Radar Presentation 01

RADAR OBSERVING AND PLOTTING -01-

Fundamentals of RADAR

ASST. PROF. DR. CPT. ENDER ASYALI

1

The word RADAR is an acronomy from the words:

RAdio Dedection And Ranging.

2

HISTORY OF RADAR -The scientist Heinrich Hertz demonstrated in 1886 that radio wawes could be reflected from metallic objects. -In 1903 a German engineer obtained a patent in several countries for a radio wave device capable of dedecting ships,but it had very limited range. -Marconi, delivering a lecture in 1922, drew attention to the work of Hertz and proposed in principle what we know today as marine radar. -Although the radar was used to determine the height of the ionosphere in the mid-1920’s, it was not until 1935 that radar pulses were succesfully used to dedect and measure the range of an aircraft. -In the 1930’s there was much simultaneous but independent development of radar techniques in Britain, Germany, France and America. -Radar first went to sea in a war ship in 1937. -Radar first used in a merchant ship in 1944

3

The requirement to carry RADAR Extract from regulation 12, chapter V of the IMO -SOLAS (1974) Convention as amanded to 1983 1- ships of 500 tons gross tonnage and upwards constructed on or after 1 september 1984 and ships 0f 1600 ton gross tonnage and upwards constructed before 1 september 1984 shall be fitted with a radar installation. 2-ships of 10000 tons gross tonnage and upwards shall be fitted with two radar installation, each capable of being operated independently of the other. 4

The echo principle -The echo is never as loud as the original blast. -The chance of detecting an echo depends on the “loudness” and “duration” of the original blast. -Short blasts are required if echoes from close targets are not to be drowned by the original blast. -A sufficient long interval between blasts is required to allow time for echoes from distant targets to return.

5

PRINCIPLES OF RADAR OPERATION Introduction Radar determines distance to an object by measuring the time required for a radio signal to travel from a transmitter to an object and return. Since most radars use directional antennae, they can also determine an object’s bearing. However, a radar’s bearing measurement will be less accurate than its distance measurement.

6

1.2.2 the range as a function of time:

The speed of radio waves is 300.000.000. metres Per second.(161,830 nm per sec) Or 300 metres Per microsecond (metres/µs). In one second radar pulse will travel around the world 7 times. Let D= the distance travelled by the pulse (metres) R= the range of the target (metres) T= the elapsed time (µs) S= the speed of the radio waves(metres/µs) D= S x T R= (S x T)/2 R= (300x T)/2 R= 150T

7

Question:calculate the elapsed time for a pulse to travel to and return from a radar target whose range is a)-50 metres b)-12 nm

8

Answer: a)-0.33 microsec b)-148.16 micro sec

9

The Timebase * the elapsed times are of the order of millionth of a second * beyond the capability of any conventional time measuring device * so an electronic device known as Cathode ray tube (CRT) is used *inventors.about.com/library/inventors/ blcathoderaytube.htm

10

RANGE SCALE (Nm) 0.75

TIME BASE DURATION (micro sec) 9.3

1.5

18.5

3

37.0

6

74.1

12

148.2

24

296.3

48

592.6 11

Cathode-ray tube Special-purpose electron tube in which electrons are accelerated by high-voltage anodes, formed into a beam by focusing electrodes, and projected toward a phosphorescent screen that forms one face of the tube. The beam of electrons leaves a bright spot wherever it strikes the phosphor screen. To form a display, or image, on the screen, the electron beam is deflected in the vertical and horizontal directions either by the electrostatic effect of electrodes within the tube or by magnetic fields produced by coils located around the neck of the tube. Cathode-ray tubes are used in television sets, computers, and radar displays.

12

Signal Characteristics In most marine navigation applications, the radar signal is pulse modulated. Signals are generated by a timing circuit so that energy leaves the antenna in very short pulses. When transmitting, the antenna is connected to the transmitter but not the receiver. As soon as the pulse leaves, an electronic switch disconnects the antenna from the transmitter and connects it to the receiver. Another pulse is not transmitted until after the preceding one has had time to travel to the most distant target within range and return. Since the interval between pulses is long compared with the length of a pulse, strong signals can be provided with low average power. 13

Pulse Length, Pulse Duration, or Pulse Width The duration or length of a single pulse is called pulse length, pulse duration, or pulse width. This pulse emission sequence repeats a great many times, perhaps 1,000 per second. This rate defines the pulse repetition rate (PRR). The returned pulses are displayed on an indicator screen.

14

The Display The most common type of radar display used is the plan position indicator (PPI). On a PPI, the sweep starts at the center of the display and moves outward along a radial line rotating in synchronization with the antenna. A detection is indicated by a brightening of the display screen at the bearing and range of the return. Because of a luminescent tube face coating, the glow continues after the trace rotates past the target. On a PPI, a target’s actual range is proportional to its echo’s distance from the scope’s center. 15

The Radar Beam The pulses of energy comprising the radar beam would form a single lobe-shaped pattern of radiation if emitted in free space. Figure 1303a. shows this free space radiation pattern, including the undesirable minor lobes or side lobes associated with practical antenna design. The beam width depends upon *the frequency or wavelength of the transmitted energy, *antenna design, and *the dimensions of the antenna. 16

17

Although the radiated energy is concentrated into a relatively narrow main beam by the antenna, there is no clearly defined envelope of the energy radiated. The energy is concentrated along the axis of the beam. The most common convention defines beam width as the angular width between half power points.

18

0.6-2 DEGRE

30-40 DEGREE 19

*For a given antenna size (antenna aperture), narrower beam widths result from using shorter wavelengths. *For a given wavelength, narrower beam widths result from using larger antennas.

20

Diffraction And Attenuation Diffraction is the bending of a wave as it passes an obstruction. Because of diffraction there is some illumination of the region behind an obstruction or target by the radar beam. Diffraction effects are greater at the lower frequencies. Thus, the radar beam of a lower frequency radar tends to illuminate more of the shadow region behind an obstruction than the beam of a radar of higher frequency or shorter wavelength.

21

Attenuation is the scattering and absorption of the energy in the radar beam as it passes through the atmosphere. It causes a decrease in echo strength. Attenuation is greater at the higher frequencies or shorter wavelengths. While reflected echoes are much weaker than the transmitted pulses, the characteristics of their return to the source are similar to the characteristics of propagation. The strengths of these echoes are dependent upon the amount of: *transmitted energy striking the targets and * the size and reflecting properties of the targets. 22

Refraction If the radar waves traveled in straight lines, the distance to the radar horizon would be dependent only on the power output of the transmitter and the height of the antenna. In other words, the distance to the radar horizon would be the same as that of the geometrical horizon for the antenna height. However, atmospheric density gradients bend radar rays as they travel to and from a target. This bending is called refraction. Types of refraction: SUB-REFRACTION SUPER REFRACTION EXTRA SUPER REFRACTION 23

The following formula, where “h” is the height of the antenna in feet, gives the distance to the radar horizon in nautical miles:

d =1.22√ h The distance to the radar horizon does not limit the distance from which echoes may be received from targets. Assuming that adequate power is transmitted, echoes may be received from targets beyond the radar horizon if their reflecting surfaces extend above it. Note that the distance to the radar horizon is the distance at which the radar rays pass tangent to the surface of the earth.

24

Factors Affecting Radar Interpretation Radar’s value as a navigational aid depends on : *the navigator’s understanding *its characteristics and *limitations. Some of the factors to be considered in interpretation are discussed below:

25

1-Resolution in Range. The ability of a radar to separate targets close together on the same bearing is called resolution in range. It is related primarily to pulse length. The minimum distance between targets that can be distinguished as separate is half the pulse length. Thus, several ships close together may appear as an island. Echoes from a number of small boats, piles, breakers, or evenlarge ships close to the shore may blend with echoes from the shore, resulting in an incorrect indication of the position and shape of the shoreline.

26

2-Resolution in Bearing. Echoes from two or more targets close together at the same range may merge to form a single, wider echo. The ability to separate targets is called resolution in bearing. Bearing resolution is a function of two variables: 1-beam width and 2-range between targets. A narrower beam and a shorter distance between objects both increase bearing resolution. 27

3-Height of Antenna and Target. If the radar horizon is between the transmitting vessel and the target, the lower part of the target will not be visible. A large vessel may appear as a small craft, or a shoreline may appear at some distance inland.

28

4-Reflecting Quality and Aspect of Target. Echoes from several targets of the same size may be quite different

in appearance. A metal surface reflects radio waves more strongly than a wooden surface. A surface perpendicular to the beam returns a stronger echo than a non perpendicular one. For this reason, a gently sloping beach may not be visible. A vessel encountered broadside returns a stronger echo than one heading directly toward or away.

29

5-Frequency. As frequency increases, reflections occur from smaller targets.

30