif it is to be viewed,owing to the relativelylow refreshrate of the fleetingbasicinformation. In olderand simplerradarsthe c.r.t. screenis phosphorwhich coatedwith a long-persistence continuesto glow sometime after the electronbeam haspassedon its way thusstoringthe information overthe scaninterval. Unfortunately.suchphosphors arenot very efficient and very high shieldingis requiredfor adequateviewingin bright conditions. The directview storagetube (d.v.s.t.)is a solution c.r.t. A meshis to the problemsof a conventional phosphor-coated the behind immediately mounted screen. Electronsarriveat the meshfrom two by the sources:a locusedbeam,velocity-modulated signal,comesfrom a conventionalelectron gun while a flood gun provides,continuously,a wide beamof electronsover the whole mesh. Sincethe modulated beamis deflectedacrossthe meshin accordancewith scannerposition and time sincetransmission,a charge pattern is written on the mesh. The pattern determines where and what fraction of incident electrons Automatic Gain Control, e.g.c. penetratethe meshand strike the phosphor. Since with desirable It is neither practicalnor indeed eachglowingelementof phosphoris excited signal by a'g.c. determined have to contour operation continuously,very bright displaysare possible. A level asin receiversfor other systems. The normal 1U

i

slow discharge.pathfor the mesh must be provided to exception of a few noisc spotsper memory update. preventsaturation;the possibility of changingthe dischargerate exists and a pilot-adjustablecontrol Scanner may be provided. On changingrangethe meshwill be There are two typesof scanneremployed to obtain 'instantly' dischargedto prevent confusion between the requirednarrowbeam,namelya directly fed parabolicreflectoror a flat plateplanararray. Of the 1ew and old screenpositionsof the targets. If the dischargepath is broken and updating inhibitrd *" two, for a givendiameterand wavelength,the flat havea frozenpicture. platehasthe highergain/narrower beam/least The modern approachto information storageis to sidelobe power,but is most expensive.Sincethe flat .. digitize the signalwhich is then storedin an plateis almosttwice asefficientasthe parabolic intermediatememory at a location dependingon the reflectorit is invariablyusedwith a modernsystem scannerazimuth angleand the time of arrival exceptwhere cost is an overridingfactor or the space measured with respectto the time of transmission. availablein the noseof the aircraft allows a larseThe p.p.i. time-basescanformat can be either . parabolicreflectorto be used. rho-thetaor X-Y as per a conventionaltelevision. The flat plateantennaconsistsof stripsof In either casememory can be read at a much higher waveguideverticallymounted.sideby side with the rate than that at which information is received. With broad wall facing forward. Staggeredoff-centre many more picturesper secondpainted we havea verticalslots are cut in eachwaveguideso as to bright, flicker-freedisplay without the need for interceptthe wall currentsand henceradiate. Several long-persistence phosphorsof low efficiency or the wavelengthsfrom the antennasurface,the energy e4pensive d.v.s.t. from eachof the slotswill be summedin space, In the caseof a rho-thetadisplay thc scanformat is cancellation or reinforcement takingplacedepending the samefor receiptand displayinformationbut, as on the relativephases.In this applicationthe phase explained,the repetition rate is different,so only of the feedto eachslot, and the spacingbetween scanconversionin time is required. With a television_ slots,is arrangedso as to givea resultantradiated type displayboth the scanformat and repetition rate pattern which is a narrow beamnormal to the plane are different, so completescanconversionis required of the plate. The greaterthe nurnberof slotsthe . from input (received)format to readout(displayed) betterthe performance; sincethe spacingbetween format. Useof the television-typedisplay makes the slots is critical we can only increasethe number multipleuseof the weatherradarindicatorrelatively of slotsby increasingthe sizeof the flat plate. simpleso we find such indicatorsable to display data The parabolicreflectorworkson a similarprinciple from other sensors (e.g.Area nav.)or alphanumeric to a car headlampreflector. Energystriking the datasuchaschecklists. reflector from a point sourcesituatedat the focus will Digitizing the signalinvolvesthe recognitionof oroducea plane waveof uniform phasetravellingin a only discretevaluesof signalintensity. ihe standard direction parallelto the axis of the parabola. The practiceis to havethreelevelsof non-zerointensity, feedin a weatherradarparabolicantennais usuallya the highestcorresponding to the contour inversion dipolewith a parasiticelementwhich, of course,is level. Although thesethree levelscorrespondto not a point source.The consequence of a dipole feed different degreesof brightnessof the paint, with the is that the beam departqfrom the ideal and there is useof a colour tube they can be madeto correspond considerablespill-over,$ivingrise to ground target to threecolours. While thereis no standardization of returnsfrom virtually helow the aircraft, the so-called the colour code as yet (1979) red is the obVious height ring. choicefor contourabletarsets. In both typesofaptenna,scanningis achievedby Perhapsthe mosl signifilant virtue of a digital r o t a t i n gt h e c o m p l e t ea n t e n n aa 5 s e m b l yt h, u sa weatherradaris the absenceof noiseon the display. r o t a t i n gw a v e g u i djeo i n t i s r e q u i r e d .A n The output of the receiver,the videosignal,is electronicallysteeredbeantis possiblewith the flat digitizedand then averaged in both time and position, platebut the considerable complications of arranging i.e. the videooutput occurringp gs after a the correctphasingof the feedto all slotshavemade transmissionis averagedwith that occurringp ps after this impracticalfor airborneweatherradarsystems. the next transmission and the video datawhich would Some of the simpler.cheaperweatherradars appearat adjacentpositionson the screenare subject employinga parabolicreflectorrotatethe reflector to a weightedaver-agingprocess. With a suitablechoice only. leavingthe feed fixed and so eliminatingthe o f m i n i r n u ms i g r a ll e v e lf o r a ' p a i n t ' u n c o r r e l a t e d needfor an azimuthroratingjoint. A disadvantage of noiseis virtuallyelirninatedfrom the displaywith the this latter systemis that the feed is not at the focus

145

rotation, memory write/read and all display circuitry. 'brain' Thus in a modern weatherradar we seethat the of the systemis situatedin the indicator while the heart'remainsin the t.r. This view is reinforcedby the fact that the pilot/systeminterface is achieved completelythroughthe indicator,both for display and control. lnstallation The scanneris a flat plate array plus associated for scannerstabilizationand.azimuthdrive. circuitry Figure9.6 illustratesa typicalinstallationin block plate t'lat is mounted on a gimballedsurfaceu'hich The form asrepresented by the BendixRDR 1200.a to pitch and roll signals allowsrotationin response with the upperend of digitalweatherradardesigned the generalaviationmarketin mind. The installation from the aircraftverticalreferencegyro (VRG). will be discussed in termsof the RDR I100 first. and Radiationis througha radomewhich ideallyis transparent to the X-bandenergybut at the same will be described. then variationsand refinements protectittnfor the scanner,preserves provides time Radome the aerodynamicshapeof the aircraftand has adequatestructuralstrength. VRG Waveguide aremadeusing Most of the interconnections N, with signal and control lines approved cables standard ---i---l Trar scanner J l ) recev e r s c r e e n e dT. h e i n t e r c o n n e c t i obne t i v e e nt . r . a n d l / losses by way of waveguide. is, of necessity. scanlrer 2 8 V d . c .t t s v ' c f f at X-band(or in co-axialcablebeingunacceptable Supply e v e nC - b a n d ) . lndrcator Whilethe aboveis a simplifieddescriptionof the BendixRDR 1200the units and tl.reircontentsare very much the samefor all moderndigitalsystems. Fig.9.6 BendixRDR 1200installation A trend in lightweightsystemsfor generalaviation aircraftis to combinethe transmitterand scannerin The transmitterreceiver(t.r.) containsall the r.f. of a two unit system, one unit. The nrainadvantage circuitryand componentsaswell as the rnodulator, plus indicator,is that the waveguide run is analogueto digital(a/d) converter t.r./scanner duplexer,IF stages, and powersupplycircuits. Unlikeolder systems,the eliminated,so reducingcapitaland installationcosts andwaveguidelosses.The argumentagainst basictiming circuitryis in the indicatorratherthan the t.r. This timing controlsthe p.r.f.,scanner combininsthe t.r. and scanneris that the unit exceptwhen the reflector axis is deadahead;the resultingdeteriorationof the beam shapemeansthe anglethrough which the beamis scannedmust be restricted.

V

Fig.9.7 Primus 200 multifunction colour weather radar. Two-unit sensor on the left, multifunction accessorieson the right (courtesy RCA Ltd)

1tt6

installedin the noseis costly in terms of maintenance cornersand flexible sections,except where necessary, strouldit require regularreplacement;thus reliability so reducingcostsand losses.A choke flangeto plain assumes evengreaterimportancein such systems. flangewaveguidejoint is normal practice,the choke Singlecnginedaircraft havenot been neglected,the flangehavinga recessto take a sealingring. problemhavingbeen tackled in two different ways, The radomeis usuallya coveredhoneycomb both of which involve a combined t.r./scannerunit. structuremade of a plasticmaterialreinforcedwith Bendix havegone for an under-wingpod-mounted fibreglass.The necessityfor mechanicalstrength, unit, while RCA have developeda wing leadingedge smallsizeand aerodynamicshape may compromise mountedunit in which a sectionof a parabolic the r.f. performance. Lightning conductorson the reflectoris used,as well as a pod-mountedunit. insidesurfaceof the radomewill obstruct the beam, Corrosiondue to moisture collection is a problem but their effect is minimizedif they are perpendicular in the waveguiderun, which may be easedby to the electricfield of the wave. Horizontal pressurization.The ideal is to havea reservoirof dry polarizationis normal sincethere is lessseaclutter, air feeding,via a pressure-reducing valve,a waveguide althoughwith moderateto rough seasthe ldvantage run which hasa slow controlled leak at the scanner; is minimal. this method is not usedon civil aircraft. Several On largeaircraft a dual installationis used. installatiorpailow cabin air to pressurizethe Obviouslythe scannercannotbe duplicatedbut both waveguiderun via a bleedervalve,desiccantand filter. t.r. and indicatorcanbe. In somecasesonly the If the cabinair is not dried and filteredmore problems indicatoris'duplicated, thus eliminatingthe needfor may be createdthan solved. Two interestingcases a waveguideswitch. Unlessone indicator is purely a that havebeenbrought to the author's attention are: slavewith no systemcontrolsother than,say, a nicotine depositon the inner wall of the waveguide brilliance,a transferswitch will be necessaryto causingexcessiveattenuation,and rapid corrosion transfercontrol from one indicatorto another. Even causedby fumes from the urine of animals if two t.r.s are fitted a transferswitchis still necessary transported by air. lfno activepressurization for scannerstabilizationon-off and tilt control. is enrployedthe pressurewithin the waveguidemay still be higher than static pressureif all joints are tightly sealed.A primary aim of waveguidepressurizationis Controls to reducehigh-altitudeflash-over. The following list of controlsis quite extensive;most will be found on all radarsbut someareoptional. The nomenclature variesbut alternativenamesfor someof the controlsarelistedwhereknown. RangeSwitclr Usedto selectdisplayedrange. Will alsochangethe rangemark spacing.Selectionmay be by pushbuttonor rotary switch,the latter possibly incorporatingrOFF','STANDBY' and 'TEST' positions. OfflStandby Pushbuttonor incorporatedin the range switch. With standbyselectedtherewill be no transmission while indicatorextrahigh tension(e.h.t.) may or may not be on.' Fig.9.8 Installation of Weather ScoutI t.r./scanner unit (courtesy RCALtd) The waveguiderun should be kept as short as possible.Inaccessibility or inadequatecoolingmay nreanthat the t.r. cannotbe situatedso as to minimizethe lengthof the run. Straightrigid waveguide shouldbe used,avoidingbends,twists.

Functibn Switch Selectsmodet-rf,operation . N O R M A L " ' C O N T O U R " ' C Y C L I C i. ,M A P P I N G " 'BEACON'. The last two of thesemodesare described later. Normaloperationallowsthe useof a . g . c(.p r e s egt a i n )o r v a r i a b l g e a i n :s . t . c .i s u s u a l l y a c t i v e .C o n t o u ro p e r a t i o ns e l e c t i o cna u s ebsl a n k i n g o f s t r o n g e ssti g n a l sa,. g . ca. n ds . t . c .a u t o m a t i c a l l . , s e l e c t e dC . y c l i co p e r a t i o nc a u s enso r n r aal i r dc . r i r t o u r presentations to alternate.Pushbuttonor rotar.y 147

'NORMAL'switch control may be used. may be omitted, this mode of operationautomaticallybeing selectedwhen a rangeswitchincorporating'OFF' and 'STANDBY' is selectedto any rangewhile 'CONTOUR' or'CYCLIC' switchesare off. (Seealso 'gain control'.) Gain Control Usedt6 set gainof receivermanually. A continuouslyrotatableor click-stopcontrol is normal. The control may incorporatecontour on-off;by rotatingthe knob pastthe maximum gain positioncontour plus presetgainwill be selected.In this latter casea separate springreturn pushbutton may be usedto turn contouroff momentarily. In other systenrsthe gaincontrol may simply incorporatea presetgainon-off switch at its maximumposition.

(usually)sectorscanangtes,e.g.Bendix RDR 1200 hast 30o or + 60o options. Contrast Control Adjusts video amplitier gain and henceallowssomecontrol of pictureas opposedto displaybrightness evenwhen i.f. ampsareoperating undera.g.c. Sonretimes calledintensity. Manual Tune Contol Associatedwith automatic liequencycontrol (a.f.c.)on-off switch. Whena.f.c. is selectedto off, local oscillatormay be tuned manuallyfor bestreturns. Generallynot usedon modernsystems.

Operation

The actual operation ofa weatherradar is quite straightforward, but to get the bestuseof the system amountof experienceand expertiseis a considerable requiredon tl'repart of the pilot. Beginnersare advisedto avoidby a wide marginany contourable ScannerStob Switch On-Off Switching. targetwithin s.t.c.rangeand any targetat all outside that range. Tilt Control Adjustment of scannerelevationangle With experiencethe pilot is able to distinguish t y p i c a l l yt 1 5 " . betweensal-eand unsafetargetsto the extent that he may be ableto penetrate,ratherthan just avoid, Brilliance or Intensitlt Control Adjustsbrightnessof weather. Severalwords of warningarein orderwhen displayto suit arnbientlighting. attemptingpenetration:one shouldalwaysselectone of the longerrangesbeforeattemptingto fly Free:e or Hold Su,itch Dara updateof display betweenstorm cellssincethe way throughmay be stopped.last updatedpicturedisplayed.Transmissionblockedfurther ahead;weatherconditionscan change and scannerrotationcontinues.Warninglamp rnay rapidlylthe limitationsof X-bandradarin so thr as be provided.Only availableon digitalsystemor signalpenetrationis concernedshould be remembered. w h e r ed . v . s . ti.s e m p l o y e d . Test Switch A specialpatternspecifiedby the replace5 weather(or mapping)picture manufacturers when test is selected.

Eraseor Trace Contol Springreturn switch to rapidlydischarge meshin d.v.s.t.,so wiping picture cleanor continuouslyvariablecontrol which alters d i s c h a r grea t e . RangeMark Cttntrol Alters intensity of rangemarks. Azimuth Marker Sv'itch Electronicallygeiterated. azimutlirnarksnraybe turnedon or ofl'. TargetAlcrt Sx'itct On-off. Whenactivatedflashes a n a l e r to n t h e s c r e e ni f a c o n t o u r a b ltea r g e ti s 'window' aheadof the aircral't(window detectedin l s 0 0 i s 7 ' 5 ' e i t h e r s i d eo f h e a d i n g s i z - feo r R C A P r i n r u 2 . l r n i n gg i v e n i i t a r u n g eo . l ' 6 0 - l i 0 n a u t i c arl n i l e s ) W rli selectedrangealsorvhenin 'freeze'mode regardless Sec'torSrurrSrl'ilci Allowsselectionof one of two 148

Finger

Scalloped

Edge

U-Shap€d

Fig.9.9 RDR 1100displays(courtesy Bendix Avionics Division)

It has alreadybeenmentionedthat a narrow paint scannerstabilizationfaults exist. With the scanner around a storm cell is a good indication of severe tilted down and stabilizationon a bright circular band turbulencebut whateverthe width the cell should be ofterrain targetscentredon the originshouidbe presented.If the band is not circularbut is severely avoided;the only questionis by what margin. The 'toppled'or strapeof the return on the screenis alsoa pointer to distorted,then most probablythe gyro is 'spilled', the spin axisis not vertical.The gyro the type of weatherahead. Targetswith fingers, i.e. hooks, scallopededgesor that are U-shapedhavebeen will be in sucha conditionif its rotor is running slow, aswill be the caseif it hasjust beenswitchedon observedto be associatedwith hail. Long hooks or indentationsmay indicatea tornado, or there is a supply or gyro motor defect. A gyro fast crescent-shaped erectswitchmay be providedwhich,when depressed, but thereareno guarantees eitherway. This chapter,and its appendix,havethus far been increasesthe supply voltageso acceleratingthe run up concernedmainly with rain, so a word is in order here to operatingspeed. The switch shouldnot be held on for more than half a minute or so, otherwisethe rotor about returns from other types of precipitation. Dry speedmay exceedthat for which it was designed,so mowfall will not be seenbut wet snowfall may be causingdamage. sen with difficulty. Fog and mist will not be To completean airbornecheck of stabilization,the detected.Hail may givestrongor weak returns:if it aircraft shouldbe bankedand pitched,within the b dry and small comparedwith the wavelength, stabilizationlimits, satisfactoryoperationbeing rctums areweak:if it is water-coated. returnsare circularterrainband. indicated by an unchanging if it is dry and with the strong; of a sizecomparable the amountof nose Note that with tilt down selected. wavelength(an extrelnecondition with 3 cm radflr) lhe echo is very strong. The increasein scatteringfor up pitch that the stabilizationcircuitscan dealwith is water*oated ice particles,hail or snow, may give rise limited. to a'bright band'at an altitudewherethe temperature Spokingis any p.p.i. presentationwhich resembles the spokesof a wheel. lt is almostcertainlycaused bjust above0"C. Lightningcreatesan ionized by a fault within the weatherradarsystem,although gaseousregionwhich may, if oriented correctly, it can be causedby unshieldedelectromagneticdevices backscatterthe radarenergy. producingstrong changingmagneticfields. There are picture on weather Severalthingsmay affect the of fault which causespoking: the p.p.i. Icingon the radomewill causeattenuation two main classes (a) video sigrraland noisespokesdue to abnormal ofthe transmittedand receivedsignal,so targets video output amplitudevariationssuchas might be which would havebeen displayedmay, under these causedby the automaticfrequencycircuits(a.f.c.) circumstances, remainundetecteduntil very close; 'innocent'precipitationwill alsoattenuatethe signal. sweepingthrough the local oscillatorfrequenciesand (b) sweepspokesdue to faulty displaycircuitry such Ground or searetums may mask a storm cell; the tilt ground asdamagedslip ringsin the time-baseresolver weather.and control shouldbe usedto separate radar. To employedin an older all-analogue targets,a difficult task in mountainousregions. (a) (b) or is the cause,the gain which of determine Interference, which takesthe form ofbroken, curved the antenna tilted to down and may be tumed p.p.i. by display,is caused or straightlineson the the fault is in maximumup; if the spokingpersists, other radar systems.The exact effect varieswith the the displaycircuitry. type of videosignalprocessing and scanconversion The aboveis only a brief discussionof someof the (if any) and with the p.r.f. of the interferingradar. factorsone.mustconsiderwhen operatingweather The older type of weatherradarwith no scan of the degreeof skill involved,a pilot radar. Because of the videooutput is conversionand no averaging new to weatherradarshouldstudy the manufacturer's particularlysusceptible to interference.High p.r.f. pilot's guidecarefully;thqy areusuallyvery good. interferingsignals,suchasGCA, givemany fine Equallyimportant,he shouldlearneachtime he uses broken radiallineson the screen.If the interfering the system;for example,if a detouris madeto avoid p.r.f. is closeto a harmonicof the p.r.f. of the radar an unusuallyshapedreturn, a simplesketchand a with, then curvedbrokenlines, beingintert'ered phone call on landingto enquireabout the weather apparentlymoving into or away from the origin. will with that targetwill add to his experience. associated result. Wheremotion is apparent,the interferenceis 'running Rememberthat to a largeextentthe body of rabbits',another commonly referredto as 'rabbit tracks'. knowledgeconcerningweatherreturnsis empirical. commonll'encountered term is What betterway to learnthan to collectone'sown Selectionof contourmay alleviateinterference results? problems. ',dbnorrnal A final and most important point needsto be p.p.i.presentations will be observedif

149

/ made,and that is that weatherradarpresentsa considerable hazard,when operatedon the ground. Detailsare givenlater in this chapter.

weatherradardirectly and rystemsof this type are still very widely used;the latter sinceit is th; currenr approachof all manufacturers.Two types of digital systemwill be considered:rho-thetadisplayand X-y display.

Block DiagramOperation We shallconsiderboth analogueand digital systems: the former sinceit illustratesthe principlesof

All Analogue System The p.r.f. generatorprovidestimb synchronizationfor the completesystem;the output is often calledthe

lAzimuthI drive I I

-ti

FT

I

1 S

Balenced mrxer.

l@ ;l

on/ I

P.R.F. gen. Mod. Tx Time base Bright up Range marks A.G.C./

s.T.c.

tl -l-,-

I

+

I

M t

T

l

,l I

Video Fig. 9. | 0 All analogue weather radar block diasram and waveforms

150

off

pre-pulse,sincethe laggingedgeis usedto triggerthe waveform coincidewith the start of the run-down. modulator. The transmitteris a magnetronkeyed by If the balancinghalf-cycleis madelargerthan the modulator which determinesthe pulsewidth. necessary we havean open centrewhereby zero range The burst of r.f. energy(main bang)ii fed from the is represented by an arc,ofnon-zeroradius,on the transmitter to the scannervia a duplexerand p.p.i. display. waveguiderun. The duplexerallowscommon aerial The gatewaveformis fed to the marker and working in that it is an electronicswitch which bright-up circuitswhich providethe necessaryfeeds automaticallyconnectsthe scannerto the transmitter to the c.r.t. for the duration of the time-base.Range for the duration of the transmittedpulse,thus marksare producedat equally spacedintervalsduring protecting the receiver. the gate,and areusedto intensitymodulatethe c.r.t. A sampleof the transmittedfrequencyis fed to the electronbeam. The bright-upwaveformprovidesa a.f.c.(automaticfrequencycontrol)mixer alongwith biaswhich preventsthe velocity of the beam berng an output from the l.o. (localoscillator).If the sufficient to excite the phosphorcoatingon the differencefrequencyis not equal to the requiredi.f. screen,except during the time-baserundown. the a.f.c.circuit appliesa controlsignalto ihe 1.o.,s<j Pitch and roll stabilizationis providedby a adjustingits frequencyuntil we haveequality. If the ryro-controlledservomechanism. differencefrequencyis outsidethe bandwidth of the a.f.c.circuit, the control signalis madeto sweepuntil Digital Weather Radar - Rho-Theta Display suchtime asthe a.f.c.loop can operatenormaliy. The radio and intermediatefrequencypart of the The main receivermixer is balancedto reducel.o. block diagramis much the samefor analogueand noise. The i.f. amplifierchainis broadband digital systems,so here only the video,timing and (bandwidth ) 2 X reciprocalof pulsewldtn) with gain control blocks will be considered.The following is controlledby the a.g.c./s.t.c. circuitsor the manual basedon the RCA Primus40. pin control. The videoenvelopeis detectedand after Analoguevideodatafrom the receiveris further amplificationis usedto intensitymodurare digitizedin an analogueto digital (a/d) converter. ( Z - m o d u l a t i o nt )h e c . r . r . The rangeselectedis dividedinto 128 equalrange With contour on, the videosignalis sampled,and if cells,for example,with 300 nauticalmilesselected abovea presetinversionlevelthe videofed to the eachcell is 300/128 = 2.344nauticalmiles,or, in c.r.t.is effectivelyremoved. termsof time, 3607 ps is dividedinto 128 time-slots The pre-pulseis fed to the a.g.c.gatewhich thus of 28.96ps. Duringeachtime-slotthe videolevelis allowsthe videooutput throughto the a.g.c.circuit first integratedthen encodedas a 2-bit word, thus o n l y f o r t h e d u r a t i o no f t h e p r e - p u l s(es a yl 0 p s ) . I n givingfour discreterepresentations from zeroto this.waythe gaincontrolline voltagelevelis madea maximumsignal.In the Primus40 a complemented function of receivernoise. The laggingeclgeof the Gray codeis usedfor the conversionbut is then pre-pulsetriggersthe s.t.c.circuitwhich reducesi.f. changedto standardbinary. gainat zero rangeand returnsit to normalafter, The scanneris drivenby a steppermotor suchthat typically,30 nauticalmiles(about 370 ps). 1024stepsaretakenfor 120" ofscan. The The laggingedgeof the pre-pulse transmitterfireson everyother stepso providing512 alsoiniriatesthe start of the time-base and gatewaveforms,the azimuthdirtictionsfrom which echoesmay be durationofwhich dependon the rangeselected.The received.Thuson eachof 512 azimuthanglesdatais time-base wavefornrl(r)is f-edto a magslip(synchro acquiredin 128 rangeincrements. resolver)in the scanner; sincethe rotor of the magslip The averaging/smoothing circuitsreducethe is drivenin synchronism with the scannerazimuth numberof lines(azimuthdirections)by a factor of m o v e m e ntth e o u t p u t sa r e1 ( r ) s i nd a n d1 ( r ) c o s0 4 to 128(= 51214)and apply a correctionto the where0.is the azintuthanglenreasurecl with reference gradientof the sigralin rangeand azimuth. The 4 to t o t h e a i r c r a f th e a d i n g U . s i n gt h e c o s i n eo u t p u t f o r I line reductionis achievedby averaging the sum of verticaldeflectionand the sineoutput for horizontal four adjacentazimuthtime cellsasshbwnin deflectionprovidesthe necessary rotatingtime-base. F i g .9 . 1 3 a n dT a b l e9 . 1 . A f t e r a v e r a g i nwge h a v e1 2 8 The start of the time-base run-downmust lineswith 128 rangecellsper line grving correspondto zerodeflectionof the c.r.t.spot. Since 1 2 8X 1 2 8= l 6 3 8 4 d a t ac e l l s .T h e d a t ai s t h e n the magslip.beingbasicallya transformer,removes correctedfirst in rangethen in azimuthas follows: any d,c. levela balancinghalf-cycleis required if in a seriesof threeadjacentcellsthe outer two cells imrnediatelyafter the time-base flybackto makethe arethe samebut the inneris different,then the inner (zero)valueof the compositetime-base average is correctedso asall threearethe same;forexample,

151

icccivcr

vidco

Range cells

7

5

3

10 1 1 12 13 1 4 1 5

127 128

(NoisGl Video levol (O thru 3)

1

I

,|

0

0

0

1

0

1

1

z

(,, --{ Complemented binarygray I

code

1

tt

Fig. 9.1I Analcgueto digital conversionin the Primus40 (courtesyRCA Ltd)

256 LINESPERFRAJVIE RATE FRAMEREFRESH l S 6 0 . 7H z ( 1 6 . 4 7 5 m S | 1 6 . 4 7 5 m-S2 5 6 = 6 4 3 5 5 , r SL I N ET I M E

DlsPtAVCELTS 128CELLS P€RLrNEX 256LTNES=32.768

,"*\

"',*",j,ffik% -urr;fll.\ .

. c4ce{

Fig. 9.12 Rho-thetarasterscanformat in the Primus40; per scan,128 linesin memory,256 512 transmissions displayedlines (courtesy RCA Ltd)

152

1

3

2

0

0

Table9.1

Four to one line averaging

'nemory

as it is received.Sincethe signallevel within eachrangecell is codedas a 2-bit word the memory Sumof four azimuth Average capacitymust be 2X 128 X 128 = 32'168bits. The adiacenttime cells memorycomprisessixteen2 X 1024-bitshift registers,so applicationof clock pulsescausesthe 0-l 0 circulationof dataprovidedthe output is connected 2-5 I to the input which is the casewhen new data are not 6-9 2 beingloaded. New datamust be loadedinto memory r0-12 3 at the correcttime in the sequence of circulating data;this timing control is providedby the new data line control circuit which synchronizesthe loading 313 would be correctedto 333 while 012 would remainthe same(seeFig.9.l4). The aboveprocesses, with scannerposition. Loadingis inhibited when the freezebutton is pressed but circulationof data togetherwith the integration of the video signal within eachrangecell prior to digitization, reducethe continues.The datais continuouslyreadout asit circulatesat a rate of about 7'772\inesper second displayednoice to negligibleproportions. comparedwith loadingevery fourth main bang,a rate Eachof the 128 lines of 128 cellsis placedin of 121'414= 30.35linesper second.The different load and readratesgivethe scanconversionin time, a-----------l a--.,--=-l leadingto a flicker free bright picture. Although 128 linesof videoarestored,256 lines aredisplayed,so it is necessary to doublethe stored After averaginglines. The line-doublingcircuit averages two adjacent storedvideolinesto generatea middleline, so giving the required256 lineseachof 128 cells(i.e. a total of 32768 displayedcells). In the averaging processthe rule is to averageup if an integeraverageis not possible.The following example,consideringpart of Fig. 9.13 Fout to one line averagingin the RCA Primus 40 Noise

E'l corrected F8. 9.14 Rargc and azimuth smoothing and correction in the RCA Primus40

153

Video from Rx

Averaging smoothing circuits

Tx trigg6r CRT Frame retrace

tI Freeze and rho-thetadisplayblock Fig,9.15 Video processing diagram

32768 bit shift register

Data out

Clock Fig. 9.15 Simplifiedmemory

by 128125= 5'12 cells,so the first mile is represented mark is locatedat cell 26 (= 5 X 5'12) with subsequent storedline...3 3 2 2 1 . . . m a r k sa t c e l l s5 l , 7 1 , 1 0 2 a n d 1 2 8 . middleline...3 3 2 2 t . . . Azimuth marksareobtainedby raisingall 128 storedline...3 2 2 I 0 . . . cellsof the appropriatelinesby I in a similar way t^o I I I 0... m i d d l el i n e . . . 2 that describedabove. The sweepis 120",so for l5storedline...0 0 0 0 0 . . . azimuthmarkerswe requireI + 120/15 = 9 linesto Rangemarks are obtainedby raisingthe appropriate be enhancedin intensity. With 256 linesthose chosenare line 2 and line 256 Nl8 = 32ly' rvhere rangecell levelfor eachof the 256 linesby l, i.e. 0 -60o markersince becomes1, I becomes2,2 becomes3 while 3 remains N= 1-8. Line 2 is usedfor the line, tltus the first'trace to the blankingis applied at 3. Thus rangemarksappearslightly brighter than -59'0625". at fact is in marker azimuth targetreturnsexceptfor level3 targets'Identification leftmosi 3 levelto a video a converts circuit contour The line is achieved of the appropriaterangecell in each of cells range sequence a If we had 0 level. video mile nautical the 25 on by a counter. For example rangethere are five rangemarks 5 nautical miles apart. 0 2 3 0 0, say,then the contouredcell of level3 would not be bordered,the sequencebeing Sincethere are 128 rangecellsper line eachnautical

three adjacent stored lines, illustrates the process:

154

0 2 0 0 0. To avoid this, the rangecell adjacentto a contouredlevel 3 cell is raisedto level 2 if necessary, thusin ourexampleO 2 3 O 0 would become 0 2 O 2 0 after contouring. Borderingis guaranteed in azimuth as a result of the line-doublingprocess since,for example,if we haveadjacentazimuth cells with videolevelsI 3 0 from'memory,then line-doublingwillgive | 2 3 2 0 and after contouring | 2 O 2 0 as required. This bordering -featwe is.necossary where the video gradientis steep, suchaswhen we receivereturnsfrom mountainsor distantweathertargets.

The rho-thetarasteris generated by tlte deflection circuitswhich are triggeredby the frame andX-Y retracewaveforms.A linearramp currentrvaveform needsto be generated for both_the)/ (vcrtical) deflectioncoilsand the X (horiz.r,ntal) deflection coilswhich form the yoke. The durationol'the ramp i s 5 3 ' 8 9p s w i t h a 1 0 ' 4 6p s r e t r a c e( f l y b a c k ) g i v r nag totalline time (time-base period)of 64.35 gs. The amplitudesof the rampwavelbrmsdeterminethe amountof deflectionin the X and X directionsand thus the particularline which is tracedon the screen; line I is at -60o, line 256 is at +60o. In.pracrice, sinceon the completionof one frameat line 256 on 0 0 vlotor-EvtLs r 2 I 3 t 2 O the right we start the next frame on the left after frameretrace,line I is blankedin order to allow the o * ( , deflectioncircuitsto settledown. The franrerateis SINAiY CODI 6O'7Hz. The X and )z ramp waveformsare initiated by thc X-Y relracepulses.The amplitudeof the I/ ramp must be a mininrumat the beginningand end of the frameand a maximumhalf-waythroughthe frame; its polarity is constantthroughout.The anrplitudeof the X ramp must be a maximurnat the beginningand ANALOGVIDEO end of the frame and zerohalf-way through the frame,when the polarity reverses.To achievethe amplitudevariationsdescribedthe X and Y ramp (uncontoured) Fig,9.17 Digitalto analogue conversion wavefolmsare amplitude-modulated by appropriately (courtesy RCALtd) shapedwaveformstriggeredby the frame retrace pulse. The c.r.t.is intensitymodulatedby one of four It shouldbe evidentthat timing and synchronization d.c.levelsappliedto its control grid. Sincethe output are all-important. We seefrom the simplifiedblock from the contourcircuit is digitalwe must employ a diagramthat the timing and controlcircuitsare digital to analogue(D/A) conversioncircuit. connectedto virtuallyall partsto ensurethe

,,1. o

Frame retrace I

I

ffi,,,

Mod

_ r l l l l l l

X-Y retrace

'--

Y Ramp

x Ramp

-

Fig. 9.18 Ramp generation

t55

necessarysynchronization. All timing signalsare derivedfrom d 4'972459 MHz crystal-controlled oscillator(period0.201 1077 ps). Of particular significance is the scannerpositionwhen new datais loadedinto the memory. A counterin the scanner drivecircuitscountseveryeighthstepin the sweep, first clockwisethen counterclockwise and so on. The counterthus givesthe memoryline numberfrom I to 128 (= 1024/8)which is usedby the new data load circuit. lt is possiblethat the scannerstepping motor may missa few beats,in which casethe count referredto abovewill not representthe scanner positioncorrectly. In order to preventcumulative errorsthe count is Jammed'at 64 wheneverthe scannercrosses the deadaheadpositiongoing clockwise.The informationrequiredfor Jam centre' operationis derivedfrom the X-axisstator output of a resolver,the rotor of which is drivenby the azimuth motor. This output variesin amplitude,and phase-reverses when tl'rescannerpases through the deadaheadpositi<-rn.. The aboveis a much-simplified descriptionof the featuresof the Prirnus40; many detailshave essential beenomitted. Other digitalweatherradarswith rho-thetadisplayssuchas the BendixRDR 1200will differ in detail but will operatein a similar way. Digital WeatherRadar - Television(t.v.) Display In the nrevioussectionwe sawthat a disital weather

radarwith a rho-thetadisplay requiredscan conversionin time only. This follows sincethe data arecollectedin the sameorderasthey are presented; only the ratesare different. With a t.v. display this is not so, thereforewe needscanconversionin both position and time. Sincethe basicdifferences betweenthe two types of digital radar are the rasters, scanconversion,and the organizationof memory we shallconcentrateon thesetopics. What follows rs basedon the RCA Primus30. The rasteris similar to a standardt.v. display except that the field and line directionsof displacementare reversedand it is quantitatively different. The rasterconsistsof 256 verticallineseach with 256 cells,thus we have256 X 256 = 65 536 displayeddata cells. Eachframeof 256linesis displayedin two interlacedfieldseachof 128 lines. The field rate is approximately107'5 per secondso the interlacegivesapproximately53'75 framesper second(fasterthan conventionalt.v.).which givesa flicker-freepicture. Azimuth drive is similar to the Primus40, the angle of scanbeing 120" achievedin 1024steps. An azimuth counter counts every fourth step so that when the count has gone from 0 to 255 there is a phasereversalof the drive signalcausingthe scanner to reversedirection. When switchingfrom standbyto a transmitmode(normal,contour,cyclic or mapping) the scanneris driven counterclockwiseto the

)'

t \

\l ',

\ \ \

t \i ! | l rl \i r t

------

\i\

ATRATI0f 10, t Hz' FlRsl t lttD RAsltRLltrt. oNt tr 126Llilts wRlTTiN t liN t o N t 0 f 1 2 8G t N f R A l tA Dl R A T0i f l 0 / t H 2 ' r l R 5 1F l t l , 0B T A NR KI T R A C 5r E t A t i rfKL Y B A CLKl N i r R O nt N D0 t f l R 5 Ir l t L oT 0 B t C I NINN G0 f S t C o l ' lfDl i t o A I R A l t 0 f 1 0 7 . H R A r t0 f l o i 5 H z ' ! ! R l I T I N A l 0 1 . 1 0 t f l ? 8 L l N t s stcoNDFliLR 0 A S T ILRI N € . KI T R A C L Il N t .o t \ t 0 r I 2 8C t f t t R A T tA0TR A T0I f 1 0 7t H ] ' s f c 0 N Dt l t L D E L A NR FLYBACKLINtfR0!,!t!D0tStCOt!0FltLDT0EtClNNlNC0frlRSIfltLDAIRAT[C[1075H. 'llt60 UMs PtR stc

Fig. 9.t9 Simplifiedrasterfor the Primus30 (only eleven linesshown)(courtesyRCA Ltd)

156

\

leftmost position, and 'chatters'there until the azimuth counter reaches255 when clockwise rotation is initiated; this ensuressynchronizationof counter and position. Digitized data is written into a random access memory (RAM) consistingof eight 4096-bit RAM chipsgivinga total of32K bit storage(lK bit = lO24 bits). Thus with a cell containinga 2-bit word there is provisionfor l6K cells(= 128 X 128). Conceptually the memory is arrangedas a grid with orthogonal ixes, so the addressat which datais to be storedmust be in X-Y format. SCanconversionis required to provide the correct addressgiventhat the data is being receivedin a rho-thetaformat.

usedasa lock-up table to give the valuesof cos 0 and s i n0 . T h e 1 2 8 X 1 2 8= l 6 K R A M c e l l sm u s tb e convertedto 256 X 256 = 64K displaycells;this is achievedby a datasmoothingcircuit. EachRAM cell is convertedinto four displaycells,the videolevelin eachdisplaycell beingdeterminedby a weightedand biasedaverage of levelsin the corresponding RAM cell and someof its neighbours.With the cellsdesignated asshownin Fig. 9.21we have: ' Ia Ib /c Id

= inr. = int. = int. = inr.

IQI6 l(2ly IQIy l(Zty

+ Is +'t7 + Is + Ir

+ It + IL + Ia + In

+ + + +

l)lal l)l4l l)l4l l)l4l

whereint. [. . . .] meansintegerpart of f . . . .1. An exampleoi the processfor one RAM cell is given in the figure. Four concentricarcs,with centreat the middleof the bottom edgeof the display,serveas rangemarks. The addresses of the displaycellsto be illuminated frrr rangemark purposesarestoredin a ROM. The (o'O) I l[-+ 8K bit ROM is time-shared with the scanconverter e AX--ri which utilizesit asa sin/coslook-uptable,asstated previously. Fig.9,20 Rho-theta to X/I scanconversion The rho-thetasectorof targetreturnsoccupies part of the X-Y display. The unusedareaof the only Assumed word hasbeencorrectlystoredat screen is usedfor alphanumerics identifyingthe (X,Y) the next word, assuminga unit range address operatingmode and the rangemarks. A ROM, used step,must be storedat (X + LX, Y + A)z) where only lbr alphanumerics, containsthe positioncode AX = sin 0 and AY = cos0, 0beingthe scanner for the bottom ofeach characteron any givenline of azimuth anglegivenby the azimuth counter (see the raster. F i g . 9 . 2 0 ) . T h e r a t eo f g e n e r a t i o n ofnew addresses Eachof the circuits providingthe functionsof is determinedby the rate of generationof new data generation, raster azimuthdrive,digitization,RAM cells,of which there are 128 for eachazimuth count. ROM addressing and transmission must be The first datacell in eachgroup of 128 corresponds and synchronized in time. All timing signals are derived to the address (0, 0). A readonly memory(ROM) is from a 10.08MHz crystaloscillator.Suitable sub-multiplesof the basicfrequencyare fed throughout the systemas triggersand clockswhich lr keep everythingin step. td lb (x+AX,Y+AY)

lr

lM

lR

la

lc

l8

Scanner Stabilazatioh

2 1

3

3

iI

2

3

2

3

3 Fig. 9.21 A RAM cell to display cell conversion in the RCA Primus 30

The needfor scannerstabilizationhasalreadybeen stated;herewe shallreviewthe implementation. There are basicallytwo types of stabilization: platform and line-of-sight.With the former the movingpart ofthe scannercanbe considered asbeing mounted cin a platform controlled,independentlyin pitch and roll, by a vertical referencegyro (VRG). With the latter, pitch and roll signalsare combined,

157

Timing and control

Control panel (rangel

tsig.9.22 XIY displayblockdiagram

taking into accountthe azimuth angleof the scanner, The aboveparagraph is the basisfor line-ot-sight the compositesignalbeingusedto control the beam stabilization. Pitch and roll signalsfronr the VRG are tilt angle. Sincethe platform systemrequires combinedin an azirnuthresolver, the rotor of which joints for azimuth,pitch and roll rotatingwaveguide is drivenby the azimuthmotor. Tl.restatorsof the movenrentplus pitch and roll motors,the line-of-sight resolverareconnectedto the pitch (P) and roll (R) systemis preferredin most modernweatherradars. outputsof the VRG, in sucha way that the rotor Only the line-of-sight systemwill be explainedbelow. output is P cos0 + R sin d, where0 is the azimuth ' Whilethe scanneris pointingdeadahead,aircraft angle. movementin roll will haveno effect on the beam The compositedernandsigrralis fed to a servo direction sincethe axis about which the aircraft is amplifierwhich alsohaspositionand velocity rotatingis in line with the bearnaxis. With the feedbackinputs. If the sunrof the inputsis non-zero, scannerpointing90" port or starboardpitch an error signalfrom the servoamplitierwill drivethe movementwill havea negligibleeft'ectsincethe axis motor so as to reducethe error to zero. The position aboutwhich the aircraftis rotatingis paralleland close feedbackfrom the tilt synchrois modifiedby the tilt to the beamaxis. Conversely with the scannerdead control so that the angleof the beanraboveor below ahead,aircraftpitch must be correctedin full while the horizontalmay be setby the pilot. Velocity the scannerat t 90" aircraftroll rnustbe correctedin feedbackis providedby a tachogenerator to prevent full by pitchingor tilting the scanner. excessive oversltoot.' 158

Antenna Elevation rotary joint

q)

Azimuth

rotary

6h I F

Azimuthmotor controllogic from indicator Pitch/roll amplifiers

L--I I I Cable and I Oear reductionl I

(T) I

Pcos0 + R . s i n0

le and

reduction

Summingpre-amplifier Position feedback

Indicator Fig.9.23 RDR 1200scannerblock diagram(courtesyBendix AvionicsDivision)

The componentsusedin the stabilizatidnsystem canvary. The positionfeedbacktransducerand tilt control may be two- or three-wiresynchrosor indeed potentiometers.Someequipmentsusea d.c. rather than a.c.motor althougha.c.is normalfor demand and feedbacksignals.On somesystemsno roll correctionis employedif the scannerazimuthangleis

restrictedto say145o, ason variousgeneralaviation systems. Unlessan azimuth steppermotor is usedthe azimuthangularvelocityof the scanneris not constant. Reversal of directionat the extremitiesmeansthat the scanneraccelerates towardsthe deadahead position and slowsdown going away from deadahead.

159

Grid vanes It follows that lesstime is availableto make correctionsat the deadaheadposition. stabilization Mapping Weather is fastenoughto copewith If the servoloop response in azimuthit will be too the nrostrapidrRovement /---=--\ time thstat the extremities.In order to vary response z'.-\"' the velocityfeedbackmay be modifiedso that it is greatestin amplitudewhen the azimuth angleis a maxlmum. With a flat plateaerialthe beamis tilted by pitchingthe plate,thus a pitch-rotatingwaveguide joint is needed.Thereis a choicewhen the system Horizontally usesa parabolicreflector,eitherthe reflectorand polarized feed feedmovein pitch or the reflectoronly. In the latter joint is usedbut the beamshape Fig.9.24 Weather-mapping facility using a parabolic caseno pitcli-rotating deterioratessincethe feed point is displacedtiom the reflector focuswith tilt applied. reflector is rotated through 90" (asin Fig. 9.24) or the directionof polarizationis rotatedby usinga waveguide rotatingjoint or a ferritepolarization Other Applications for Weather Radar twister. The cosec2beamis difficult to achievewith a flat AJthoughthe primary function of a weatherradaris specificallyfor a pencil beam. to detectconditionslikely to giveriseto turbulence, plate array designecl beamcanbe obtainedby variousother usesfor the systemor part of the system However,a fan-shaped reversingthe phaseof the r.f. energyfed to the slots havebeen;and continueto be, found. Thesewill be in the top half of the plate. briefly described. Whenselectedto nrapping,rivers,lakesand areclearlyidentified,so allowing coastlines Mapping Virtually all weatherradarsoffer a mappingfacility. confirmationof position. Built-upareasand mountainswill givestrongreturns.An interesting At its most crude,selectionof mappingmerely pilot the phenomenonmay be noticedoverthe plainsof the whereupon the tilt beam s.t.c., can removes United States:sincefences,buildingsand powerlines down to view a limited regionof the ground. At its beam, tend to be laid out with a north-southor east-west bestthe beamis changedto a fan-shaped wherebyreceivedechoenergyis constantfrom all orientation,returnsfrom the cardinalpoints are partsof the illuminatedgroundregion. In the strongest,thus givingnoticeablebright lines on the to north, south,eastor west. Appendixit is shownthat the receivedpoweris radarcorresponding inverselyproportionalto the squareof the rangefor a beam-fillingtarget(A9.9). alsoif the beamis Drift Indication at an angle@to the horizontal the range With downwardtilt the retumedecho is subjectto a depressed R = ft cosec@wf "e /r is the aircraft height. So for Doppler shift due to the relativevelocity of the equalreturnsfrom ground targetsat different aircraft alongthe beam. The spectrumof Doppler depressionangles(hencerange)the transmittedpower shift frequenciesis narrowestwhen the beam is needsto be distributedon a cosectd basis.sincewe alignedwith the aircraft track. The Doppler signal will thenhave(Pr)o(PtlR2i". lcosec2Qlh2 cosec261= canbe displayedon a suitableindicator(A-type display)where,due to the spectrum,it appearsas Q l h ' ) , i . e . P , i s i n d e p e n d e notf r a n g e . With a parabolicreflectoran approximatecosec2 noiseelevatedonto the top of the returnpulse.With manualcontrol of the azimuth position of the scanner beamcanbe obtainedby useoi a polarization'noise' the pilot can adjustuntil the Doppler sensitivegrid aheadof the reflectorsurface. In the (spectrum)is at a minimum, when the drift anglecan to the beam weathermodethe grid is transparent to the conducting be readoff the control. This option is rarely found. sincethe E field is perpendicular vanesof the grid while in the mappingmode the grid a Beacon Interrogation reflectspart of the beamenergydownwardsincethe The transmittedpulsefrom the weatherradarcan be E field is parallelto the vanesand thereforedoesnot usedto triggera suitably tuned beacon(transponder) satisfythe boundaryconditions.To achieveremote on the ground. The beaconreplieson 93 l0 MHz, switchingbetweenweatherand mapping.eitherthe

I,

160

so a weatherradar with a local oscillatorfree.rencyof 9375 MHz and a transmit frequencyof 934i MHz will producea differencefrequencyof 30 MHz for normal returnsand a differencefrequencyof 65 MHz for the beaconreply. Two differently tuned i.f. amplifiers can be usedto separatethe signals.As an alternative two local oscillatorsmay be used. On somesystetnsthe selectionof beaconeliminates the normal returns from the display;on others it is possibleto show weatherand the beaconresponse. The easewith which this facility can be uied to find offshore oil rigs makesradarsoffering beacon operationan attractiveproposition for helicopters supplyingthe rigs. Such radarsusually havea

short-rangecapability;for example,the Primus50 offers 2 nautical mile rangeusinga 0.6 ps pulse,thus givinggood rangeresolution. Multifunction Display The weatherradarindicator is increasinglyusedfor purposesother than the displayofweather or mappinginformation. X-I rastersin particularmake the displayof alphanumericdata straightforward, henceall the major manufacturersnow offer a 'page-printer' option with one or more of the radars in their range. Similarlydisplayof navigationdatais availableasan option with the latestcolour weather radars.

nrotr

xon* rftoffi

Fig. 9.25 Primus 30 with page-printeroption (courtesy RCA Ltd)

161

A page-printeroption is normally used to display checklists.The alphanumericdata is arrangedand storedin pageson EPROMs,either in the indicator or in an external auxiliary unit. EachBagemay be button. Pages calledup in turn usinga page-advance containingthe normal checklistindex and emergencychecklistindex are particularlyimportant, and usually havededicatedbuttons usedto call them up for display. Havingdisplayeda pageof one of the indexesa line-checkbutton may be usedto advancea cursor(line highlightedby displaying,say,black alphanumericson greenbackgroundrather than greenon black asfor the other lines).With the cursorset,the chosenchecklistcanbe displayedusinga list button. Apart from checklistsother alphanumeric information which may be listed includeswaypoints, or indeedany pilot+ntered dataif pagesare allocated to this facility. One method of allowingthe pilot to enter data is to usea calculatorkeyboard; Hewlett-Packardand TexasInstrumentsmake calculatorswhich can be modified to interfacewith the page-printersystem. Display ofnavigation data from externalsensors suchasVORTAC, Omega,INS or Loran is achieved through an interface unit. Typically the pilot is able to displaywaypointsjoined by track lines,together with the weatherdata. As an examplethe RCA Data Nav. trI systemallowsthe displayof current VORTAC frequency,rangeand bearingto current waypoint and up to three correctly positionedwaypoint symbols from twenty which can be stored. An additional featureof the Data Nav. I I is a designatorsymbol which may be set to any desiredlocation on the screen:this location can then be enteredas a new waypoint to replacethe current one, so providingan alternativeroute if stormsareobservedon original intendedcourse. Progressin this areais rapid' ln 1979,with the appropriateinterface,the weatherradar could display projectionsof aircraft position on straightor curved paths,ETA at waypointsand warningsof sensordata failure in addition to the data referredto above.

WeatherRadarCharacteristics ARINC Characteristic564-7 allows the designermore freedomthan do most other suchdocuments; howeverit is quite clearwhat performanceand facilities are to be made available. The following is a summaryof someof the more significantand/or interestingitems. Range At least 180 nautical miles for subsonicaircraft and 162

300 nauticalmiles for s.s.t.is usually demandedby customers. Rangemarks at 25 nautical mile intervalsup to 100 as are the availablerange nauticalmiles are suggested, of30/80/lS0 or 30/100/300or 30/80/180/ selections of a continuouslyvariable 360. The advantages 30 nautical miles to maximum from range displayed arestated. Displayed Sector nt ieast + 90" but not more than t 120". Displayed rangeat + 90" to be not lessthan 60 per cent of maximum range. The scanmay be reciprocatingor circular. (Note in the caseof the latter, sincethe scannerrotatesthrough 360' RAM (radar absorbent material)screeningis neededon the nosebulkheadto prevent excessivelystrong receivedsigrrals.) Radio FrequencY C-Band 5400 MHz x2OMHz (nominal) X-Band 9375MHzt 20 MHz (nominal), or 9345 MHz L 2O MHz (nominal) Bandwidth Minimum bandwidth = l'216 where the pulsewidth 6 must be lessthan l0 Ps' Dispby AccumcY ^ Azimuth angle:t 2-. Range:the greaterof t 5 per cent or I nauticalmile. Sensitivity Time Control Range2law from 3 nautical miles to the point where a 3 nauticalmile target ceasesto be beam filling. Two-wirelogic from scannerto set s.t.c.maximum rangein accordancewith antennagain (and hence bearnwidth). Scanner Stab and Tilt Two-wire pitch and roll sigrralseach(E/2300) V + 2 per cent per degreewhereE is nominal I 15 V 400 Hz referencephase(50 mV per degreebut in terms of supply). The phaseof signalsis expressed specified. Dummy load of 20 kQ where signalnot used. Pilch and roll rate capability 20o per second. Line-of-sight(two-ais system):combinedroll, pitch and tilt freedomt 35" with accuracy+ lo, manual tilt i 14". Split axis (three-axissystem):roll t 40', pitch t 20o,manualtilt I 14",combinedpitch and tilt !25",accuacy t 0'5". Droop nosesignal(tr/575) V per degree,samephase asnosedown. (Two-wire signal,20 mV per degree

EnrouteNavigation.

fry

1 ) T h e i n t e n d e dt r a c k l i n e o r i g i n a t i n gf r o m l h e a i r c r a f t s y m b o ld i s p l a y sl h e p r o g r a m m e dr o u t e o f f l i g h t .W a y p o i n t s a n d t h e i r n u m b e r sc a n b e d i s p l a y e do n t h e t r a c k l i n e .W h e n D a t a N a vi s u t i t i z e dw i t h R N A V ,t h e a s s o c i a r e dV O R T A C s y m b o l i s d i s p l a y e da l o n g w i t h i t s f r e q u e n c ya s i l l u s t r a t e d W h e n u s e d w i t h a V L F / O M E G Ao r I N S s v s l e m .t h e D a t a N a v w i l l d i s p l a ys i m i l a ri n f o r m a t t o nW . h e n w e a t h e ri s e n c o u n _ tered lhe current Waypoint may be ottsel by using RCA'S e x c l u s i v eD e s i g n a t o rf e a t u r e T h e D e s i g n a t o rc a n b e m o v e o to any location on the screen by means of the Waypoint

3) The aircraft has now completed the turn and intercepted t h e n e w l r a c k l i n e w h i c h w i l l s a f e l yc i r c u m n a v i g a l et h e d a n g e r o u sw e a t h e r .l f d e s i r e d ,y o u c a n r e l u r nt o t h e o r i g i n a l Waypointand track line by pressing lhe cancel button on t h e D a t a N a vc o n l r o l o a n e l .

o f t s e tc o n t r o ls h o w i n gt h e D e s i g n a t o sr y m b o la t 3 5 " r i g h t , 46 nm. 2 ) W h e n t h e D e s i g n a t o ri s a t t h e d e s i r e dp o s i t i o n ,t h e n e w W a y p o i n tc a n b e e n t e r e di n t o t h e N a v i g a t i o nS y s t e mb y m e a n so f t h e " E N T R " b u t t o no n t h e D a t a N a vc o n t r o lp a n e , . T h e c u r r e n tW a y p o i n tw i l l b e m o v e dt o t h e n e w l o c a t i o na n d a n e w t r a c k l i n e e s t a b l i s h e dT. h e R a n g ea n d B e a r i n go f t h e a i r c r a f lt o t h e V O R T A Cs t a t i o nw i l l a l w a y sb e d i s p l a y e di n t h e l o w e r r i g h t h a n d c o r n e ro f t h e d i s p l a y .

4) Waypoinl Listing Mode can be sebcted by means of-the "MODE" b u t t o no n t h e D a l a N a vc o n t r o lp a n e l .I n f o r m a t i o n f o r 3 W a y p o i n t sc a n b e d i s p l a y e do n t h e s c r e e n .T h e c u r r e n l W a y p o i n lw i l l b e d i s p l a y e di n y e l l o ww i t h a l l o t h e rd a l a i n g r e e n .A l l t h e W a y p o i n t sc a n b e d i s p l a y e di n g r o u p so f "Waypoint lhree by moving the Offsel Control" left or right.

FiE 9.26 En-route navigation with the (colour) Primus 200 (courtesy RCA Ltd)

for SSTaircraft where scannermount is droppedwith nose.) Scalerequiredon scannerwherebytilt anglemay be read.

Ilaveguide C-Bandtype ARA I 36 or WR I 37. X-Bandtype RG-67U. Ridgedwaveguide rejected;v.s.w.r. m a x i m u ml . l : l . 163

Magnetron Magnctic Field No more than lo compassdeviationwith sensorl5 ft from the t.r. The t.r. shouldbe mounted at least2 ft from the indicators,other t.r. units and other devices sensitiveto magneticfields.

width. It is normal to calculatethe m.p.e.l. assuming a stationaryscannerand a point source'in which case an averagepower of 6 X F X Pt is spreadover an area of n X D2 X 1Jin2(e l2)i thus the distanceD in rnetres for an exposurelevel of l0 mW/cm2is givenby:

'l

^

Maintenanceand Testing Safety Precautions There are two hazardswhen operating weather radar, namely damageto human tissueand ignition of combustiblematerial. The greaterthe averagepower density the greater the health hazard. A figure of l0 mW cm' is a generally acceptedmaximum permissibleexposure level(m.p.e.l.). Among the most vulnerableparts of the body are the eyesand testes. The greaterthe peak power the greaterthe fire hazard. Any conductingmaterialcloseto the scanner may acf as a receivingaerial and have r.f. currents induced. There is obviouslya risk, particularly when aircraft are being refuelledor defuelled. An additionalhazard.which doesnot affect safety but will affect the serviceabilityof the radar,is the possibilityof very strongreturnsif the radaris operatedcloseto reflectingobjects. The result of these'returrlsis to burn out the receivercrystals which are of the point contacttyp€' The following rulesshouldbe observedwhen operatingthe weatherradaron the ground: l. ensurethat no personnelare closerto a transmittingradarscannprthan the m'p.e.l. boundary, aslaid down bY the sYstem manufacturer; 2. nevertransmit from a stationaryscanner; 3. do not operatethe radarwhen the aircraft is being refuelledor defuelled,or when another aircraft within the sectorscannedis being '4. refuelledor defuelled; do not transmitwhen containersof inflammable or explosivematerialare closeto the aircraft within the sectorscanned; 5. do not operatewith an open waveguideunless r.f. power is off; neverlook down an open waveguide;fit a dummy load if part of the waveguiderun is disconnected; 6. do not operatecloseto largereflectingobjects or in a hangarunlessr.f. energyabsorbing materialis placedover the radome(RAM cap)' The safedistancesfor radarsvary widely, dependingon averagepower transmitted and beam 164

I

|-

6FPt

D=m6 hffil

o't-

*

L ' J where: 6 is pulsewidth in seconds; F is pulse repetition frequency in pulses per second; P1 is peak Powerin milliWatts; 0 is beamwidth.

Thus consideran older airline standardweatherradar (Bendix RDR IE/ED) usingmanufacturers'nominal figuresfor the longestrangeoption, i.e.6 = 5 ps; F = 2 O O ; P t = 7 5 k W ; 0 = 3 " ; w e h a v eD = l 8 ' 6 6 m (ev60 ft), while for a modern generalaviation radar (RCA Primus20) where6 = 2'25 ps, F = 107'5, = = & = 8 k w , 0 8 6 w e h a v eD l ' 1 2 m ( e , : * f t ) ' To ensuresafety precautionsare observedconsult manufacturers'data for safedistancesthen, if operatingthe radar for maintenancepurposes,place radiationhazardwarning noticesthe appropriate distancefrom the nose. Whenworking by the scanner with the radar on standby a notice should be placed 'do not touch', or better still by the controlsstating the transmittershouldbe disabledor the waveguide run broken and a dummy load fitted. If the radaris switched on prior to taxiing, the transmitter should not be switchedon until clearof the apron' X-ray emissionis a possiblehazard when operating the transmitter with the caseremoved, such as might be done in a workshop. The likelihood of dangeris small but the manufacturers' data should be consulted. Check for Condition and AssemblY Obviouslythe weatherradar systemas a whole is subjectto the samerequirementsas all other airborne equlptn.nt in respectof securityof attachmentand condition; however,certainpoints need to be highlighted. -Thi waveguiderun should be the subject of fairly frequentinspectionsand shouldbe suspectedifpoor performanceis reported. Wheninspectingthe waveguide,corrosionand physicaldamagesuchas cracksand dents are obviousthingsto look for' Flexible waveguidecoveringsare subject to perishing, crackingand a detachedmechanicalbond at the flanges.lnternal damagein flexible waveguidecanbe founa Uy gently flexing while listening and feeling for clicks. There shouldbe no more than minor bendsin

l. Checkwarm-uptime for magnetron. the H plane of flexible waveguideand the radiusof bendsin the E planeshouklbe greaterthan about Zlrn.2. Checkfan motor (a light pieceof papershould 'stick' to the filter). to dismantlethe If it is thoughtnecessary 3. Checkinternalpowersupplyvoltagesand currents waveguide,un1o curryoui an internalinspection, with built-inmeter,if fitted. or testmeterconnected care needsto be replaced, or if i pieceof waveguide to testsocket,if supplied. joint Choke flanges must be takenwhen re-installing. NB'. when disconnectinga test lrleter from the plain flanges.Seaiingor O-iingsmusi must mate to test socketit is vital that the shortingplug be fitted to achieveu pr.rrut. seal. The E planesof otherwisecrystalearth shouldbe replaced, piecesshouldbe paraliel.Undue adjoiningwaveguide for measurement, current broken returns, either forceshouldnot be usedin aligningwaveguide, will not be made' within the run or at the endsof the run. All waveguide supportsshouldbe secureand undamaged. 4. Checktest facility: patternshouldbe asspecitied by manufacturer.In particularcheckthe pattern If an internalinspectionusinga probelight reveals is centredand neitheroverfillingor underlilling dirt or moistureit may be possibleto cleanby pulling screen,and that the rangemarksareequallyspaced througha cleansoft cloth andior blowingout with an (lineardeflection,rho-tlieta).symmetricalarcs line. Caremustbe takennot to scratch air pressure (lineardeflection,X-Y)andcorrectin number sincethiswould the insidesurfaceof the waveguide (time-base duration,rho-theta)'A rho-thetaraster renderit scrap,aswouldsignsof corrosionor deposits may showa smallopencentre(about{ in'; but no which cannotbe removedis suggested above. tail. run shouldbe Any drain trap in the waveguide 5. Observegroundreturnson all ranges.Operateail moisture checkedfor blockage,and accumulated controlsand ensurethe desiredeffcct is achieved. in the shouldbe removed.Filtersand desiccators pressurization feed,if fitted, shouldbe inspectedfbr Notes (filter) and colour(if desiccator is pink it cleanliness The aboveis only a brief outlineof the checksto be is unserviceable). alwaysusethe manltfacturers' carriedout. OneshtlLrld of The scannershouldbe checkedfor freedom when tcsting.When procedures movementaswell asgeneralconditionand securityof recornmended if is necessary carryingout itenr(5) above.experience the attachment.In carrying()ut scannerinspcctions o f t h e c o n d i t i t l n w i t h c e r t a i n t y , a n y i s t o s t a t e , o n e dishor plateshouldnot be turneddirectlyby hand but throughthe gearing.Backlashin the gearscanbe the systenr.The pictureobtainedwill dependon the headingof the aircrali;asan extremeexantplean checkedby gentiyapplyingforwardand backward aircraf'tpointingout to seawill givevery diflerent movementto the edgeof the dishor platein both d ith an g r o u n dr e t u r n so t t i t s r a d a rw l t e nc o m p a r e w pitch directions:in a 30 in. diameter azimuthand aircraftpointingin the directionof a rangeof hills. imtenna,movementof ; T at the edgein
NAHOOTIEACKGROTIND rSISE PIAKS

(uGr{Tt{olsE}

r/tFfs TrsTsAt{ffi

coHroun

TE'T BAND

il

TESrilfffE B*lw { ilff{*s $iltctr} -lT{o EFFscr { *fffiIcrD sAlH}

F t g . 9 . 2 7 R D R I E / F t e s t p a t t e r n ( c o u r t e s yB e n d i x A v i o n t c s Division)

scannerand radomeaswell as the t.r. If both attenuatorsreadlessthan normal then the fault must if only the input be in the commonr.f. components; attenuatorreadslow then transmitterpower output is down;while a low output attenuatorsettingindicates ivity. poor receiversensit Functional Test - ScannerStabilization ObserveSafetyPrecautions 1. Removeradome. Switchto standbywith stabilizationon and tilt zero. Check,usinga spirit level, that the flat plate or the plane acrossthe Fig 9.2E Simplified radarsystems tester rim of the reflectordishis verticalwith the scannerdeadaheadand at the extremitiesof the sectorscan. centreposition. A co-axialcableconnectsthe horn to the testerwhich is insidethe cockpit. The tester 2. RemoveVRG from its mounting and fit on a tilt so that its surfaceis horizontal tableadjust.ed servesasa beaconand returnsa signalwhich should alongboth axes. appearat zerodegreeson the p.p.i. displayat a range determinedby the testerdelay. An input attenuator . 3 . I n c h s c a n n etro t h e d e a d - a h e apdo s i t i o n .W i t h . - \ stabilizationon, adjusttilt controluntil scanner ) can be adjustedso if the testerjust fails to be give is vertical. attenuator will a measure of the triggered,the 4. Simulatea suitablepitch angle@noseup. Using radiatedpower. An output attenuatorcan be 'echo' just visible(radar a protractorspirit levelensurescannerelevation is on adjustedso that the down. Repeatfor nose by @degrees presetgainwith standardized changes brilliancesetting),thus down. of system the attenuatorwill givea measure 5. Sinrulateaircraliroll, ensureno changein sensitivity.The output attenuatormay alsobe used . s c a n n eer l e v a t i o n . to checkthe contourinversionlevelsetting. 6 . I n c hs c a n n etro p o r t e x t r e n t i t ya, z - i m u tahn g l e0 . Note the testerwill checkthe wavesuiderun. 166

Simulatea suitableroll anglea port wing down and ensurescannerelevationchangesby o sin 0 up. Repeatfor starboardwing down. Simulateaircraft pitch angle@and ensurescanner elevationchangeis @cos0 in oppositesense. Repeat(6) and (7) but with scannerat starboard extremity noting senseof roll correctionis reversed. Inch scannerto deadahead. Simulatesuitable pitch angle. Switch stabilizationfrom on to off and back to on, and ensurescannermovesin elevationwithout excessive overshooting. Switch stabilizationoffreplace gyro in aircraft mounting. Carry out final checkof tilt controlbefore switchingoff and replacingradome.

a measureof waveguidelossescan be obtainedfrom the differencesin forward power at either end. Practicaldifficulties occur in fitting the directional 7couplerin a rigid waveguiderun. In some installationsa couplermay be permanentlyfitted at 8. one end, usuallyat or closeto the t.r. In order to measurethe v.s.w.r.of the waveguide the scannershouldbe disconnectedand a dummy 9. load fitted in its place. ARINC 564-7 specifiesa maximumv.s.w.r.of I .l : I for a new installation. Whenmeasuring the v.s.w.r.of the scannerthe radomeshouldbe removedand the scannertilted up 10. to maximum to avoid returns. Figuresobtained shouldbe checkedagainstmanufacturers'data. On I l. fitting the radomethe v.s.w.r.at the antennawill deteriorate,measurementat variousazimuth and tilt angleswill givean indication of the radome performance. Notes: The most likely causeof lossof radomeefficiency The aboveappliesto a typical line-of-sightsystem. as a transparentmaterialto r.f. is ingressof moisture For any particularequipmentthe manufacturers' through small pinholesand crackswhich may appear procedureshould be followed, observingstated on the outsidesurfaceof the radorne. Such pinholes tolerances. A platform systemcheckoutvariesfrom the above and cracksmay be seenby shininga light on the outsidesurfaceand viewingfrom the inside. A in that changesin elevationare independentof moisture detectorcan be usedto measurethe azimuth angle. resistance betweentwo adjacentpoints on the In items (6), (i) and (8) an azimuth angleof 90" radome. The detectorhastwo probeswhich are strouldbe chosenif possible,sincethis will make pressedfirmly againstthe insidesurfaceof the cos 0 = I and sin 0 = 0, so simplifying checking. Pitch and roll correctionpotentiometers areoften radome. Wherethe resistancemeasuredis lower than normal it may be due to an ingressof moisture. accessible in which casepitch correctionshould be carriedout with the scannerdeadaheadand rolBench Testing correctioncarriedout to balanceany errorsat the There is not spacehereto considerbenchtesting'in extremities. depth,but mentionshouldbe madeof a special A gyro simulatormay be usedfor a checkoutof the completesystemlessthe VRG. The VRG is disconnected and the simulatorconnectedin its place. The procedureis then similar to that above, with the appropriatepitch and roll anglesbeing selectedon the simulator. Sincethe VRG is not testedthis is not a full functional test but is useful for fault-finding. The signalsfrom the simulatorshould correspondto the VRG output signalstandards (e.g.ARINC).

.c.frd

Check of v.s.w.r. If the waveguiderun or scanneris suspect,v.s.w.r. checkscan be carriedout to find the faulty itcnr. To carryout sucha checka directionalcouplermust be fitted in the run, and power in the forward and reversedirectionsmeasured using,for example,a thernristorbridgepowermeter. The checkshouldbe carriedout at both endsof the wavesuide run so that

Fig.9.29 RD-300weather radartestset(courtesy IFR. Electronics Inc) weatherradartest set,the IFR RD-300which, togetherwith an oscilloscope,can be usedto perform all the commonradartestswithout the proliferation of signalgeneratorsand other instrumentsnormally found on a radartest bench. 167

accurate'For possibleand reasonably measurement signalsarecompared the received rangemeasurement 'standard'to giveso-called pseudorange.The Beinga relativelynew developmentthe Stormscope with a ratlgelrteansthat particularly methoduscdto deterntirte is, asyet, not to be found in servicein anythinglike asmany aircraft asis weatherradar' Sincespaceis at strongsignalsappearto be closerthan they actually sincethe source are.which is not reallya disadvantage a premium the coverageof the Stormscopein this activity electrical severe detailed region of a more is a to allow signals such limited of been book has coverageof radar. The numberof pagesallocatedhere andhenceturbulence. appearsasa bright greendot on the Eachdischarge reflectsthe importanceof eachof the systemsto of staff and aircrewtoday. The situation circulaldisplayscreenat a positionrepresentative maintenance An likely, aircraf't. or, more to thc future position relative in the reversed the source well be may aircraftsymbolis locatedat the centreol-tlredisplay equalized. w i t h r a d i a l i n e sm a r k e da t 3 0 " i t l t c r v a last t dt w t t As statedin the introduction to this chapter,the on detecting equallyspacedrangerings. The rangeol'tlre outer dependsfor its success Ryan Stormscope receiver' with turbulence. ring is asselected on the panel-mounted associated which is activity electrical Sincethe radiatingsourceis naturalonly a receiveris 40, 100 or 200 nauticalmiles. Sincetlre outer ring is overweatherradar. not at the peripheryof the displaythe rnaximuttl required;an immediateadvantage is of the orderof 260 nauticalrniles. rangeavailable To obtaindirectionalinformation,useis madeof an is only a momentaryeventso Eachdischarge ADF loop and senseantenna,both eitherdedicatedto with ADF. storagein memoryis required.The trlemorycan installationor time'shared the Stormscope a displayunit storethe positionof 128 dots,thesebeingdisplayed The restof the installationcomprises to form a mapJikepicture. Whenthe 129thdischarge unit. and a computer/processor planning the oldestdot in memoryis replaced;in this since occurs taken in installation be Caremust lrom generators, way the imageis continuouslyupdated. lf the is proneto interference Stormscope or a new rangeis selected from aircraftheadingchanges motors,strobelights,etc. Interference positions are incorrect until all I 28 havebeen dot by inhibiting the is avoided transceivers communication which cantake up to 25 s on a updated,a process the Stormscopewhenevera transceiveris keyed'

RyanStormscope

displayunit andcomputer/ Fig.9.30 RyanStormscope (courtesy RyanStromscope) processor

Operation Signalsfrom the two orthogonalloops and the sense antennaareutilized to giverangeand bearingof the sourceof electricalactivity received.The propertie:. of the loop and senseantennamakebearing lSe

stormyday (only 5-10s with tomadoactivitywithin range).On quiet daysa dot may remaina long time erased.A'clear' but after 5 min it is automatically button allowsthe pilot to erasethe displaystarting to the newestin with the oldestdataand progressing

a total erasetime of I s. Another button allows the display of that activity which is forward of the aircraft with, of course,the full memory dedicatedto thesesignals. The 128 dots appearin clusterson the display, indicatingwhere bad weathercan be expected. As the weatherbecomesmore severethere is a spreading inward towardsthe centre due to the pseudorange reducingas a result of the strongersignals.Witll increasingstorm intensity the display becomesvery animated. A test facility is built in whereby when the appropriatebutton is presseda dot cluster appears nearto a positionof 45o; 100 nauticalmiles. In addition, a test set is availablewhich can simulate at variousrangesand azimuth anglesfor sigrrals systemcheckoutand calibration.

Appendix FactorsAffecting WeatherRadarPerformance TheRadarEquation If powerP1 is radiatedfrom an omnidirectional antennathen the power density(power per unit area) decreases with range. At a rangeR a sphereof surfacearea4nR2is illuminatedby the e.m. wave, thus: Powerdensityfrom omni antenna=#

(A9.1)

Sincea directionalantennais usedwith gain G (over an isotropic antenna,i.e. perfectly omnidirectional) we have:

Powerdensity from = PrG(Ae.2) directionalantenna 4rRz Comparison of Stormscope and Radar It would appearthat, purely from a functional point The targetwill interceptpart of the radarbeam,the of view, i.e. ability to avoid turbulence,there is little sizeof the part dependingon the radarcross-section to choosebetweenthe two systems.Someadvantages ofthe target o. Sincethe reflectedpower is subject ofStormscopeover radar are: to the samespreadingout in spaceasthe incident l. no moving parts and no transmitter,hence powerwe have: meantime betweenfailures(m.t.b.f.) should be Power density of _ P,GO higher; (Ae.3) (4rR2)z echo at aircraft 2. simplerantennainstallationwhich keepsdown installationcosts; The radar antennainterceptspart of the echo signal, 3 . only the cheapestradarscomparein capital cost; the sizeofthe part dependingon the effectivecapture 4. fully operationalon the ground with 360" field areaA, so receivedecho powerPr is givenby of view. of Stormscopecomparedwith Somedisadvantages radarare: l. sensitiveto interference; 2. limited rate and accuracyof data acquisition although it would appearfrom operational evidencethat this doesnot preventthe Stormscopefrom being used for efficient and safeweather avoidance; 3. lacking in any other applications such as mapping, navigation data display or page printer option. Two things should be pointed out here; firstly the extras with weather radar have to be paid for; secondlythe Stormscopeis a relatively new developmentby a company small in comparison to the giantsofweather radar. It should not be beyond the ingenuity of the designersto develop the system so as to eliminate some, if not all, of the disadvantages.

(Ae.4)

Pr=

The relationshipbetweenantennagainand effective capture areacan be shown to be 4nA

G =f

(Ae.s)

of thee.m.wave.So where), is thewavelength equation(A9.4)becomes: -

o P,G2).3

& =1ffi

.

(Ae.6)

ReplacingP, by the minimum detectablesigtal pow.er rn and rearranging,we have the well-known radar rangeequation:

=ffi R*"*t

(Ae.7)

The Radar Equation for Meteorological Targets With meteorological targetswe have a large number of

169

independent of radarcross-section scatterers o; so; providingthe targetfills thebeam,we may represent thetotal radarcross-section bv: o = V42oi

lS0l,t'6P,G02 c6 n3

P, = -

(Ae.l3)

'7ff;n-

as the echo power receivedfrom a beam-filling rain

(Ae.8) cloud. (Note that 180 x2OO lkl2).

where: Xoi is the averagetotal backscatter of the particlesper unit cross-section volume; V^ is the volume occupied by the radiated pulsewhich can be approximatedby ( n l 4 ) R 20 2c 6 l 2 ; 0 is the beamwidth(equalin horizontal and verticalplanesfor a pencil beam); c is the velocity of propagation: 6 is the pulseduration.

Minimum Detectable Signal E q u a t i o n( A 9 . 1 l ) g i v e st h e m a x i m u mr a n g eo f a weatherradar in terms of the minimum detectable signalfrom a non-beam-fillingtarget. A threshold levelmust be chosenwhich is greaterthan the r.m.s. valueof the noise occupyingthe samepart of the frequencyspectrumas the signal. If the signal exceedsthe thresholdit is detected:if not it is missed. Too low a thresholdwill give rise to falsealarms. In choosingthe thresholdlevel the interpretation (A9.6) for o we Substituting in have: of the operator is significant,particularly in P t G 2 ^ , 20 2 c 6 2 o i (Ae.e) conventional(analogue)weatherradars. In digital 'rD _- -SIZFFthe choiceof thresholdis taken out of weather.radars or eyes, of the operators. Noise spikes the hands, Equation(A9.9) is applicablewhere the targetis which do not occur in the sametime-slotafter beam-filling,for examplea sphericalcloud of 3 transmissions severalsuccessive are not displayed; nauticalmiles diameterwill fill a 4" beam up to about they are said to be averagedout. As a result in a 43 nauticalmiles. For a ta-rgetoutsidethe beam-filling digitalweatherradara lower threshold,or minimum rangethe proportion oi'beam filled can be shown to signalto noiseratio (s.n.r.),can.betoleratedwith be (Dl0R)2 whereD is the targetdiameter.So I improvementin maximum range. consequent equation(A9.9) would become,for sucha target: In introducingthe factorsrelatingto noiseinto the P , G 2 ) yc26 D 2 2 o ; equationit is convenientto usethe noisefigure (Ae.r0) radar r, = *-:;.IFFFF grvenby an equationin which the fourth power of R occursas in the basicradarequation(A9.6) and contrasting with equation(A9.9) wherewe havethe squareof R. Againwe can producea rangeequationassuming non-beam-fillingtargets: R*a*o

= P r G 2 ) t 2c 6 D 2 2 o i 5l2r2m

(A9.1l)

If the wavelengthis long comparedwith the diameterd of a scatteringparticlethen it can be shownthat: ns lklz>d;6 zoj = --Ia=

(Ae.t2)

where lkl2 is relatedto the dielectricconstantand hasa valueof about 0'9 for water and 0'2 for ice. It is helpful to replace2d16 by an expression involvingrainfall rate, suchan expressionis provided by 2di6 - 200rr'6wherer is the rainfall rate in mm/h. lt shouldbe noted that this is an empirical relationship,the constantsbeingsubjectto variability from one experimentalobservationto another. Replacing2 o; in equation(A9.9) and usingthe relationshipbetween,dir we have 170

p ' = *l* S"IN"

(Ae.l4)

where: S;/N; is the input s.n.r.; Sofly'ois the output s.n.r. The input noisecan be taken askTB where:k is Boltzrrrann's constant,= l'38 X 10-23 joules/degree: in degrees Kelvin; ?nistemperature .Bis the noisebandwidth(differentfrom 3 dB bandwidthbut often approximated bYit). equation(A9.14) and substitutingfor So rearranging ly'; we have: Si = kTBF SslNe

(A9.15)

Substitutingfor m in equation(A9. I I ) gives: o n2).3c62o;

R."*' = n#ftngl;i;;

(Ae.r6)

a single Equation(A9.16) resultsfrom considering pulse;ho.vever many pulsesarenormallyreceived from a targetduring one sweepof the aerial. The numberof pulsesreturnedfrom a point targetis

0 X p.r.f.

n = +

(n

564 togetherwith an empirical fornrula (Ae.r7)Characteristic relatingPI and maximum range. The primary purpose

of the PI is to enablea comparisonto be made wherec^.r is the scanningrate in degreesper second; betweendifferent radarsratherthan effect the p.r.f. is the pulserepetition frequency. accuratecalculationof maximumrange. We can utilize someor all of the n pulsesto improve Atmospheric Effects detectionin a processknown as integration. Use of a Therearethree effects.namelyattenuation,refraction c.r.t.,togetherwith the propertiesof the eye and and lobing,which can degradeor evenenhancethe brain, constitutesan integrationprocess.Digital of a radaroperatingin the earth's techniques,whereby the signaloccuning in successive performance atmosphere. correspondingtime-slotsis averaged,is also a form of primarily Attenuationdue to absorptionby gases, integrationin this sense.We can define the integration water vapour, will reduce the oxyBen and maximum efficiencyas follows: particlesalsoabsorb rangeattainable.Precipitation the e.rn.energyand caus.'scattering.The scattering is csscntiallirr the opelutionof weatherradarbut will decrelsethe rangeand reducethe absorption = where(S0/)r s.n.r.of a singlepulsetbr a $ven a b i l i t yo l ' t l r er a d a rt o p e n e t r a tcel o u d si n o r d e rt o probability of detection; 'see'what is bc.r"ond.Ernpiricalresultsareavailable (S/ffh = s.n.r.per pulsefor the same ri increases we nraysirnplystatethat degradation but probability of detection when n the of I.rencc descriptions C-band with frequency. pulsesare.integrated. radarand X-band equipmentasweather-penetration The integrationimprovementfactor nEn canbe radar. asweather-avoidance includedin the rangeequation. is not Sincethe densityof the atmosphere The averagepower P of the radaris relatedto the uniform, refractionor bendingof the radarwaves peakpowerP; by: may take place. Watervapouris the main contributor will normally be bent ( A e . l 8 ) to this effect. The radar.waves P = PtX6 Xp.r.f. aroundthe earth sincethe atmosphericdensity Substitutingfor Ps in equation(A9.16) and altitude, thus leading with decreasing usuallyincreases incorporatingthe integrationimprovementfactor we to an increasein radarrange. have: Lobing is the arrivalat the targetof two radar waves,one via the direct path and one by way of PG2l,2o3c EnDo; (Ae.le) reflectionfrom the earth'ssurface. Depend'ingon the R*"*o 5t2r2 VTBF Q (S/l/), relativephasesrangemay be enhancedor degraded. ln an effort to simplify the rangeequation In an airborneweatherradarwith smallside lobesthis EUROCAEand RTCA havederiveda Performance doesnot createthe sameproblemaswith lndex, PI, from the basicradarequation. Detailsof ground-based radars. the calculationsinvolvedare givenin ARINC

- = (s/.^/), L" reET,.

ltl

10 Dopplernavigation

Introduction

300 knots. Thus after I rnin the systemgivesa readoutof position asbeing 2995 nautical milesfrom A Doppler navigatoris a self containeddead.-reckoningB on trackt after I h 2700 nautical miles from B; after systemgivingcontinuousreadoutof aircraft position l0 h the indicatedpositionis B. usuallyrelatedto waypoints. Military aircraft have madeuseof suchequipmentsincethe mid- 1950s while bivil usefor transoceanicnavigationcommenced 3OOOn.m. in th6 early 1960s. In recentyearsihr ut. of Doppler navigatorsin long-rangecommercialaircraft has by inertial navigators,triple largelybeen superseded systemsbeing fitted to airlinerssuchas the 747 and l" -' Concorde. Military developmentsinclude composite lo Doppler and inertial systemsand we may expect to seesuchsystems'gocivilian' in the future. There are still many civil aircraft carryingDoppler navigatorsbut theseare older, long-rangeairlinersand assuchthey are fitted with equipmentwhich, althoughnot in any way primitive, doesnot employ Fig.l0.l Effectofheadingerror the very latest techniques.A classof civil aircraft for which there is a continuingneedfor Doppler With a + | per cent error in computedspeed,the navigatorsis surveyaircraft which, by the very nature on the position readoutafter 1 min, I h and tolerance areas not covered by of their work, operatein l0 h will be I 0'05. + 3 and t 30 nauticalmiles ground-based navigationaids(exceptOmega)and respectively.With a + 1" error in the heading requireaccuratepositionalinformation. Use of we have the situation givenin Fig. 10.r . information Doppler navigatorsin helicoptersis not unusual;there see that d = 2 X 300/ X sin 0'5, thus after We can aircraft. for such specifically designed is equipment I min, I h and l0 h the aircraftmay be up to 0'087, The advantageof Doppler navigationlies in the 5.24 and 52'4 nauticalmilesaway from the indicated fact that it is a self-containedsystemwhich doesnot aids and can operatein any part position. In both casesthe absoluteerror increases rely on ground-based with distanceflown. In practiceit is the heading of the world. This advantageis sharedby inertial information which usuallylimits the accuracyof the of navigationwhich alsosharesthe disadvantage wslem. degradationof positionalinformation as distance of information The degradation flown is increased. arisesfrom the fact that startingfrom a known position subsequentpositionsare computedby sensingthe aircraft velocity and integratingwith respectto time. Errors,oncethey are introduced, can only be eliminatedby a position fix. A simplisticexamplewill illustratethe build-up of error. An aircraft takesoff from A to fly to B, 3000 nauticalmiles awayin a direction due west from A. Usingheadinginformation from a directionalsensor suchasa gyromagrleticcompassthe Doppler navigatorsensesthat the iircraft is flying due west at Fig. 10.2 Doppler effect - transmission

J ,l

172

Doppler Effect

ct

l.

,l

l l r l ln 1842 the AustrianscientistChristianDoppler predictedthe Dopplereffect in connectionwith soundwaves. It was subsequentlyfound that the effect is also applicableto e.m. waves.The Dpppler effect can be describedas the changein observed Aircraft Ground frequencywhen the source(transmitter)and observer (receiver)arein motion relativeto one another. The Fig. 10.3 Doppler cffect - reception noiseof moving trains and road traffic is a demonstrationof the effect commonly observed. (10.2) fr=(c+v)l\. The applicabilityto e.m. wavesis illustratedby the useof police radarspeedtraps,to the cost of Again we seey = 0 leadsto c = )\,f and the aircraft offenders. moving away from the ground targetgives In an airbome Doppler radarwe havea transmitter f r = ( c - y ) / I . which, by meansof a directionalantenna,radiates We must now considerboth effectssimultaneously. energytowardsthe ground. A receiverreceives the The wavelength tr in equation(10.2)is the echo of the transmittedenergy. Thus we havethe wavelengthof the echowhich must be ).' in equation situation whereboth transmitterand receiverare (10.1). .Thussubstituting).' for X we have: movingrelativeto the ground;consequently the ^ k+v\ ^ original frequencytransmittedis changedtwice. The Ir = (-fi I. differencebetweentransmittedand received frequenciesis known as the Doppler shift and is very We are interestedin the Dopplershift, /2r, which is nearly proportionalto the relativemotion between the differencebetweentransmittedand received the aircraft and the ground alongthe direction of the sigralsthus: radarbeam. - _((c+v) .) 2v Considerthe transmissionof e.m. energytowards f o = f , - -f = ' f( {( c- _ - vl l} = = . f .c - v " t the ground. Let the relativevelocity of the aircraft in the direction of the beambe r, the frequencyof the Now c = 186000 milesper secondso I is obviously radiation/ and the speedof the electromagnetic very small comparedwith c, so with negligibleerror wavesc (= 3 X tO8ms-l ). Referringto Fig. 10.2we we may write: seethat in t secondsthe wavewill havemoveda (10.3) fo = 2vflc. distancect to b while the aircraft will havemoveda distancevt to a. The wavesemitted in time r will be This equationis the basisof a Doppler radar. bunchedup in the distancebetweena andb which is Observing,as above,the conventionthat y is positive ct - vt. The numberof wavesemitted will be /r for movementtowards,and negativefor movement cycles. Thus the wavelengthobservedat the target, away,the groundtargetgivesan increasedreceived ).', is givenby: frequencyon a forward beamand a decreased t r ' - ( c r- v } l f t = ( c - v ) l f ( 1 0 . 1 ) receivedfrequencyon a rearwardor aft beam. From Figs 10.4and 10.5we seethat the relative We can seethat if the transmitteris stationarywith velocity of the aircraft in the direction of the beam respectto the groundthen y = 0 and equation(10.1) centroidis v = V cosI cosc where Iz is the masnitude. ' reducesto the familiar relationships =/tr. If the of the aircraft velocity with respectto the grou-nd. transmitteris movingaway from the ground target, So equation(10.3)becomes:' that is the beamis directedtowardsthe rear,then (10.4) fp = (2Vf cos0 cosc)/C. r in equation(10.1)becomes-y andwe have \'=(c+v)lf. It is at this stagethat the studentis often convinced Now considerthe receivedsigtal. In a time / the that a Doppler radarcould not possiblywork due to aircraft would receiveall the wavesoccupyingspace the smooth earth paradoxand the mountalnparadox, cf in Fig. 10.3. Howeverin this time the aircraft which arehopefully dispensed with below. i movesa distancer/ and hencewill receivethe number It is falselyarguedthat if an aircraft is moving of wavesoccupyingct + vt in f secondsor (c + r)/tr parallelto flat ground then thereis no changein range wavesin I second.Thus the receivedfrequency,fr, betweenthe aircraft and the groundand thereforeno is given by: Doppler shift. That this is falsecan be seenby

r

t

r

l

l

l

173

Vi the verticalvelocity component asshown in Fig. 10.6. Thesevelocitiesare in antenna co-ordinateswhich, with an antennafixed rigidly to the aircraft, are airframeco-ordinates.As the aircraft pitchesand rolls the antennamoveswith it and hence Vil, Vn' and Vf will not be the velocitiesin earth co-ordinatesrequiredfor navigation.

vir

vi

Fig. 10.4 Airborne Doppler single-beamgeometry in thc vertical Dlane

Fig. 10.6 The resolved velocity vectot

geometryin the Fig. 10,5 AirborneDopplersingle-beam plane horizontal consideringthe actualtargetswhich produce of the energy. Thesetargetsare backscattering irregularlyshapedscatteringobjectssuchas pebbles and there is. of course.relativemotion betweenthe aircraft and individualtargets- hencea Doppler shift. If the illuminatedareawere perfectly smooth no reflectedenergywould be receivedat the aircraft. The othdr falseargumentconcernsslopingterrain. If the aircraft is flying horizontally abovea slope then its rangeto the ground along the beam is changing and thereforethe Doppler shift will be affected. Again this falsehoodis exposedby the fact that the 'slope' actualtargetsare individualobjectswhose with respectto the aircraft is randomand henceis not related to the slope of the ground.

AntennaMechanization The aircraft velocity has three orthogonal @mponents: Vfi the headingvelocity component; V) the lateral velocity component; and 171

Fig. 10.7 Drift angleandgroundspeed One conceptuallysimple solution is to stabilize the antennain pitch and roll, in which casethe earth velocitiesVg, V1 and VV are co-ordinate-related equal to Vi, Va' and Vpi respectively.If the lateral velocity V4 is non-zeroit meansthat the movement of the aircraft is not in the direction of the heading and a non-zerodrift angle,6, existsasin Fig. 10.7. The resultantof Vg and Va is the velocity vector with magnitudeequalto groundspeeds, and direction that of the aircraft'strack. It is convenient for navigationpurposesto presentthe pilot with ground speedand drift angleinforrnation rather than Vs urd V1. With movingantennasystems,the antennais stabilizedin pitch and roll and alsoalignedin azimuth with the track of the aircraft,that is to say track-stabilized.The drift angleis givenby the angle betweenthe antennaand aircraft longitudinalaxes measuredin the horizontal plane. SomeDopplersuse pitch but not roll stabilizationsinceerror due to roll is small for smalldrift anglesand furthermoretends to averageout over the flight.

Fixed antennasystemsmust compute the velocitiesVry,Vn and Vv eachbeinga function of yi, V;, Vf R andp, whereR and F are roll and , pitch anglesappearingin the expressions as trigonometricfunctions. The relationshipsarederived in the Appendix.

Doppler Spectrum The beamsare of finite width, henceenergywill strike the ground alongdirectionsof different relative velocities.As a consequence a spectrumof Doppler shift frequencies is receivedasshownin Fis. l0.g wherethe effectsof sidelobesareisnored.

Wilh A? typically 0.07 radians(4") and I typically 70- we haveAfp lfp rypically 0.2. Sincebackscatteringfrom the illuminatedtarget areais not constantover the whole areathere is a random fluctuation of the instantaneousmean frequencyfp. To determinethe aircraft'svelocrty accuratelythe time constantof the velocity-measuring circuitsmust be sufficientlv lone to smooththis fluctuation,but not so long asio beunableto follow the normal accelerations-of the aircraft.

BeamGeometry

Sincethere are three unknowns Vi, Va' and Vf , a minimum of threebeamsarerequiredto measure them. ln practicethree or four beamsare usedin a configurationinvolvingfore and aft beams;assuchit is known asa Janusconfigurationafter the Roman god who could seeboth behind and in front. The beamsradiatedcan be either pencil.asin Fig. 10.10or narrowin elevation(nO) Uut wide in %power azimuth(Ac) asin Fig. 10.9. The hyperboliclines -3 dbs fo, h , f*, f-u are linesof constantDoppler shifts calledisodopsand are drawn assuminga flat earth. Whenwide azimuth beamsare useda fixed antenna systemwould leadto the beamcrossing a wide rangeof isodopsunderconditionsof drift, resultingin an excessivelywide Doppler spectrum. Consequently suchbeamsvirtuallydictatea track-stabilized anrenna. fd Dopplershift A wide azimuthbeamhasadvantages in that smaller Fig.10.8 TheDopplerspectrum antennaareasare requiredand roll performanceis improvedin the casewhereno roll stabilizationis Sucha phenomenonis undesirable but providing employed. the spectrumis reasonably'peaky'the meanDoppler Figure 10.9 showsthat for a fully stabilizedsystem shift is easilymeasured.If Z is the anglebetweenthe the Doppler shift on all four beamsis the same. beamcentroidand the aircraftvelocityvectorthen Without roll stabilizationsmallerrorsareintroduced e q u a t i o n( 1 0 . 3 )b e c o m e s : which tend to average out. Stabilizationcan be achievedby servoloops which drive the antennaso as 2Vf "otr. ( 1 0 . s ) to equalizethe Doppler shifts. Alternativelypitch lo =; information (and possiblyroll) can be fed to the Differentiation with respectto 7 givesa first Doppler from a verticalreferencesuchasa gyro or approximationto the half-powerbandwidth of the evena mercury switch leavlngazimuthstabilizationto spectrum: be achievedby equalizationof Doppler shifts. Typically in suchradarsground speedand drift angle 2A gy.rinT Lfo = ( r 0 . 6 ) are the only outputs wheregroundspeedis givenby equation(10.4) and drift angleby the amountof azimuthrotationof the antenna.Headine whereA7 is the half-powerbeamwidth. The ratio of information is usuallyaddedto the drift ingle to give Lfp to fp is thusgivenby dividingequation( 10.6) aircraft track. by ( l0.s). Figure 10.10showsthat for a fixed aerialsystem Lfp = the Doppler shifts on all four beamsare,in general, -f; a7.tan7. (10.7) not equal. It is shownin the Appendix to this 175

Fig. t0.9 Moving aerialsystem- typical beamgeometry

chapter that the velocitiesin airframe coordinates, Vri, VA' and Vf , dependon (f2 - f l),(f2 -/3) and (.f | + f3) respectively,where/l ,f2 andf3 are the Dopplershifts on beamsI ,2 and 3. Relationshipsfor thesevelocitiescan alsobe derivedusing/4 and two other Doppler shifts. With this redundancywe have or continuedoperation the possibilityof self-checking after one beam failure. Computationof groundspeedand drift anglein a fixed antennasystemcan be dividedinto three parts: firstly computationof Vi , V; and Vl, usingthe Dopplershifts from three of the four beams;secondly computationof Vg, V4 and Zy usingpitch and roll information and the previouslycomputedairframe co-ordinatevelocities;and lastly, drift angle= uctan (ValVn) and ground speed= (Vn'+ Vnzlo's (seeFigure 10.7). 176

TransmitterFrequency The choiceof r.f. is, asever,a compromise.The advantageof using a high frequency is that the sensitivityof the radarin Hertz per knot is high, as can be seenfrom equation(10.3); furthermore,for a givenantennasize,the higherthe frequencythe narrowerthe Dopplerspectrum. If, however,the radiated frequency is too high atmosphericand precipitationabsorptionand scatteringbecomemore of a problem. Another considerationis the availabilityof componentsfor the variousfrequency bandswhich might be considered.Most Doppler radarsoperatein a band centredon 8'8 GHz or 13'325 GHz, the former, to date,beingperhapsthe most common for civil aircraft use.

Heading

Fig. 10.10 Fixed aerial system - typical beam geometry

Modulation At first sight it would appearthat no modulation is necessary, indeed c.w. Doppler radarshavebeen built and operated,a great attraction being simplicity. Difficulties,however,arisein transmitter receiver isolationand discriminationagainstreflectionsfrom nearbyobjectsin particular the dielectric panel (radome)coveringthe airframeopeningfor the antenna. At other than low altitudes unwanted reflectionsare comparablein amplitude to ground returns. Noiselike variationsin vibrating radome echoeswill more than likely be in the same frequency band as the gxpectedDoppler shifts, and thus indistinguishableexcept where the s.n.r. is sufficiently high at low altitudes. To overcome the above problems both pulsed and

frequencymodulated(f.m.c.w.) radarshavebeen used. The earliestDopplerswere pulsedso that echoesfrom nearby objectswere receivedduring the recoverytime of the diplexer and hencewere not processed.In the so-calledincoherentpulsesystemsa Doppler signalis obtainedby mixing receivedsignals from fore and aft beams;thishas two undesirable consequences.Firstly the returnson the fore and aft beamsmust overlapin time if mixing is to take place, this meansstabilizationand/or wide beamsmust be used. Secondly,the Doppler shift on the individual beamsis not available,hencethe senseof direction of the velocity vector (forward or backward)and the verticalvelocity cannot be computed. With modern radarsf.m.c.w. is the most common type of transmission.The spectrumof the transmitted sigrd consistsof a largenumber of sidebandsas well

177

asthe carrier. Theoreticalanalysisof f.m.c.w. reveals an infinite number of sidebandsspacedby the modulation frequency f,n amplitudeof individual sidebandsbeing determinedby Besselfunctions of the first kind of order n and argumentrn where n is the sidebandconcerned(first, second,third, etc.) and rn is the modulationindex,i.e. ratio of deviationto f.. By usingthe Doppler shift of a particular sideband,and choosingan appropriatevalueof m to give sufficient amplitudeof the sidebandconcerned, suppressionof noisedue to returnsfrom the radome and other nearby objectsis achieved. A problemcommonto both pulsedand f.m.c.w. Doppler radarsis that of altitude holes. In a pulsed systemifthe echo arrivesback at the receiverwhen a subsequent pulseis beingtransmittedthen it is gatedout by the diplexer and no Doppler shift can be detected.Similarlywith f.m.c.w.,if the round-trip travel time is nearly equal to the modulationperioda deadbeatwill occurwhen mixing transmittedand receivedsignals,and again no Dopplershift will be detected. If a low modulatingfrequencyis use'dthe first altitude hole may appearabovethe operating ceiling. Howeverlow p.r.f. in pulsesystemleadsto low efficiency and the possibilityof interferenceif the p.r.f. is in the rangeof Dopplerfrequencies expected(eudio). For f.m.c.w.givena choiceof sidebandused,typically third or fourth, and modulationindex,typically2l or 3, suchasto avoidradomenoise,the modulatingfrequencymust be fairly high to allow a reasonabledeviation. A fairly high modulatingfrequencyis usually varied eithercontinuously(wobble)or in discretestepsto avoid altitude holesat fixed heishts.

Over-WaterErrors Doppler navigatorsmeasurethe velocity of the

Fig lO.l I Frequencymodulatedcontinuouswave transmittedsignaland receivedgroundechospectrum

174

aircraft relativeto the surfacebelow them. When flying over water that surfaceitself may be moving due to seacurrentsor wind-blownwater particles. Randomseacurrentsare of speedsusually a good bit lessthan half a knot, and this small el'fect averages out sincethe currentsare in random directions.Major seacurrentsdo not exceed,say, 3 knots and sincedirection and approximatespeed areknown they can be compensatedfor. Wind-blowndropletswould give an error lessthan the wind speed,about 3 knots error for l0 knots wind with the error varying as the third root of the wind. On long flights such an error will be reduced by averaging. . o i o-

k___ Overwater caltoralon : I shrfterror

Frequency Fig.10.12 Over-water calibtatiory shift Whenflying overland the beamsilluminatean areacontainingmany scatteringparticles.Generally the backscatteringcoefficientsover the whole illuminated areawill be of the sameorder givingrise to the Dopplerspectrumshownin Fig. 10.8. Over smooth seathe situation is different; a larger fraction of the incident enerS/ will be returnedon the steepestpart of the beam sincethe surface backscatteringcoefficient will dependon the angle of incidence.The net resultis to shift the Doppler spectrumasshownin Fig. 10.12,so that the mean Doppler shift is lessthan it should be for the aircraft velocity. The error introduced,which could be up to 5 per cent,is known as over-watercalibrationshift error. The narrowerthe beam widih the lesssignificantthe error,so someDopplersaredesignedto produce beamsnarrow enoughto keep the error within acceptablelimits. Other Dopplershavea manual land-seaor seabias switch which, when in the seaor on position respectively,causesa calibrationshift in the oppositesenseby weightingthe response.of the Doppler shift frequencyprocessingin favour of the higherfrequencies.For a carefully chosen compensationshift the error can be reducedby a f-actorof about ten.

Antenna axis

N(ml

Ground track o

Lobc switched beams

]

o

.L

I

t '

Frequency

Fig. 10.13 Lobe switching

Lobe switching is a more successfulmethod of reducingcalibrationshift error. The beam angleof incidenceto the ground is switchedby a small amount periodically. The illuminated areasfor the two switchedanglesoverlap,as do the Doppler spectra.The Doppler shift frequencyused for velocity measurementis where the spectracross. This crossoverpoint correspondsto the retum from the samegroup of scatterersat the sameangleof incidenceand is thus not affectedby over-water flight. Figure 10.13 illusrratesthe technique,which porks far better when track-stabilizedantennassince then the lobe switchingis at right anglesto the isodops.

Navigation CalculationS

:

The ground speedand drift angleinformation is normally presentedto the pilot but in addition is used,togetherwith headinginformation,to givethe aircraft position relativeto a destinationor forthcomingwaypoint. To achievethis the pilot must set desiredtrack and distanceto fly before take off. In Fig. l0. | 4 the pilot wishesto fly from A to B, a distanceof 50 nauticalnriles,with desired track 090. Thc aircraft has flown for 6 min at a speedof 500 knots on a headingof 100 with a drift o[ 27" starboard,thus the total distanceis 50 nauticalnrilesand the aircraftis at point C. The trackerror is 37-, tlre alongdistanceto go is X = l0 nauticalmiles:the acrossdistanceis Y = 30 nauticalnriles. In order to seehow the Doppler navigatorarrives at the alongand acrossdistances indicatedto the pilot we must considerthe informationavailable: ' Groundspeed(s) and - Doppler radar dril't angle(6) - gyromagneticcompass Heading(I/) Desiredtrack (Til)and - pilot Distance(D)

Fig. 10.14 Navigationcalculations

To arriveat along distanceto go (X) and across distance(y) the true track (?') and track error angle (E) areneeded. We have,assumingdrift to starboard aspositive:

T =H+6 E = T-Td x = D -JjScosf,'dr Y = tssinEdt

(r0.8) (10.e) (10.10) / ( 1 0 . l1)

where I is the time of flight, and the senseof the acrossdistanceis positiveto the right.

Block DiagramOperation Moving Antenna System Figure 10.15 illustratesa block diagrambasedon the MarconiAD 560, a systemintroducedin the mid-I960s and usedon a'varietyof civil aircraft. It is still to be found in service. The sensoris an f.m.c.w. type employing wobbulation of the modulatingfrequencyf^ to avoid altitude holesand usingthe Nth sideband (tr/ = 3 in the AD 560) to avoid unwantedinterference due to radomevibrations. For the choice.ofthe third sidebanda suitablemodulationindex is 2.5. obtained by usinga deviationof t I MHz on the 8800 MHz carrierand a modulatingfrequencyof 400 kHz. Two mixer stagesgive the Doppler shift frequency

179

I

Transmitter/receivcr

Aerial unit p--

Heading

F----+ Drifi

Set tiack

Autopilot Steering ind.

Ground speed

Trackcr

+ Memory

Set zero Set distance

=TTo diStance

i Computer I !

Displayunit

:

Fig. 10.15 Moving aerial Doppler block diagram based on the Marconi AD 550

fo. The first mixes the receivedsignalwith a sample of the transmittedsignal,the requiredsideband (1200 kHz in the AD 560) beingselectedby the intermediatefrequencyamplifiers. The secondmixes .fy'times f^ with the selectedsidebandto extract fi.; by meansof a low passfilter with a cut-off frequency ' ofabout 20kHz. There are two transmit and two receivelinear slotted arrays. Anti-phaseand in-phasearraysare usedfor transmit and receive,an arrangementwhich can be shown to compensatefor changesin wavelength.The arraysareconnectedto appropriate inlets/outletsby an r.f. switch(varacterdiodesin the AD 560). Fore and aft beamsareobtainedby providingfor connectionto eitherend of eacharray, while port and starboarddeflectionis achievedby use sequence is of sidereflectors.The beam-switching 180

important where pitch and azimuth drive of the aerial is concerned;the AD 560 sequenceis port forward, starboardaft, starboardforward, port aft, a complete cycle taking I second. The Doppler spectrumis fed to two loops: one coarseand one fine. The searchloop providesfor coarseadjustmenlof a ground-speedmeasuringshaft which determinesthe frequency of a voltagecontrolled oscillator(v.c.o.) in the tracking loop through a feedbackpotentiometer. With the searchloop nulled the v.c.o. frequency is approximatelyequal to the meanDoppler shift frequency,a discriminatorwithin the tracking loop is then able to apply an error signal to the speeddrive so as to position the ground-speed shaft accurately. The Doppler can then be saidto be locked on, and any changein ground speedwill be trackedby the fine trackingloop.

drift angle(azimuth drive) and so give track to another differential synchroin the displayunit. The Sterboard rotor of the seconddifferentialsynchrois set by the drift operatorat the desiredtrack angle,hencethe output is the differencebetweentrack and desiredtrack, Timo i.e. track angleerror6. The resolverin the computerunit resolvesground Noseup speedS into its alongand acrossspeedcomponents S cos.EandS sin^Erespectively.In the AD 560 a ball 1 second resolveris used,thus givingmechanicalanalogue a tracking in Dopplershiftwith aerialmisalignment computing. The ball is drivenby Fig 10.16 Change oscillator-fedsteppermotor, hencethe rate of rotation is proportional to ground speed. The axis of rotation dependson the angleof the drive wheel If the antennaaxis is not aligred with the track of the aircraft, either in pitch or in azimuth, the Doppler which is set by a servoposition control systemto be equal to the track error angle. Two pick-offwheels strift will changewith beam-switching.With mounted with their axesat right anglesrotate at a occurs in Doppler the change misalignmentin azimuth rate dependingon the alongand acrossspeeds.These at a rate of I Hz, while if the misalignmentis in pitch the rate is 2Hz. This follows from the beam-switching rotations are repeatedin the displayunit by meansof servodrivesand causethe counlersto rotate. The and rate (seeFig. 10.16). Reference sequence alongdistancecounter is arrangedto count down from pitch and to the waveformsof 2 and I Hz arefed the initial distanceto the waypoint until it reaches any misalignment respectively; circuits azimuth drive zero when the aircraft will be on a line perpendicular of the antennawill be detectedby the drive circuits, to the desiredtrack and passingthroughthe waypoint. resultingin the antennarotating so asto align itself If both alongand acrossdistancesreadzero shift with the aircraft track. At this stagethe Doppler simultaneouslythe aircraft is over the waypoint. accurately and four beams on all is the same representsground speed,alsothe anglebetweenthe Fixed Antenna System aircraft and antennalongitudinalaxesis equal to the Figure10.17may be usedto explainthe principlesof drift angle. a fixed antennasystemto block diagramlevel. The and search the in both s.n.r. is measured The antennaconsistsof planararraysof slotted waveguide tracking loops. If the searchloop checkis not satisfactoryfor three out of the four beamsthe three or printed circuit, separatearraysbeingusedfor transmissionand reception. Beam'switchingis servodrivecircuits are disabled.The sigrralto noise to to.go achievedusingvaractordiode or ferrite switchesto the system causes loop tracking the checkin couple the transmitterand receiverto the memory if it is not satisfactory. In memory the appropriateport. shaft are fixed and a antennaand ground-speed The receivedsignalis mixed with a sampleof the drift and memory flag appearsin a ground'speed transmittedsignaland the wanted sideband give the f.m.c.w. to continues angleindicator. The sensor filtered out and amplified. If only ground speedand ground'speedand drift anglefor as last-measured drift angleare requiredfurther mixing may take place long as the poor s.n.r.continues. to extract the Doppler frequenciesasin the moving Ground-speedoutput from the sensoris in two antennacase;howeverthe senseof the shift (positive to coupled forms.- A synchrorotor is mechanically or negative)is lost. If the three velocity vectorsin the The feed' three'wire giving a shaft the ground-speed direction of the aircraft cq'ordinatesare required,an trackingv.c.o. frequency,which is proportional to intermediatefrequency,feis retainedwhich will be ground speed,is fed to a sine'cosineresolverin the dependingon computer and alsoto a frequencydividerwhich scales reducedor increasedby an amount radiated. is being beam which per nautical pulse and shapesthe sigral so that one The time-multiplexed Doppler shifted ,fq siSnals mile is fed to a distance flown indicator (integrating are separatedby a demultiplexerdriven by the counter). control and feedingfour tracking beam-switching gives a A synchrotransmitterin the antennaunit oscillatorsbecomelocked Voltage-controlled loops. to the drift angleoutput sincethe body is bolted frequencies the incoming to fO ! fo by sweeping driven by is rotor the and part antenna ofthe fixed lock on. The four they until through their range the azimuth motor. A differentialsynchro and differenced,as summed then are outputs tracker transmitteris usedto add heading(from compass)to Pf

SA

181

Heading

Attitude

x o x o

= E

] z E Air speed

Variation initial position and waypoints

Fig 10.17 FixedaerialDopplerblockdiagram

appropriate,in a combiningnetwork to provide ground-speedand drift-angleoutputs and, with a signals/r, fy and/2 proportional to aircraft compassinput, will providenavigationdata to a CDU co-ordinatevelocities(seeAppendix, Al 0.3). givinga two-unit Doppler navigationsystem. Digital Within the computer/display unit (CDU) the outputsareprovidedin accordance with ARINC 429 aircraft referencedvelocity componentsare (DITS) and ARINC 582, thereis alsoan optional transformedinto track-orientatedearth-referenced synchrooutput for drift angle.This unit, introduced componentsusingattitude signals(pitch and roll) n 1979,may heralda comebackfor Dopplerin from a verticalreferencegyro (seeAppendix, A10.4). airline servicesinceit has beenorderedby Boeingfor With a headinginput and waypoints,in terms of installationin their 727s and,737s. to be usedin desiredtrack and distance,set in by the pilot the conjunction with Lear Siegler'sperformanceand computercan integratethe alongand across navigationcomputer system(PNCS). velocitiesto givedistanceto go and across-track error (distance)respectively. The CDU may offer latitude and longitude rcadoutof position by transformingthe aircraft velocitiesinto north-orientatedhorizontal components. True north, as opposedto magneticnorth, may be usedas the referenceif the pilot is ableto enter the variation. With attitude, headingand true air speed inputs the output from an airspeedtransformation circuit or routine may be comparedwith the velocity transformationto give wind speedand direction.

Installation TheDopplernavigator system illustrated in Fig. 10.I 5 requiresfive units asindicated;i.e. antenna, transmitter-receiver, tracker,computer and display unit. The Marconi AD 560 comprisesthe five units mentionedplus a junction box, ground-speed and drift-angleindicator, distance-flownindicator and a control unit (or simply a panel-mountedswitch). The weight of the AD 560 is about 30 kg which shouldbe comparedwith Marconi'slatest Doppler the AD 660 which weighs5 kg (sensoronly). The AD 660 is a single-unitDoppler sensorgiving Fig. 10.18 AD 550 (courtesyMarconiAvionicsLtd) '182

:1 I

i t .

{:,-:"* . l::

ril,:.: tri l

i ,

i f

Fig. 10.19 Doppler7l antenna/electronics unit (courtesy the DeccaNavigatorCo. Ltd)

Figure10.18showsthe AD 660 with cards removed.Note the useof large-sca1e integrated circuitscommonin all modernsystemsaswe approachthe 1980s.The antennais a printed circuit microstripproducingfour beamstransmitted sequentially.The transmitteris a f.m. Gunn diode oscillatorgeneratingover 200 mW at a carrier frequencyof 13.325GHz. Computationand control is achievedthrougha microprocessor. The DeccaDoppler7l and 72 aredesisnedfor v./s.t.o.l.(vertical/shorttake-offand landing)aircraft operatingbelow 300 knots and fixed-wins aircraft operatiggup to 1000knots respectively. these Efstemshavehad civilian saleslimited to aircraft with specialneedssuchas certainhelicopteroperationsand surveying.Figures10.19-10.22 showunits of a typicalDeccaDoppler7l installation.Interconnections aresimple;all three indicatorsbeingdriven directly from the antenna/electronics unit. A headinginput is requiredfor the PBDI which, togetherwith the antenna/electronics unit, forms a basictwo-unit system,the two metersbeingoptional. Another lq._19.?0 Doppter7l position,bearinganddrift indicaror optional unit is an automatic chart display driven by (PBDI) (courtesy theDeccaNavigator Co.I_tay the PBDI or a more sophisticatedreplacement,a TANS computer.

183

error areusually error and track-angle Across-track availableasoutputsfrom a CDU fbr useby an autopilot. Obviouslywarningsignalsmust alsobe providedto indicatethe integrityof the steering signalsto the userequipment.

Controlsand Operation We shallconsiderthe Doppler71 as an example, variationsexist. althoughobviouslyconsiderable P.B.D.I. Indicator,controllerand generalpurposeprocessor w i t h p r o g r a m m cea p a c i t yo f 1 5 0 0 1 6 - b i tw o r d s . memory. Battery-protected

and drift meter Fig. 10.21 Doppler7l ground-speed (courtesythe DeccaNavigator Co. Ltd)

Switches l. DOP TEST: groundcheckingof sensor; ST BY: inputsinhibited,displaytlashes; LAND/SEA: allowscorrectionfor overwater calibrationshift error to be switcltedin. 2. LMP TEST: checkof displayand lamps; HDG/VAR: displayof headinginput and insertion of m agneticvariations; FIX: positiondisplayedis fixed; Doppler arestored;warninglamp incrementaldistances flashes:slewswitchesoperable: POS:aircraftlatitudeor longitudedisplayed; GS/DFT: groundspeedand drift angledisplayed: BRG DIST: bearingand distanceto next waypoint displayed; waypointlatitudeor longitude. WP: selected 3 . W A Y P O I N TI t o l 0 : a l l o w s f o r w a y p o i n t selection.Waypointscan be insertedor changed at any time. latitude,longitudeor 4. LAT LONG: three-posttion both (alternately)displayed. 5. SLEW:two switchesusedfor insertingvariation. and presentposition,waypointco-ordinates resettingthe numericdisplaysas required. \Displays

l. Numeric:twb groupsof threeseven-segment filamentsshow data selectedby seven-position switch. 2. Sectordisplay:indicateslatitude no*fh (N) or south(S), longitudeeast(E) or west(W). The Doppler 70 seriessystemsare c.w., three-beam displayshowingtrack error, (not switched)K-bandradars.Adequatedecoupling 3. Trackerror: analogue to the selectedwaypoint. in degrees, betweentransmitterand receiveris inherentin the desigr,so allowingthe useof c.w. A localoscillator 4. Warningindicators:incorporatedin analogue displayto givewarningof Doppler failure signalis usedfor mixing, so providingan intermediate (or memory),computerfailureor test mode frequencyt the Dopplershift. Furtherdetailsof the selected. systemareincludedbelow. (courtesy Fig.10.22 Doppler7l hovermeter theDecca Co.Ltd) Navigator

184

Ground-Speedand Drift Meter Drift anglerange:i 39.9o. Displayofground speedup to 300 knots and drift Altitude: 45 000 ft abovesround level. angle+ 30o (expandedscale). Supply2 : 8 V d . c . , 2A . Powerfailure and memory warning flags. Manualsettingof drift and ground speedprovided for. Testing Hovcr Meter Displays: ModernDopplershavea considerable amountof along-heading velocity- range-10 to +20 knots: built-in test equipmentwith which to carry out across-heading velocity- ranget l5 knots; checks.Syntheticsignalsmay be generated by verticalvelocity - ranget 500 ft min-r. switchingantennasat a much higherrate than normal, thus leadingto the memory flag clearingand a given readingpossiblyappearingon the ground-speedand Characteristics drift-angleindicator. This might be the effect of pressingthe test switch on the ground under memory conditions.If airborneand in memorya similar ARINC Characteristic540, airborneDoppler radar, wasissuedin 1958 and lastprinted in January1960; checkcould be carriedout, but ifin the signal condition,i.e. Dopplershift presentand s.n.r. it is no longermaintainedcurrent. For this reason satisfactory, then a goodand easycheckis to operate detailsof a currently availablesystem,the Decca the slewingswitchesto offset ground-speedand Doppler7l arelisted. Sincethis is primarily a helicoptersystema'very brief data summary for the drift-anglereadings;on releasethe readingsshould MarconiAD 660 systemaimed at the airliner market return to their original positions. This check will causea small error in the computedposition but this hasalsobeenincluded. DeccaDoppler 7l Power: 100 mW Frequency:13'325and 13.314GHz. Intermediatefrequency:10.7MHz. Beamwidth: 5o in depressionplane, I lo in broadside plane. Depressionangle'.67". Modulation:none,c.w. Number of beams:three continuous. Along-headingvelocity rangeto computer: -50 to +300knots. Across-heading velocity rangeto computer: ! 100 knots. Supply: I l5 V, 400 Hz, single-phase. Altitude range:0-20 000 ft over land or over water when surfacewind ) 5 knots. Accuracyof sensor- lessthan O'3Voor 0.25 knots (whicheveris the greater(overland)). Acquisitiontime: within 20 s. Indicatedaccuracy,ground speedand drift meter: 3'5 at 100 knots; 5 at 300 knots, drift t 0'5o. Indicatedaccuracy,hover meter: along and across velocitiesI I knot, verticalvelocity t 40 ft min-I. Marconi AD 660 Power:200 mW. Frequency: 13'325 GHz. Modulation:f.m.c.w. Number of beams:four, sequential. Velocity range: l0-800 knots.

End

Simulated flight path

151 Sta$ 2

065

WPT 2

Stage 1

Start Fig, 10.23 Test conditionsfor simulatedflight to check computer(seetext)

185

can be eliminatedby sleivingin the oppositedirection by the sameamount to producea cancellingerror. To check the computer/displaypart of a Doppler navigatora coursemust be simulatedby setting compassheading(d.g., directionalgyro, selected), drift angleiground speedand waypoint courseand distanceusingappropriateslewingcontrols. Having set everythingup the computeris switchedon for a timed run, at the end of which the displayedreadings d-rouldbe asindependentlycalculated. Usually a figuresfor written procedurewill give the necessary sucha checkbut in any casethey are reasonablyeasy to work out. As an example,startingwith the following: Waypoint I

track 335"

Waypoint 2

track 065"

Appendix Aircraft and Earth RelationshipsBetrrveen Co-Ordinates A s i n F i g .A l 0 . l l e t i ' , i ' , k ' b e o r t h o g o n aul n i t vectorsdefining a right-handedco-ordinatesystem with the positivedirection of the axis spannedby i' beingforward along the aircraft'slongitudinal axis and ihe positivedirection of the axis spannedby i' beingstarboardalong the aircraft'slateral axis.

distance15 nauticd miles distance15 nautical milqs

OlO" Heading l0o starboard Drift Ground speed 600 knots at the end of the simulatcd two-hg flight the across distanceshouldbe zeto, the distancefTown42 nauticalmiles and the time taken 4 min l4'5 s, all to within the tolerancelaid down for the system. Ramp test setshavebeenproducedfor Doppler systems,usually purpose-builtby the manufacturerof as are, for example, the radar and not general-purpose one will sets. Sometimes etc. test ILS, VOR, DME, switcheswhich can be find meterswith associated usedto monitor variousinternal voltagesandior currentsbut this is more likely on older multi'unit equipment. It is important for accuracyto ensurethe antenna is alignedwith the aircraft'slongitudinalaxis. The Doppler will interpret any slight misalignmentasa drift-angleerror. lnitial alignmentof all antennasis importantbut with a fixed antennasystemoncethe hole is cut in the airframe,correctly aligned,the only causefor concernafterwardsis that the antennais fitted the ccrrectway round. With moving-antenia systemsan alignmentprocedurefor the antenna mounting is carriedout initially by usingsightingrods on the mountingand the aircraft' Viewingthe rods from a distanceto ensurethey arein line, and then tiglrteningthe securingbolts throughthe slotted holesin the mountingplate,will ensurethat the changedwithout a need. antennacan be subsequently one shouldbe although for an alignmentcheck.inspections. on rnajor out carried

r86

FE. At0.l Aircraft co-ordinates As in Fig. A 10.2let i, i , k be orthogonalunit vectorsdefining a right-handedco-ordinatesystem with the positivedirection of the axis spannedby f beingforward alongthe aircraft'slongitudinalaxis projectedon to a plane parallelto the ground and the positivedirection of the axis spannedby 7 being starboardalong the aircraft'slateral axis projectedon to a planeparallelto the ground.

k Fig.A10.2 Earthco-ordinates Further le! positivepitch be nose'upand positive roll be starboardwing-down,then from Figs.A10.3 and A10.4 we have: i'= k'=

icosP-ksinP isinP+kcosP

i'= TcosR+/rsinR k'= -i sinR+kcosR

sinceZ is thevectorsumof Vd,VA' and l/y'. Thus:

vn = hvi+av{+vvl of the projeition whereft, a andv arethe magnitudes system ofz on to eachaxisof the co'ordinate i . e . u= h i + a i + v k FromFigsA10.5,A10.6andA10.7we seethat for: b e a mI beam2 b e a m3 beam4

h= -H h= H h= H h=-H

a= A a= A a= -A a=-A

v v v v

= = = =

V V V V

whereI/ = cos0 cosa;.4 = cos0 sina; Z = sind.

k' F8. A10.4 Aircraftroll Thus the matrix of transition from l, i, k to i', i', /c' is givenby:

'[i lrrhn] :-il F::

Fig. A10.5

f cosf =10 l-sin P

sin P sinR cosR cosP sinR

Velocities in longitudinal/lateral plane

vi{

sin P cosRl -sinR l=U cosP cosRJ

l!1, If the aircraftvelocityvectorl/ hasco-ordinates Vt, Vv with respectto i, i, k andVfi , Va', Vf with lo i',i',k'we have: respect

w=Mxv,,\

V(/ Fig. A10.6 Velocitiesin planenormalto longitudinal/lateral plane

(Al0.l)

i.e. Vy = Vrt cosP + Vj sinPsin R + Vi sinPcosR - Vl sin R Va' cosR V.e= = -Vi + Izf cosP cosR + Va' cosP sin R Vv sin P

The Doppler Shifts for a Four-Beam Janus Configuration

Fig. A10.7

Aircraft velocity vector

Now the Dopplershiftsaregivenby/2 =2Vnf lc' Themagnitude of the relativevelocityvector24, in thedirectionof anybeamis the innerproductof the. therefore: aircraftvelocityvectorV andthe unit vectoralong fi = 2f (-H Vl +,a Va'+ Y Vl)lc a. Thus: thebeamcentroid, Vx'+ V Vf)lc

VR= V'u =(Vi+V71'+Vv).u

fr=2f(HVil+A f s = 2 f ( H V r i - A vA'+ v vi)lc fc = 2f(-H vi 'l v; + v vi)lc

(Ar 0.2)

187

The Aircraft Velocity in Earth Co-Ordinates Expressedin Termsof DopplerShifts - From equations(A10.2) we have:

fe)

c(fz + fq) v l = c ( fi +ft) - __4TT

(Ar0.4)

where Kp = 47cof .", "

v1i= c(fz;fr\=+*-fr c(h v; =c(20p.=-rrr

W=MxWir;:itil

(A10.3)

c i,4 = AEos;1il-; "! Y

__:_

a

4f sin 0

are known constants.

rl4 is obtained from pitch and roll signalsandf1,f2,fg are the Doppler shifts measuredon beamsl, 2 and 3. Substituting for Vfi, Vt' , Yi from equations (A10.3) Note beam4 is redundantbut could be usedfor (Al0.l) we have: checking purposes. into equations

188

11 Radioaltimeter

Introduction The meaningof the terms aircraft altitude or height is complicatedby the variousreferencesused from which the height can be measured.A barometric altimeter sensesthe static pressureat aircraft leveland givesa readingdependenton the differencebetween {his pressureand the pressureat somereferencelevel. For aircraft flying aboveabout 3000 ft, the reference of paramountimportanceis that levelcorresponding to a pressureof 1013.25mbar(29-92in.Hg),the so-calledmean sealevel. The other barometric referencesusedare local sealevel and airfield level. The pilot is able to set the referencelevel pressureat l0l3'25 mbar,QNH (localsealevel- regional)or QFE (airfield level),the Q codesbeingusedin communicationwith air traffic control(ATC). Converselythe radio altimeter measuresthe height of the aircraft abovethe ground. lf an aircraft is in levelflight the barometricaltimeter readingwill be steadywhile the radioaltimeterreadingwill be varying unlessthe aircraft is ffying over seaor plain. It follows that radio altimetersare most usefulwhen closeto the ground, say below 2000 ft, and particularly so when landingproviding the final approachis over a flat surface. As a consequence, radio altimetersdesignedfor usein civil aircraft are low-levelsystems,typical maximum rangesavailable being 5000, 2500 or even500 ft in the caseof usein automaticlandingsystems.Military aircraft can utilize high-levelradio altimeters.

BasicPrinciples Radioheightis measuredusingthe basicideaof radio ranging,i.e. measuring the elapsedtime between transmission of an e.m.waveand its receptionafter reflectionfrom the ground. The heightis givenby hAlf the productof the elapsedtime and the speedof light:

necessary in orderto 'mark' tlie time of transmission. both f.m. and pulsedtransmissions areused. The methodof time measurement dependson the type of modulationusedand the complexityof the airborne equipmentwhich is acceptable.Threebasictypesof altimeterare marketed:pulse,conventionalf.m.c.w. (frequencymodulatedcontinuouswave)and constant diflerencefrequencyf.m.c.w. The bgsicprinciple of a pulsedsystemis simple, sincethe transmittedand receivedpulsesclearly representeventsbetweenwhich the time can be measured.With f.m.c.w.thereis no singleevent duringone cycleof the modulatingfrequency;however specifictimesduringone-halfcyclecanbe identified by the instantaneous frequencybeingtransmitted. Sincethe transmitterfrequencyis continuously changing,the receivedsignal,which hasbeen subject to delaydue to the round-triptraveltime, will be different in frequencyto the transmittedsignalat any instant in time. The differencefrequency,fi, can be shown to be proportional to the height as follows. Assumea triangularmodulatingwaveformof frequencyfm and amplitudesuchthat the carrier,'/", is modulatedovera rangeA/. This situationis illustratedin Fig. I I .l . Th€ two-waytraveltime is 2Hlc where11is the heightand c the speedof light. The magnitudeof the rateof changeof transmitted frequencyis 2 . A/. f^ (= 0.5 Lf lO10.25fil. The productof the elapsedtime and the rateof changeof frequencywill givethe diflerencein frequency betweentransmittedand receivedfrequencythus:

f n = 2 . L f. f ^ . 7 , = 4 . A f. f ^ . H l c

( rl . r )

Tltus the measurement of the beatliequencv determinesthe heightsince4 .Lf . f^jc is a known constantfrrr any particularsystem. The beatfrequencyis constant,for triangular modulation,exccptat thc turn-aroundregiontwice

& & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & & &p&e& ( r?b.egisnwget h e e l a p s e d t i m e i n r c&y&c & l e&. & & &S&i& n& c e&t& u& r n&- & a r&o&u& n& d t&a&k & e s&p=l 4a 9c 2 e ti n

microseconds). Energyis radiatedat a frequencyh the band 42OO'44O0 MHz. Modulation of the carrieris

havethat the average beatfrequencyfor 2Zps is fnl2 l'orthe restof the moclulating cycle;thebeat frequencyis constantat fi . If the averagebeat

189

Thfi will result in'a vertical dhplacetnent of thc graph of the receivedsigral in Fig. I l.l. The effect for a descendingaircraft (positiveshift) is shown in Fig. I1.2. As canbe seenthe beatfrequencyis fn - fa andfi, + f6 for equalperiods. If we take the averageover half a ntodulationperiod we get

Transmittcd

Un +fa +fn - fil2 =fh asrequired.Thisassumes

fn ) fa which will be the casewith a radio altimeter. Received

o

o C o t ct o E o .9 ! o G

Fig.I l.l A FMCWradioaltimeter- frequency/time relationship frequencyover one modulatingcycle is detectedwe will havea measuredfrequencyfi givenby:

C'

o

= (((tlf; -2r)fn + rfnl2)fm fn '= fm(r - 3Tfmlz)

o

(l1.2) o

With the aircraft at about 2000 ft andf^ = 2OOHz we have: fn = fn (l _ 0.0012)

Q'

6

Fig.I 1.2 Theeffectof Dopplershifton beatfrequency

i.e. an error of about 2.4 ft at 2000 ft. On the ground therewill still be an elapsedtime, due to built-in tlsingequation(l l.l) we seethat for a systemwith delay,ofabout 0.12 ps corresponding to a residual A.f = 100 MHz (42504350 MHz) andf ^ = 200 Hz altitude(seelater)of 57 ft, say. Whenwe use the beat frequencyfi = (4 X 100X 106X 2OOH)/ (984 X 106)= 8011sothe rangeol fp lor a 0'12 ps andf^ = 2OOHzin equation(l 1.2)we Illlll= find that the erroron the grounddue to the averaging 0-2500ft altimeterwith a residualaltitudeof 57 ft kHz. Thiswide bandrvidthcan of the non-constant would be 4'56-?.04'56 beatfrequencyis about 0.002 ft which is insignificant. be reducedby' maintaininga constantdifference In practicea perfecttriangularmodulating frequencythusimprovingsensitivity.Sincefi frequencyis difficult to achieve,someroundingat dependson both Lf andf * we canvary eitherin the turn-aroundtakingplace. In fact with any inverseproportionto the heig.htso keepingfi reasonably shape-modulation waveform,it canbe constant. servoloop nrustmonitor the shownthat the average beatfrequencywhen usedin The necessary equation( I I .l ) will yield a heightreadingwhich is frequencyand control the modulatorso as diff'erence c o r r e cw t ithinacceptable i m i t s . A p p e n d i xI L I to changeLl'or frt accortlingly.The control signal provesthis for sinusoidalfrequencymodulation,whichrepresents the heightof the aircraft, Sincethe is easierto obtainsincethe rateof changeof carrier controlledvariableis madeinverselyproportionalto frequencyis boundedin magnitudeby unity. heightit becomesdifficult to employthe technique lf thereis relativemotion betweenthe aircraftand at lower heights.particularlywhenA/ is varied. This of b1'the contpromise the groundimrnediatelybelow the receivedfrequency latterproblenrcanbe overcorne will experience a Dopplershift,/a (seeChapterl0). c o n v e n t i o n a l al yt l o w a l t i t u d e sa n d r v i t h operating 190

constantj[ otherwise,or by modifying feedbackin the loop such that fp varieswith any changein altitudebut not oversucha greatrangeas it would with no feedback.The Frenchlnanufacturer TRT producealtimeterswith constantfi at all heights, control beingachievedbyvarytngf^; the tolerance + I ft or I per centis met.

FactorsAffecting Performance Ihe accuracyof a radioaltimeterdepends fundamentallyon the precisionwith which the time of transmission is marked. For precisetiming a wide transmittedspectrumis requiredsincethis would lead to a steepleadingedgein a pulsedsystemor, in the caseof f.m.c.w.,a largefrequencydeviationwhich effectively givesan expandedscale. Ofcourse a finite spectrumis transmittedand in fact is limited to a total spreadof between4200 and 4400 MHz by (the band 1600-1660MHz internationallegislation hasalsobeenallocatedfor useby radioaltimetersbut is no longerused). In an f.m.c.w.systema countermay be usedwhich the numberof cyclesor half-cycles measures in one periodof the modulation.Sincethe counteris unable to measurefractionsof a cycle,other than possiblya half, the count is a discretenumber. The process resultsin a quantizationerror calledstepor fixed error. We may rewriteequation( I I .l ) as:

H=&,=#

(l r.3)

where,rV= fnlfn is the number of beat frequency cycles(to nearestintegerbelow)in the modulation cycle. Sincely' is an integerthe height measuredwill be subjectto a quantizationerror calledsteperror equalto cl4Al'which with A/= 100 MHz is2.46 ft. In practicein any one rnodulationperiodthe actualcount will dependon the nurnberof positive-going zerocrossings of the beat frequency, this could be N or N + I dependingon phasing.So if the phasingvariesthc count will jurnp betweeny'y'and Iy'+ | and backso avertgingout the steperror p r o v i d i n gt h e t i r r r cc o n s t l n t< l ft h e i n d i c a t i n g c i r c u i ti s l a r g ec o r n p u r ew ' t li t h t l t 0 t i r r r eb e t w e e nc o u n t f l u c t u a l i o n s l. t c l n b e s h o w t rl h a t t h ec o u n tw i l l changelirr a charrgcin lrcightol'u quarterof a w a v e l c n g tohl t l r c r . l ' . A t t l r el ' r c q u e n cuys e df o r r a d i oa l l i r r r c t c ri .st( l u i t r t cor l ' a w a v e l c n g tilsr l e s st h a n I i n . l n d l r c n c cr r r r l c sl lsy i r r go v e ' vr c r ys n t o o t h s u r l a c etsh c l l L r c t t r l t i r r g l d i o l r c i g hw t ill cause a v e r a g i nogr r l o l ' t h c e r r o r . l ) e . l i b e r a t ewl yo b b l i n gt h e

phaseof the modulation frequencywill alsohavethe desiredeffect providingthe wobbulation rate, say l0 Hz, canbe filteredout. lf a frequencydiscriminatoris usedto measure frequencythe steperror is not presentsince is continuousratherthan discrete.With measurement conventionalf.m.c,w.altimeters. however.the ranse of frequenciesto be measuredis largeand discrirninatorcircuitssufficiently stableand linear are difficult to achieve.With a constantdifference frequencyf.m.c.w.altimetera discriminatorcan easilybe usedto detect the smallchangeswhich occur in l7r. The receivedsignalstrengthvarieswith the height ofthe aircraft. From the radarrangeequationthis variation is as the fourth power of the range;however with an altimeterthe greaterthe range(height) the greaterthe areaof the target(ground)is illuminated. For radio altimeterstherefore.the variationis as the squareof the range.The gainfrequencycharacteristic of the differencefrequencyamplifier(f.m.c.w.) should be suchthat the higher frequenciesare amplifiedmore than the lower. Sucha characteristic helpsby reducingthe dynamicrangeof the frequency-measuring circuitand alsoreduceslow frequencynoise. Part of the two-waytraveltime is accountedfor by the aircraftinstallationdelay(AID). Since,ideally, the radio altimetershouldreadzero feet cln touchdown,the residualaltitudewhich accountsfor the AID is madeup of cablelength,multipliedby a factor (typically l'5) to allow for propagationspeed, and the sum of the antennaheights.It is important that the systemis calibratedso asto allow for AlD. One method which hasbeenr.rsed for an altimeter utilizedfor blind landingis to measurethe difference frequencyon touchdownon a numberof flight trials of a particulartype ofaircraft. The frequencyarrived at empiricallycanbe injectedinto the difference frequencyamplifieron the benchand the setcan then be adjustedto givea zero feetoutput. A mole commonmethodis to calibrateby cuttingthe antennafeedersto an appropriatelength;this will be describedunderthe heading'lnstallation'. Transmitter-receiver leakagecan be viewedasnoise which limits the receiversensitivityand alsomay transmit causean erroneousreading.Useof separate will givespaceattenuation,a and receiveantennas figurein the regicnof about75 dB shouldbe aimed e l t i m e t eor u t p u t i s f o r i n a n i n s t a l l a t i ow n here,tha usedin critlcalsystemssuchasautomaticIandingand g r o u n dp r o x i m i t yw a r n i n g( g . p . w . s . ) . Signalsoccurringat nearzerorangearecausedby reflectionsfrorn landinggearand other appendages, 191

(al

(cl

Tx

Multipath

r.caraf \ \ \

t Time (pulsed). Freq.(f.m.c.w.)

performance Fig,I1.3 Somefactorsaffecting aswell as the leakagesignalreferredto above. In pulsedsystemsand lowest frequencytracking (spectrumfiltering)in f.m.c.w.systemsis usedto both constant-difference frequencyand pulse altimetersa trackingloop is employedto follow the retain accuracy. changesin altitude. Initially, the altitude will be The altimeter can be designedwith a responsetime unknownso a searchmode is enteredwhich,while in the orderof a few milliseconds; howeverone does seekingthe correct altitude, will vary receivergain. not normally wish to follow the snrallestvariationsof As previouslydiscussed, at low altitudesthe gainwill the groundbelow. The output is usuallyfilteredwith be low, thus while searcfuing in the frequency a time constantof saya few tenthsof a second. (f.m.c.w.)or time (pulsed)regionof the unwanted signalsthe largegain reductionensuresthey are weak. The groundreturnis relativelystrongensuringlock-on Block Diagram Operation to the correctsignaland henceindicationof the actualradioaltitude. As alreadymentionedtherearethreemain approaches Multipath signalsarisesincethe first-time-around to radio altimeters.Most altimetersareof the f.m.c.w, echowill be reflectedfrom the airframeback down type, the majority of thesebeingconventionalfor the to the groundand returnasa second-time-around sakeof simplicity. Althoughtrackingf.m.c.w.and echo. Whilethis multipathsignalwill be considerably pulsedsystemsare more complex they do have weakerthan the requiredsignalthe height-controlled advantages overconventionalf.m.c.w.aswill be gainwill in part nullify this favourable situation. In from the previousparagraphs.Simplified appreciated trackingaltimetersthe initial or subsequent search block diagramsfor the threetypeswill be considered. canbe in the directionof increasing altitudeso lockingon to the correctsignalfirst, a similar Conventionalf.m.c.w. Altimeters approachto outboundsearchin DME. The transmitterin a modernequipmentcomprisesa Aircraft pitch and roll will meanthat the beam solid-stateoscillatorfrequencymodulatedat typically centreis no longervertical;howeverif the beamis 100-150Hz rate. Whilemost of the power fairly broad,at leastpart of the transmittedenergy (0'5-l W) is radiatedfrom a broadlydirectional will take the shortestroute to the ground. Provided antennaa smallportion is fed to the mixer to beat receiversensitivityis adequatetherewill be sufllcient with the receivedsignal. energyreceivedfrom the nearestpoint for accurate The echois mixedwith the transmittersamplein a measurement. strip-linebalancedmixer to producethe beat A consequence of broad beamsis that in flying frequency.Useo{ a balancedmixer helpsin reduction overroughterrain,reflectionswill be receivedfrom of transmitternoisein the receiver.The gainof the anglesother than the vertical.Sincethe non-vertical wide bandbeat frequencyamplifierincreases with pathshavea longertwo-waytraveltime the spectrum frequencyto compensate for the low signallevelof of the differencefrequencywill be spread(ground (highaltitude). Signallimiting the high frequencies diffusion). The spectrumshapewili be steepat the removesunwantedamplitudevariationsand givesa low frequencyend corresponding to the correci suitablesignalform for the counter. altitudemuch the sameasthe pulseshapein a pulsed frequency-measuring circuit A cycle-counting systemwill havea steepleadingedge,(see Fig. I l.3b providesa d.c. signalto the indicator. Basically and c). This spectrumwideningis increasedby suitableswitchingcircuitscontrol the chargingol'a aircraftroll and pitch. Leadingedgetrackingin capacitorso that a fixed amountof chargeis 192

Fig. I1.4 ConventionalFMCW altimeterblock diagram

Ft. | 1.5 Constant difference frequency FMCW altimeter block diagram pnerated for each cycle (or half-cycle) of the

rmknownbeat frequenry. With the simplesttype of indicatorthe total chargeper second(current) is hdicated on a milliammeter calibratedin feet. Condant Differencc Frequency f.m.c.w. Altimeten This approachis similar to a conventionalf.m.c.w. dtimeter at the r.f. end. The beat frequency anplifier is a narrow band with gain controlled by &e loop so that it increaseswith altitude. A tracking discriminatorcomparesthe beat frequency,/6, with ainternal reference,fr;if the two are not the same, tr crror signalis fed to the loop control. The outputs of the loop control circuit are usedto -t the modulator frequency,f^ (or amplitude if &viation, Al is controlled), to set the gain of the anplifier and to drive the indicator. The changein f- (or A/) is such as to make/6 = /' . Obviouslyany dage in height will lead to a changein/6 and

consequentloop action to bring/6 back to the requiredrate;in doing so the indicator feedwill change.lf f6 andf, are far removed,searchaction is instigatedwhereby the modulator frequency(or A/) is made to sweepthrough its rangefrom low to high until lock on is achieved. hrlsed Altimeters A simplifiedblock diagrarfiof a typical pulsed altimeteris shownin Fig. I 1.6. Suchaltimetersare manufacturedby, amongothers,Honeywell;the figuresmentionedin the descriptionof operation which follow arefor the Honeywell7500Be series. A p.r.f. generatoroperatingat 8 kHz keysthe transmitterwhich feedsthe antennawith pulseof r.f. of 60 ns duration and frequency4300 MHz. The radiatedpeakpower is about 100W. A time reference signal,/6, is fed from the transmitterto initiate a precisionramp generator.

19:t

reliablesignal(tive or six pulses)is receivedwhen the track loop becomesoperational. Tx During track the overlapofthe track gatepulse and videopulsedeterminesthe magnitudeof a current which is comparedwith a reference(offset) current. to Wherethe two currentsareequal,the output of the rate circuit (integrator) is zero, otherwisea positiveor negativevoltageis fed to the rangecircuit. The range circuit (integrator) adjustsits output voltage,Vp, if Ramp its input is non-zero. Since I/p determinesthe timing of the track gatepulse any changein /p will cause I I the previouslymentionedoverlapto alter until such 1r6ck gate | pulse time as the loop is nulled,i.e. overlapcurrent= | I N reference current. Any changein height will Video rcturn thereforeresult in a changein Zp to bring the loop back to the null condition. Automaticgaincontrol (..g.c.)and sensitivitytime Fig.I 1.6 Pulsealtimeter blockdiagram control(s.t.c.)are fed to the receiverwherethey control the gain of the i.f. arnps. During searchthe The ramp voltageis comparedwith the range a.g.c.circuitsmonitor the noiseoutput of the receiver voltage,/4, which is proportionalto the indicated and adjustits gainso asto keepnoiseoutput constant. height. When the ramp voltagereachesVp atrack The s.t.c.reducesthe gainof the receiverfor a short gatepulseis generatedand fed to gateB and an time, equivalentto say50 ft after transmission, and elongatedgatepulseis fed to gateA. The detected then its controldecreases linearlyuntil a time video pulseis also fed to gatesA and B. A further equivalentto say200 ft. This actionprevents gatepulseis fed to the a.g.c.circuits. ircquisition of unwantedsignals, suchasleakage, Unlessa reliablesignalis detectedwithin the duringthe searchmode. elongatedgatepulsethe trackisearch circuitwill During track the a.g.c.maintainsthe video signalin sigralthe commencement of a searchcycleand break the a.g.c.gateat a constantlevel. This is importantto the track loop by removingits reference currentfeed.. ensureprecisetracking of the receivedsignalsinceany During searchthe searchgeneratordrive to the range variationin amplitudewould causethe areaof overlap circuitensuresthat I/p, startingfrom a voltage to track gateand videosignalsto change.At low representing zerofeet,runsout to a voltage heightson track the a.g.c.reducesthe receivergain, representing 2500 ft. The searchcyclerepeatsuntil a so helpingto avoidthe effectsof leakage.Whenthe

-JI||l--

r

194

l

\

heightincreasesthe leakagesignalis, of course,gated out. givingtime discrimination.

height readingof the radio altimeter it is likely that therewill be a signalfailure detected. Sincethis is due to attenuationbecause ofexcessiverange,and not a failure or degradationof the altimeter,it is desirablethat no warningof failure is given and that Monitoring and Self-Test the pointer on the indicator is parked out of view. A cruise-monitoring circuitmay be incorporated The integrity of the radio altimeter output is vital, particularlyin automatic landingapplications. Circuit which, usinga delayedand attenuatedfeed between transmitterand receiver, checkscontinuing redundancyand comparisonis an effectiveway of dealingwith the problem. For exampletwo separate satisfactoryoperationin the absenceof a detectable receivedsignal.Any warningto the autopilotmust dtitude-measuringcircuits may accepta feed from not be affectedby cruisemonitoring;it shouldbe the mixer and independentlyarrive at the aircraft's activewheneverthere is a lossof r.f. or when any height. Should the two heightsbe different by more than an acceptableamount, a warning signalis sent to other failureis detected.Contraryto the above, many radioaltimetersreactto a lossof signalby the indicator and any other systemsto which the parkingthe pointer and displayingthe flag. height inlormation is fed. A disadvantage of using The antennaand feeder,if not properlymatched redundancyand comparisononly is that no attempt ii to the transmitter,will give rise to a reflectionwhich madeto eliminatethe causeof failure, so information may causeproblemssinceit will be delayedwith is lost. respectto the transmittedsignal.A directional Self-calibrationis an approachwhich is able to couplingcircuit may be usedto monitor the reflected compensatefor small errors. If a part of the v.s.w.r. and so givea warningof excessive signal transmitteroutput is passedthrough a precisiondelay The monitoring and self-calibration circuits vary line the resultingsignalcan be usedto provide a 'check a checkis height'. For examplein a conventionalf.m.c.w. greatlyin detail and in how comprehensive systemthe delayedtransmittersamplemay be mixed carriedout. All, however,on detectinga failurewill providea warning signalto operatea flag in the with an undelayedtransmittersampleto give a beat indicatorand a similar,but usuallyseparate, warning frequencywhich may be comparedwith a suitable signal to other systems, dependent on radio altitude frequency. lf the two frequencies are reference information. In particularif a tie-inwith autopilot different the modulatingsignalis adjustedto bring the warningsignalwill control them in line; for example,if the beat frequencyis too hasbeenestablished, within the autopilot systemwhich an interlock circuit A/ by low, equation( I I . I ) tellsus that increasing prevents erroneous information dictatingthe flight will, the modulating signal amplitude in increasing providelatchedindicator turn, increasethe beat frequencyas required. Similar path. Somemanufacturers ideasmay be appliedto constant-differencefrequency lightson the front panelof the transmitter-receiver which give an indicatioq of the areaof failure. f.m.c.w.and pulsedaltimeterswheresuitable A self-testfacilityis usuallyprovidedwherebya parametersare adjustedasnecessary. delayline, ideallybetweenantennas, is switchedin, A self-calibrationloop suchas describedabove thusgivinga predetermined readingon the indicator. may operatecontinuouslyusingwhat is essentially A disadvantage of sucha facility is that it introduces redundantcircuitry. Howeverin equipmentwhere devicessuchasco-axialcablerelays the altitude is measuredby meansof a loop, suchas a electromechanical which are,of course,somethingelseto go wrong. It servoedslope(controlledf^) f.m.c.w.system.we may be arguedthat checkingthat the readingon the wherethe may havesequentialself-calibration groundbefore take-off is somespecifiedIigure near measuringloop is switched,perhapsthree times per zerois adequate, but nevertheless, someform of mode. second.into self-calibrate in-flighttest facilityis usuallyrequired. Checkingreceivedsignalquality is a featureof On pushingthe self-testbutton, providingthe most altimeters. In a pulsedsystemthe presenceof detectedreceivedpulsesin a gatepulseis feasible;in a equipmentis operatingcorrectly.the failurewarning frequencyaltimeter the presence output shouldbe activeso causingthe warningt'lagto constant-difference of a spectrumcentredon the requiredbeat is checked. appearon the indicatorand,moie important. preventingthe autopilotutilizingradioaltimeter With a conventionalf.m.s.w.altimeterone cannot an interlock information. As an extra safeguard. checka particularpart of the time or frequency shouldbe providedso asto preventself-testoncethe domainbut signalplus noiseto noiseratio may be autopilotor any other systemhasbegunto makeuse monitored. of the radioaltimeterheightoutput. Whenthe aircraft is flying abovethe maximum 195

Indicator As mentionedpreviously,a milliammetermay be used to indicateheightbut an altemativeis a servo-driven pointer. A decisionheight(DFI)facility is also provided.The pilot setstlle DH bug to the required heightreadingand in doingso determinesthe voltage V1, fed to a comparator.The other comparatorinput is a d.c. analoguealtitudesignalwhich if lessthan Iz4 w i l l c a u s et h e D H l a m p t o l i g h t ,s o w a r n i n gt h e p i l o t that the aircraliis flying below the DH setting. A b l o c kd i a g r a mo f a n i n d i c a t o irs s h o w ni n F i g . I 1 . 7 . whereisolationamplifiershavebeenomitted for simplicitv.

usedparticularly laterally to avoid roll error in which caseleadingedgetracking(pulsed)or spectium-filtering (servoedslopef.m.c.w.)must perform adequately. The antennasmust be mountedsufficientlyfar apart to avoidexcessive leakagebut not so far apartasto producea largeparallaxerror at touchdown,a spacing between20 in. and 8 ft may be required. lf the spacingbetweenantennas,mounted longitudinally,is 8 ft and the midpoint of the line joining the antennasis, say,7 ft abovethe groundon touchdown,then half the shortestdistancebetween the antennasvia the groundwill be (42 + 72)o'sx g 11 givinga parallaxerror of I ft on landing. This may be takeninto accountwhen calculatinethe residual

t

:

l E a-L-$oH set \ E Comparator DH bug or index Differential amp

Fig. I 1.7 Simplified servodriven indicator

Indicator

Installation

--TSelf

lFla( FigureI 1.8 illustratesa singleradio altimeter test I installationshowinginterfaceand selectionlinks. Supply Co-axialfeederspassr.f. energyto and from the Transmitterreceiver transmitand receiveantennasby way of separate ato switches.Whenself-testis activatedthe transmitted L r n K sfl M OD energyis fed to the delayunit whereit is attenuated anddelayedbeforebeingfed back to the receiver. For a particularinstallationfeederlengthand delayis known so the correctreadingon self-testmay be and enteredin the pilot checklist and calculated functionaltest procedure. The antennasarebroadlydirectional,flush-mounted hornsoften beingemployedgivinga beamwidth Fig.I1.8 R a d i o a l t i m e t e r i n s t a l l a t i o n betweenabout 20o and 40o. Broaderbeamsmay be

196

To other systems: autopilot, GPWS, flight director

Fig. I1.9 ALA-5lA (courtesyBendixAviohicsDivision)

DH ind.

lrt Lamp\

Alt display tape DH Select -_''Buq

Flag

DH ref. symbol

AlC ret. symbol

DH tao

Mast(_

DH adjust and self test button

./

oH select/' Test Knob

FiE I l.l0 KI 250indicatorfor usewith KRA l0 radio (courtesy KingRadioCorp.) altimeter in respectof leading altitude. Good performance edgetrackingor spectrumfiltering will make parallax in antenna error worse. A further consideration reflectionsfrom positioningis avoidingexcessive in this suchasthe undercarriage; protuberances respectsidelobe energymust be kept low (say 40 dB down). The transmitterreceivermust be mountedwithin

Fig. ll.ll

Typical moving vertical scale indicator

reachof the antennarsincethe feederlength is critical. Sincethe transmiiteris relativelylow power, coolingis not a severeproblembut forced-aircooling energy ableto copewith up to 50 W of dissipated may be required.The powersupplywill be I l5 V a'c' 4OOHz if the equipmentis to ARINC specifications but 28 V d.c.equipmentswill be found. Aircraft Installation DelaYAircraft installationdelay(AID) is the elapsedtime 197

betweentransmit and receivewhen the aircraft is at touchdown,and is due to the delay in the feedersand the height of the antennasabovethe ground. In order that the indicator will readzero feet on landins the installationmust be calibrated. Variousmetho-dshave bien used,by far the most common being that laid down in ARINC 552,{; AID is definedby the formula:

interchangeability of transmitter-receivers between different installations. In practiceon a new installation,havingdetermined suitablepositionsfor the antennas, a minimum cable length for feasiblet.r. location will be found. Equation( I I .4) cannow be usedto decidethe AID and to calculatethe cablelengths.ARINC 552A and, installationmanualsprovidea AID = P+K(Ct+Cr) ( r 1 . 4 ) usually,manufacturers' graphfrom which cablelength can be readoif. Ar un where: . e x a m p l ec o n s i d e r P =l 0 f t , m i n i m u mt o t a l c a b l e P is total minimum path length between transmit l e n g t h =l 0 f t a n d K = 1 . 5 .W e h a v e and receiveantennasvia the ground when the P + K (Ct + Cr) = 25 ft so the 20 ft AID cannot be aircraft is in the touchdownposition (minimum used. If we choose40 then total cablelenethis path length is specifiedto avoid parallaxerror); (40 - l0)/l .5 = 20 ft, whereaswith 57 *.luu. K is the ratio of the speedof light to the speedof 3l'3 ft. One shouldbe careful,when usinga graph, propagation of the co-axialcable(typically 1.5); to ensurethat it corresponds to the type of cable C1is the transmitterfeederlength; (RG - 9/U in ARINC 552A) and further beingr"rsed C, is the receiverfeederlength. check the axeswhich may be total cablelength or (AID is not in fact aircraftinstallationdelay eachcableand total path or antennaheight(each sinceAID is an elapsedtime whereasthe cableys.antennaheightin ARINC 552A). right-handsideof ( I I .4) is in feet. A more accurateterm would be residualaltitude.)

Calibrationis achrevedby cutting the cablesto a lengthwhich givesan AID of 20, 40 or 57 ft (figures of 40,57 and 80 fr arequoted in draft proposaisfrom ARINC in 1978). The transmitter-receiver is bench calibratedfor the 57 ft AID standard.Groundingone of threepins on the t.r. plug by meansof a jumper externalto the unit selectsthe appropriatezero-bias adjustmentto give the 20,40 or 57 ft AID as required. The result of choosingthis method is

(J

+ o

P-------> Fig.ll.l2 198

An AID calibration chart(& = 1.5)

Interface Wereit not for the usewhich is madeof radioaltitude informationby other systems, it is doubtful whether many civil aircraftwould carry radioaltimeters.The outputsavailableareheigirt,rateof changeof height, trip signalsand validity (l1agor warning)signal. Some of thesewill be fed to the autoland/autopilot system, the g.p.w.s.and a flight director. Most frequentlyusedarea d.c. analogueof aircraft heightwherevera systemneedsto continuously monitor radioheightand,essential, a switched, lail-safevalidity signal(invalid low). The rate signal, i.e. rate of changeof height,may be derivedin the systemutilizingthe heightsignal,but if providedwill take the form of a phase-reversing a.c.analoguesigral (ARINC 552A). The trips areswitchabled.c. voltages, switchingtaking placewhen the aircraft transits througha pre-setheight,the DH bug is sometimes calleda pilot set trlp. Againtrip signalsmay be generated in thosesystemsusingthe heightanalogue sigrral. Autolandor blindlandingsystemsmust haveradio heightinformationwhich will be usedto progressively reducethe gain of the glideslopesignalamplifier (not radio)in the pitch channelafter the aircraftpasses overthe outer marker,and will alsobe usedto generatetrip signalswithin tlte autoflarecomputer. The followingis a brief summaryof eventswith radio heights: 140 ft (a) radioaltimeterinterlockswitchedin;

(b) changes in response to glidepathsignals; 120 ft (a) preparatoryfunctions; 90 ft (a) check 140 ft operation: 50 ft (a) glidepathsignaldisconnected; (b) throttle closureinitiated; (c) pitch demandmaintainscorrectdescent rate; 20 ft (a) rudderservodisconnected; (b) aileronscentred.

used. Whetherdual or triple installationsare used dependson the probabilityofan undetected degradation; consequently dual radioaltimeterswill only be usedwhere monitoring, self-calibrationand redundancyare deemedsufficiently comprehensive and reliable. As soon as'onefits more than one radio altimeter to an aircraftthe possibilityofinterferenceexists. If. the number I systemwere to receivea leakagesignal from number 2 a falseheight readingmay result. Also This sequenceis applicable,for example,to a sincethe antennasarebroadlydirectionaland all -l BAC I I series500 aircraft usingan Elliott series facingdownwardsthe echo from number I will be ll00 auto touchdownsystem.The 140and 90 ft receivedby number 2 (and 3) and vice-versa. trips are fed from the radio altimeter;the othersare Various safeguards areemployed. Minimum generated in the autoflarecomputer. coupling may be achieved by separating the pairsof The g.p.w.s.needsradioheighttrips on all modes antennassufficiently (at least8 ft, say) and possibly profile sincethe changes abruptlyat differentheights. ensuringthat the E fieldsof adjacentpairsare at right ' Mode2 operationisexcessive terrainclosurewarning and so dependson the rateof changeof radio height. angles(seeFig. I l.l3). As a iurther precautionmultiple-installation The trip signalsand the rate signalwill normally be altimeters will employ different modulation generated within the g.p.w.s.using validheight frequencies.As an exampleof how this helps, irrformationfrom the radioaltimeter. considerFig. 11.14wherewe havetwo altimeters,one Somemulti-functionflight directorinstruments operatingwith a modulation frequencyof 100 Hz, havea risingrunwaysymbolwhich movesup to meet an aircraft symbol as the aircraft descendsto touchdown. Operationis typicallyoverthe last 100 ft. The verticalmovementof the rising runway t l dependson the height analoguesignalfrom the radio --rj t Faltimeter,while its lateralmovementis controlledby t t l l-+: o the ILS localizeroutput. Failure of either radio c fb iA o { f ' c altimeteror localizercauses the risingrunway to be o obscuredby a'RUNWAY' flag. r I

IOO MHz

Multiple Installations All-weatherlandingswill only be safeif information fed to the autolandsystemis reliable.To achieve reliabilityof radioheighta multipleinstallationis

til

i

til >20in No. I

E

l.- *l

>20 in No. 2

1/105 1/100 F i g . I l . l 4 D u a l - i n s t a l l a t i o nm o d u l a t i o n l i e q u e n c i e s (100 and 105 Hz)

Er-

m

til

-l

>20 in

F

No. 3

Fig. I l. | 3 Triple-instaltation aerialarrangements

199

..]:GJ,:

the other 105 Hz (e.9.CollinsALT 50). Assumethat (b) Two trips: singlemake contactsswitchingsupply at one instancein time 're' in one of the receiverswe from userequipmentbelow pre-setheight. havetwo signalsboth at fc F 4300 MHz) and both Adjustable0-2500 ft t 6 per cent and increasingin frequency;one at 100 Hz rate,the other 500-1500ft I 6 per cent. at 105 Hz rate. One-hundredth of a secondlaterthere will be a non-zerobeat frequencyf6 glen by rateof Optional (additional to above) (a) Synchrooutput representingheight to be changeof frequencyof the most rapidly changing signalmultiplied by the time lag of the slowesr. So: employedfor displaypurposes.Sameaccuracyas (a) above. = f n ( L f x 1 0 6 x1 0 0 x 2 ) x ( / ) (b) Altitude rate 400 Hz phasereversing,200 mV = ( 1 0 0x 1 0 6 x 1 0 0x 2 )x ( l / 1 0 0- l / 1 0 5 ) 1 0 0f t - r m i n - l = 9'5MHz. Accuracy:greaterthan t 20 f.p.m. or f l0 per Thus after one cycle of f^ the interferingbeat is well c e n tu p t o 5 0 f t ; t 3 0 f . p . m .o r t l 0 p e r c e n t out of rangeof the differencefrequencyamplifier 50-500ft. (c) Additional trips: three at 0-200 ft t 3 per cent t bandwidth. The beat will changeat a 5 Hz rate, rcachinga nraximumof 100 MHz. 3 ft; two at 0-500 ft t 3 per cent t 3 ft; one at The different modulation frequenciesare selected 1000-2500ft t 6 per cent. i jumper in a sinrilarway to AID, i.e. by a between appropriatepins,the jumper beingpart of the fixed installation.A similartechniquefor pulsealtimeters Ramp Testing and Maintenance could be to employ sufficiently different p.r.f.s to 'height' ensurethat the changedue to an,interfering It is importantto stressthat due to the extensive pulsewould be at a rate fast enoughto prevent interfaceit is essentialthat the radio altimeteroutputs lock-on by virtue of the altimeter time constant. are compatiblewith thosesystemswhich it feeds. If we alsoconsiderthe programpins usedto select modulation frequencyand AID and, further, critical Characteristics feederlengths,it is clearthat replacement of units or partsof the fixed installationmust only bd carried The following are selectedand summarizedfrom out when completecompatibility,both internaland A R I N C5 5 2 4 . external,hasbeenestablished. A functional test on the ramp is quite straightforward. Input I Output r.f. Coupling The radio altimetershouldreadnearlyzero feet when 50 f,) RG-9/U (or equivalent)co-axialcable. switchedon. If the antennasaremountedforwardof Cable+ antennas.w.r.lessthan l.l : I over frequency the main wheelsthe readingwill be lessthan zero; rcnge4210-4390MHz. if aft of the main wheelsgreaterthan zero. The flag shouldclearshowingthe r.f. path is not brokenbut Altitude Range not provingthe loop gainis sufficient;attenuation From 2500 ft to a'few feet'belowtouchdown. shouldbe introducedto checkthis but is unlikelv to be calledfor. Loop Gain Whenself-testis operatedthe correctreading Sufficientto ensureproper operationup to 2500 ft shouldbe obtainedand the flag should appear. While assuminga total feedercablelength of 30 ft of keepingthe self-testswitch pressedthe DH bug may RG-9/U,a groundreflectioncoefficientof 0'01 and be adjustedfrom.a higherto a iower readingthan the with an additional9 dB loop gain for contingencies height pointer, the DH lamp being first lit and then (e.g.longeror differenttype ofcable). the pointer. asthe bug passes extinguished Special-to-typetest setsare availablewhich allow Outputs variationof simulatedaltitude:this is usefulfor checkingtrip signals.On somealtimetersoperating Basic the self-testcausesthe pointer to sweepagaindue to a (a) d.c. altitudeanalogue:V = 0.2h +0.4 below variationin simulatedaltitude. 480 ft; V -- l0 + l0 In ( (/, + 20)/500)above 480 ft (ln beinglog to the basee). Accuracy:greaterof ! 2 ft or 2 per cent up to 500 ft, 5 per cent thereafter.Time constant0.1s. 200

Appendix

Sinusoidal Frequency Modulation

Alsozlz= Lf l2f^, whereA/is total range'od7 / l frequencyvariationso: vl,,= kV1V, sin (nAlI + 2nfrT)

cos(2nf^t * n f*T) (A I I .4)

If the carrier Vs sin 2nf.t is frequencymodulatedby The beat frequencymay be found by differentiating a sinusoidalwaveform, V* sin 2rf^t, then the the argument(angle)in (Al 1.4)with respectto time output of the transmitter,vs,and the receivedsignal, and dividing by 2z to give vr, are givenby: fn = -((n LfT)(2nf^) stn(2nf*t - nf^T))l2r = nLfTfln sin(2nf^t -nf*T+n) vt = Vtsin(2rfrt +rnsin2nf*t) (All.l) (All.5) rr = Vr sin (2nf, (t - f) + m sin 2nf* (t - T)) Note the minus sign resultingfrom the differentiation ( A l 1 . 2 ) of the cosineterm hasbeenreplacedby a phaseshift of n radians.The average beatfrequencyoverhalf a where ?nis the two-way travel time and m is the modulatingcycle,l 12fm, is: modulationindex (constantin this application). If receivedand transmittedsignalsare fed to a 7n= 2f* Ullrr^1a,1 multiplicative mixer we have, after some = r LffmT cosnf^T (Al l_6) manipulation,a differencefrequencysignalof: AgainsinceT 4 | I fm, cosn/rrI= l, so: v1 = kV1V, sin(2m sin(nf*T)X 7n o 2Lff^r cos(2nf*Q - fl2)'1 + ZtrfrT) (All.3)

= 4Lff^Hlc

wherek is a constant of proportionality. Since T is much smallerthan I I f^ we may write:

(Al1.7)

This is the sameasequation(l l,l) derivedassuming a linearmodulatingwaveform.

sin nf*T x f^T

201

12 Area navigation

controlled airspaceis lacking in detail but is sufficient to make clear the disadvantages.With heavy traffic we havemany aircraft occupying a relativelysmall Beforeradio aidswere available,pilots navigatedby in particularthe scheduled visualcontact with the ground and were responsible proportionof the airspace, the airways. To free aircraft in aircraft air transport for their separationfrom other aircraft. With the a navigationsystemwhich " one needs airways the from adventof radio and improvedinstrumentationit area,hence area a large used over be safely can becamepossibleto fly in situationswhere the ground tied to fixed points, irrevocably and not navigation, could not be couldno longerbe seenand separation on the ground. But one beacons, VORTAC as such guaranteed.From suchbeginningsthe need for airon ,',cango too far; the thought of aircraftconverging navigationaids and controlled and ground-based from all directionsis frightening.Whatis an airport regionsbecameapparent: 'dog legs' is new airwayswhich can remove In the 1930sthe airspaces':rroundingcertain busy needed thus shorteningroutesand ones, parallel existing and controlledzoneswith airportsbeganto be designated times. relatingto weatherand to qualifications, flight restrictions, benefitsof areanavigationare not alwayseasy The placedon thosewho wantedto enterthe zone. With possibleto realize. For examplein the United even or growth of a the growth of air traffic has come the the areasmost usedby air traffic of the Kingdom system. worldwidecontrolledairspace 'shape'ofthis transport type is largely coveredwith scheduled was controlledairspace The areasand zonesalreadyand an control airways, influencedby the introductionof one of the earliest area navigationwhich will benefit to extension ground-based navigationaids,radio range. This to achieve. However,in difficult is equipment,introducedin the 1930s,gavefour beams economically regionssuchasBritain, busy small but geographically which suitably equippedaircraft could follow. Beam is an extremelyusefulbool equipment navigation area flying wascontinuedand intensifiedwith the adoption aircraft which general aviation of number large to the a system of controlled VOR and so of confirmed in uncontrolled and the airways beneath potter around airwayswhich radiatefrom ground stations. The but with possible before, was generally. It airspace airwayslink control areaswhere a number of airways easier. much now it is equipment correct the convergeover c€ntresofhigh-density traffic. Neither the airwaysnor the control areasstart at ground level, asdo control zoneswhich arecentredon one or a groupof airports. Generalized Area Navigation System The airwavs.control zonesand coritrol areas constitutecontrolledairspacewithin which instrument Area navigationequipment is not new although the flight rules(lFR) are in force. Only.instrument-rated acronym RNAV. is fairly recent. In fact RNAV could havebeenimplementedin the 1950shad the choice pilcts flying aircraft fitted with a minimum for an internationalstandardbeen the Decca iquipment complementcan usecontrolled airspace for navigationalpurposesalthough,for non-conformingNavigator. Other equipmentsproviding navigation facilitiesover a wide areaand not tied to fixed points flights, i specialvisualflight rules(VF'R) clearance are Loran and Omegaof the ground-basedsystems canbe obtained from air traffic control (ATC) to enteror cross.Within controlledairspaceseparation and Dopplerand InertialNavigationSvstemsof the variety. Unfortunatelyall of these is the responsibilityof ATC, whereasin uncontrolled self-contained expensivethan VOR/DME which is more are pilot who, systems airspacei1 is the responsibilityof the use. The adventof airbornecomputers in widespread serviceif in an however,canbe givena separation navigation hasnow madepossiblesophisticated advisoryserviceareaor on an advisoryroute. systemsincludingan RNAV systernbasedon The abovedescriptionof the structureof Development of Airspace Organization

m2

R/DME. The trick is to 'shift' the position of the headingand drift angleor co-locatedbeaconsto a phantom beaconor waypoint track angleand ground speedor location chosenby the pilot. The pilot useshis wind directionand speedor VOR/DME instrumentationh the sameway as cross-track distanceand track error,etc. before,exceptthat steeringcommandsare ielatedto Analoguepresentation on HSI: a waypoint remote from the beacon. headingand track The computer power, of course,allows much more coursedisplayand setting than the generationof steeringcommandsto phantom desiredtrack beacons,severalnavigationsensorand air data lateralsteeringcommand. outputsmay be mixed to provide a meansof lateral Analoguepresentation on attitudedirector: and verticalnavigationand a display of data which pitch and roll steeringcommands. can take many forms. The all-purposesystemis Analoguemap presentation: illustratedin Fie. 12.1. route,beaconand waypointdata.

Flight data storage unit

Automatic data entry untt

Control and display unit

Navigation computer unil

Conventional instrumentation

Electronic, moving or proJected Map display

Possible sensor inputs

t

II

Aar data inputs Fig. l2.l

Generalareanavigationsystem

The computer. usingstored data and inputs from a No attempt hasbeenmadehere to give a definitive list rzriety of sensors.calculatesthe aircraft position of displayeddatasincethereis considerable variation. absolutelyin termsof latitudeand longitudeand also The datarequiredfor the computerto performits relativelyin terms of deviation tiom the desiredflight functionareof threetypesand canbe input to the path. A variety of display formats may be usedas systemin threedifferentways. For regularlyflown follows. routes'hard' datasuchaslocation,elevationand frequencyof VORTAC beaconsand airports, Digital read-outon display and control unit: standarddepartureand arrivalroutes(SIDSand presentposition, latitude/longitudeor STARS) etc. will be storedin a flight data storage

203

unit (FDSU), typically on magnetictape. Waypoint position,'soft' data.may be enteredor amendedin 'scratchpad' flight by meansof a keyboardand displayon the control and displayunit (CDU). Real-timedatafrom navigationand air datasensors arecontinuouslyavailablefor input from a varietyof sources. 'soft' the in The data relatingto waypoints are sensethat they canbe amendedbut they may be 'hard' dataon a magneticor punchedcard storedas and input via an automaticdataentry unit (ADEU). This facility is usefulsincean operatorcould havethe waypoint data for all regularlyflown routes recorded on cards,the correctone being chosenfor a particular flight. {r Sincethe information from the sensorsis in analogueform analogueto digital conversation (A/D) is necessary beforethe computercan handleit; in the areanavigationsystem be may A/D circuits itselfor in the systemswhich feedit. The form of areanavigationsystemsis by no meansfinalized,and with the varietyof inputsand outputspossibleit seentsunlikelythat functional to the sameextent as will be achieved standardization it haswith other systemssuchaslLS, VOR, ADF, eLc- Onecoukl write a bttok on those RNAV and availablenow VNAV (verticalnavigation)eqttipttrents

(1979) but herespacewill only allow a brief RNAV. with of VOR/DME-based discussion 583'l ' Future and ARINC Characteristic examples, and of microcomputers use on depend developments displaysystems(see utilizationof flexiblec.r.t.-based ChapterI 3).

RNAV PrinciPle VOR/DME-Based The basicidea is simple;signalsfrom existingVOR and DME co-locatedbeaconsareusedto giverange and bearing,not to the stationbut to a waypoint ;pecifiedby its rangeand bearingfrom the station. To achievethis the RNAV triangle(Fig. 12.2)hasto be continuouslysolved. We have: pr: distancebetweenbeaconand aircraft; 0 1: magneticbearingfrom beaconto aircraft; p2: distancebetweenbeaconand waypoint, 02: magneticbearingfrom beaconto waypoint; p3: distancebetweenaircraftand waypoint; 03: maflneticbearingfrom aircraftto waypoint. The quantitiesp1 and 0I areknown from normal VOR/DME operation.the quantitiesp2 and02 are enteredby the pilot, hencetwo sidesand an included

V O R/ D M T

-\ t'z

/

Waypotnl

Fig. 12.2 RNAV. trirnglc

204

/"

1,3

(

angleof the RNAV triangleare known, so p3 and 0 3 can be found.

h

= ((iz-xr)2 + (y, - yr)')6t

b, = i;:' rdi-i',l4,i:it wherexL = p1-sin0 p yk=pkcosok

Q2:)

k=1,2

lf (y, - y r)l(x, - x,) ) 0, 0. is in eitherthe north-eastor south-westquadrant;if Oz - y t)l@z xr) ( 0, 03 is in eitherthe north-west or south-east quadrant,if yz - y, = 0, 0g is either 0 o r l 8 0 " , w h i l e i xf z _ x r = 0 , 9 : i s e i t h e r g 0 o r 270". The ambiguitycanbe resolvedby observing that 03 will only changeby a smallamountfor successive calculations.An exampleis givenby Figure12.5wherewe have:

41!s

,,,LK Fig. 12.3 Vector solution of RNAV. triangle

01 x1 x2 pr 0r

= = = = =

9 0 . 0 2= 1 8 0 p , r = 4 0 , p 2 = 3 0 1s o : 40 sin 90 = 4O,yr = 40 cos90 = 0 3 0 s i n l 8 0 = 0 , ! z= 3 0 c o s l 8 0 = - 3 0 ( e 4 q ' + ( - 3 0 ) 2 ) o ' s= 5 0 tan-r (-30/-40) = 36.87 0r. 1 8 0+ 3 6 ' 8 7= 2 1 6 ' 8 7.

The solution of the trianglecan be found by analoguemethodsasin one of the earliestRNAV computersfor the generalaviationmarket, the King KN74. The vectorsP{0 1 un6Pzf02 ^t" representedby squarewaveslilfroseamplitirdesare If the previouslycalculatedd3 was 216 then the new proportional to p1 and gz and whosephasesrepresent 0 " is 216'87. to 01 and 02 respectively.From Fig. 12.3we see that:

P r l t = =p z l 0 z : p r l t l

( r 2 l. )

wherethe minus sigr indicatesvector subtraction. Thusif we reversethe phaseof the squarewave representingOrl0: anAadd this to the squarewave representingpz l!2 we will havea waveform the fundamentalof which represents p3 and 03 in amplitude and phaserespectively. Y axis (N)

Fig.12.5 RNAV.triangle example

(X, Y) andpolar(p, 0) co-ordinates Fig.12.4 Cartesian The solution may also be found by a digital computer. Expressions for p3 and 03 canbe found by convertingto cartesianco-ordinates (seeFig.12.4) then revertingto polar co-ordinates.Thus:

&-

The programfor the solution of the RNAV trianglecould be basedon the aboveor someother formulation of the problem. Sincethe programis fixed it will be storedin read-onlymemory (ROM). The trigonometricfunction valuesmay alsobe stored in ROM to speedup calculations.Note that complete sine and cosinetablesneednot be storedsince sin d = cos (0 -90), also c o s 0 = - c o s | 1 8 0- 0 l i f 9 0 1 0 1 2 7 0 a n d c o s0 = c o s ( 3 6 0- 0 ) i f 2 7 0 < 0 < 3 6 0 .

Thus, for example,a cosinetable for angles0 to 90 only is sufficient. A look'up table for the inverse tangentfunction is more problematicaland can be avoidedby a reformulationof the equations(12.2) to be solved,for exampleby usingthe cosinerule althoughhere ambiguity is introducedin the solution for 0 3 which is slightly more complicatedthan that arisingfrom (12.2).

DME/VOR a Mean sea level

N(m)

Fig.12,7 Slantrangetriangle RNAV. vector

Waypoint

tl3

Course deviation distance

Inbound course selected

(o8s)

Fig. 12.6' Deviationtriangle

distance(pt) *e needto solvethe slantrangetriangle shown in Fig. 12.7. The beaconelevationmust be fed into the equipmentfrom a FDSU, ADEU or by meansof a keyboard. Aircraft altitudeis obtained from an encodingaltimeter. Usingthe notation of Fig. 12.6 we have: Pr = (@r), _ (A_E)\o3 Sim.ilarcalculationsare necessaryif the systemhas VNAV capability,i.e. if steeringcommandsin both pitch and roll are obeyedthe aircraft will achievea specifiedaltitudeat the activewaypointor at a specifieddistancefrom the current waypoint.

NP-2041A BendixNav.ComputerProgrammer

Introduction The NP-2041Ais a ten waypointRNAV computer. may be enteredfrom a The waypointparameters keyboardon the front panelor from a portable reader.Bearingand distanceto the magnetic-card activewaypoint are found by solvingfirst the slant rangetrianglethen the RNAV triangle.ln addition the unit can be usedfor frequencymanagementfor both v.h.f.communicationand navigation. ( r2.3) The completeRNAV systemcomprisesan N P - 2 0 4 1 Aa, C N - 2 0 1l A c o m m . / n a vu.n i t , a '-p3sin(0"-0t) t.p DM-2030DME, an IN-20l44' electroniccourse Ifp is negativefrom (12.3) then the aircraft is to the deviationindicatorand an bncodingaltimeter. of HSI and RMI is achievedthroughan Presentation left of the desiredinbound course. For examplein IU-2016Ainterfaceunit. The abovepackagecanbe F i g . 1 2 . 6i f 0 3 = 2 7 0 " , 0 c= 3 0 6 ' 8 7 "( t o ) a n d to with an ADF and a transponder pr = 50 thenp = 50 sin 36'87 = 30 nauticalmiles complemented (a 3 ,4,5 triangle)with aircraft to right of courseby makeup a BX 2000 system.Other optipnsarea reader if 03 = 306'87" znd0c.=2700 weatherradarinterfaceand a magnetic-card 30 nauticalmiles,whereas HP-67 (modifiedTexasSR52 or Hewlett-Packard then p = 50 sin (-36'87) = -:0 nauticalmiles,i.e. aircraftowners scientificcalculator).Largebusiness aircraft to the left of courseby 30 nauticalmiles. or nearlyfull full for a possible customers be would In the abovefor accuratenavigationthe RNAV may be aircraft singlecngined while small package, plane; triangle;hould be in the horizontal c o m m . / n a va.n d V F R s y s t e m : i . e . t h e b a s i c w i t h f i t t e d the from beacon DME to the distance unfonunately an indicator. aircraft is givenas slant range. To obtain ground

Coursedeviationcalculationinvolvesthe solution of another triangleshown in Fig. 12.6. It is normal with RNAV to give the deviationin terms of distance rather than angle,at leastout as far asa specified range. Solution of the deviationtriangleis possible sinceone side,p3,.andall anglesareknown. So using the sine rule:

#=m&'r

206

RNAV.system Fig.12.8 BendixNP-2O4lA-based Although herewe are concernedmainly with the RNAV computer,a brief descriptionof the other unitswill be given. The CN-201lA (Fig. 2.2) is a panel-mounted unit containingtwo v.h.f. two transmitter-receivers, communications an audioselectionpanel, VOR/LOC receivers, glideslope receiver(optional),markerreceiver (optional)and varioussystemcontrols(a less comprehensive CN-2012Amay be usedwith the and NP-2041A).The IN-2014,{indicatoris discussed illustratedin Chapters4 and 5 (Figs4.7 and 5.3). interfaceunit The IU-20164 remote-mounted outputsto levels convertsVOR/LOC and glideslope and that satisfyHSI and/orRMI requirements performsother functionsnot of interestin this context. The DME, encodingaltimeter,HSI and RMI requireno commenthere,havingbeendealt with elsewhere.The calculatorsare simply modified to attacha plug-inconnector.

KTS/TTS: displaysground speed(BRG/KTS window) and time to station in minutes(DST/TTS window) to waypoint in RNAV or APR modd or to VOR/DME stationin VOR/LOC mode. The SBY and ACT windows displaythe number (0-9) of the standbyand activewaypoint. The 'IN' legendis illuminatedif courseshown(CRS window) is inboundwhile'OUT'legend(below 'IN' legend)is illuminatedif courseshown is outbound. The Mode selectorcontrolsthe mode of operation asfollows:

OFF: self+xplanatory. VOR/LOC: conventionalnavigation,the waypoints arethe stations. RNAV: waypointsare remotefrom the associated stations.Lrft/Right coursedeviationis linear within 100 nauticalmiles,full-scaledeflection (f.s.d.) being 5 nauticalmiles,from 100 nautical Display and Control miles out deviationis angular. Figure 12.9 illustratesthe front panel of the APR: as RNAV but linear deviationup to 25 nautical digitaldisplays NP-2041A.Therearesevenseparate miles,f.s.d.being l'25 nauticalmiles. indicators. 'TEST': specifieddisplayfor satisfactoryoperation. eachemployinggas-discharge seven-segment The quantity displayedin eachof the windows Data is enteredby meansof the keyboardor by dependson the position of the DisplaySelector magneticcardreader.Of the l6 keys 1l aredual witch and,for someof the displays,the Mode functione.e.FREQ./I , NAV.2/. (decimalpoint), etc. Selectorswitch. With the DisplaySelectorset to: Data entry must alwaysbe in the correctsequence parameters FREQ., asfollows: shown in waypoint SBY: standby B R G / K T S .D , ST/TTSE , L X l 0 0 a n dC R S l . p r e s sS B Y W P T ,F R E Q . ,C O M . l , C O M . 2 , windows. parameters for SBY. B R G ,D S T ,E L , C R S ,N A V . I , N A V . 2 ,A D F shownas ACT: activewaypoint key asrequiredto select or XPR (transponder) BRG/DST:displaysbearing(BRG/KTSwindow) and for data; appropriateaddress distance(DST/TTSwindow) to activewaypointin 2. pressnumberkeysto enterdata; RNAV or APR mode or to VOR/DME stationin window and if correct VOR/LOC mode. 3. checkdata in app.r-opriate

207

\__',,

Fig. 12.9 NP-2041A (courtesy Bendix Avionics Divrsion)

press'ENTER' key. An annunciatorlight indicateswhen a key is being pressed. As an examplethe sequencefor the entry of NAV.I frequencyis: l. selectKBD on COM.iNAV. unit; 2. setmodeselectorto VOR/LOC; 3. pressNAV.I key, ensuredot in FREQ.window flashes; 4. pressappropriatenumberkeys,e.g. 109'80, ensurereadoutin FREQ.window (scratchpad) is correct: 5. press'ENTER' key. Frequencywill be transferred from FREQ.window to NAV.I window in COM./NAV.unit within which the NAV.I receiverwill be tuned to that frequency. A lurther exampleis given by the insertion of a waypointparameter, saystationelevation200 ft for with waypoint3: beaconassociated l. modeselectorto any positionother than'OFF' or'TEST': 2. displayselectorto SBY; 3. pressSBY WPT key; 4. pressnumber key 3, ensure3 appearsin SBY u,rndow: 20a

5. pressEL key, ensuredot in EL X100 window flashesl 6. pressnumberkeys0 and 2 in that order,ensure 02 appearsin EL Xl00 window (scratchpad); 7. press'ENTER'key. i.e. FREQ.,3RG, The other waypoint parameters DST, CRS may be enteredin a similarway. To enter outbound coursewe pressthe CRS XFR key having previouslypressedCRS key and enteredinbound course. Two keys not previouslymentionedareWPT XFR, which transfersSBY waypoint number to ACT, and LPP,which loadsthe presentposition of the aircraft into waypointzero. Block Diagramdperation Microcomputer The heart of the NP-2041Ais a which comprisesa central processor microco-rnputer unit (CPU),systemcontroller,ROM, RAM, system clock and hput/output (l/O) ports. The CPU is an the associated chips 8080A 8-bit microprocessor, being drawn from the same8000 seriesfamily e.g. 8224 clock generator8228 systemcontrol and bus peripheralinterfaceetc. driver,8255 programmable The microcomputeracceptsdata in a suitableform

fRot{t PAr{tt0tSPLAYS

0tsPtAY 0ArA coitIR0t

CARDRTADTR INPUT

M I CROCOM PUITR

v(n mf lilPUl

PARAtttt I U NI N C DAIA

|Il v^t NPUT

ll,AY POllil 8 I A RI N G IURI

t I N I A RI Z T D WAYPOINT v A RI A8 r t

t?.t9 Np-2041Ablock diagram(courtesyllendir lq. AvionicsDivision) from the keyboard. selection-switch,carrl

reader, nav. receiver, DMEreceiver anoatti_ei.r.-ii"ln"o", a",, long asa memoryhold voltage(external)is maintained.In additionthere is subjected to arirhmeric onatugj.oi'oo.r;tii3", is 2K bits of storein _o thenoutputin a suitablef*;;i"",h.. *o]r whie-h provid.,, ,".;;;;riJlug" ro, itoi,'Bra, l:li,:1. oara trom the CpU. disranceto comm./nav. andInterlaceunits.Theoperations *aypoini. L."rringto on waypoint,qtc. Volatii dataareperformedby rr,r.cpti-*iri.r, lost(
"t,i*n.,'i"." ild;:i:i'*:f?f int"iu"

menrory ofe6Kbits(e6x l0l4 = ,816;

The card-readirinterfaceraises the signallevelto that suitablefor TTL operation,"d s;;;;;;; rn'i"irrrrp, signalw-hichcauseithe microprocessor basic ;fdsi,.'**OV service loopsVOR/LOC, to break the l"o service loop in prosresswhile it ,.;;;; APR..The totatnon-votatile [eM i, zriiir' i"ii*onr. Datafronr tne VOR receiver r,?.rage for is in rhe form ol two the. te rs fo gar1m9 r I 0 way poin lir^"td_t-lg. rs. constantamplitudesquare. Inedarain rhenon_volatile waves, RAMin onlyr"iuii,.O ,, phaseand variable(var.)phase. reference(ref.) Th. ;L;;;;;;;*"*

friri. rr,o storagespaceis usedfor rhe prograr's tbr the four

zop

waypoint are serially shifted out to the ECDI lbr between the ref. phaseand var. phasesignals Ospiayin the courseand distancewindow. VOR A the station. to representingthe bearing Comm./nav.frequencymanagementis achievedby voltagecontrolled oscillator(v.c.o.) is phase-lockedto parallel b.c.d.addressand dataoutputsfor tuning -OR type phase the ref. phaseusingan exclusive purposes. comparator. (If two squarewaves,of equal frequency 'l' '0' The bearingto the waypoint is fed to the ECDI are applied and phase,switchhg between and and alsothe interface unit in the form of the RNAV to an exclusive-OR gatethe output will be zero since ref. phaseand RNAV 30 llz var. phasgderived Hz 30 0@0 = 0 and I @l = 0; if not in phasethe mean level waypoint bearingoutput LSI and waypoint from the of the output will be non-zero). The ref. phase-locked d/a converter. The feed to the waypoint variable signalis fed to the var. phase-lockloop where it is from the I/O ports is in digital format. LSI bearing with the var. phase,the difference phase-compared processesthe RNAV 3QHz ref. and 30 Hz ECDI The controlling the repetition rate of pulsesfrom a second var.phasesto produce left/right deviationsignalsto integratedcircuit) v.c.o. The VOR LSI (large-scale 'bar'. The interfaceunit sirnilarlyprovides drive the countsthe pulsesfrom the variablerate v.c.o.to left/right deviation signalsfor the HSI (and possibly obtain the bearingwhich is fed to the l/O ports as a autopilot). four-digitb.c.d.numbertwo digitsat a time (since The displayintensity control setsthe intensity databus is only 8 bits wide). of the sevensegmentindicatorsand front panel level The DME distancedata input is in the form of a with the settingof the in accordance annunciators pulsepair where the time interval betweenpulses ' D I M ' c o n t r o l o n t h e c o m m . / n a vu' n i t . A c o m m o n the distance(12'36 ps per nauticalmile)' represents 'DIM' control is usedfor all units of the BX 2000 The DME ISI convertsthis time-intervalinto a systemto ensureuniform intensity of lighting. four-digitb.c.d.numberwhich, asin the caseof the VOR LSI output, requirestwo readingsfo transfer the data through I/O Ports. King KDE 566 An encodingaltimeterfeedsdatavia buffers' (interface)to setlatchesin the I/O ports. The data in 500 ft incrementssincethe C1, C2 and C4 Introduction changes linesfrom the encodingaltillleterarenot connected The KDE 566 is an automaticdatainput/output system(ADIOS) usedwith the KCU 5654 control (seeChapter8). u n i t ( F i g . 1 2 . 1 2 )f o r m i n gp a r t o f t h e K N R 6 6 5 The flag signalsof externalequipment(e.g.nav. and DN{E)aremonitoredby the flag interface.If an digitalRNAV systemillustratedin Fig. 12.11. The completesystenr,which may havemore units than invalidsignalis detectedthe flag interfaceoutput is thoseshown.will not be describedsincea systemwith computer. ports to the l/O through transferred hasalreadybeendiscussed' similarcapabilities Finally,the mode and displayselectorinterface an ADEU in any detailso not considered We position of have translatethe informationrelatingto the 566 follows. In f'act of the KDE description brief that are a logic levels the appropriateswitchesinto rather than an ADEU an ADIOS is called the unit ports. I/O the to applied sincethe magneticcardsmay be recordedby the unit usingdata frotn the KCU 5654.memoryaswell Outputs Data to be displayedon the front panel is data asprovidingthe dataentry or input function from display ports to the transferredthroughthe I/O cards. pre-recorded b . c . d . a d d r e s s , b . c . d . o f d a t a c o n s i s t s T h e control. The magneticcardsare about the sizeof a business dataand decimalpoint. The displaydatacontrol (frequency, cathode cardand can storethe waypointparameters decodesthe dataand providesthe necessary waypoint and outbound, course inbound, course gas displays' discharge and anodedrivesfor the ten The appropriatedatawill not be displayedbut will distanceand bearingfrom the beacon)for up to if a flagsignalis detected.For waypoints. A numberof cardscanbe preparedfor be replacedby dashes (from-to) can examplethe BRG/DST flag in RNAV mode will show frequentlytravelledroutes. The route top right corner and the on each card DME noted search' be flag, DME for any of the following: nav. cannot be they so that the data to fix of off 1, loss clipped with nav. paired frequency not test,DME nav.or DME input signalsor ILS frequencyselected. changed. by the computer The RNAV flagsignalgenerated must alsobe fed to appropriateexternalequipment. Block Diagnm Operation The KDE 566 operationis bestexplainedin termsof The selectedinboundor outboundcourseof the the to its modesof operationwhich aremonitor, record, distance the computer and waypoint active 210

Card slot

KCU 565A memory/control/display

DME Tune Digital distance DME dist.

KDr571 dist. spd/tts indicator

Digital course glideslopedev. and flag RNAV./VOR/LOC dev. and ftag KN 581 RMI

Fig. l2.l I King KNR 665 RNAV. system

thereis a synchronizing sequence of 6 bits set to I followedby 2 bits at 0. The datacycleconsistsof l0 X 5 X l6 = S00 bits sincethereire l0 waypornts, 5 waypointparameters and l6 bits for each parameter.A further96 bits followingwaypoint9 'test aredesignated waypoint'. Thuswe havea total of 904 bits from the KCU 5654 which arestoredin four shift registermemoriesprovidinga more than adequatestoragespaceof 1024bits. In this mode the KDE 566 memoryis a mirror imaseof the K C U 5 6 5 , 4m e m o r y . U p d a t i n go . . u r c e v e r yI 1 . 3m s . RECORD The recordmodeis activatedwheneverthe recorcl button is pushed.The modewill not be entereduntil Fig.12,12KCU565A(courtesy KingRadioCorp.) the end of a memory-refreshing cycleassignifiedby tl.reoutput of the memorysynchronizer to the mode enterand error. The mode control circuit nronitors control circuit. the recordand enterbuttonsand the belt position On insertinga cardin the slot a microswitchis detectorto determinewhich mocleshoul,ibe actlve closedby the cornerof the cardunlesspreviousiy andso instructthe restof the svstem. clippedoff. The recordbutton switchis in series w i t h t h e m i c r o s w i t c ht ,h u sw h e nt h e b u t t o ni s p r e s s e d MONITOR t h e n t e m o r yi s t e m p o r a r i l yf r o z e n .t h e m o t o r d i i v e & r i a l d a t ai s c l o c k e di n f i o m t h e K C U 5 6 5 A t h r o u g h systentis activatedand the cardbegins to travelout the memorydatagatingand voltagetranslation of the slot providedrhe cardis whJe and fully circuit.the clock pulsescomingfrom the rnasterclock inserted.The motor.drivesa belt which has small in the KCU 5654. lmmediatelyprior to a data cycle holesin it at appropriatepointsallowinglight from a

211

KDE 566

KCU565

!KDE 566

Mastor clock in

Front panel

Serral data out Serial data in Read/write

Enter button I

Record button

1 4 vI tov I to all l.C.'s s v j

PWR switch

Fig. 12.13 KDE 565 block diagram(courtesyKing Radio Corp.)

lamp to shinethrough onto a photoresistorin the belt p o s i t i o nd e t e c t o ri n p u t . - Whentl.recardhasbuilt up speedthe mode control is notified by the positiondetectorthat all is ready for recording.The datais clockedout of the four to four magreticheadswhich record shift registers the digitaldataat the appropriatepointson the card

212

thc ferromagneticoxide in one of tlyo by magnetiz-ing on whether0 or a I is toie--'' directions.depcrrdilrg recorded.A systt:nicounteris advancedone count eachtirlc thc nremoryis clocked. Bctweenwriting4 and previously bits (at a lirrre)the card advances recordeddatais erased.After 256 countsthe data hasall beenrecordedand the systemreturnsto the m o n i t o rn t o d e .

ENTER A card, on which a set of data are recorded,is pushed fully home in the slot closinga microswitch so positionedthat it will closeeventhough the corner of the cardis clipped. Whenthe enterbutton switch,in serieswith the microswitch,is pressed,the motor drive systemis startedand the card travelsoutward. When the card reachesa position slightly before where datarecordingbeganthe belt positiopdetector notifies the mode control when then'enablesall enter circuitry. The magneticheadsread data from the cardssince the changingmagneticfield, asthe card passesover a magnetized part, will causea currentto ilow in the coil wrappedaroundthe headcore. Because of the way in which the datawererecordedthis current occursin pulses,positiveor negativedependingon whether I or 0 was recorded.Eachof the four heads feedsan amplifier and thencethe thresholddetecrors which providedigitaldataoutputs. The digitaldata arefed to the datadecoderswhich enterthe datainto the correctmemory channelsequentially.The four-channelcount multiplexergathersthe counts from all four channelsproducingone count ourpur for the counterand decoder. Whenthe cardhastravelledpastthe end of the datatracksthe belt positiondetectorinitiatesthe error checkphasevia the mode control. The counter and decoderoutput is examinedto determineif 4 X 256 = 1024bits havebeencounted. If the count is correctthe mode control gatesthe masterclock to memory,actuatesthe read/writeline to the KCU 565A and entersthe contentsof memoryinto the KCU 565.4memory. After datatransferis complete the KDE 566 returnsto the monitor mode. ERROR If the count from the counterand decoderis not 1O24the mode control initiatesa flashinered error light and returnsto the monitor mode. lio attemptis madeto enter datainto the KCU 565A.

Standardization The first meetingsof the AEEC areanavigation sub-committee wereheld in 1969to discussan ATA statementpreviouslyprepared.Threepossible systemsfor airlineusewereproposed:a simpleMark l, a sophisticated Mark 2 and a Mark 3 which involved an expansionof INS. The ARINC characteristics lor the Mark I and 2 systemswerepublishedin i970: however,beforepublicationof the Mark 3 characteristic it wasdecidedthat the Mark I system,

which by this stagewas no longer '$mple', and the Mark 3 systemshouldbe combined,hence the publicationin 1974of the ARINC Characteristic 583-l Mark l3 areanavigationsystem.The remainder of this sectionwill be usedto briefly describetne Mark 13 system. The Mark 13 is a three-dimensional systemdesisned for usein all typesof commercialtransportaircrafi. The basicinformationrequiredfor lateraland vertical navigationis derivedfrom a mix of VOR/DME and INS dataplus altitudefrom an air dlta iomputer or sinrilarsource.If INS dataarenot available VOR/DME fiing with air data/magnetic heading smoothingis usedin which caselossof VOR/DME dataleadsto an air databaseddeadreckoninsmode. The parameters of at leasttwenty waypoiits are providedfor usingmanualor automaticentry. Two successive waypointsdefine a greatcircle leg with respectto which navigationand steeringcommand or deviationsignalsarefed to conventional indicators to givelateraland verticalcommandsand in addition areproducedfor useby the AFCS. Both parallel track and verticalpositioningat any point on the track arecapabilities providedby the system;in the latter casevisualand auralaltitude alert sisnalsare generated. The Mark l3 systemcomprises two units, a navigationcomputerand a controland displayunit, with the possibleaddition of a flight data storage unit. Processing of the inputsand generating the outputsshouldbe performedby the NCU while the CDU providesthe pilot/systeminterfaceincluding control of the INS when usedasan RNAV source. The sys.eminputs are as follows: VOR omni-bearing:analogueor digital; DME slantrange:pulsepair,variablespacing; INS: presentpositionlatitudeand longitude; INS: true headingand velocity(NiS and E/W); altitude:analogueor digital; baronretriccorrection:only if analoguealtitude uncorrected; TAS: synchro,a.c. analogueor digital for data smoothingand d/r; magneticheading:synchro; VOR warning:discretehigh or low level; D M E w a r n i n g :d i s c r e t e ; altitudewarning:discrete; TAS warning:discrete; magneticheadingwarning:discrete; progralncontrol: pinswired to chooseinput rlpiions: phase:26 V 400 Hz; a.c.reference go-around:discretefrom AFCS,

213

altitude alert cancel:discrete; Mach number: synchroor digital; IAS: synchro; ILS localizer/glideslope deviation:d.c. analogue; localizerfailure: discrete; glideslopefailure: discrete; AFCSengaged: discrete; autothrottle engaged:discrete; digitalclock input: ARINC 585. The systemoutputs are as follows:

omni-bearingto waypoint: sin/cosfrom four-wire resolver; relativebearingto waypoint: synchro; crosstrackdeviation:high- or lowlevel d.c. analogue; vertical track deviation:ascrosstrack; lateral track angleerror: synchroor'digital (b.c.d.); drift angle:synchroor digital (b.c.d.); lateral track changealert:.28V d.c.; vertical track changealert: 28 V d.c.; lateralsteering(roll command):a.c. or d.c. analogue;

\

(

Fig 12.t4 TIC T.34A RNAV. test set(courtcsy Tel-lnstrumentElectronicsCorp.)

214

vertical steering(pitch command):as lateral; to/from: high-or low-leveld.c.; desiredlateraltrack: synchroor digital(b.c.d.); track angleerror plus drift angle:synchro; distanceto waypoint:digital(b.c.d.)0_399.9 nauticalmiles; presentposition(lat./long.):digital(b.c.d.); groundspeed:digital(b.c.d.)0-2000knoti: t i m e t o g o : d i g i t a (l b . c . d . 0 ) - . ] 9 9 . 9m i n : crosstrackdistance:digital(b.c.d.),O-3gg.g nauticalmiles; lateraltrack angle;digital(b.c.d.); desiredaltitude:digital(b.c.d.)0-50000 ft; secondsystemdata: two-wiredatabus: RNAV systemfailure:high-or low-leveldiscrete: altitudealert failure:discrete; digitalbus warning:discrere; a u t o t u n ev a l i d :d i s c r e t e ; altitudealert: discreteauraland visual.: VOR frequency : 2l 5 selection; systemstatusannunciation:28 V d.c. onceper second; high deviationsensitivity:28 V d.c. when selected; VOR frequencyalert: 28 V d.c.when discrepancy; paralleloffset track alert: 28 V d.c.when selected: speederror: d.c. analogue; speederror warning:discrete.

TestingRNAV

An RNAV systemis simply a computerwhich actson data from externalsensors.In order to check for standardoperationthesesensorsmust give known inputs to the RNAV system. For examplewe could useappropriateVOR and DME test setsto give a bearingand distancefrom beaconof lg0'anA fO nauticalmiles respectively,then with a waypoint positionof 090" and .10nauticalmilesfrom the beaconthe RNAV bearingto the waypoint shouldbe 53.13" and the RNAV dislance50 nautlcalmiles sincewe haveset up a 3 : 4 : 5 triangle. This simple examplecan be extendedto incorporatethe other inputs demandedby more sophisticatedsystems. Flagoperationmust be thoroughly tested. A self-test which checksthe displayand displaydrive and perhapsother circuitsis usuallyprovided. Tel InstrumentElectronicsproducean RNAV test set,the T-34A,which is in fact a combinedDME/VOR ramp tester. The convenience of suchan arrangement is obvious.The DME sectionis virtually the sameas the TIC T-24A (Chapter7) the VOR sectionis similaFffffie TIC T-278 (Chapter4) but with additionalfeitures,i.e. 108.05MHi r.f. ind b e a r i n gosf 4 5 " , 1 3 5 o 2, 2 5 " ,3 I 5 o . It shouldbe remembered when trouble-shooting that, if the systemincludesan ADEU or FDSU, mechanicalproblemsmay occur affectinginput of data. Poor contactbetweenreadingheadand The precedinglist of input and output signalsis magneticcardor tapecancausea lossof data. If the ratherlengthybut illustratesthe computingpo*., cardor tapednve slowsdown but the readine -magnetic availablein modernequipment.Detailsof ihe sienal frequencyremainsthe same then the rateof characteristics canbe found in ARINC 5g3. The last flux is slowerand hencethe headoutpui is lowered. eightof the inputsand the lasttwo of the outputsare Whenrecording a slowdrivecauses biis to be wntten designated asbeinggrowthinputsar outputs. The on top of eachother (pulsecrowdinglosses) while if method of achievingthe necessary outputs, giventhe the drivespeedsup the bit is recordedovera longer inputs, is left to the designerand will vary in both lengthof track leadingto incorrectheadoutput if hardwareand software. replayedat normalspeed.

215

13 Gurrentand future developments conditionswhere previouslythe aircraft would have to be grounded. The reluctanceto replacean equipmentwhich is Changesin aircraft radio systemsoccur more and perfbrmingadequatelyreducesthe sizeof the rnarket more frequently due to the improvingstateof the for the radio systemmanufacturer.Of coursethereis art. The first airborneradio equipnrentsused problem with new aircraft which will have the no thermionicdevices, cat'swhiskerdetectorsand large latestprovenequipmentfitted. Paradoxically, the parallelplate tuningcapacitors; power,weightand situationwe haveis that the aircraftfitted with sizewere restrictionson the developrnentof such equipmentemployingthe lateststateof the art are equipments.In the 1950stransistorized equipment more likely to be in the generalaviationcategory, beganto appearalthoughnot completely sincethat marketis very much biggerthan that for transistorized, the r.f. stages beingreluctantto conrmercial airliners. succumbto solid state. Even now the thermionic Completelynew systemsdo not appearvery deviceis still with us in the shapeof the magnetrortr frequently,althoughwhen they do it is often and the c.r.t.but not, I think, for very long. Claims the improvementin the stateof the art has because concerningan all solid-state weatherradarweremade made the impossiblepossible.An airborneOmega aboutmid-1979,a commerciallyviableequipment receiverwasnot a viablepropositionuntil the appeared in 1980(e.g.CollinsWXR700). The c.r.t. will remainwith us for many yearsbut will, I'm sure, computerpowerand memorycapacitynecessary could be economicallymade availablein a box of eventuallybe replacedby a matrix of reasonable size. electroluminescent elements. SyqtemssuchasVOR, DME, ILS, etc. require Transistorizedequipmentis of coursestill enorrnouscapitalinvestmentand so once adoptedon marketed,but many of the transistors, diodesand a largescaletend to last an extremelylong time. resistors now appearon integratedcircuits. The During and immediatelyafterWorldWar II many emergenceof first small scaleintegration(SSI) then but only a few airborneradiosystemsweredeveloped medium scale(MSI) and now.largescaleintegration new systems developed since the 1950shave survived; (LSI) of evermore componentson one chip has replacements for been internationally agreed not revolutionized the designof air radiosystems.In existingsystemsbut providedcompetitionfor them. particularusingLSI techniques to produce The microwavelandingsystem(MLS) which will microprocessors opensup a wholenew world. succeedILS will be the first replacement system,as The rate of developmentin the last decadeor so for to competing system, decades. opposed that means many aircraft fly with a rangeof It must be mentionedin the introductionto a technologiesrepresentedin their electronicsystems. chaptersuchas this that the changeswe are seeing, It is not inconceivablethat an aircraft could be in haveseenand will be seeing,are to a largeextent due servicewith a valveweatherradar,a transistorized to vastexpenditureon defenceand spaceresearch. ADF and an RNAV systememployinga Havingstatedthe obvious,I will now briefly give my microprocessor, or someother combinationwhich would make it a flying electronicsmuseum. That this thoughts,occasionallysupportedby facts, on what is to come. happensand will continueto do so is the company accountant's choicenot the engineer's or the pilot's.. The replacementof one systemby another performingessentiallythe samefunction must be The State of the Art justified in termsof increasedsafety,increasedpay load,increased reliabilityor an improvementin and other LSI circuits are usedin The microp*ocessor performancewhich allowsflights to be made in the currentgeneration of radiosystems(1979). lt lntroduction

216

seemsclearthat such circuits will be used more and r.f. field continueswith perhapslessobviousresults. more for succeedinggenerationsnot only for A logicaloutcomeof the development of low-noise, computingpurposes,in the conventionalsense,but reliable,small,solid-state r.f. amplifiersand sources alsofor control of virtually everything. Often, in must be to mount t.r.s adjacentto antennadownleads equipments,a microprocessor will be under-utilized so that r.f. cablesand waveguides longerthan a couple of inchesare a thing of the past. Reliability is the key but nevertheless it will still be a cheaperapproach in this latter developmentsincesucht.r.swould be than usingthe minimal amounfof hard-wiredlogic relativelyinaccessible. and furthermore spareprocessingpower is available for expansion. All one needsto do is add the necessary software. I doubt if special-purpose LSI circuitswill appearin greatvariety sincethe volume The Flight Deck ofproduction required for a reasonableunit cost is very large. Havingsaid that, we alreadyseespecial The flight deck will be radicallydifferent in future ISI chipsfor VOR and DME signalprocessing. with flexibleelectronicdisplaysreplacing The cqmpleteall-purposeairborne computer is conventionalinstrumentation and a keyboardwith 'completely not with us yet and may neverbe. By alphanumeric and dedicatedkeysreplacinga massof all-purpose',I mean a computer systemwhich rotary and toggleswitches.The trend hasalready monitors all sensors,drivesall displays,performs all in Chapters9 and started,examplesbeingdiscussed control functions and is the only equipment with that conventional 12. This is not to suggest which the pilot can communicatedirectly. Such a completely; display/controlwill disappear computercould be built today but there are will be neededas instrumentation electromechanical problemssuchas reliability, duplication or back-upfor the electronicdisplaysand toggle triplication being necessary,the need for performing switcheswill alwaysbe usedfor certain functions. As severaltasksat the sametime and the huge I/O examplesof the way thingsaregoing,work by Boeing interface. Certainly in the near future, and indeed and British Aerospacewill be discussed. now, airbornecomputerswill work in specific function areassuch as navigation,flight control, flight Boeing767 management,etc. although someof thesefunctions Thereare a number of featuresof the 767 flight deck visibility may be combinedand performedby one computer. worthy of mention, such asspaciousness, Interwiring is beginningto changeradically in and comfort; our main concernhere,however,is the aircraftinstallationsdue to our ability to handle large displayand control of radio systems.Rockwell'Collins quantitiesof fast time multiplexed digital data. The are to provide their multi-colour EFIS-700electronic prospectof a main serialdigital data highway with flight instrument systemwhich includestwo shadow spursout to sensors,for incoming data, and control maskc.r.t. indicators:an electronicattitudedetector and/or displayunits, for both incoming and outgoing indicator (EADI) and an electronichorizontal data,is a very real one. Again we havethe problem of situationindicator(EHSI). Computationand reliabilitybut the savingin interwiring will be alphanumericdisplaywillbe providedby the Sperry significant.This is happeningnow in certain flight managementcomputersystem(FMCS). There functionalareassuchas frequencycontrol and will be an EADI, EHSI and FMCSfor eachpilot, passenger entertainmentsystems,and will be extended. while an additionalcentrallyplacedelectronicdisplay In this context mention should be made of the useof will be usedaspart of the cautionand warning optical fibreswhich can handledata at a very much system. fasterrate than conventionalcables(15 X 106 bits per The EADI presentationis similarto a conventional second;cf.2400 bits per second,say). In additionto attitude-directorindicator-one of which is fitted as a the increasedcapability the cheapness of the raw back-up. Blue and black fields representsky and materialsand the immunity to interferenceare earth respectivelywith a white line, separatingthe advantages which make it virtually certain that we two fields,as the artificial horizon. There are scales (left), speeddeviation shallseehbre optics usedin the future and would see for roll (upper),glideslope them now were it not for the difficulties encountered (right) and pitch (verticalcentreline).In addition in connectingthem together. there is a risingrunway symbol and a radio altitude The increasein the use of digital sigrals and the digital readout. The information displayedfrom ability to processthem has a most noticeableimpact radio aidsis glideslopedeviationagainsta scale, in the aircraft in that controllers,displaysand of a symbolic localizerdeviationby lateral.,movement facilities are all changed,meanwhile progressin the runway and radio altitude by verticalmovementof 217

q-"ff W:.

(courtesy Fig.l3.l Boeing767flightdeckmock-up Boeing Commercial Aeroplane Co.) the symbolic runway and by digital readout. Decision height selectedand operatingmodesare also displayed. The facility exists,with an EADI, for blankingof scalesnot in use. The flexibility of a computer-drivenc.r.t. displayis fully utilized in the EHSI by providingfor operation in three modes:map display,full compassdisplayor VOR/ILS modewith a lull or partialcompassrose. Weatherradardata can alsobe displayedmaking a dedicatedweatherradardisplayan optional extra. Whenpresentingdata in map form the display is orientatedtrack up; a verticaltrack line with range marksjoins a symbolicaircraftat bottom centreto a boxeddigitalreadoutofthe track at top centre. Headingand presetcours€are givenby distinctive 218

pointen on a partial scalenear the top of the display. trend Ground speed(from INS), a three-segment vector(projectedpath), wind direction and speed, plannedflight betweennamedwaypoints,vertical operatingmode and weather deviation,range.scale, radardata are all shown. A variety of coloursare usedto avoid confusionin the interpretationofthe largeamount of data in a 4'7 X 5'7 in. display. A conventionalRMI providesback-up. The FMCSwill mix storeddaia and data from severalradio and non-radiosensorsto provide position fixing, optimized flightpath and speed guidance,drive for the EADI and EHSI and also hardwareand softwaremonitoring to assistin trouble-shooting.A four-million-bit disk memory

systemwill be employed to provide storageof data suchaslocation of airportsand VOR stations, selected companyroutes,standarddepartureand arrivalroutes(SIDSand STARS)and aircraft/engine parameters.Communicationwith the systemis providedby a l4-line c.r.t. display with24 characters per line, for display of navigationand performance data,and alsoa full alphanumerickeyboard plus dedicatedkeys. British AerospaceAdvanced Flight Deck Work by the British Aircraft Corporation and Hawker SiddeleyAviation on the evolution of flight decksinto a form which would include integratedelectronicdisplaysbeganback in the early 1970s. The resultswere sufficientlyencouragingto commence,jointly, an advancedflight deck programin January1975. Electronic displaysand controls havebeen studiedwith regardto engineeringfeasibilityand human factors. The programhascome up with an exciting view of the flight deck of the future. The main display consists of seven9 in. c.r.t.ssplit into two discrete sub-systems. Threedisplays(Sl, 52 dnd 53) centrally locatedon the instrument panel presentaircraft systemsand engineinformation while four displays (Fl, F2, F3 and F4) split two eachsideof the panel, presentflight information. ln addition,two further c.r.t.s(Dl and D2) areprovided,one eachsideof the maininstrumentpanel,for documentation,e.g. checklist,performancedata,etc. The flight information displaysare an EADI and of informationis in a m EHSI. Presentation conventionalformat, the EADI being similar to an ADI but with information in both analogueand digital form surroundingthe ADI earth/groundcircle. The EHSI canpresenta conventionalHSI type displaywith full compassroseand lateraldeviation bar or a map-likedisplaycompletewith route and weatherradardata. Like the EADI the EHSI has flight information either sideof the main displaybut in this casein digital form only. A facility to switch the displayfrom one c.r.t. to the other will help shoulda malfunctionoccur. In additionthereare back-upinstruments. conventional

easierand saferinterpretation. Work hasbeen done by NASA usinga Boeing737 in which the EADI displayformat is suchas to give the pilot the next best thing to the VFR view when landing(Flight, I I September1976). As conventionalelectromechanical instruments giveway to c.r.t.sso c.r.t.swill one day giveway to solid-state devices.Litton Systemsof Toronto have recentlyannounced(1979) a 3 X 4 in. displaymade up of nearly 50 000 LEDs. The matrix is computer-drivento providethe requireddisplay. The advantages over the c.r.t. are reducedsizeand longerlife (m.t.b.f.). A displaytechniqueparticularlysuitablefor ILS approaches is that of a head-updisplay. Such displaysare commonplaceon modern military aircraft.Whenlandingusingpanel-mounted instrumentsthe pilot must look up to establishvisual contact. If suchcontactis not possiblelookingdown againto return to instrumentscould createproblems of fastassimilation of the dataon instruments.With a head-updisplay the approachguidancesymbolsare of opticaldevicesso projectedby somearrangement that they canbe viewedthroughthe windscreen. Civil airlinerswill be fitted with suchdisplays(e.g. Airbus4300) althoughit shouldbe noted that with two pilots one couldbe eyes-upand one eyes-down duringapproachto avoidan abortiveattemptto acquirevisualcontact.

Packages Multi-System

and the With the adventof micro-electronics smallunit size,it is possibleto bring consequent varioussystemstogetherin one package.In the days of valveswe had one system- many boxes,whereas now it is possibleto think in termsof onebox - many examples qystems.In fact we havealreadydiscussed in Chapter12. As a further, of multi-systempackages -implemented, example,considera andyet-to-be antenna.Thereis no reasonwhy the radar Doppler for the Dopplerradar,an inertialsensor, electronics sensor,srichasLoranor Omegaor a radio-navigation VOR/DME,and a navigationcomputershouldnot all be mountedon top of the fixed Dopplerantenna Altemative Instrumentation forminga singlepackage.Control,displayand radio developments The Boeingand BritishAerospace sensor antennaswould all needto be remote in suchan abovedo not depart from the conventional discussed installation. displayformat for the main part of the EADI and the EHSIdisplay.Obviouslyin usingelectronicdisplays symbolsa wide rangeof showingcomputer-generated Data Link possibilitiesfor display formats exist; however,any departurefronr conventionwould requirepilot A two-waydigitallyencodedautomaticinformation retrainingand would haveto be justified in termsof

219

::'$ .

m#G

iG*-

I

Fg. tf.Z Experimentaladvancedflight deck (courtbsy BritishAerospace)

link hasbeenthe subjectofperiodic discussion by variousworkinggroupsfor over30 years.If and whenthesystemwill be implemented andwhatr.f. dtannelswill be usedif any arestill unknown. ARINC ?,20

project paper 586 providesdetailedinformation on a possiblesystemwith an entertainingappendixon the history of automatic communicationsfor aircraft by the then Chairmanof the AEEC. W.T. Carnes.

lFr

In fact an ATC automaticdatalink doesexist in the form of the secondaryradarsurveillance radar system(Chapter8) which will probablybe extended sometime in the future(seeADSEL/DABSbelow). Herewe areonly concernedwith a two_way automaticdatalink utilizingv.h.f.,h.f. or Satcom for universaluse. Participatingaircraft

b \ \ \ \ \ \

\ r

, / // t| \\\ \

, /

r ' /

I

\ \

Subscriber terminals line or radio(microwave) link Fig.13.3 DataIink system The datalink is essentjally ground-controlled. Aircraft participatingin the syitem are .polled, by the groundstationwhich transmitsa sequence of digital signalssplit into messages eachof which containsa particularaddress (aircraftregistrationnumber)and suitabletext. The airborneequipmentresponds if it recognizes its addressin the pollingsequ.n.. which is repeatedat a rate determinedby the giound station. Messages will be eitherto/from ATC 6r to/from the airlinecompanyand may relateto clearances, altitudechanges, positionreports,flight_planchange, weatherdata,etc. The text of the messages will be routedto/from terminalson the grounaita a polnt-to-pointcommunications network interconnecting the groundstationand the terminals.

ADSEL/DABS The problemsof fruit and garblingwerediscussed in Chapter8; suchproblemsincrease with trafflc d e n s i t y .A d d r e s s e l e c t i vS e S R( A D S E L )a n d discrera e d d r e sbse a c o ns y s r e m( D A B S )h a v eb e e n developedin the UK and USA respectively in an effort to postponethe dateby which the iCAO SSR systemwould becontesaturated.In additiona new

systemwill provide for a transferof more informationthan is possiblewith currentSSR,enable ntoreaccurateand reliabletrackingby ATC and so help in the development of automatecl approach control systems(CAAS _ cornplrterassisie.t approach sequencing) and further.form an indispensable part of a proposedbeacon-based collisionavoidance s y s t e m( B C A S ) . A memorandumof understanding wassignedby the FAA (for the USA) and the CAA (for tie UK) earlyin 1975to allow for future deveiopment of selectively addressed SSRsystemson a co_operative basis.We arestill someway off an ICAO standard system,but wheneverit comesit will be compatible with SSRso that existingairborneequipm.ni may continueto be used. A R I N C C h a r a c t e r i s t7i c1 8 ( N o v e m b e rl 9 7 g _ draft) laysdown a specification for a transponder which lorms part of rhestandardSSRsvstem (ATCRBS- ATC radarbeaconsystemjandDABS. pl , p2 and F: pulseparameters, In that characteristic frequencies and SLSprovisionsur. ui fu, itandard SSRbut the ability to respondto modesA and C interrogations only is requiredratherthan A, B, C and D. - Two typesof interrogationwill be possiblewith the new system,an ATCRBS/DABSail_callor a DABS only. The all-callinterrogationwill consist of threepulsesPl, P3 and p4 togetherwith a SLS control pulseP2. The pl, p2 and p3 pulseparamerers are as for the ICAO SSRwhile p4 is a 0.g pulse the irs leadiirgedgeof which is 1.5gs after the leadingedge A" ICAO transponderwill ignorep4 *f.,it. :f.P: a DABS transponder will recognize the interrogationas all-calland respondwith the all-callreplv. A DABS interrogationconsistsof pt and p2 preamblepulsesand a datablock. The datablock is a singler.f. pulseeither 15.5or 29.5ps longemploying differentialphaseshift keyed(d.p.s.k..y midutation. With this type of modulationa lg0" phasechangeof the carrierat a databit phasereversal position represents a 'l'while no phasechangerepresents a .0,. The first phasereversaloccurs0.5 pi after the leading edgeof the datablock,this is the sync.phasereversal. Subsequent phasereversal positionsoccurat time 0'25l/lrs (N>2) after the sync.phasereversal.The maximumvalueof iy'is 57 or I l3 giving56 or I l2 databits transmittedat a 4 M bitls rate. The trailing edgeof the datablock has0.5 ps of r.f. addedto ensurethe demodulationof the lastbit in the dara block is completedwithout interference. An optionalP5 may be radiatedasan SLS control pulsein the sameway that p2 is radiatedfrom an ICAO interrogator.p5 will be transmitted0.4 ps

Preamble3.5us 2rs

t

Data Block 15'5or 29'5rs

I

0.5 0.5 0.25

H

0'8 rs

ffi

53 54 55 56 57

'

Sync. phase reversal

56 or 112 phase reversalpositions

o'l------{l* O'aj ".- o.r FS

ll

ll

FS

pS

rf = t03O MHz

-

Phasereversal 'l Positionbit :

Phase reversal position bit : 0

Fig. 13.4 ADSEL/DABS format interrogation before the sync.phasereversal.For an aircraft fitted with a DABS transponderthe receivedP5 amplitude will exceedthe amplitudeof the data block hencethe transponderwill not decodethe d.p.s.k.modulated sigral. An ICAO transponderequippedaircraft will not reply to a DABS interrogationsincethe P2 pulse will triggerthe SLS suppression circuit. A DABS transponderwill generateICAO replies (12 information pulses)in responseto ICAO interrogationsand DABS repliesin responseto all-call and DABS interrogations;DABS transpondersalso generatesquitterat randomintervalsto allow acquisitionwithout interrogation(similarto DME auto-standby, Chapter7). A DABS replyis only similarto an interrogationin so far asit containsa preamblefollowedby a data block of 56 or I l2 bits. The preambleconsistsof four 0'5 ps pulseswith the spacingbetweenthe first pulse andthe second,third and fourth pulsesbeing 1.0,3.5 from leadingedge and4'5 prsrespectively, measured to leadingedge. The data block begins8 prsafter the 222

leadingedgeof the first preamblepulseand usespulse positionmodulation(p.p.rn.)at a data rateof I M bit/s. In the I gs intervalallottedto eachdatabit a 0'5 gs pulseis transmittedin the first half il the data b i t i s a ' l ' a n d i n t h e s e c o n dh a l f i f a ' 0 ' . T h e d a t a block is thus 56 or I l2 ps long. The r.f. is 1090MHz asfor the ICAO SSR. Thereare four typesof interrogationfrom an ADSEL/DABSinterrogatorall of which havethe samepreamble.The all-calldatablock containsa of 28 onesin a 56-bitblock,the all-callreply sequence detailsof data containsthe aircraftaddress, equipmenton boardand parity bits for interchange purposes.A surveillance interrogation error-checking and parity bits and alsoa ol 56 bits containsaddress dn t h e r e p e aot f t h e h e i g h ti n f o r m a t i o nr e c e i v e o ground. An aircraftrecognizing the address in a interrogationwill replywith a 56-bitdata surveillance block containingthe altitudeor identity. The of dataarein I l2-bit blocks remaininginterchanges both ways,a comm.-Ainterrogationgivingriseto a

5 6 o r 1 1 2p s

Preamble

br o.5
o.5


l+-)

Data Block

o.5 {+

l'','l',,r1

r-r-r:l-T-'l

I'lolrlolrl

P.P.M.cxample lol ....0 . 11

Fig.13.5 ADSEL/DABS replyformat comm'-B reply and a comm''c^inte.rrogation giving. simplexsystememployingfrequencymodulation riseto a comm'-Dreply' A-B inteichang!involves *orld b.-rr.d with ujtir,l una ao*ntinr. frequencies .The altitudeor identity aswell asother data,anican be separated by between4 and l0 MHz. Aircraft usedlbr trackingpurposeswhile the c-D interchange Saicom.unt.nnu,would be broadlydirectional, contansan extended-ren_gth message segmentof g0 possiblywith switchablelobes. bits in both directions'one thingnot yet decidedis The accuracyof any Satnav.systemwill dependon the methodby which datawill be traniferredinto the knowledgeof satellitepositionand so a numbe.f and out of the transponder from varioussensors and trackingstationsare requiredon the ground. Since to variousdisplaysvia a suitableprocessor.Two. the airb"orne equipmenimusthaveth! oatarelatingto areproposedfor the interface,firstly using the satelliteporition a link must be established T:tl99t ARINC 429 digitalinformation transferryrt*r betweenthe trackingstation and the aircraft, most (DITS) format, secondly.a synchronousI M bitisec probablyvia the satellite.Knowingthe positionof interfacewhich would allow datarequested in an the satellitethe airborneequipmentmustestablishthe uplink to be containedin the next downlink. aircraft'spositionrelativeto the satellitein orderto The systemhasonly beenbriefly described: the obtaina dx. readeris referredto ARINC 718 for further details. The variousmethodsby which a fix canbe obtainedinvolvesomecombinationof measurement of angularelevationof a satellite,rangeof a satellite Satcom. and Satnav. and rateof changeof rangeof a satellite(Doppler shift). Directionof arrivalof a signalat tire saiellite Therearemany satellites ih orbit aroundthe earth may be methodswhereby -foundusinginterferonteter beingusedfor relayingtelephoneand television the satelliteantennas aremountedon long booms signals,.weather sensing, observation and military (say50 ft) and the phasedifferencein signals arrrvins navigationand communication.(For a comorehensrve at the antennasis measured.Rangemeasurement reviewseeFlight,28 October197g.) Unforiunately may be obtainedin a similarfvay to that employedin noneso far areusedby civil aircraftand it is not DME. In a range-rate systemthe Dopplerstrilt tf a known (by the author)when suchusewill occur. signalfrom the satelliteis recordedovlr a periodof However,brief commentscanbe madeon the say l0 min, then the aircraftpositioncan be computecl principlesinvolved. from the time of zeroDopplershift and the slopeof A possiblev.h.f.Satcom.systemis described in the frequency/time graphat zeroshift. ARINC Characteristic 566. The satelliteis simplya The range-rate m.ethodcanprovidean accuratefix repeaterfor voiceand datacommunications beiween aboutonceevery lI h, usinga satellitein a 500-mile a i r a n dg r o u n d . I f t h e s a t e l l i t e isat synchronous circularorbit, obviouslyfor aircrafta largenumberof altitude(22 000 nauticalmiles)the service areawould satellites must be usedto reducethe tirne_interval be 4l per centof the earth.ssurface.a OouUle_cfrannel betweenfixes. The aircraftvelocityand altitucle

must be accuratelyknown during the time taken to obtain a fix asthe satellitemakesa passover the line of closestapproachto the aircraft. The US Navy use sucha systemflorsurfaceship navigation. Angle-only,rangeonly or angle-range methods may be used. In thesesystems,by usinga two-way link betweenthe aircraft and a ground station via the satellite,the computationof aircraft position can be carriedout on the ground,the databeing sent to both aircraft and ATC. A range-onlysystemwhich should comeon line in the 1980sis the Global Navigation System(NAVSTAR.) using24 satellites;however use may be restrictedto military aircraft and ground personnel. The frequenciesinvolvedfor Satnav.are likely to be v.h.f.or around1.6GHz. It will obviouslybe advantageous if a group of satellitescould be usedfor both communicationand navisation.

simulationsshowedthat both systemswould do the job and the choiie must havebeendifficult. One factorwhich helpedswingthe vote musthavebeena reductionin cost of the TRSB systembroughtabout by the developmentof cost-minimizedphased-array techniques(COMPACT)by Hazeltine.A conventional electronicbeam scanningarray consistsof many radiatingelementseai,hof which is fed by an electronicphaseshifter. Changingthe phaseof the r.f. energyradiatedby eachelementcausesthe far field beam to scan. With COMPACTthere is nearly a 4: I reductionin the numberof phaseshifterseach of which feedsall radiatingelementsthrougha patentedpassive network. The resultis accurate bearnscanningwith low sidelobesat a reducedcost. The principlesof TRSB are quite simple. Consider a radio beam scannedrapidly to and fro; an appropriatelytuned receiveron an aircraft within rangeof the beamsourcewould receivetwo pulsesin one completescan,as the beamswept past twice. The pulsespacingis relatedto the anglemade Microwave Landing System (MLS) betweenthe centroid of the scannedsectorand the line joining aircraft to beamsource. Note that the The long, controversialand heated argument about systemas describedis ambiguoussincethe computed which systemwill be adoptedas the successor to ILS anglecould be either sideof the centroid of the endedin 1978 with the choiceof a time-referenced scanning beam(TRSB)system.The requirementwas scannedsector. Ambiguity may be removedby, 'to' and which is the 'fro' for a landingsystemthat would allow for a variety of knowing which is the scan, curvedor straight-lineapproacheswithin a large or by knowing the scancycle start time, i.e. the 'to' volumeof airspaceand that would not suffer to the half cycle. For accuracy commencementof the sameextent as ILS from multipath effects. precisiontiming circuits must be used. The main contendersby the time the final decision For lateraland verticalguidanceazimuth scanning wasmadewere the USA with TRSB and the UK with and elevationscanningbeamsare required. Preamble a commutatedDoppler. Demonstrationsand instructionsmust be usedto identify the beamswhich.

Scan cycle

tt related to {l Fig. I 3.5 Principlesof TRSB

224

are synchronizedso that one time dift-erencefor each and C2. The particularmodc useddependson the beamis measuredin a 150 ms frame. Back beamand aircral'tllt and the type of groundstaiion. flare measurements are alsopossibleon a t'ull system which will be usedin conjunction with a high-accuracyMode AB Angular and rangeinformation is available DME. Reflectedbeamreceptioncan be elinrinatedby usinga standardland-based MADGE station. The time_gating(echo suppression).The r.f. employed direction from which the aircral'tis interrogatineis will be in the Ku band. nreasured by the groundstationinterferom-eterfthe The battle lbr ICAO recognitionis not the whole subsequent reply containinghorizontaland vertical story of MLS, systemshavingbeendevelopedior deviationfrom an approachor overshootpath specialapplications;MADGE (microwaveiircraft determinedby the sitingof the grounduni.nno arrays. digital guidanceequipment) is an MLS-based The azimuth deviationwith respectto the approach on-groundderivedinterferometry, developedby the path centreline is displayedon a purpose-built British company MEL(see below); SCAIvILS(small precisionrangeand azimuthnreter(pRAM). while on communityairport MLS) is a Hazeltineproduct using a standardcross-pointer deviationindicatorazimuth their COMPACTantennasfor the TRSRsystem. and elevationguidanceis givento an approachpath selectedby the pilot. The overshootpath is selected Microwave Aircraft Digital Guidance by the pilot in azimuthonly, elevationdeviationis Equipment(MADGE) with respectto the ground-dellned overshootpath. Rangedata,derivedby measuringthe elapsedtime betweeninterrogationand reply.is displayedon a Introduction PRAlvl.With an appropriatelink in the wiring of the MADGE is a microwave,secondary radarsystem airborne installationor on receiptof a commandin which providesguidanceto a landing site for any ground the stationreply the rangedatais sentto the suitablyequippedaircraftwithin a rangeof abour l5 interrogations. miles. As well as providing flexibility in the approach groundinterlacedwith subsequent The two typesof air-to-ground transmission are path,asone would expectin any successor to ILS, a referredto asA channel- interrogation:B channeltwo-way data link is establishedwhich resultsin rangedata. deviationand rangeinformation being availablenot only in the air but alsoat the landinesite. Mode C Two alternativesfor Mode C are available, The MEL EquipmentCompanyllmited (A PhilipsCompany)havewon sufficient military orders both providingguidanceto a point offset horizontally from the landingsite,by at least200 metres. Such to assurethe future of MADGE. The first extensive guidanceis suitablefor helicoptersoperatingto civilianusewill be in the North Seaoil fields where. offshore platforms. In addition to azimuth ind one platform is alreadyequipped. elevationand rangedataone of two arrowson the PRAM indicatesto the pilot on which side of his Basic Principles aircraftthe landingsiteis situated. The systemprovidesboth rangeand angulardata. The rangeis derivedin the sameway as with DME, Unlike Mode AB the pilot-selected angularoffset of the approachpath is not available i.e.measurement takesplacein the airborne with Mode C. equipmentafter an air-to-groundinterrogationand a The particularapproachpath usedis ground-defined, subsequentground-to-airreply. The air-derivedrange henceMode C is known asthe groundcontrolled may be containedin the interrogations, subsequent to mode(GCM). The platform installationis rotated the recognitionof valid replies,in order to make this to the correctcompassbearingwithin one of four information availableon the ground. sectorschosenby the pilot, the choicebeingrelayed Azimuth and elevationanglesare ground-derived, b y r l t . usinginterferometryor, in the caseof elevation,radio The two ModeC alternatives areMode Cl and C2 altitudeand range. If the latter is the caseboth range which differ in the way elevationguidanceis derived. and radioaltitudedata must be transmittedfrom With Mode Cl radio altitudeand rangeare air to groundin order that the elevationtrianglemay transmittedto the landingsite.wherethe elevation be solvedfor elevationangle.The angulardatais anglecan be computedsincethe sineof the angleis containedin the reply to the airborneequipntent's equalto the ratio of the radioaltitudeto the (slant) interrogation. range.With ModeC2 elevationinformationis derived asin Mode AB. Modesof Operation Mode C I allowsgreaterflexibiJityof approach. Therearethreemodesof operation,namelyAB, Cl The aircraftcan be guideddown a glideslopeto a point

225

remote from the landingsite (say 0'5 nautical miles away) from where the final approachis completedin levelflight. The parametersof the approachpath are under the control of the ground station for both ModeCl and C2. lnterferometry A basicinterferometerconsistsof trvo antennas feedingreceivers,the outputs of which are compared in phase. If the radiatedwavearrivesfrom a direction other than normal to the planeof the two'antenna array then the energyarrivingat one antennawill havetravelledfurther than that arrivingat the other by a distanced. The phasedifferencebetweenthe antennasignalswill dependon d which, in turn, dependson the direction of arrival. A two-antennainterferometeris of little usesince an ambiguousmeasureof the direction of arrivalis obtained. For exampleconsiderthe antennasspaced I cm apart and a wavelengthofthe radiatedwave equalto tr cm. lf d = 0, I, 2),,etc. the measured to phasedifferencewill be zerocorresponding with respectto the directionsof arrival0, measured

" .r" i 't,i

Fig. 13.8 MADGE,hardware(courtesyMEL Equipment Co. Ltd)

226

Fig 13.7 Two antennainterferometer normal, givenby sin 0 = 0, tri Q,2ltlQ, etc. Thus with I = 100 cm, say, and tr = 6 cm, then for zero phase d i f f e r e n c0 e c o u l db e 0 , 3 ' M , 6 ' 8 9 , e t c .d e g r e e s . To resolvethe ambiguity severalantennasmust be usedin a lineararray. Phasedilferencemeasurements can be madebetweenany two antennasignalsto give directionof arrival. collectivelyan unambiguous With the spacingof the antennaschosento be in the ratio2:4 : 8 : l6 : 32 the derivedangleword canbe codeddirectly in binary.

Conbrlls and Instrumentation A Mode AB controller has the following controls: l. 'OFF/STANDBy/ON'. Whenin standbythe input to the modulator is inhibited wirh a . 3-min warm-up delay for the transmitteris initiated. 2. GROUND/AIR control. Four thumbwheel switcheswhich selectthe ground and air addressand the frequencyof operation. 3. Angle offsets. Three rotary switcheswhich allow the pilot to selectone of a variety of elevationand azimuth anglesfor approach. For overshootazimuth offset only is provided. 4. 'TEST'. A push switch which activatis the built-in test equipment(b.i.t.e.).

The length and message content of the interrogation word, which is assembled in the logic unit, depenis on the mode of operation. For A channel(M;de AB) a 25-bit word requestingguidanceinformation rs transmittedat a jittering meanrateof 50 Hz. B channel,containingrangeand other data in a 60-bit word, can be interlacedwith A channelinterrogations a t a f a c t o r ys e tr a t eo f 2 . 5 , 5 , l 0 o r 5 0 H z . C m o d e s useonly one interrogationword 60 bits long at a jittering meanrate of 100 Hz. -BothCl and C2 interrogations containguidanceinterrogationdata and range,but in additionmodeCl transmitsradio altitude information. Appropriateair and ground addresscodes,asselectedon the controller, form p a r t o f t h e g u i d a n c ien t e r r o g a t i o n . The interrogationword pulsetrain,suitably In addition, if the controller is dual mode i.e. Mode processed in a line receiverand pulsemodulattr, 'ANGLE AB and C an OFFSETS'/,SECTOR GCM' controlsthe grid voltageof a travelling-wave_tube five-positionswitch is provided to allow the pilot to which amplifiesthe r.f. from a solid-statesource selectwhich of the four approachsectorshi desires phase-locked to 56 timesa referenceoscillator for ModeC (GCM) operation or, if usinga Mode AB crystal. A 4-bit parallelcode,determinedbv the station,the angleoffset controls may be enabledby frequencyselectionat the controller,selectiwhich of s e l e c t i n g ' A N G LO E FFSET'. four crystalsis to be usedasa reference.The pulse The PRAM and crossed-pointer deviationindicator codeanrplitudemodulatedfrequencyof between havebeenmentionedpreviously.In additionthree 5 ' 1 8 2 5a n d 5 . 2 0 0 5G H z i s f e d v i aa l o w - p a sfsl i t e r optionalindicatorsmay be fitted: and circulatorin the microwaveassemblyto one of l. Overshootwarning. Only activeduring Mode two antennasselectedby a switchwhich is controlledby the logicunit. AB operation. Thereare two activestites of Receivedsignalsareamplifiedand detectedin a this ntagneticindicator.in one of which double-superhet receiver,then passedto the logic indicationof the serviceability of the ground unit. lncoming noise pulsesarecombatedby overshootinterferometeris given;the other reducingthe receiversensitivityasthe rate ofreceived givingovershootwarning. .Off is displayed pulsesincreases.Interference from multipathechoes when the indicatoris not active. 2. Low-fly warning. Similarto ( I ) aboveexcepr is avoidedby settinga thresholdlevelin accordance with the amplitudeof the first pulseof the incoming that the activestatesshow eitherthat the word. The weakermultipathechoeswill be unlikely elevationfailurewarningflagis pulledin to exceedthis thresholdwhich returnsto zero at the (elevationsafe)or that the aircraftis low. end of eachreceivedword. 3. Excessazimuthdeviation.Only usedin The reply is clockedinto a shift registerand then Mode C. A lamp which flasheswhen the checkedfor validityand parity. Validity is aircraftis more than 60 metresfrom the determinedby comparingair and groundaddress approachpath and within t nauticalmile of codes receivedwith thoseselected.A ranseclock. the landingsite. w h r c hw a ss t a r t e dw h e n . t h ei n t e r r o g a t i oI o nok place, is stoppedon completionof .asuccessful validitv and p a r i t yc h e c k . Block Diagram Operation The systemfunctionsin eithersearchor track. The installationcomprises an interrogatorset(logic U n t i l t h e r a t eo f v a l i d a r e rde p l i e si s a c c e p t a b lteh e u n i t) . a n i n t e r r o g a t osre t( t r a n s m i t t e r - r e c e i v ie r ) . logicunit causes the t/r output to switchbetween controller,an antennaselector,two antennasand up forwardand rearantennasat a 0.5 Hz rale. llavins to five dilferentindicators,possiblyduplicated,as acquired a r e l i a b l er a n g ev a l u e a, t r a c k i n gg a t ei s described previously.All electroniccircuitry is generated so that only thosereplieswithin I gs c o r r t a i n ewd i t h i n t h e l o g i cu n i t a n d t r a n s m i i t e r - r e c e i vbefore. er and 2 ps after,the expectedtime of arrival (ti r) with tl"reexceptionof that containedin the areaccepted.With repliesregularlyfallingwithin the PRAM. trackinggatethe rangeof changeof rangeis

227

|E

E o O

E

P

€ !

-

E ,

E P

3 , " 3 9

l-

s g E ; -8€sE € f 5';

=--l e

o o

< o o O x o

o

g)

a

o

G

o o

;

;

> 6 4

l o l o , t s

r b t t__

o o

i

I

o

G :

o

- = - ;

q)

o

n

N

C b

o.-

c.l o

r

= E

o

C

;

!

d

0)+

q

:

o

d !

tro P C O O

6

g s

a O

o o

F 9 = 6 s ;

= ^ AE

o o

*

I

__J

i

< 9 2

o , C C O

- ;

[

228

6

o

= i

E! . 8 , ' n C @ v , o 4

r

d

- : d 6 i !

; g

O c ( f o

. ,..t oo Fl

ii>

computed. If the rate exceeds30 knots antennasvitchingceases, the forwardantennabeingselected for decreasingrange,the rear antennafor increasing range. Each antennahas a polar diagramwhich is 180" wide in the horizontal plane and 40o wide in the vertical plane. With validity and parity confirmed rangeand angle data is fed to the output control logic circuits. Digital rangeand azimuth information is fed to the PRAM, the azimuth information beingunaffectedby the selectionof angleoffsets. Azimuth and elevation information is comparedwith the selectedangle offsets,the resultingdigital differencesare then convertedinto analogueguidanceoutputs which are fed via matchingresistorsin the interfacejunction box to the cross-pointer deviationindicator. The d/a convertergainis progressivelyreducedas the range decreases from I nauticalmile. This reduction in eain is known as beam-softening and resultsin meter deflectionbeingpropoitionalto lateralratherthan angulardisplacementfrom the approachpath. Warningsignalsaregenerated within the logicunit. Flag drivesare fed to the cross-pointerinstrument being28 V for validinformation,0 V duringa fault condition. A low-fly warningis provided in Mode AB, or an excesselevationdeviationsignalin Mode C. Azimuth warningsare the excessazimuth deviationin Mode C and the overshootsignalin Mode AB. The PRAM display is enabledby the azimuth flag signal. For Mode Cl a radio altimeterinterfaceunit is requiredto digitize the analogueheight signalfrom the radio altimeter. As a checkthe digitizedheightis convertedback into analogueform, then compared with the height input. A fault signalis fed to the logic greaterthan 1 25 ft is detected. unit if a discrepancy The fault signalis also generatedif the radio altimeter'sflagoutput showsa lault condition. With a fahlt detectedthe elevationflag showsand the heightinformationpresentbit in the interrogation word is negated. Selected System Parameters Transmitterpeakpower: 150W nominal. T r a n s m i t t efrr e q u e n c yc h a n n e l s5: 1 8 2 ' 5 ,5 1 8 8 ' 5 , 5 1 9 4 ' 5 . 5 2 0 0 'M 5 Hz. Receiverfrequencychannels:5004'5, 5010'5,5016'5, 5 0 2 2 ' 5 .5 0 2 8 ' 5 .5 0 3 4 ' 5M H z . lnterrogationword rate: 50 Hz mean(A channel); 100 Hz mean(C channel). Word jitter: + 2 ms (A channel);+ I ms (B channel). M e s s a gbei t r a t e : l ' 0 1 1 2 5 M H z n o m i n a l . lnterrogationword length:25 bits (A channel); 60 bits (C channel). Reply word length:60 bits (all channels).

Message format: non-retum.to-zero(n.r.z.). Height data, down link: I I bits (binary). Rangedata,down link: l6 bits (b.c.d.)., Elevationdata, up link: 9 bits. Azimuth and flag data,up link: l5 bits. + 2'5 to t l0o Analogueazimuthguidance:selectable full scale. Analogueelevationguidance:selectablet 0'5 fo t 5" full scale. Azimuth coarseoutput: t 45o full scale. Elevationcoarseoutput: t25" (A channel):-5 to +20' full scale(Cl channel). Rangeoutput: 25 nauticalmilesat l0 V;30 nautical m i l e sa t l 2 V . Velocity output: I 200 knots full scale. Digitaloutputs: I '01 125 MHz, bi-phasen.r.z. Anglesand flags:28-bit word at reply rate. Rangedata: l8-bit word at interrogationrate. Digital resolution:azimuth0'235'; elevationO'144": range9'26 metres.

Collision Arioidance hasbeenthe Up until 1979collisionavoidance responsibilityof ATC tbr aircraft flying under IFR while pilots are responsiblefor their own safety underVFR. This situationis likely to continuelbr sometime in the future eventhough a workable collision avoidancesystem(CAS) hasbeen developed. A self-containedsystemwould protect the ofwhether other aircraft equippedaircraft regardless were similarly equippedor an ATC servicewas available.Sucha systemcould be built measuring of change,and directionof all aircraft range,range-rate within a certainvolume of spacearound the protected aircraft. Receiveddatacould be usedto compute projectedpathsso that the risk of collisioncould be a warninggivenor evaluatedand,if necessary, initiated. The costof sucha automaticmanoeuvre systemprovidingthe accuracyrequiredis prohibitive, at leastfor the time being. As an alternative,we may havea co-operative systemand this is an areain'which work hasbeen exist: done. Two possibilities radar l. an interrogator/transpondersecondary systemwhich could measurerangeand range-ratein much the sameway as DME does but with obviousproblemsin crowdedairspace wherethe systemis most needed; 2. a time multiplexedsystemin which all aircraft transmitin turn without interrogation. The calculationof the rangeat nearestapproach

229

q (miss distance)is complicated and requiresprcsent relativepositions,ipcluding altitude, and the speed and track ofeach aircraft involved. One approachis to usethe componentof relativevelocity perpendicularto the line joining the aircraft asshown in Fig. 13.10whereV1 , Va, Vn arethe velocity vectorsof aircraft A, aircraft B and B relativeto A respectively,while X is the measureof the risk of collision.Another,simplermethodis to usethe rangedividedby the range-rate,measureof the risk beingknown as tau (r). In both methodsa minimum valueof the risk measureis set,below which evasive actionis taken. With the latter method,however, when the closingvelocity is small r is not a good measureof risk and a minimum rangecriterion shouldbe includedin the system.

The CurrentGenerationof ARINC Characteristics

ln 1979a numberof new ARINC Characteristics (700 series)were adoptedby the AEEC. The systems coveredincluderadio altimeter,DME, ILS, VOR, ADF, Selcal.,p.a. amp.,v.h.f. comms,weatherradar detailthe and ATC transponder.Thesecharacteristics for radioequipmentin the 1980s. airlinestandards between Thereareobviouslymany rninordifferences and somemajor onessuch new and old characteristics asthe incorporationof DABS into the ATC transponder.However,the most significantchangeis the switch to serialdigital signalsfor both system outputsand control. The detailsof the digital information transfersystem(DITS) are givenin 429-2publishedI March 1979. ARINC Specification Additionalinformationis givenin projectpapers453 and72A,the very high speed(VHS) bus and digital tively, frequencyifunctionselection(DFS) respec althoughthesepapershad not beenadoptedby May 1979. for the transfer the standards The DITS describes Aircraft of digitaldatabetweenall avionicssystenls, not just I / radio. Data flow is one way only, via twistedand Aircraft shieldedpairsof wiresat a rateof l00K bits per A-L s e c o n d( K b s )o r l 2 t o l 4 ' 5 K b s( 1 0 0 0K b s f o r V H S The systemis basedon 32-bitwords. The bus). threatgeometry Fig.13.10 Collision encodinglogic for eachbit is basedon a voltage 'Hi' stateat the beginningof a bit As an exampleof a systemwe will briefly consider transition,a 'Null' statebeforethe end of was workable as long ago as which to a CAS Bendix returning the interval 1969. The systeminvolvesan airborneclock, t h a ti n t e r v arle p r e s e nat sl o g i c ' l ' s i m i l a r l ya ' L o ' t o '0'. The transmittervoltage while on the ground 'Null' represents a logic computerand transmitter-receiver + 1 0 ! I V . 0 1 0 ' 5 V a n d- 1 0 1 I V f o r t h e and clock accuracyof the systemis syncl.rroniz-ation l e v e lasr e ' N u l l ' a n d ' L o ' s t a t e sr e s p e c t i v e l y . ' H i ' , During a 3 s of an atontic clock. by means assured lnlornrationis codedin one oi severalways:binary intervalknown as att epoclt,eachparticipating aircrafttransmitsduringone of 2000 time-slotseach codeddecirlal(b.c.d.).two's contplementfractional of 1'5 ms duration:while one is transmittingthe rest binary(b.n.r.)or the InternationalStandards (lSO) alphabetNo. 5. Codel'or graphics of altitudeand other data(e.g. organisation listen. Transmission yet beendefined. Discretedata suchas rnade. not is has data heading) 'code'. Wordsare The groundstationkeepsall aircraftsynchronized on/off switchingis simplya I -bit 4 and hence by a gapof at least bits betweenwords, synchronized in time;and frequencyof transmission propagation by the fact that thereis no voltage time and gap from is recognized a rangernaybe measured is carriedout b1'one parity r ntay be Errcir checking shift. Thus Doppler rate fiom transition. range 'bit per word, the requiredparity is odd. i.e. tlte total onceduringeachepochfor all reporting calculated numbeo r t ' ' l s ' i n a w o r di s o d d . aircraft. Commandsto the pilot basedon the of eachword is label. The basicorganization computer'sevaluationof the risk are: aircraftabove, identifier(SDI), dirtafield. t e l o w ,p r e p a r et o c l i m b , source/destinatic-rn p r e p a r et o d i v e ,d i v e .a i r c r a f b are givenby clinrband l'ly level. The comrnands s i g n / s t a t ums a t r i xa n df i n a l l y ,b i i 3 2 , p a r i t l . T h e f i r s t 8 b i t s o f e a c hw o r d a r ea s s i g n etdo a l a b e l u h i c h b a c k - l i g h t el e d g e n dosn o n ei n d i c a t o r . identifiesthe informationcontainedin the data fleld, Any CASwhich may eventuallybe adoptedwill e . g .v . h . f .c o m m .f r e q u e n c yD, M E d i s t a n c es,e l e c t e d be like the Bendixsystem,but not necessarily c o u r s ee. t c . B i t s9 a n d l 0 a r eu s e df o r t h e S D I w h e r e discussed and techniques certainlysomeof the tactors a word needsto be directedto a specificsystemof a abovewill be relevant. 230

:g

multi-systeminstallationor the sourcesystemof a multlsystem installationneedsto be identified. The SDI is not availablefor alphanumericwords (lSO alphabetNo. 5) or where the bits are usedaspart of the data field when the resolutionrequireddemands matrix it. Bits 30 and 3l areusedfor the sign/status no t, to, test, right, North, which represents wordsareusedto computeddata,etc. If several too long for one word then the transmit a message signistatusmatrix is usedto indicatefinal word, intermediateword, controlword or initial word. The aboveideasare best illustratedby examples: l. ADF. typicalword 0 0 0 I I 0 I 0 0 0 0 0 0I I 0 I 0 0 0 0 l 1 0 0 0 0 0 1 0 0 0 '0 The label 0 0 I I 0 I 0' is followed by SDI '0 0' the all-callcode. Data field bits I I ,12 and l3 areb.f.o. - on/off (0 for off), ADF/antenna mode (0 for ADF) and sparerespectively.Data field bits 14-29arethe frequencyselectbits with ' bits 21-29the 1000kHz selectiod,bits 23-26the 1 0 0k H z , b i t s l 5 - 1 8 t h e I k H z a n db i t 1 4 t h e 0'5 kHz selection.The exampleshownreading f r o m b i t 1 4 .i s 0 0 0 1 1 0 0 0 0 0 1 l 0 l 0 l + 800 +1000 + l0 0.5+5 = l8l5'5 kHz. herebut in any case is not applicable Sign/status 00 canbe readaspositive.The final bit is setto 0 to make total one count odd.

distancewith the most significant bit (m's.b') of the most significantcharacter(m.s.c.)beingbit 29. The eximple shown readingback from bit 29 is 1 0 0 0 0 l l 0 0 l l l 0 1 0 o l 0 l 2 5 7 8 6 nautical miles DITS words reprcsentingparametersare generated in units such asVOR receivers,DME interrogators' etc. and will terminatein navigationcomputers' display driversor indicators. The rate of transmissionof suchwords varies,for examplethe minimum rate for DME distanceis 6 times per secondwhile radio height is 20 times per second; DITS radio controlwordssuchasexamples(l) and (2) aboveare transmitted9 times per second'

ConcludingRemarks

currentradiosystemswith This book hasconsidered examplesof a rangeof technologies.The final chaptir hasattemptedto showdirectionsin which radio systemsmay progress.We will certainlysee Ut-S and most probablyADSEL/DABS' Flight decks will makefull useof flexibleelectronicdisplaysbut a instrumentsand minimalcomplementof conventional indefinitely. will remain controls Modulatingsignalswill probably remainanalogue in propagatingsystems,sinceto go digital would requlreishifi up the frequencyscaleto accommodate the wider bandwidthsrequired. Eventhough the 2. ATC transPonder,tYPicalword basisof the systemswill be analoguesigrals,they will 'go digital'asearlyaspossiblein the circuitry' 0 0 l I I 0 l I 0 0 0 0 1 0 0 0 0 0 0 0I 0 1 0 1 0 1 0 0 0 0 1 Cornputlngpower will increasebeyond that which we order are The datafield bits I l'17 in ascending can employ; this hashappenedalready,the-power is altitudereportingon/off (0 for on), inertial there,all we needto do is think of the applications' computer, referencesystem/flightmanagement after which there will be yet more computingpowei' for future i.e. IRS/FMCinputswhich arereserved Perhapsthe biggestquestionmark is ov-erthe use(0 for IRS),ident. on/off (1 for on)' altitude future of VOR/DME asthe standardICAO datasourceselect(0 for No. l), IFR/VFR which navigationalaid. Possiblereplacements arev'l'f'/Omega' for future use(0 for IFR) and X-pulse Satniv. in one form or another'or evenLoran C; the is reserved on/off (0 for off). Datafield bits l8 to 29 of thesebeingthat they arelo.ng.range' advantages codefor Mode A replies representpilot-selected A poisiblescenariois an aircraftfitted with a with bits l8-20 the D codegroupselect,2l-23 varietyof deadreckoningand positionfixing nav' the C,24-26 the B and 21'29 the A codegroup aidsin a minimumnumberof boxesbackedup by select.The exampleshownreadingback from accurategroundtracking,immenselypowerful datalink' bit 29 is groundcomputersand a corpprehensive 0 0 r 0 l 0 l 0 l 0 0 0 CAS and MLS of the required u -ultiple ivith r a 0 =code 5 I J accuracythe aircraftof the future couldgo from ramp t; rampwithout the interventionof the pilot (b.c.d.) 3. DMEdistance andln almostperfectsafety. This could be 1 0 0 0 0 0 0 1 0 0 0 11 0 0 0 0 1I I I now, but would be cost prohibitive; developed 0 1 0 1 0 0 1 0 0 0 0 who wantsa robot flying the'plane? anyway and for the DME exclusively datafieldis used

The

Recommended reading

As the readeris probablyawarethereis a dearthof books on avionics;those that exist and concern themselveswith aircraft radio are perhapsa little dated.In contrastthe numberof booksivailableon electronics, computersand radiois staggering. I have chosento list most books availableon avionicswhich I think are worthy of the reader's attention. For sourcesof backgroundmaterialon basictheoryof electronics and radio the readeris perhapsbest advisedto, visit a suitablelibrarv or bookshopand pick out thosewhich seemto suit him or her best. Sincethis is a fbrmidabletask. I have listedsomebookswhich I think may be useful. A briefnote is givenasa guideto contentand level of each.

A very good book but the emphasis is on operationaluseso the theorygivenis brief. C o m p l e m e n t a rt yo t h i sb o o k . 8. S.E.T.Taylor and H.A. parnrer.Grountl Stutlies for Pilots. Volurne l, Radio Aids. Granada ( 3 r d e d i r i o n ,1 9 7 9 ) . Coversthe needsof prospective contnrercial pilots in so far asthe useof raclioaidsis concerned. 9. J.L. McKinleyand R.D. Brenr.Electricitt,and Electronicsfor Aerospatc Vchit.les.MeGruw-t{ill ( 2 n de d i t i o n t, 9 7 l ) . Coversfundamentaltheoryand brielly d e s c r i b easv i o n i c s y s t c n ) 5B. t s i c . 10. D.C. Green,Transmissiott,st'slarrs, ll. pitnran l. M. Kayton and W.R. Fried (Editors\, Avionics (1978'1. NavigationSystems.John Wiley and Sons(1969). I l . D . C .G r e e nR , a d i o S y s t e n r lsl ,. p i r n r a n( l 9 7 g ) . Authoritativebook which.isstill of 1 2 . D . C .G r e e nR , a d i o S l t s t e m sI l,l . P i t r r r a (n1 9 7 9 ) . considerable interest.Coversradio and 1 3 . D . ( ' . G r e e nE , l e c t r o n i c s , l l .p i t m a n( l 9 7 g ) . non-radioaids. Postgraduate 1 4 . D . C .G r e e nL, ' l e t , t r o t i l c ' s , l lpl .i t r n a n( 1 9 7 g ) . engineerlevel. 2. G.E. Beck(Editor),NovigationSysterns.yan s o o k sa r ew e l l w r i t t e na n d A l l o l ' G r e e n 'b NostrandReinhold( 1971\. i l l u s t r a t e dT. e c h n i c i alne v e l . Similarto Kayton and Fried.alsoconsiders 1 5 . J . E . F i s h e ra n d I l . B . G a t l a n dL, l e c . t n t n i cf sr i t m marinenavigationsystems.Equallyworthy Theoryinkt Prac'tice.I)ergrrnon ( lrrd ediiion. but not, perhaps,ascomprehensive t9761. sinceit has almost300 l'ewerpages. Mainly concerned with designtif circuits. 3. B. Kendal.Mqnualof Atktnics. Granada'l9791. U n d e r g r a d u a t e a n d p r a c t i s i negn g i n e e r s . ' Biasedtowardsair tral'ficcontrol aspectsof 16. G.A. StreitrnatterarrdV. I:iore.Microprocessors aircraltoperations.Complementary to this Thcoryanl Applit'utr,lrs.Reslorr( lg79). book. O n eo f t h c b e s to l ' a r c c e n tn u n r b e o r f books 4. E.H.J. Pallert,Airuaft ElectricalSystems. on mlcroprocessors. P i t r n a n( 2 n d e d i t i o n .1 9 7 9 ) . f 7 . A . J . B a d e nF u l l e r M , i c r o w a v e sp. e r g a m o n 5. E.H.J.Pallett,Aircraft Instruments. pitman ( l n d e d i t i o n .l t ) 7 t ) 1 . ( 2 n d e d i t i o n .t q Sl ) . L l n d e r g r a d u al cl e' v e l .U s e f udl e s c r i p t i o nosf 6. E.H.J.Pallett,Automatic Flight Control. c o l n p ( ) n e nat sn dd e v i c e s . Granada( 1979)'. 18. Telec'onntwic'utlons S-r,s/enls Llnits, l-6. Course All of Pallett'sbooksarewell written with ( - h a i r r n a nG: . S m o l . T h e O p e nU n i v e r s i t yp r e s s goodillustrations.Very usefulto avionics ( l(r76). engineers.May be considered ascomDanion 19. L.'lect rrtrnag,tct i( s and L'Iect ron ics. Cou rse v o l u m e st o t h i sb o o k . C h a i r r n a nJ:. J . S p a r k e sT. h e O p e nU n i v e r s i t l , 7. N.H. Birch and A.E. Branrson, night Briefing P r e s(sl 9 7 J) . J'rtr .&/ors,Volume3,Radb Aidsto Air Nayisatiort. Both l5 and l(r arehighlyrecomnrended as P i t m a n( 4 t h e d i r i o n .1 9 7 9 ) . c o u r s en t a t c r i a l .

232

'

I am sureI have omitted many books which are equalin merit to those listed. The editions referred to ( I st unlessotherwiseindicated) are those with which I am familiar; the readeris rCvisedto check that theseare the latest editions. I havenot mentioned any mathematicstextbooks. but for thosewho wish to study aircraft radio systemsin depth, considerablemathematicmaturitv is needed. Many books havetitles which are variations on the theme 'Mathematics,forTechnicians'most of which will be useful. The Open University is againa

usefulsourcefor thoserequiringappliedmathematics at undergraduatelevel. A huge sourceof materialon avionicscomesfrom organizations which arenot publishinghouses.The readeris advisedto consultthe publicationslistsof nationalaviationauthorities,suchas the CAA and FAA, and alsoARINC and ICAO. Aircraft and equipmentmanufacturers producecomprehensive manualsof varyingstandardswhich will be consulted by the reader,asa matterof course,in the execution of his dutiesin the aircraft industrv.

83

Glossary

a.c. - Alternating current: current flow which navigation rystems. changesdirection periodically. Altitude trip - A discrete signal from a radio Acquisition - The recognitionof a signal. altimeter which changesstateas the aircraft passes ACU - Antenna CouplingUnit. through a pre-determinedaltitude. A/D - Analogueto Digital conversion.,, a.m. - Amplitude modulation: meaningfulvariation Address- A"Brq,qpof pits which identify a particular of the amplitude of an r.f. carrier location in memory oi'idiri'e other data sourceor Analog,analogue- A quantity or signalwhich varies destination. continuouslyand representssomeother ADF - Automatic Direction Finder: a systemcapable continuouslyvarying quantity;hence an analog of automaticallygivingthe bearingto a fixed radio circuit which processes suchsignals,an analog transmitter. computerwhich performsarithmetic operationson ADI - Attitude Director Indicator: an instrument suchsignals. which demandsattitude changeswhich, if AND gate - A logic circuit which givesan output of I if, and only if, all its inputs are l. executed,causethe aircraft to fly a path determinedby radio or other sensors. Angle of cut - The anglebetweentwo hyperbolicor ADSEL - AddressSelectiveSSR: British circularl.o.p. at their point of intersection. developmentof SSR,compatiblewith DABS. Antenna,aerial - A devicespecificallydesignedto a.f.c. - Automatic frequencycontrol: automatic convertr.f. current flow to electro-magnetic tuningof a radio receiver. radiation,or vice versa. AFCS - Automatic Flight Control System. a.o.c.- Automatic overloadcontrol: a circuit which a.g.c.- Automatic gain control of a radio receiver. preventsan excessive rateoftriggeringofa AID - Aircraft InstallationDelay: the time elapsed transponder. betweentransmissionand receptionin a Radio APU - Auxiliary Power Unit: a motor-generator Altimeter installationwhen the aircraft is in the fitted to an aircraft for the purposeof providing touchdownposition. ground power and startingthe main engines. Air Data Computer- A unit which senses, evaluates A/R - Altitude Reporting: automatic coded and outputs quantitiesassociated with altitude, transmissionof altitude from aircraft to ATC in an airspeed, verticalspeedand Machnumber. SSRsystem. Air speed- The speedof an aircraft relativeto the air Array - A group ofregularly arrangeddevices,for massthrough which it is flying. example,antennasor memorycells. AIS - Audio IntegratingSystem:the electronic ASP - Audio SelectionPanel. interfacebetweencrew membersand audiosources Associatedidentity - The identification of co-located and destinations. VOR and DM!. beaconsby synchronized Algorithm - A sequenceof stepswhich alwaysleads transmissions of the sameMorsecodecharacters of a conclusion. from eachbeacon. Alphanumerics- Displayedcharacterswhich may be Astable- Havingno stablestate. lettersor numeralsor both. ATC - Air Traffic Control. Altimeter- Pneumatic:a pressure measuring device ATE - AutomaticTest Equipment. calibratedin feet;alsobarometric,pressure and ATU - AntennaTuning Unit. servoaltimeter. Radio:a systemwhich measures A-typedisplay- A c.r.t. displayin which the timebase the heightof an aircraftabovethe earth'ssurface. deflectionis horizontaland the signaldeflectionis Altitude hole - [,ossof signalin an f.m.c.w.radardue vertical. to round trip traveltime beingequal to the Automatic standby- seeSignalcontrolled search. modulationperiod:of importancein Doppler 234

-

:!

.d - l

:{

g

Backlash - PleyGdfrdrani:l linkage such as scannerdriles, Balun - Balancedto unbelancedtransformer: used. for example. rn conncstint a co.axial feeder (unbalanced)to a (xntrc fed half wavedipole antenna(balancedl. - The referencepressurelevql of a Barometric di barometricdtimeter asset by the pilot. Base- The integral value of the number of sym[ols in a countrngsystem. The central regionof a bipolar transistor. Baseline- The line joining two ground stations in a hyperbolicnavigationsystem. b.c.d. - Binary coded decimal: a positional code in which eachdecimaldigit is binary coded as a 4-bit word. Beamsoftening - Progressivereduction in gain of demandsignalchannelin landing systems. Bearing- The angle,measuredin a clockwise direction,betweena referenceline through the aircraft and a line joining the aircraft and the object to which bearingis being measured.The referenceline may point to magneticor true Norfh or be in line with the aircraft'slongitudinal axisfor magnetic,true or relativebearingrespectively. Beat frequency - The difference frequency resulting when two sinusoidsare mixed in a nori linear device. b.f.c. - Beat frequencyoscillator: an oscillator,the output of which is mixed with an incoming c.w. signalin order to producean audiblebeat frequency. Bidirectional - Refersto an interfaceport or bus line which can transferdata in either direction. Binary number system - A counting system using 2 as its baseand employing the symbols0 and l. Binary signal- A signalwhich can take on one of two states,one lepresentingthe bit 0, the other the bit l. Bipolar transistor - A solid state device utilizing two typesof current carriers:holesand electrons; capableof amplifying or switchingfunctionspvhen usedin suitablecircuits. Bistable- Havingtwo stablestates. Bit - A singlebinary digit, i.e. the symbol 0 or l. BITE - Built In Test Equipment. Blade antenna- A rigid quarter wave antenna,the bladeshapeof which givesoperationover a wide band of frequencies;electricalcomponehtsmay be housedwithin the blade for the purposeof improvingthe performance. BNR - Signalrepresentinga binary number. Bonding- Electrical:interconnectingmetal parts with conductorsin order to eliminatepotential

differences.Mechanical:joining parts to one anotherby methodsother than thoseinvolving bolts,screwsand rivets. Booleanalgebra- The algebraof two state,or binary, variables. Buffer - A circuit used for isolating or matching purposes. Bug - A fault, usually in software. A mark, fixed or set,on a meter face. Bus - One or more conductorsusedas an information path. A conductorusedto cany a particularpower supply to varioususer equlpments. Byte - A specificnumber of bits (usually 8) treated asa group: 8-bit word. c - Standardnotation for the speedofpropagation of e.m.wavesin freespace:c = 3 x 108 metresper sec= 186000 milesper sec= 162000 n.m. per sec. CADC - CentralAir Data Computer. Capsule- An evacuatedairtight containerusedto detect changesin pressure. Capture- The sensingof a radio beam suchas occurs in ILS. CAS - CollisionAvoidanceSystem. CDI - CourseDeviationIndicator:an instrument which presentssteeringsignalsto the pilot which, if obeyed,causethe aircraft to follow a particular flight path. CDU - Control DisplayUnit. Cell - A circuit or deviceusedfor storingone characteror word, the locationbeinggivenby a particularaddress.A singlechemicalsourceof electromotiveforce(e.m.f.). electronic Chip - A collectionof interconnected componentsformed on a singlesiliconwafer. joint which.by Chokeflange- A type of waveguide givesa good of a half-wave stub, use short-circuited electricalconnectionacrossthejoint. CIWS- CentralInstrumentWarningSystem. Clarifier- Tuningcontrol for the insertedcarrier oscillatorneededfor s.s.b.reception. Clear- To placeone or more storagelocationsin a particularstate,usuallyO. circuit in a system; Clock - The basicsynchronizing the waveformfrom sucha circuit. Clutter- Unwantedradarreturns. Co-axialcable- A pair of concentricconductors by an insulatingmaterialand usedfor separated of r.f. currentsup to about line transmission 4 GHz. Code - A systemof symbolsand rulesusedfor represe nting informationsuchasnumbers,letters and control signals.

2!F

Colocation- VHF navigationand DME beacons sharingthe samegeographicalsite; such beacons will operateon pairedfrequencies and use a s s o c i a t ei d e n t i t y . Commutator - A mechanicalor electronicrotating contactdevice. Compassrose- A circular scalemarked in degrees and usedfor indicatingaircraft heading. Computer - A machineor systemwhich performs arithmeticand logicalfunctions;maybe analogue or digital,electronicor mechanical. Contour - Blankingof the strongestsignalsin a weatherradar. Control bus - A bus usedto carry a variety of control signals. CPU - CentralProcessing Unit: a devicecapableof executinginstructionsobtainedfrom memory or other sources,a term often usedfor a microprocessor. Crossmodulation - Modulation of a desiredsignalby an unwantedsignal. Crosstrackdeviation- The perpendiculardistance betweenaircraft position and the desiredtrack. c.r.t. - Cathoderay tube: an evacuatedthermionic devicewhich hasan electrongun at one end and a fluorescentscreenat the other;the electronbeam from the gun writes a pattern on the screenwhich is a function of analoguesignalsappliedto the device. Crystal- A frequencysensitivedeviceusedto determineand maintain the frequencyof oscillatorsor establishnarrowpassor stop bands within closelimits. A point contactdiode which may find useas a mixer or rectifier in microwave systems. CVOR - ConventionalVOR (beacon). CVR - CockpitVoice Recorder. c.w.-Continuous wave:continuoustransmission of unmodulatedr.f. duringthe time the transmitteris keyed. Cycle - One of a recurringseriesof events. D/A - Digitalto Analogueconversion. DABS - DiscreteAddressBeaconSystem:American development of SSR;compatiblewith ADSEL. Ilata - That on which a computeror processor operates; singular:datum. Databus - Usuallyeither8 or l6 bi-directionallines capableof carryinginformation to and from the CPU,memoryor interfacedevices. Datasavememory - A memory devicewhich does not lose the data storedin it when power is switchedoff. May requirea battery. dB - Decibel:unit of relativepoweror voltage 235

measuredon a logarithmic scalewith multiplying factorsof l0 and 20 respectively. dBm - Unit of power: decibelsrelativeto I rnW. d.c. - Direct current:currentflow in one direction only. d.d.m. - Differencein depth of modulation: refersto the 90 Hz and 150 Hz modulating frequenciesused in ILS. Deadreckoning- Calculationof position using vehiclespeedor acceleration,time in motion, direction and the known co-ordinatesof the initial position. The absoluteerror in a dead reckoning systemincreases, without bound, with distance flown. Deccanavigator- A c.w. hyperbolic navigation system. Decimalnumber system- A counting systemusing l0 asits baseand employingthe symbolsO,1,2, 3,4,5,6,7,8,9. Decometer- A phasemeter usedin the Decca navigationsystem;threeareemployed:purple, red and green. fletector - An electroniccircuit which de-modulates amplitude or pulse-modulatedwaveforms. D/F - DirectionFinding. DH - DecisionHeight: the height at which the runway shouldbe in view when on an approach. Differentiator - A devicewhich givesan output proportionalto the rate of changbof input. Digital - A systemor deviceusingdiscretesignalsto representparticular valuesof a varying or fixed quantity numerically;a signalin sucha system. Diode- A semiconductor or thermionicdevicewhich preventsflow of current in one direction. Dipole, half wave - An antennaconsistingof two co-linearlengthsof conductoreachone quarter wavelengthlong at the desiredfrequencyof operation. The two polesmay form a 'V' shapeif a more omni-directionalpolar diagramthan a figure of eight is required. Direct access- Capability of readingdata from a particularaddressin memory without havingto access throughprecedingstoragearea. Disc - Storagedevicegivinglargecapacity and almost direct access.' Discretesignal- A signalcharacterizedby being either'on'or'off'. Discriminator- A circuit which convertsfrequency or phasedifferencesjnto amplitude variations. DITS - Digital Informition TransferSystem: A R I N C4 2 9 . DME - DistanceMeasuringEquipment: a secondary radarsystemcapableof measuringthe slant range of a fixed transponder.

Doppler effect - The changein frequencynoted , nearest100 ft. when a wave sourceis moving relativeto an EPROM - ErasableProgrammableROM: a ROM observer. which, usingsuitableequipment,canbe erasedand Doppler navigation system * A dead reckoning re-programmed. systemconsistingof a Dopplerradarand a computer which, with headinginformation from a Fan-in- The numberof inputsthat canbe handled, compass,calculatesthe position of the aircraft. usuallyby a logiccircuit. Doppler radar - A primary radar systemwhich Fan marker - A position fixing aid for en route utilizes the.Dopplereffect to measuretwo or more airwaysnavigation:alsoZ marker. of groundspeed,drift angle,longitudinalvelocity, Fan-out- The numberof circuitswhich canbe driven lateralvelocity,verticalvelocity. from an output terminal,usuallyof a logiccircuit. Doppler shift - The differencebetweentransmit and Fasterect- The applicationof a largervoltagethan . receivefrequenciesin a systemsubjectto the requiredfor normalrunningto a gyro in orderto Dopplereffect. reducethe time takento achieveoperatingspeed. Doppler spectrum- A band of Doppler shift f.e.t. - Field effect transistor:a solidstatedevice frequencies producedby a Dopplerradarwith a utilizingone type of curretrtcarrier(c.f. bipolar finite beamwidth. transistor). DPSK - Differential PhaseShift Keying: a form of Fetch - The part of a digital computercycle during digitalrnodulationin which a phasereversal which the locationof the next instructionis 'l'. indicatesthe binary digit determined,that instructionis takenfrom memory thift angle- The anglebetweenheadingand track. and enteredinto a register. d.s.b.- Doublesideband: transmission of both side Filter - A circuit which selectswanted or rejects bandsof an a.m.wave,the carrierbeingsuppressed. unwantedsignals, usuallyon the basisof frequency. Duplexer - A devicewhich permits sharingof one Firmwave- Instructionsstoredin ROM and hence not easilyamended. circuit or transmissionchannelby two signalsin particularuseof one antennafor receptionand Flag- A signalwhich hastwo discretestates,one of which (low) usuallyindicatesfailure. transmission. DVOR - Doppler VOR (beacon). Flagbit - The softwareequivalentof a flip flop which may be usedasan indicator,for example,to d.v.s.t.- Directview storagetube: a type of c.r.t. indicatethe beginningor end of a pieceof data. with a high intensity display. Flare - The final phaseof a landingduring which the Dynamic RAM - A type of RAM in which data rdte of descentis reducedwith height. storedwill fade unlessperiodicallyrefreshed. Flashover - Dischargethrough air between conductorsacrosswhich a largepotentialexists. EADI - ElectronicADI: similarlyECDI and EHSL EAROM - ElectricallyAlterable ROM:seeEPROM. Flight level - With a pneumaticaltimeterreference s e ta t 1 0 1 3 . 2 5m b a ro r 2 9 . 9 2i n . H gt h e i n d i c a t e d Earth - seeGround. ECL - Emitter CoupledLogic:logiccircuitsemploying height,to the nearesthundredfeet,is thc flight to mode .bipolar transistorsgivingvery fast operation, level;thereply from an ATC Transponder C interrogations. reasonablefan-in and very good fan-out. e.h.t.- Extra high tension:a sourceof e.m.f.,usually Flight log - A devicewhich recordsan aircraft's flight path in the horizontalplane,for example,a measuredin kilovolts,usedasa supplyfor c.r.t.'s power transmitters. rollermap. and high Flip flop - A circuit havingtwo stablestates,often Elapsedtime - The time betweentransmissionand referredto asa bi-stable.The circuit remainsin receptionin a radarsystem. Electroluminescent- A property of deviceswhich one stateuntil triggered,two triggersbeing convert electricalenergyto light. requiredto revertto the originalstate.A e.m. wayes- Electromagneticwaveswhich include mono-stablemay be referredto as a flip flop. representation of a Flowchart- A diagrammatic radio and light waves. e.m.f. Electromotiveforce. sequenceof operationswhich are often algorithmic. f.m. - Frequencymodulation:meaningfulvariation Enable- A signalwhich allowsa circuit to give an of the frequencyof a carrier. output; alternativelya gate(signal). Fruit - UnwantedSSR replies. Encodingaltimeter - A pneumaticaltimeterwith a parallelcoded output of9 to I I bits representing Frequencypairing - The permanentassociationof frequenciesin different systemssuchasVOR/DME the aircraft'sheight abovemean sealevel to the

237

and Localizer/GlidesloPe. Freeze- Not allowing updating of a weather radar picture. f.s.d. - Full scaledeflection. Garbling - Receivedsignalsoverlappingin time. Gate - A circuit, the output of which dependson certaininput conditionsbeingmet; for example AND, OR, NAND, NOR gates.A switching waveform. The regionof an f.e.t. which controls the output current. Gimbal - A frame in which a gyro is mounted so as to allow freedomof movementabout an axis perpendicularto the gyro spin axis. Glidepath/glideslope- The vertical plane approach path to a landingsite. That part of ILS which providesverticalguidance. GPWS- Ground Proximity WarningSystem. Gray code - A one bit changecode. Grey region- A condition of uncertainty. Ground - A point of zeropotential;alsoearth. Ground plane - A surfacewhich completelyreflects e.m. wavesand which, at the frequencyof interest, behavesasif it extendsto infinity in all directions. Ground speed- The speedof an aircraft projectedon to the earth'ssurface. Ground wave - A radio wave which follows the earth'ssurface. Gunn diode - A solid statedeviceutilizing the bunchingof current carriersand finding ' applicationasan oscillatorin microwavesystems. Gyroscope,gyro - A spinningmassfree to rotate about one or both of two axesperpendicularto one anotherand the axis of spin. In the absenieof externalforcesthe spin axis direction is fixed in space.

Hexadecimalnumber system - A counted system using l6 asits baseand employingthe symbols o , 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9A ,, B , C , D , E , F . High levellanguage- A vocabulary,togetherwith grammaticalrules,in which eachstatement correspondsto severalmachinecode instructions so making the task of programmingleSstedious; examplesare BASIC, FORTRAN, ATLAS, PASCAL. etc. Hot mic. -'A microphonewhich is permanentlylive irrespectiveof crew operatedswitch positions; live output is fed to the CVR. HSI ' Horizontal Situation Indicator: an instrument displayinginformation from the compassand v.h.f. navigationaids,the latter being in the form of deviationsignalswhich, in the caseof VOR, relate to a courseselectedon the sameinstrument. Hyperbolicnavigation- A meansof navigationusing a co-ordinatesystemof hyperboliclines definedby ground basedradio transmitters.

i.c. - Integratedcircuit: a devicecontaining electroniccircuitsinseparablyfabricatedas an integralpart of the deviceitself; often referredto as a chip. i.f.- Intermediatefrequency:the fixed frequencyat which most of the amplificationand selection takesplacein a superhetrodynereceiver. IFF - Identification Friend or Foe: military version of SSR. IFR - Instrument Flight Rules: apply when VFR are excludedby virtue of flying in controlled airspace or lack of visibility. IIS - InstrumentLandingSystem:the current standardICAO approachaid. Impatt diode - Impact avalanchetransit time diode: a silicon p-n junction reversebiasedto its avalanchethreshold;can be arrangedto act as a Handshake- Electricalverificationthat a data in microwavecircuits,henceits negativeresistance transferhastaken place. oscillators. use in with in Hard data Data which remains memory Impedance- The total oppositionto the flow of power removed. varieswith current;in generalimpedance Hardware-- The sum total of componcnts of a system frequency. which havea physicalexistence. in.Hg - Inchesol mercury: a unit of pressure Hard uV - Hard micro volts: the voltageacrossan measurement;the height of a column of mercury load. open circuit beingmeasured; supportedby the pressure Headup display- Equipmentwhich allows 29.92 in.Hg= 1013.25mbar= I standard information to be visuallypresentedto the pilot atmosphere,i.e. the pressureat meansealevel. while looking through the windscreen. - A signalwhich prevents,aparticularcircuit Inhibit of the aircraft nose direction tail to Heading The from performingits function. in degrees clockwise longitudinalaxismeasured INS - Inertial NavigationSystem:a non radio from eithermagneticor true North. navigationaid which computesthe aircraft'sposi Height ring The ground return from a vertical tion by deadreckoningusingthe measuredaccele' gives a bright height sidelobein a WeatherRadar of an airbornegyro stabilizedplatform. ration the origin. p.p.i. on centred the on ring 238

Lenc - A region bounded by lines of equal phasein trctmction - Coded rnformation which causesa the Deccaor Omeganavigationsystems. computer to perform an operationusuallyon data latch - A circuit that may be locked into one of two availableat an addresswhich forms part of the particularstableconditions. instruction. lnstruction set - The sum total of instructions which Leading edge- The edgeof a pulse which occurs first in time; cf. laggingedge. can be executedby a particularcomputer. leakage - Unwantedcouplingbetweentransmit and lntegrator - A device,the output of which is receiveantennas. proportional to the sum of past inputs. LED - Light Emitting Diode:'a semiconductordiode Intensity Modulation - Variation of the velotity of which emits light when forward biased. the electronbeamin a c.r.t. so asto causea Limiter - A circuit which limits the voltageexcursior correspondingvaliation in intensity of the of a waveform. brightnessof the display. Interface- The point at which two parts of a system Linear array - A one-dimensionalarray of antennas arrangedto producea beam which is narrow in or two systemsmeet. one plane,for example,a slotted waveguide. lntprferometer- An antennaarray, togetherwith l.o. - Local oscillator:the circuit usedto provide an phasediscriminators,capableof measuringthe output which is mixed with an incomingr.f in directionof arrivalof an e.m.wave. order to producethe i.f. in a superhetrodyne lnterlace Time multiplexing of modesof in radar system; receiver. in a secondary interrogation particularmodesA and C may be interlacedin Load - A devicewhich drawscurrent; to connect sucha device. SSR. Interrogator- The independentpart of3 secondary Localizer- That part of ILS givingazimuth guidance. binary radarsystem. Logic circuit - A circuit which processes signalsin accordancewith the rulesof Boolean lntemrpt - The suspensionof the executionof a usedin digitalsystems. algebra;extensively currentroutinewhile a computercarriesout an alternativeroutine; the signalwhich triggerssuch L,ook-uptable - A circuit, the output of which is the function valuecorrespondingto the input which anactron. a ROM which, for the argument;usually represents I/O - lnput-Output. example,might storethe sine(function values)or a IRS - Inertial ReferenceS)'stem:the heart of INS' largenumberof differentangles(arguments). Isodop - The line joining thosepoints on the earth's antenna-' An antenltaconsistingof a coil of waves. Loop e.m. which reflected from surface wire, usuallywound on a ferritecore,which, originatingfrom an airbornetransmitter,suffer the ideally, reactsonly to the changingmagneticfield sameDopplershift. in an e.m.wave;an ADF or an Omegaloop antenna coils. two mutuallyperpendicular in has employed frequency; variation of Jitter Random l . o . p .- L i n e o f p o s i t i o n . DME and MADGE in orderthat wantedreplies l.oran - Long rangeaid to navigation:a pulsed may be recognized. hyperbolicpositionfixing system;LoranA obsolete,LoranC - current,Loran D - short Key - To turn on a radiotransmitter. rangeversionof Loran C (Loran B - never Keyboard- A devicewhich allowsan operatorto operational). input-informationinto a computer;the keysor or LSA diode - Limited Space- chargeAccumulation characters switchesmay representalphanumeric diode:similarto Gunn diode. dedicatedfunctions. l.s.b.- Leastsignificantbit in a binaryword- Lower Klystron - A thermionic deviceemployingvelocity. sideband:the sidebandof an a.m.transmission nrodulationof an electronbeamand capableof which is of lower frequencythan the carrier. oscillatingor amplifying continuouslyat - LargeScaleIntegration:a largenumber of ISI microwavefrequencies. circuits(usually1000or more)on a singlei.c., Knot -- The unit of speedusedin air and marine similarlySSI(small),MSI (medium)and VLSI navigation:I knot = I nauticalm.p.h. (very large). Laggingedge- The edgeof a pulsewhich occurslast Lubber line - A referenceline againstwhich a movlng scaleis measured. in time, i.e.the right-handedgeif the pulseis or drawnagainsta time viewedon an oscilloscope to the right. scaleincreasing

239

Machinelanguage- The basicinstructions,in binary characteristicsof a carrierwave in order to impress code,executedby a computer;maybe written informationon it. down in hexadecimal Monostable- A deviceor circuit with one stable or octal code. condition to which it will return, after a specified MADGE - MicrowaveAircraft Digital Guidance delay,when disturbed. Equipment: a secondaryradarsystemwith civil Morsecode - Combinationsof dots and dashes applicationasan approachaid to offshore to lettersof the alphabetand to numerals. assigned platforms. FET: an Magnetron- A thermionic deviceemployingvelocity MOSFET- Metal-Oxide-Semiconductor f.e.t.wherethe gateconnectionis insulatedfrom modulation of an electronbeamin a magneticfield the drain to sourcechannelby an oxide of silicon. and capableof oscillatingat microwavefrequencies m.p.e.l.- Maximumpermissible exposurelevel:in with very high power output for short periods. relation to microwaveradiation. Magslip- A four wire synchroresolvercommonly usedto resolvea timebasewaveforminto sine and MP mode - Multipulsemode: a transmissionsequence which allowslane ambiguitiesto be resolvedin a cosinecomponentswith respectto the azimuth DeccaNavigationSystem. angularposition of a radarscanner. m.s.b.- Most significantbit in a binary word. Main bang - The transmissionfrom a pulsedradar. m.s.l.- Meansealevel. Main lobe - The predominantcigar-shaped part of a MTL - Minimum TriggeringLevel; the signal directionalantennapolar diagram. amplituderequiredto initiatean action;of Marker beacon- A radio aid transmittinga vertical significancein secondaryradarsystems. directionalbeam so allowinga pilot to fix his Multiplexing - Transmittingmore than one signal position on an app.roach path or airway. over a singlelink; signalsmay be separatedin time mbar - Millibar:a unit of pressure measurement; or frequency. 1013.25mbar = I standardatmosphere i.e.the pressure Multivibrator - A circuit which can be rn one of two at meansealevel. states,neither,one or both of which may be stable, Memory - A devicervhich storesinformation for future use. henceastable.monostableand bistable Microcomputer- A contpletedigital computing multivibrators. systemthe hardwareof which comprises a NAND gate - A logic circuit which givesan output of microprocessor and other LSI circuitssuchas 0 if, and only if, a.llits inputs are l. memoryand input/outputports;a singlechip with NDB - Non-DirectionalBeacon:a radio transmitter circuitscapableol' providingcontrol.arithmetic/ at a known geographicalsite for useby ADF. logicoperations. nlentoryand input/output. logic - Representationof the bit I by a lowNegative Microphone,mic. - A transducer which converts voltageand the bit 0 by a high voltage. soundwavesto electricalsignals. n.m. - Nauticalmile: the lengthof a minute of arc ot - A chip which providesthe control Microprocessor longitudeon the equator;approximately6076 ft and arithmetic/logic operationsrequiredby a o r 1 8 5 2m . digitalconrputer;a chip caprbleof processing Noisefigure - The s.n.r.at the input of a receiver informationin digitaltbrrn in accordance with a dividedby the s.n.r.at the'output:a measureof codedand storedsetof instructionsin order to receivey,'noisiness'. c o n t r o lt h e o p e r a t i o no l ' o t h e rc i r c u i t r yo r Nor\-retumto zero - A binary signalwhich makesa equlpment. transitiononly when the bit I needsto be Microstrip - Transntissionlinesand p3i\ive represented. componentsformedbv depositingmetalsrripsot' Non-volatilemembry - A memory that holds data suitableshapesand dinrensidns on one sideof a afterpowerhasbeendisconnected. dielectricsubstrate. the other sideof which is NOR gate - A logic circuit which givesan cutput of I completelycoatedwith metalactingas a ground if, and only if, all its inputsare0. plane. Null - A no error signalcondition in servosystems. Microwaves- A.vagueterm usedto describeradio frequenciesabove1000 MHz. Indicator. OBI - Omni-Bearing MLS - MicrowaveLandingSystem:replacementfor Selector:the controlwith OBS- Omni-Bearing ILS. which the pilot selectsthe desiredVOR radial. m.o. - I\'lasteroscillator:the circuit which provides Octal number system- A counting systemusing8 as the frequencyreferencein a radio transmitter. its baseandemplo.,ingthe symbols0, I , 2, 3, 4, 5 ,. Modulation - Variation of one or more 6.7. 240

Omega- A c.w. hyperbolic navigationsystemgiving worldwide cover. Omnidirectionalantenna- An antennawhich radiates in or receivesfrom all directionsequally; impossible to achievein three dimensions. Omni-station- A VOR ground transmitter. One bit drange coile : A binary code in which only one bit in the word changeswith each count, for example,Gray code. One shot - A monostablemultivibrator. On-line- Refersto equipment in direct interactive communicationwith a computer. Refersto the installationand commissioningof a systemor the stateof a systemwhen operational. ONS - OmegaNavigationSystem. OBencentre- A p.p.i. display in which the radial timebaseline has its origin on an arc of non zero radius,the arc representingzero n.m. range. OR gate- A logic circuit which givesan output of 0 if, and only if, all its inputs are 0. Orrgn - A point with zero co-ordinates;the illuminatedspot on the screenof a c.r.t. at the start of the timebase. Orthogonal- At right angles(.in two or three dimensionalspaces). PA - Passenger Addresssystem. Power amplifier. Page- A number of words treated as a group; within memory typically 4096 consecutivebytes; for displaypurposesa portion of memory which can convenientlybe displayedon a VDU. Pairedfrequencies- seeFrequencypairing. Parallaxeror - The readingerror resultingfrom viewingan instrument or display from other than headon. Paralleloperation - Used by a digital systemin which one line or circuit dealswith only one bit in a word. Parity - A bit or bits addedto a group of bits such as to make the sum of all bits odd or even,hence'odd or evenparity which can be checkedfor error detectionor correction. p.e.p.- Peakenvelopepower: a measureof power in s.s.b.systems;sincecarrierpower cannotbe quoted the r.f. power dissipatedat the peakof the modulatingwaveform is given in specifications. Performanceindex - A global measureof the quality of a WeatherRadar system;relatedto maximum range. Peripherals- Units or devicesthat operatein conjunctionwith a computerbut arenot part of it; more generally,units or systemsconnectedto a but not part of the systemunderconsideration system.

Phantombeacon- A waypoint, in an RNAV system basedon VOR/DME, at which no actualradio beaconexists,its positionbeingdefinedin terms of bearingand distancefrom the nearestin-range beacon. Photosensitive- A devicewhich changesits electrical characteristicswhen exposedto light; for exarnple photocell,photodiode,phototransistor. diode:may.be PIN diode - P-type/lnsulator/N-type frequencies. power at high switch usedas a higtr on an Pitot pressure- The dynamicair pressure aircraftcausedby its movementrelativeto the air masssurroundingit; dependenton both air speed and staticpressure. Planararray - A two-dimensionalarray of antennas arrangedto producea beam which is narrow in two planes,for examplea flat plateslottedarrayas usedin WeatherRadarsYstems. p.l.l. - Phaselock loop: a circuit which, by usinga an error signal, phasediscriminatorto generate controlsan oscillatorso as to makeits output equal in phaseand hencefrequencyto an input or demandsignal. Polardiagram- A plot of points of equal field strengthwhich givesa diagrammaticrepresentation of the directionalpropertiesof an antenna. Polarization- The planeof the e-fieldin an e.m. wave. Polling- lnterrogationof circuits,units or systemsto determinetheir stateof readinessto receiveor transmitinformation;scanninginterruptlinesto determinewhich, if any, requireservicingby a computer. Port - A circuit providingelectricalaccess(in or out) to a system,usually a computer system. Positionfeedback- A signalrepresentingthe position of the output of a positioncontrol servosystem which may be comparedwith an input or demand signalso as to producean error signal. Positionfixing - Finding the position of a vehiclein relationto a groundfeaturesuchasa radio beacon. Positivelogic - Representationof the bit I by a high voltageand the bit 0 by a low voltage. Potentiometer- A three'terminalvariableresistor; betweenthe wiper terminaland the resistance eitherof the end terminalsvarieswith adjustment; the resistancebetweenthe end terminalsis fixed' p.p.i. - Planpositionindicator:a radardisplaywhich showsthe relativepositionof targetsin a plane; targetson the samebearingwill be superimposed on the display if they havethe sameslant range. of p.p.m.- Pulsepositionmodulation:transmission (in of position time) tEe varying by information pulseswithin a grouP' 241

p.r.f. - Pulserepetition frequency: the number of pulsesper second;may be usedto quantify pulse groupsper second. Primary radar - A radar (radio detecting and ranging) systemwhich detectsthe reflectionsof its own transmissions from an uncooperativetardet. Programpins - A group of connectorpins Jomeof which may be groundedto selectparticular modes of operationof a systemfrom severaloptions available. hogrammable - The capability of accepting data which altersthe electricalstateof internal circuitry so as to be able to perform one of severalpossible tasks. hogramme counter - A CPU registerwhich holds the addressof the next instruction to be fetched from memory; automaticallyincrementedafter a fetch cycle. hogramme, progrtrn - A set of instructions. arrangedin an orderedsequence, which determine the operationscarriedout by a computer. PROM - ProgrammableROM; programmedafter manufactureaccordingto the user'ssilecifications; generallynot reprogrammable. p.r.p. - Pulserepetition period: the reciprocalof p.r.f. p.t.t. - Pressto transmit or pressto talk. Pr.rlse compression- A techniqueusedin radar systemswhich allowsa relativelywide pulse to be transmittedand a narrowpulseto be processed in the videocircuits.

Radome- A detachableaircraft nosecone'madeof dielectricmaterial;more generallyany dielectric panel or antennacover. RAM - Random AccessMemory: a memory which affords immediateaccessto any location whereby information may be written in or read out. Raster- The pattern tracedby the electron beamin a c.r.t. RBI - RelativeBearingIndicator: displaysthe relativebearingof an NDB. Read- To senseinformation containedin some devicesuch as memory or an input port. Real time - Computation relatingto a processduring the time that the processoccurs;the resultsof such computation may be usedto control the relatedprocess. Refreshing- The processof restoring the chargeof capacitorswhich store the contentsof dynamic RAM: at intervalsof, for example,slightiy lessthan I ps cellsare automaticallyread, the resultsthen being written into the samecells. Register- A memory devicewith minimal access time usedfor the temporary storageof binary coded information; usually a collection of flip flops,onefor eachbit which can be stored. Resolver- A devicewhich can give signals representingthe sine and cosineofan angle. r.f. - Radio frequency. Rho-Rho-Rho- A position fixing systemwhich relieson measurementof distanceto fixed points: rho-rhosystemsgiveambiguousfixes. Rho-theta- A position fixing systemwhich relieson Q+ode - A code usedin R/T operationsto identify measurementof distanceand bearingof a fixed the nature of commonly usedmessages, for point. example:QFE - atmosphericpressureat airfield Rising runway - A runway symbol on a flight level;QTE- true bearingfrom groundstation; director driven laterally by localizersignalsand QNH - atmosphericpressureat local sealevel. verticallyby radio altimetersignals. QE - QuadrantalError: the errorintroducedin ADF RMI - Radio MagneticIndicator: an aircraft due to re-radiationfrom the airframe. instrumentwhich indicatesrelativeand magnetic Qfactor - A measureof the selectivityof a tuned bearingsderivedfrom VOR and ADF. circuit. RNAV - Area navigation:navigationwhich doesnot Quadrature- At right angles:a 90o phasedifference necessarilyconfine the aircraft to a fixed airways betweensignals. system. Quarter-waveantenna- One of the conductorsof a ROM - ReadOnlyMemory: containspermanently half wavedipole mounted on a ground planewhich storedinformation written in during manufacture; servesto 'reflect' the quarter-waveconductor so as random accessis availableto all stored to produce,effectively,a dipole. information. Routine - A list of correctly sequencedcomputer Radarmile - The time taken for an e.m. warc io instructions;the terms routine and programare travel I n.m. and back,approximately12.36 ps. often interchangeable but the former is usually Radial - One of a set of straighthalf linesterminating appliedto a commonly usedset of instructions at a fixed point; a line of radio bearingfrom a which may be calledby other programs. 'VOR station. R/T - Radio telephony: speechcommunicationby Radio, radaraltimeter - seeAltimeter. modulatedradio waves. 242

when squitter is received;known also as automatic Scanconversion- In position: the conversionof standby. co-ordinatesfronr rho-theta(angleand distance)to X-Y (orthogonalgrid). In time: the conversion Skin effect - The tendencyof a.c. at h.f. and above to avoid the centreof a conductor so reducingthe betweenrate of receipt of data and (usually faster) useful crosssectionalareaand henceincreasing rateof display. resistanceto'current flow. Scanning- The processof causinga directionalbeam of e.m.radiationto sweepthrougha sectorof Skywave-- A radio wave refractedback to earth by the ionosphere. space.The processof polling interrupt lines or Skywavecontamination- Receptionof a skywave devices. and groundor spacewavesimultaneously;an Scott-T transformer - Used in synchro systemsfor exampleof multipathpropagation. three-wireto four-wire conversionor vice versa. Slant range- The actual rangeof a targetin a plane Scratchpad- Memory, often on the CPU chip, in horizontal. which is not necessarily which data neededfor subsequentoperationsmay be temporarily stored. Display on which data may SLS - Side Lobe Suppression:a techniqueusedin be checkedbeforebeingenteredinto the main SSRto preventinterrogationby sidelobes. s.m.o.- StabilizedMasterOscillator. memory. s.n.r.- Signalto noisepower ratio. Search- The processleadingto acquisition. Soft data - Data which is lost when power is removed. Secondtrace echo An echo or return of a radar - Programs,languages and proceduresof a Software transmissionwhich givesa falserangereadingsince part of software hasa no system: computer transmission receiver after a at the it arrives ' physicalexistenceother than as written down on subsequentto the one givingrise to the echo. by the state paperor storedin codeasrepresented Secondaryradar - A radar systemwhi,chrequiresa of a signalor device. by an target;a radiolink is established cooperative - A radio wavewhich travelsin a straight Spacewave interrogator,the return or reply being suppliedby line being neither refractednor reflected. on receiptof the interrogation. a transponder Spectrum- The sum total offrequency compon€nts Selcal- Selectivecalling:automaticalertingsystem of a signal. usinga v.h.f.or h.f. groundto air link to a SPI - SpecialPositionIndicator: an additionalpulse particular aircraft or group of aircraft. of r.f. which may be radiatedby an SSR Sequentialaccess- Readingdata from a particular transponderfor identificationpurposes. addressin memory havinggone through preceding Squitter - Random transmissionof pairs of pulsesof storageareain order to find that address;for r.f. from a DME beaconas requiredfor the example.storageon magnetictape. ' in which operationof signalcontrolledsearch. system by a digital Used Serialoperation s.s.b.- Singlesideband:the transmissionof one one line or circuit dealswith all the bits in a word sidebandof an amplitudemodulatedwave. sequentially. Radar:a secondary Servoloop - A systemin which a signalrepresenting SSR- SecondarySurveillance transponder an airborne employing system with an radar for back comparison output is fed the which transmitsinformationrelatingto identity the amplifieddifference,or error input reference, and/or altitude;rangeandbearingis availableby signal,beingusedto control the output. elapsedtime and usinga directional measuring indicator Sevensmall bar-shaped Seven-segment interrogation. light sourcesarrangedin a figure of eight pattern, suclithat activatingparticularcombinationsof the Stable- A mechanicalor electricalstatewhich is automaticallyrestoredaftera disturbance. sevensourcescausesa characterto be displayed' used memory- Memorywhich storesinformation commonly Static Shadowmaskc.r.t. A type of c.r.t. in sucha way that it doe'snot needrefreshing. for colourdisplays. serially Static pressure- The air pressuredue to still air; Shift register- A registerwhich is accessed with height. decreases both in and out; variationscan giveparallelto serial s.t.c.- Sensitivitytime control:variationof receiver o r s e r i atlo p a r a l l ecl o n v e r s i o n . gainwith time so asto makethe output amplitude Sidebands- Thosebandsof frequencies.either side independentof the rangeof the receivedsignal of the carrierfrequency,producedby modulatioit. source. Sidelobe- Thosepartsof a directionalantennapolar Subroutine- A smallprogramor routinewhich may diagrameithersideof the main lobe. be calledby a largerprogramor routine to perform Signalcontrolled search- Allowing DME to only a specificoperation. transmitting and searching hence commence

243

,ar(

Super flag - A high level warning signal providing sufficient current at 28 V d.c. to energizea relay so indicatingvalid or no-warningstatus. Superhetrodynereceiver,superhet- A radio receiver in which the receivedsignalis mixed (hetrodyned) with a tuneablelocally generatedsignalin order to produce a constanti.f. Sweptgain - An alternativeterm to s.t.c. s.w.r. - Standingwaveratio: seev.s.w.r. Synchro devices- A type of transducerwhich convertsangularposition to an electricalsignalor vice versa;all synchrosare transformerswith both rotatableand stationarycoils. Three wire devices establisha unique relationshipbetweenthe rotor angleand the voltagedistribution in the three coil stator; four wire devicesestablishvoltageswhich dependon the sine and cosineof the rotor angle and are thus termed synchroresolvers. Designationsare STRX or TR: synchro torque receiver;STTXor TX: synchrotorque transmitter; DTTX or TDX or CDX: differential synchro torque transmitter;CT: control transformer;RS: synchro resolver. Synchronization - Changesrelated in frequency, time or position.

Track - The actual direction of movementof an aircraft projectedonto the earth and measuredin degreesclockwisefrom magrreticNsrth. Track angleetror - The angulardifferencebetween track and desiredtrack. Transducer- A devicewhich convertsinput energy ofone kind into output energyof anotherkind which bearsa known relationto the input. Transponder- The triggeredpart of a secondary radarsystem. t.r.f. - Tuned radio frequency: a basicradio receiver in which selectionand amplification of the modulatedsignalis carriedout in the r.f. stages, there being no i.f. Trigger- A signal,usually a pulse,which initiatesa circuit action. Tri-statebuffer - A buffer which can assumeone oi three states:0 or I when requiredto feed a load, high impedanceotherwise;the high impedance stateexistsin the absenceof an enablesignal. TRSB - Time ReferencedScanningBeam: adopted by the ICAO as the techniqueto be employedby MLS. TTL - Transistor-Transistor Logic: logic circuits employing bipolar transistorsfabricatedon i.c.s. giving fast operation and good.fan-inand out. TACAN - Tactical Air Navigation:a military sysrem Tunnel diode - A type of semiconductordevice which givesrho-thetanavigation;therangingpart which can be made to exhibit a negativeresistance has the samecharacteristics as DME. characteristic undercertainconditions. Tape,magnetic- Storagederriceusingsequential TVOR - Terminal VOR: a low power VOR station access. situatedat an airfield. Telephone,tel. - A transducerwhich converts Two.from five code - A code commonly usedfor electricalsignalsto soundwaves. frequencyselection;any two from five wiresmay Time base- A waveformwhich changeslinearly with be groundedgiving ten possiblecombinations,ohe time; the term is normally appliedto the waveform for eachof the digits 0-9. which causesregulardeflectionof the electron beamin a c.r.t. so as to trace a line representinga USART - UniversalSynchronous/Asynch ronous time axis, the line alsobeing referredto as a Receiver/Transmitter:a devicewhich interfaces timebase. two digital circuits the timing of which may or Time constant- A measureof the degreeof may not be related;similarlyUART and USRT. resistanceto change:if a systemis subjectto an u.s.b.- Upper sideband: the sidebandof an a.m. externalinfluencewhich causesit to changefrom transmissionwhich is of higher frequencythan the one stateto anotherand it wereto executethat carrier. at WEROM - Ultra-violetEraseable ROM: an EPROM. ' change a rate equal to the initial rate it would completethe changein a time equal to the time constant. Varacterdiode - A voltagecontrolled variable To/From - Refersto selectedVOR radialsor capacitance;the capacitancevarieswith reverse omni-bearings; if the pilot complieswith VOR bias. derivedsteeringcommandshe will be flying v.c.o.- Voltagecontrolledoscillator. towardsthe beaconif a 'to' flag is in view and Velocity feedback- A signalwhich is proportional to away from the beaconif a 'from' flag is in view. the rate of changeof positionof the output of a Topple - The effect of allowingthe angularvelocity servosystem;in position control systemssuch of a VRG to fall below that at which it exhibits feedbackis usedto limit hunting, i.e. an excessive the propertiesof a gyro. number of overshoots. 24

VFR - Visual Flight Rules: apply in uncontrolled Wavelength- The distandebetween points of airspacewhen visibility allows;limitations on pilot identicalphaseangle;wavelengrh), = c/f. qualificationsand equipmentfitted are minimal Waypoint- A significantpoint on a route. under VFR. Wheatstonebridge - An electricalmeasuringcircuit Video signal- The post detector signalin a radar consistingof four impedance.rrn, .onn.-.tedin a receiver. closedchain;with an excitationsupply,connected Volatile memory - A memory that losesstoreddata to two oppositeterminalsin the chainthe currenr when power is disconnected. drawn from the other two terminalsis determined VOR - VHF Omni-Range:a systemgivingthe bearing by the ratio of the impedances. to a fixed groundradio beacon. Whip antenna- A quarter-wave antennamadefrom a VORTAC - VOR and TACAN beaconson the same t h i n r n e t a rl o d . geographical site,i.e. co-located, are termed Word - A groupof bits treatedasan entity; it may c o l l e c t i v e lay V O R T A Cb e a c o n . representan instruction,address or quantitv. VRG VerticalReference Gyro: a gyro to which Write - To recordinformation in somedevicesuchas gravltycontrolledfbrcesareappliedby an erection a memory or output port. systemsuchas to maintainthe spin axisin lhe verticalplane;usedto givesignafs proportionalto X-Y display- A p.p.i.displayon which rarget pitch and roll. positionis determinedin termsof horizontal(X) v.s.w.r.- Voltagestandingwave ratio: the ratio of the randvertical(Y) diSplacement from a datum point: maximum to minimum voltageof the standing it may be referredto as t.v. display. waveset up on a mismatchedline; = (l + (pr/pr)o.ty(l _ prlpr)o.s) v.s.w..r. > I 7*nner diode - A diode operatedwith reversebias so equalityindicatinga perf'ectmatch,p, and p1 that breakdownoccurs,the breakdownvoltage beingthe reverse(or reflecteci) power and forward remainingconstantfor a wide rangeof ,.urrri (or incident)powerrespectiveli. currents. Zone - A regionboundedby hyperboliclines - A hollow, round or rectangular, Waveguide metal separatedby a distanceequal to half a wavelength tube which is usedto transmite.m.energyat of the fundamentalfrequencyof a Deccachain. microwavefrequencies.

245

Exercises

2. Draw a block diagramof an f'm' transmitter' The following exercisesare givento test the reader's 3. Draw a block diagramof a superhetreceiver' this knowledgeaid understandingof the cont€nt of 4. Describefour differentways in which binary volumeiihis aim will be best achievedif the questions in electroniccircuits' digitsmay be represented arenot readuntil one is ready to attempt them' usedfor airborne commonly 5.- Desiribe fwo codes Havingworked through eachchapterthe exercises selection. radio frequencY associitedwith that chaptershouldbe attempted' 6. Compareanalogueand digital representationof attempted be should given which are papers Six test data. It is only aftir ihe whole book hasbeen read' test each 7. Discussbriefly the following:I'C'A.O', ARINC' to devoted be recommendedthat one hour found ATA 100, national aviationauthorities' oapertthe answersare not givenbut are to be papers *iittin ttte relevantchapters' Ideally the t€st reader Joufd be markedindependently,however'the subjective' albeit strouldbe able to givean assessment, Ghapter2 of his attempt at the written answer-typepapers' can be achievedfrom test An accurateassessment io l. Describetypicalantennatunitrgarrangements oaper6 by givingone mark for eachcorrectly system' comms h'f' a generalaviation20 channel utt.rnpt.a qu.ttion, deductinghalf a mark for each List and statethe functionof typicalaudio t: ' incorrectly attemptedquestion,leavingthe-score aircraft' systemson a largepassenger unchangedfor eachquestionnot attempted,then when a crew member hupptnt *hui Describe i. multiplying the resultby 10/6 to givea Percentage' transmitson v'h.f. comms' Someof the questionscan be usedto Senerate Draw the block diagramof a CVR showing 4. drawing with others,for example,thoseconcerned the sourcesof the inPuts' ramp tests,listingcontrols,etc' could clearly block diagrams, typicalv.h.f' commsantennas' Discuss 5. apply to any of the systemsdescribedherein' Even a howl; e:. itr- to mii' feeduackin an AIS leadsto it is *iittun extlnded set of questions,assuggested, fault? the isolate how would You unlikely that the syllabusfor any coursewill be prospective example, For completelycovered. aircraft radio maintenanceengineerswill be required to satisfyexaminingbodiesand/or employersin the Chapter3 following areas: ADF l. List the soulcesof errorsaffecting operation' how it 2'. Describequidrantalerror and explain maYbe ctlrrected. swingts 3. Describehow an ADF groundloop out. In addition they must show evidenceof havinghad - carried RMI Draw a situattondiagramand a dual pointer 4. of responsibilities the to assume experience sufficient 200"(M) ol a heading on piesentationfor an aircrait an engineer. anotherat *lth un NDB due north of the aircraftand are ADFs 2 and I numbers which to :OO; ,.tutiu. Chapter1 tunedresPecttvelY' basicprinciplesof ADF' of bandwidthin an 5' Exp[ainthe 1 . C o m m e n ot n the significance 6. Draw an ADF block installationdiagram' information link.

basicelectricaland electronicprinciples . legislation ramp,hangarand workshoppractices readingwiring and schematicdiagrams fault findingskiils,etc.

246

Chapter4 l. Describethe differencesbetweenthe radiated signalsfrom Doppler and conventionalVOR stations and explain why airborneequipmentoperation is not affected. ?. Explain the terms automaticand manualVOR. 3. Draw a situation diagramfor an aircraft on a headingof 090'(M) with i selectedradial of 2g0; and with a fly right demand and from flag showing on the flight director. 4. Draw a dual VOR block installationdiasram. 5. Describehow information derivedfrom"a VOR receiveris presented to the crew. 6. DiscusstypicalVOR antennas.

a DME ihterrogator (assumeswitched on and any warm up time expired). ?. Explain how echo protection can be achievedin DME. Why might a DME interrogatorreceivelessthan ?l^ IOO%replies'! 4. Describethe arrangements for colocated beacons. 5. Describe,in generalterms,how you would carry out a ramp test of DME. 6. Draw a simplifiedDME block schematicdiagram.

Chapter8

Explain the needfor and the implementationof l, SLS in SSR. 2. Explain the terms fruit and garblingas appliedto SSR. l. Explain why a marker sensitivityswitch is 3. What is successive required. detectionand whv is it 2. Describethe needfor and a typical implementa_ necessaryin an ATC transponder? 4. Draw a block schematicdiagramand explain the tion of loadingcompensation fo, an ILS installation. actionof a decoderin an ATC transponder. Draw a block diagramof a glideslope receiver. 1 5. Describehow barometricaltituie may be Describethe outputsfrom iLS to the aircraft,s t. encodedinto a form suitablefor selectingthe reply to instrumentationand to other systems. a mode C interrogation. 5. Describe I L S a n dm a r k e ri h a n n e l l i n g 6. Draw a typicalATC transponder arrangements statinghow selectionis made. controller. statingthe purposeof eachcontrol. 6. Describe,in generalterms,how you would carry out a ramp test of an ILS. Chapter 5

Chapter9 Chapter 6 Compareplatformand line of sightstabilization. l. l. Explainhow, in distancerelatedphasemeasuring 2. Describe,briefly, video signalprocessingin a digitalweatherradar. navigationsystems, errorsdue to changes in clock 3. Describehow a p.p.i.displaycan be usedto offset can be minimized presentinformationrelatingto weatheraheadof the 2. List the factorsaffectingpropagationof Omega aircraft, signalsstatingfor each,how, if atill, compensation is 4. How doesa weatherradarflat plateantenna made. achievea narrowdirectionalbeam? 3. Describebriefly the generalprocedurefor skin 5. Describethe safetyprecautions mapplngprior to decidingthe positionof an Omesa to be observed when operatingweatheriadar, statingthe possible antenna. consequences 4. Describehow Deccachainsaredesignated. ofnot doingso. 6. Describe 5. Explainhow laneambiguities you would checka waveguide how run in De"cca may be for condition resolvedby usingthe Mp mode. 7. Discusscontour,STC and AGC in a weather 6 Describethe characteristics of the radiatedsignals radar. fromaLoranCchain. 8. Describehow rangeand bearingresolutionmay . be improvedin a weatherradarstatinethe d i s a d v a n t a goefst a k i n gs u c hm e a s u r e i t og i v e Chapter7 rmprovement. 9 Explainthe basicprinciplesof operationof a l. Describethefour possiblemodesof operation of Ryan Stormscope. 247

Chapter10 l. Describethe Doppler effect asutilized in an airborneDoPPlerradar. 2. Explain how a moving antennaDoppler radar measuresdrift angle' 3. Discussfactorsleadingto a choiceof f'm'c'w' for Doppler radars. 4. Explain the needfor a land/seaswitch' 5. Driw a simplifiedblock diagramof a Doppler navigationsYstem. 6. Describe,in generalterms,how you would carry out a ramp test of a Doppler navigator.

the airlinerof the 1980sand beyond,payingparticular of informationfrom attentionto the presentation radio sensors. 2. Explainthe basicprinciplesof how an automatic datalink usingh.f. and/orv.h.f. commscould be set up. 3. CompareADSEL/DABSwith currentSSR. could be 4. Explainone way in which satellites usedfor navigationPurposes. 5. Describe,briefly, a TRSBMLS' 6. Explain how a collision risk measuremay be arrivedat. 7. CompareDITS with currentmethodsof informationtranst'er. 8. Explainthe principlesof interferometry.

Chapter11 Distinguishbetweenbarometricand radio l. altitude commentingon the usefulnessof both. 2. Explainthe basicprinciplesof an f.m.c.w. altimeter. 3. Why doesDoppler shift havea negligibleeffect on a radio altimeter? 4. List the sourcesof error in radio altimeter systems. of using constant 5. Explain the main advantages differencefrequencyaltimetersoverconventional f.m.c.w.altimeters. 6. Draw a block schematicdiagramof a pulsedradio altimeter. 7. Which systemsrequiresignalsfronl a radio altimeter? What are the signalsinvolved?

Chapter12 l. Draw the block diagramof a generalarea navigationsystem. 2. Explainthe basicprinciplesof RNAV basedon VOR/DME beacons. 3. Draw and labeltypical RNAV, deviationand slantrangetriangles. 4. Describethe functionsperformedby a typical digitalnavigationcomputerbeingpart of a VOR/DN{E basedRNAV system. 5. Explainthe actionof a typicaldataentry/record unit. 6. Describe.in generalterms,how you would carry out a ranlp testof a VOR/DNIEbesedRNAV system.

Test Paper1 l. Compare,briefly, the differenttypesof antenna which may be found on aircraft. 2. Draw a simplifiedblock diagramof a computer and statebriefly the functionof eachblock. 3. Draw and explaina simpleanti'crosstalk network. 4. With the aid of a block diagramexplainthe actionof an h.f. ATU. 5. Describehow informationfrom an ADF is to the pilot. presented 6. Explainhow displayednoiseis reducedin a digitalweatherradar.

Test Paper2 Describetwo waysof modulatinga c.w. carrier' l. 2. Discussnavigationusingradioaidsunder the deadreckoning,rho'theta,rho'rho-rho, headings, theta-thetaand hYPerbolic. 3. Draw a simplifiedblock diagranrand explainthe payingparticular actionof a frequencysynthesizer to se,lection. attention 4. Draw a block diagramof a VOR receiver' 5. Definethe termsjitter and squitter. 6. Describe,in generalterms,how you would carry out a ramp testof a line of sightscanner stabilizationsYStem'

Test Paper 3 Chapter 13 l. 244

Describein generaltertns,the instrumentationof

Describethe modesof propagationusedwith l. airbclrneradioequiPment.

2. Define the terms hardwareand software. 3. Describe,with the aid of a sketch,a typical h.f. wire antennainstallationpaying particular attention to safetyfeatures. 4 Sketcha typical error curve for ADF statingeE, loop and field alignmenterrorsfor your curve. 5. Draw and explain a simplified ATC transponder block diagram. 6. Draw a typical weatherradar control panel statingpurposeofeach control.

Test Paper6

l. An e-m. waveof frequency30 MHz will havea wavelength of (a) l0m, (b) lOcm,(c) l0 ft. 2. A loop antennais usedf or (a) VOR and ADF, (b) ADF and Omega,(c) Omegaand VOR. 3. Above30 MHz propagationis by (a) space wave,(b) sky wave,(c) ground wave. 4. Fadingat l.f. and m.f. may be due to (a) poor receiversensitivity,(b) atmosphericattenuation, (c) simultaneousreceptionof sky and ground wave. 5. A carrierof amplitude5 V is amplitude Test Paper4 modulatedby a signalof amplitude3 V, the percentage modulationis (a) 157o,(b)tbJ%,(c) 60Io. l. Describehow a capacitive type antenna operates; 6, A constantamplitudemodulatingfrequencyof 500 kHz causes list systemswhich might usesuch an antenna. a carrierto vary between8798.5MHz 2. Explain how an interrupt signalmight be usedto and 8801.5MHz, the modulationindex is h\ ll3. (b) 3, (c) 6. achievea data transferfrom a radio sensorto a 7. Which of the following is not equivalentto navigationcomputer. 2 3 ' s ? ( a ) l 0 l I 1 2 ,( b ) 2 7 s , ( c ) 1 5 , u . 3. Describebriefly the basicprinciplesof ILS. 8. The b.c.d.equivalentof 3C16is (a) 0l l0 0000, 4. List the facilitiesprovided by a typical general (b) I I I 100,(c) 001I I 100. aviationAIS. 9. Which of the following, wherethe l.s.b.is an 5. Draw a simpleinterlock arrangementfor a dual odd parity bit, represents h.f. installation. 68,e? (a) 10001001, ( b ) I 1 0 0 0 1 0 0(,c ) 1 0 0 0 1 0 0 0 . 6. Describehow the possibility of lnterference is 10. An addressbus usuallyconsistsof (a) l6 minimizedin a multiple radio altimeter installation. bi-directionallines,(b) l6 unidirectional lines, (c) both bi- and uni-directional lines. I l. Rho-thetanavigationis the basisof Test Paper5 (a) VOR/DME, (b) Omega,(c) ADF. 12. To avoid earth loops in audio systemscable l. Describe,briefly, the fetch-decode-increment. screensshould be (a) earthed at both ends, executecycle of a computer. (b) not earthedat eitherend,(c) earthedat one end 2. List sourcesof interferenceto aircraft radio only. systemsand statemethodsusedto minim2e the 13. An aircraft v.h.f. communicationstransceiver effectsof suchsources. will provide(a) 720 channelsat 50 kHz spacing, 3. Discussthe term squelch. (b) 360 channelsat25 kHz spacing,(c)720 channels 4. Explain the principlesof lane ambiguity at 25 kHz spacing. resolutionin ONS. 14. An aircraft at flight level l0O will be able to 5. Describe,in generalterms,how you would carry communicatewith a v.h.f.groundstationat 100 ft out a ramp test of a VOR. abovem.s.l. at an approximatemaximum rangeof 6. At a point in a 180 n.m.leg of a flight the (a) 123 n.m.,(b) 12.3n.m.,(c) 135 n.m. following situation exists: 15. The minimum 1000Hz, 30%modulatedsignal heading 090"(M) levelto achievean output s.n.r.of 6 dB from an drift 10"port airline standardv.h.f. receiveris (a) I pV, (b) 3 gW, distanceto go 80 n.m. ( c ) 0 . 1 8x l 0 - r 2 W . desiredtrack 085"(M) 16. A typical a.f. responseof a v.h.f. transceiveris Draw the situation diagramand calculatethe across (a) 500 to 2000 Hz, (b) 300 to 25OOHz, distancereadingif the wind and headinghave (c) 300 to 4000 Hz. remainedunchangedfor the leg so far. (Assume 17. Typical radiatedpower from an airline = 0.017 radians/degree and that sin 0 0 if 0 < 0.2 standard v.h.f. commstransmitterwould be radians). ( a ) . 1 0W , ( b ) 3 0 w , ( c ) 5 0 w . 18. In an airline standardh.f. installationthe ATU would reducethe v.s.w.r.of the antennaand ATU

249

combinedto betterthan (a) l.l: l, (b) 1.3:I, ( c ) 1 . 5 :l . 19. An ARINC standardh.f. commssystemhas a typical power output of (a) 400 W p.e.p.,(b) 700 W

transmitin the range(a)962 37. TACANbeacons to l2l3 MHz,(b) 1030to 1090MHz,(c) 978 to

l2l3 MHz. 38. DME gives(a) range,(b) slant range,(c) ground speed. (c) 1000W p.e.p. p.e.p., is codedby (a) the 39. If a DME is in track subsequentlossof signal 20. A Selcaltransmission will causethe equipmentto (a) search, numberof r.f. bursts,(b) thepulsespacing, (b) automaticallystandby,(c) go into memory. (c) the modulatingtonesused.' (a) radiated reduces 40. Mode A and C pulsespacingare, respectively An anti-crosstalknetwork ii. (a) 8 and 2l gs, (b) 12 and 36 ps, (c) 8 and l7 ps. interference, (b) reduces conducted interference, 41 . Selectionof 5237 on an ATC transponderwill (c) preventstransmissionon both h.f. systems givethe following pulses,in order of transmission simultaneously. 2 2 . A i r l i n e s t a n d a r d A D F s w i l l , a f t e r Q E c o r r e c t i o(na, ) F l A l A 4 B . 2 C l C 2 D l D 2 D 4 F 2 , (b) Fl cl Al c2 A4Dl B2D2D4 F2, havean error bound of (a) 3', (b) 5", (c) 8". 23. The averageof the absolutevaluesof the peaks (c) Fl Cl C2 Al A4B2Dl D2D4 F2. 42. The output of an encodingaltimeter for an of an ADF error curvegive(a) field alignmenterror, (c) altitude of 7362 ft would give the code (b) loop alignmenterror, QE correction. phasc (a) reference Al A2 A4 Bl B2 C2C4D2,(b) Al A2 A4B/., phase the leads Itthe varlable )4. (c) A2 ,44 Cl C2. by 30" the magneticbearingto the VOR station will 43. The -3dB bandwidth of an ATC transponder Ue1a):O', (b) 210",(c) 150b. the is (a) 6 MHz, (b) 3 MHz, (c) 12 MHz. 090' and of receiver 25. With a selectedomni-bearing An ATC transpondershould not reply if phase by 280'. 4. referenci phase lagging the variable (a) Pl > Y2.+ 9 dBs,(b) Pl > P2 + 4.5 dBs' the flighi director will show (a) fly right; from, (c) Pl ( P2. (b) fly right;to, (c) fly left; to. 45. An X-band weather radar will operate at is VOR receiver of a 26. The frequencyrange (a\9375 MHz, (b) 5400 MHz, (c) 8800 MHz. ( a ) 1 0 8t o I 1 7 . 9 5M H z ,( b ) 1 0 8t o 11 1 . 9 5M H z , Secondtraceechoesare avoidedby 6. ( c ) I l 8 t o 1 3 5 . 9 5M H z . (a) a p.r.f. greaterthan someminimum, choosing is at 27. The VOR audio identificationtone a p.r.p.greaterthan someminimum, (b) (c) choosing 1020 Hz. (b) 1000 Hz, (a) 1350Hz, )6. Which of the following is a localizerfrequency? (c) increasingeither or both of the receiver sensitivityand transmitterpower. (a) I10.20 MHz, (b) 109.15MHz, (c) I12.10 MHz. glideslop e . The pilot reportspronouncedground returnstc does 47 )9. In which oi tire following bands (c) sideof the display,the most likely causeis (b) u.h.f. one (a) v.h.f., h.f., operate? (a) systempermanehtlyin the mappingmode, 3b. If the 90 Hz tone predominatesin a localizer (b) scannertilt faulty, (c) gyro toppled. receiverthe deviatiorrindicatorwill show (a) on (c) (b) fly right. 48. Broken radial lines are observedon the weather left, fly course, 31. The v.j.w.r. of a localizerantennashouldbe no radarindicator, the most likely causeis (a) a.f.c. circuit sweeping,(b) dirt in the magslip, more than (a) 5: I , (b) 3: I , (c) I .5:I . for (c) interferencefrom anotherradar' 32. An ONS, usingsoftwarecorrection 49. A weatherradarwith a p.r.f. of 200 and a duty predictableerrors,will giveaircraft position to an cycle of l0 x 10-3 would havea bandwidth of u..uru.y (r.m.s.)of (a) l-2 nm, (b) 0-l nm, approximately(a) I MHz, (b)_500kHz, (c) 3 MHz. (c) 2_inm. facilities gives navigation worldwide 50. A typicalmemory sizefor a digital weather 3t. Omega I 1.33 and on 10.2, transmitting (a) stations radaremployingan X-Y displayis (a) 4 kbit' five using (b) 8 kbit, (c) 32 kbit. 13.6 kHz, (b) eight stationstransmittingon.10.2, placedstations 51. If Pand R aretheVRG pitch and rollsignals 11.33and 13.6kHz,(c) strategically signals. respectivelyand 0 is the azimuth anglethen the transmittingfrequencymultiplexed (a) demandsignalfor a line of sight stabilization of a mastet usually consists 34. A Deccachain pat, (c) independentsystemis (a) Psin0+ Rcos0,(b) Pcos0+ Rsin0, and three slaves,(b) a master-slave (c) Pcos0x Rsin0. statlons. 52. An X-bandDoppler radarshowsa ground 35. The usablenight rangeof Deccais about of 400 knots, a reasonableestimateof the speed (c) (b) 360 nm. (a) 120 nm, 240 nm, shift would be (a) 5 kHz, (b) 500 Hz, Doppler 100 kHz, (a) pulsed at r.f' radiates C Loran 36. kHz. ,c) 12 (b) pulsedr.f. at 14 kHz, (c) c.w. at 100 kHz. 250

53. An f.m.c.w. Doppler radar operating at a frequencyof 8800 MHz, modulatedat 500 kHz with a depressionangle of 60' will have altitude holes at multiplesof approximately(a) 500 ft, (b) 2000 ft, (c) 8000 ft. 54. Wobbulationin a Doppler radar is usedto overcomethe effects of (a) reflections from the dielectricpanel,(b) overwatercalibrationshift errors, (c) altitude holes. 55. A radio altimeterwould not be connectedto (a) MADGE, (b) a flight director, (c) an ATC transponder. 56. The DH lamp comeson when the aircraft is (a) over the outer marker, (b) below a pilot set barometricaltitude, (c) below a pilot set radio altitude. 57. It is found that the most suitable positions for a and antennas radio altimetertransmitter-receiver

leadsto a minimum total feeder length of 9 ft and an path length of 8 ft; what antenna€round-antenna would be a suitableAID setting? (a) 20 ft, (b) 40 ft. (c) 57 ft. 58. A phantom beaconis (a) a co-located VOR/DME beaconwith no identity transmission, (b) a TACAN beacon,(c) relatedto a VOR/DME beaconby pilot set distanceand bearing. 59. An aircraft is 20 n.m.'and045'(M) from a VOR/DME beacon;the rangeof the current waypoint, which is due eastof the beacon.is shownas 20 nm; approximatelyhow far is the waypoint from the beacon?(a) 20 nm, (b) 30 nm, (c) 40 nm. 60. MADGE mode Cl deriveselevationinformation by using(a) radio altitude and slant range, (b) interferometry,(c) a directionalbeamnarrow in elevation.

:!,

251

Index

Acquisition,108 ADSEL,22I Aircraft installationdelay, l9l, 197 Airwaysmarker,72 Altitude hales.178 Angleof cut, 8l Antennaeffect,5 I Antennatuning unit, 33 Antennas.3 Area navigation display and control, 207 generalized system,202 RNAV computer,206 2I 3 standardization, principles,204 VOR/DME-based ARINC, 19,230 Associated identity, I l1 ATC 600A (rFR), 120,136 ATC transponder block diagramoperation,128 characteristics, I 35 coding,123 controlsand operation,127 encodingaltimeter,132 falsetargets,125 installation, 126 interrogation, l2l principles, 121 ramp testing,135 rcply,122 sLS,125,128,133 Attitude director indicator,'l4, 217 Audio integrating system,37 Audio selectionpanel,38 Auto standby,106 Autoland,198 Automaticdata input/output, 210 Automaticdirectionfinder block diagramoperation,47 calibrationand testing,55 characteristics, 55 ' controlsand operation,54 installation.52 principles,45 systemerrors,49 Automaticoverloadcontrot. t28 Azinruthmarks,154 Balancinghalf cycle,15I Balun,63 Baseline, 8 l. 95 Basicrate,l0l Bearingresolution,143 Beatfrequencyoscillator,48

252

Bendix

BX-2000,206 cN-2011,2l NP-2041A,206 RDR lE,164,166 R D R 1 1 0 0 ,r 4 8 RDR 1200,146 Boeing 7 4 7 ,t 4 , 3 7 767,217 British Aerospaceadvancedflight deck, 219 Cabininterphone,37, 4O Centilane,80 Clock offset, 82 Coastalrefraction,50 Cockpitvoicerecorder,38,42 Coding,8 Codingdelay, l0l Collins Alt 50, 200 E F I S - 7 0 02, 1 7 wxR700,216 Collision avoidance,229 Colocatedbeacons,ll0 Conductivitymap,85 Conto.ur,143, 154 160 Cosec'beam, Cossor555, 67, 78 Coursedeviationindicator,63 nrosstalk,r.3,33 i ,bi,221 Datalink, 219 D e c c aA t \ C 8 1 , 1 0 2 DeccaDoppler 70 serie.,183 Deccanavigator a m b i g u i t i e9s7, , 9 9 antenna, 99 chain,95, 96 installation,99 Mk 1S/Danac,99Mk 19,99 positionfixing,96 signals,96 Decisionheight.69, 196 Decometer,95,96 Deviationindicator,70 Directview storagetube, 144 DITS,230 Diurnaleffect, 85 DME I 14 analogue, ' block diagramoperation,ll2

channel arrangements, I l0 characteristics. I I 7 controls and operation, I l2 digital, I l4 g r o u n d s p e e d ,1 0 9 i d e n t i f i c a t i o n , 10 6 i n s t a l l a t i o n ,I I I interrogation, 109 principles, 105 ramp testing, 119 reply,109 time to station, 109 Doppler navigation system antenna mechanization. I 74 beam geometry, I 75 characteristics, I 85 controls and operation, 184 Doppler effect, 173 Doppler shift, 173, 187, 190 Doppler spectrum, 175 f i x e d a n t e n n a s y s t e m ,1 8 1 installation,182 movlng antennasystem,179 navlgation calculations, I 79 o v e r w a t e re r r o r s , 1 7 8 testing,185 t r a n s m i s s i o n ,1 7 6 Drift angle,174 Drift indication (weathcr radar). 160 Echo protection, 108, 125 Electromagnetic propagation, 4 E l e c t r o m a g n e t i cr a d i a t i o n , 2 Electromagnetic spectrum, 4 Encoding altimeter, 132, 210 Fan marker, 72 Field alignment error, 52 Flat plate antenna, 145 Flight interphone, 37, 38 F r a m e p u l s e s ,1 2 2 F r e e z e ,1 4 8 F r e q u e n c yp a i r i n g , 7 1 , I l 0 Fruiting,125 Carbling, 125 Geomagnetic field, 85 Ground conductivity, 85 G r o u n d c r e w c a l l s y s t e m ,3 8 , 4 2 Group count down, 128 H e a d u p d i s p l a y( H U D ) . 2 1 9 Hc4}rt ring, 145 ILF- ;cmms rn:enna, 30 t{c
-:- 35 :

tt[IO,rl llcltd

l*.uil*: ::Cr:). 148 -5lI{ f series. 193

Horizontalsituationindicator,63, 217 Hyperbolicnavigationprinciples,80 ldentification,friend or foc, l2 I IFR ATC 600A, 120, 136 NAV 40IL, 67 NAV 4O2AP,78 R D 3 0 0 ,1 6 7

ILs

a n t e n n a s ,7 6 block diagram operation, 72 categories,69 characteristics, 77 controls and operation, 76 coverage,70,72 difference in depth of modulation, 70 fiequency pairing,7l 'glideslope, 70 identification, 70 installation, 74 loading compensation, 75 I o c a l i z e r ,6 9 matker,72 principles, 69 r a m p t e s t i n g .7 8 I n d e x i n g , 1 0 2 ,1 0 4 Instrument flight rules, 202 Intensity modulation, 140 Interference, I 3 lnterferometer , 223, 226 I n t e r r o g a t o r , 1 0 5 , 1 2 1, 2 2 7 Isodop, I 75 Janus configuration, 175, 187 Jitter,106 Khg

K C U s 6 5 A ,2 1 0 K D E 5 6 6 ,2 1 0 Kr 20416,'ts KMA20,75 K N 7 2 .7 3 ,7 5 KN 74,205 K N 7 5 ,7 5 K N R6 6 5 , 2 l l K N S8 0 , 1 2 ,l l l K P I5 3 3 ,l l l K P r 5 5 2 ,6 4 KX 1758,75 KY 196,2r,23 Laneambiguities,86, 97 Lanecount, 80 Laneslip,80 Lanewidth, 80 Laning,80 Line of position,79 Litton LTN 211,87 (lLS), 75 Loadingcompensation Lobeswitching.179 Loop alignmenterror,52 Loopantenna,3,45 ' Loop swing,55 LoranC block diagramoperation,I 03

rli

253

"&t

LoranC Gont'd) chain,101 installation,102 principles,102 signals,101 MADGE block diagram opention, 227 controls and instrumentation, 227 parameters, 229 principles,225 Mapping,150 Marconi A D 5 6 0 ,1 7 9 A D 6 5 0 , 1 8 2 ,1 8 5 Master,80 Maximum permissibleexposivelevel, 164 Microcomputer,9, 26, 94, 208 Microwavelanding system'224 Middlemarker,?2 Modal interference,85 Modeinterlace,122 Modulation,5 Mountain effect, 50 MP mode,97 Multiplexing, T N A V 4 0 1 Lo F R ) , 6 7 NAV 402AP(tFR),78 Night etfect,50 Noisefigure,170 effect,85 Nonspheroidal Notching,99 Omega broadcastpattem, 84 characteristics,95 controls and oPeration,90 hardware,93 installation, 87 interface.89 position fixing, E6 ramptesting,95 signalpropagation,84 skin mapping,8T software,90 stations,83 Omni-bearingselector,63 Opencentre,15l Outboundsearch,108 Outer marker,72 Overwatercalibration shift error, l?8 38, 40 address, Passenger entertainment,38, 4l Passenger reply, 108 Percentage index, 171 Performance Phantombeacon,58, 203 Phaseoffset,82 Pictorialnavigationindicator,63, I I I Ptanposition indicator, 140 Planararray, 145 Polar cap disturbance,86 Polardiagram,3 Precipitationstatic,l3 Programdiscretepins, 89

2g

14l Pulsecompression, Pulsecrowding,215 Pulsewidth limiter, 129 Quadrantalerror, 5 I Quadrantalerror correctioncuwe,57 Quadrantalerror corrector,5 2 Radarrangeequation,169 Radarsystemstester,165 Radio categorization,4, I I historical develoPment,I principles,2 receiversand transmitters,7 Radio altimeter block diagramoperation, 192 200 characteristics, factors affecting performance,19I indicator,196 installation,196 installationdelay,l9l, 197 interface,198 monitoring and self test, 195 multiple installation,199 principleq,189 ramp testing,200 sinusoidalfrequencymodulation, 201 Radiomagneticindicator,53 Radome,147 Rangemarks,151,154, 157 Rangeresolution,l4l Rate aiding, 86 RCA AVQ 85,114 DataNavlI, 162 Primus20. 154 P r i m u s3 0 , 1 5 5 ,1 6 l Primus40, 151 Primus50, 16l Primus200, 146,163 WeatherScoutI, 147 RD 300 (IFR), 167 Reciprocalsearch,108 Residualaltitude, 190, 19l, l9E P$o2,Rho3navigation,79, 8l 58, 105 Rho-thetanavigation, '14,199 Risingrunway, 139,158 RyanStormscope, 223 Satcom/satnav, Scannerstabilization, 143, L57 Scott-T-transformer,.93 Seabias,178 Search,107 Secondtraceechoes,142 Secondarysurveillanceradar, l2l Selcal,35 Senseantenna,45 Sensitivitytime control, 143,194 interphone,38, 40 Service -sia;i;fibe

;;tression,'I 25,r28,Bt

Signalactivatedsearch,105 Skin mapping,87 Slantrange,105,206 Slave,80

Solareffects,85 SperryFMCS,217 Spikeeliminator,129 Spoking,149 Squelch,22, 23 Squitter,106 50 Staticinterference, Staticmemory,108 50 Stationinterference, Stationrate.101 139, 168 Stormscope, L-band,ll1, 126 Suppression, Sweptgain, 143 TACAN, IO5 TerminalVOR.58 TIC T24A, ttg T268, T288, T298,78 T278,67 T 3 0 B ,6 7 , 7 8 T 3 3 8 ,T 4 3 B ,1 3 7 T50A, 120 Time referencedscanningbeam,224 Track,107 105,12l, 160 Transponder, TRT radio altimeters,191 Velocity memory,108 Vertical effect, 5 I VHF comms block diagramoperation,23 28 characteristics, controlsand operation,22 installation.20 principles,20 ramp testing,29

Visual flight rutes,69, 202 VLF comms,87

voR automatic,manual,61, 65 block diagramoperation,55 characteristics,65 controlsand operation,65 conventional,58 doppler,6l identification,59 installation,63 o u t p u t s , 6 36, 6 principles,58 ramp testing,5T VSWR check,weatherndat,167 Weatherradar analogue,l50 beaconmode, 160 162 characteristics, conditionand assembly,164 controlsand operation,147 digital,rho-theta,15I digital,t.v. (X-Y), 156 installation.146 multifunctiondisplay,161 other applications,160 principles,140 ramp test, 165 safety precautions,154 scanner,145 scannerstabilization, 143, l5'l Wobbulation,f 78, l9l Z-matker,72 Zone,Decca,96

255