Report Of The Presentation

INTRODUCTION TO RADAR SYSTEM (REPORT)

Submitted Submitted by:-

to:-

Dr. Ying Himanshu Khanna

Wang 10

0411947 1.

Introduction to RADAR ststem

The term RADAR is an acronym made up of the words Radio Detection and Ranging. It is an electronic device used to determine the presence, direction, height and distance of both fixed and moving such as aircrafts, ships, motor-vehicle, weather formation etc using electromagnetic waves. It is something that is in use all around us. Air traffic control uses RADAR to track planes both on ground and in air, to guide planes in for smooth landing. Police use RADAR to detect the speed of passing motorist. NASA uses RADAR to map the Earth and other planets, to track satellites and space debris. The military uses it to detect the enemy and to guide weapons. Meteorologists use RADAR to track storms, hurricanes and tornadoes. So, RADAR is a very useful technology used today almost everywhere.

1.1 How RADAR works? The arrow diagram shown below is self explanatiry that how does it work. Electronic magnetic wave os produced by a transmitter and is allowed to propogate in space. The radiated EM wave is reflected by the object in the space(Target). The EM wave is reflected in all the directions and EM Wave

Reflected by target

Echo received

Decision making

some of the reflected wave is received by the RADAR receiver whcih is called as an ECHO of the signal. Using the echo signal various parameters like range, direction, size etc of the target can be determined.

1.2

Block diagram of a RADAR system

Below shown is the general block diagram of a RADAR system. It consists of two blocks, Transmitter and a Receiver. The recevier used in a RADAR is a superhetrodyne recevier.

Basic parts Power amplifier(Transmitter):- It generates suitable waveform for a particular job the RADAR has to perfoem. Most RADAR use a short pulse waveform so that a singke antenna can be used on a time shared basis for transmission and receiving.

Duplexer:- It allows single antenna to be used at a time by protecting the sensitive receiver from burning out while transmitter is perfroming its function and by directing ECHO signal to receiver than to transmitter. Antenna:- It is the device that allows the transmitted signal to be propagated into the space and collects the ECHO energy on receive. Almost always a DIRECTIVE antenna is used that directs the radiated energy into a narrow beam to concentrate the power as well as to allow the determination of teh direction of the target. Superhetrodyne receiver:- A low noise amplifier is used to amplify the received ECHO to a desired level and adds very less noise to the signal. Local oscillator produces another signal which is mixed with the ECHO signal using a mixer. The mixer used in superhetrodyne receiver doesnt multiply the tow frequeicies of the signal, but its produces a sum and difference of two frequences. This is the principle on which a superhetrodyne receiver works. Mtched filter maximizes the output signal to noise ratio. Finally the decision is made by the 2nd detector which decides whether the ECHO received is to be realised or not. Finaly it is displayed on the screen using a video amplifier.

Antenna

1.3

Classification of RADAR

Duplexer

RADAR is basically classified in three types:1) Monostatic RADAR:- It is a type of RADAR in which same location is used for transmitting and receiving the EM wave. 2) Bistatic RADAR:- In this type of RADAR, different transmitting and receiving locations are used. 3) Multistatic RADAR:- This is the 3rd type of RADAR in which transmission is done from one or more locations and receving is also done at one or more locations. 1.4 Types

Low no amplif

of RADAR:- Some of the various types of RADAR used are mentioned below:-

1. 2. 3. 4. 5. 6. 1.4

Pulse radar High-resolution radar Pulse compression radar Continuous wave radar FM-CW radar Surveillance radar

7. Moving target indication(MTI) 8. Pulse droppler radar 9. Imaging radar 10. SAR(Synthetic radar 11. Weapon control radar 12. Weather observation

Information available from RADAR

RADAR can be used to extract lots of information about ab object(Target) in air or ground like its range, radial velocity and angular direction. These aspects are discussed in brief below:1) Range:- It is the most distinguishing feature of RADAR. It is measured by the time taken

by the signal to propogate in space at speed of light and come back to the RADAR. The accuracy of range measured depends upon the RADAR. 2) Rdial velocity:- Radial velocity is the velocity of target in the direction of LINE OF SIGHT(LOS). It can be obtained from the rate of change over a period of time. It can also be obtained from the measurement of doppler frequency shift. 3) Angular direction:-The one method of determining the direction of a target is by determining the angle where the magnitude of the ECHO signal from the sacnning antenna is maximum. It requires an antenna with narrow bandwidth.

4) Size and shape

1.4

If the radar has sufficient resolution capability in range or angle, it can provide the measurement of target. By sending and receiving repetitive pulses, size of the target can be obtained and if there is high range resolution, even shape of the target can be realized.

Importace of bandwidth in RADAR

Bandwidth basically represents information, hence it is very impormant in many RADAR application. Two types of bandwidth are there in RADAR:1) Signal BW:- It is determined by the signal BW. Large BW is needed for resolving targets

in range , for accurate measurement of range to a target and for providing a limited capability to recoganize any type of target from another.

2) Tunable BW:- It offers the ability to change(tune) the RADAR signal frequency over a

wide range of the availabe spectrum. This can be used for reducing mutual interference amongst RADARs that operate in the same frequency band. The higher the frequency, the easier it is to obtain wide signal and wide BW. 1.4

Operating with more than one frequency

We can use multiple frequencies in two ways:1) Frequency agility:- use of multiple frequency on a pulse to pulse basis. 2) Frequency diversity:- use of multiple frequency that are widely separated ,sometimes in more than one RADAR band. Following are the benefits of using more than one frequency:1) Elevation null filling:- Operation of radar at single frequency can result in lobed

structureto the elevation pattern of the antenna due to the interference b/w the direct signal(RADAR to RADAR) and the surface scattered structure. There will be a formation of lobbed structure in which there will be reduced coverage at some elevation angles(nulls) and increased signal strength(lobes) at other. A change Oof frequency wil change the location of nulls and lobes, whcih results in filling of the nulls and RADAR will be less likely to lose a target ECHO signal. 2) Increased target detectability:- With only one frequency, there could be some small ECHO signal and missed detection. Using multiple frequency will minimize the chances of missing the ECHO 1.4

The doppler shift in RADAR The change of frequency of a wave for an observer moving relative to the source of the wave is called doppler effect. it is used in radar to separate moving targetd from clutter(unwanted echos received)

Fd =Doppler frequency

Vr = V cos Ѳ = relative velocity of the target λ= radar wavelength 1.5

RADAR equation

Pr= Power received; σ= RADAR cross-section;Pt=power transmitted; Gt= gain of natenna; R= Rnage; Ae= effective area of antenna

Using this equation we can find the range of the targer and all the other parameters which we have already discussed above.

2 SAR ANTENNA There are two types af antenna, one is Real aperture radar( RAR) where the antenna is a physical object that first emits and collects, the radiation. The other one is Synthetic aperture radar(SAR)

where the antenna moves to cover a synthetic radar. SAR is the capability of long-range propagation characteristics of radar signals and the complex information processing capability of modern digital electronics to provide high resolution imagery.Two terms which are of prime importance when we try to undertsand the working of an SAR antenna are:1) Range resolution or downrange resolution or cross track resolution:- It is defined as

the resolution along the line of sight(LOS) from the radar to the target region. 2) Cross range resolution or azimuth resolution or along track resolution:- It is the resolution perpendicular to LOS and parallel to ground. With respect to SAR resolution, the preffered terms are Fine and Course. Better resolution is finer, not greater; poorer resolution is courser, not less. In this way ambiguity in termilogy can be avoided. Of course, in practice the terms high resolution (fine resolution) and low resolution(coarse resolution) are often used without ambiguity.

2.1 How does SAR works??? This is just an intuitive feel of how an SAR works. Consider an airborne SAR imaging perpendicular to the aircraft velocity as shown in the figure below. Typically, SARs produce a two-dimensional (2-D) image. One dimension in the image is called range (or cross track) and is a measure of the "line-of-sight" distance from the radar to the target. Range measurement and resolution are achieved in synthetic aperture radar in the same manner as most other radars: Range is determined by precisely measuring the time from transmission of a pulse to receiving the echo from a target and, in the simplest SAR, range resolution is determined by the transmitted pulse width, i.e. narrow pulses yield fine range resolution. The SARs other are dimension is called azimuththan (or Since much lower in frequency along track) and is perpendicular to range. It optical systems, even moderate SAR is the ability of SAR produce physically relatively resolutions require an toantenna fine azimuth that differentiates it larger than canresolution be practically carried by an from other radars.antenna To obtain fine azimuth airborne platform: lengths several resolution, a physically large antenna is hundred meters long are often required. needed to the transmitted received However, anfocus airborne radar couldand collect data energy into a sharp beam. The sharpness of while flying this distance and then process the the asbeam the azimuth resolution. data if itdefines came from a physically long Similarly, optical systems, such antenna. The distance the aircraft flies as in telescopes, require large aperture synthesizing the antenna is known as the synthetic aperture. A narrow synthetic beam width results from the relatively long synthetic aperture, which yields finer resolution than is possible from a smaller physical antenna. Achieving fine azimuth resolution may also be described from a doppler processing viewpoint. A target's position along the flight path determines the doppler frequency of its echoes: Targets ahead of the aircraft produce a positive doppler offset; targets behind the aircraft produce a negative offset. As the aircraft flies a distance (the synthetic aperture), echoes are resolved into a number of doppler frequencies. The target's doppler frequency determines its azimuth position.

2.2 SAR antenna requirements and design The SAR Antenna requirements include several RF, electrical, mechanical, and thermal parameters. The most important are the required beam shaping and steering capabilities, which implies the necessity to consider an active-phased array instead of alternative solutions such as multi-feed reflector antennas, or passive-phased array, which do not have the required flexibility. ACTIVE PHASED ARRAY is a group of antenna in which relative phases of the respective signals feeding the antenna are varied in such a way that the effective radiation pattern of array is reinforced in one direction and suppressed in other direction which provides good beam shaping capabilities which is why Active phased array are used in SAR. Main RF requirements to be considered are as following:a) The beam size in azimuth and the minimum elevation beam-width. b) The beam shaping capability. c) The azimuth and elevation steering angles. d) The central frequency and the band-width. e) The EIRP f) The noise figure g) Dual polarization. As a consequence the following aspects have to be fixed:A) The technology of the radiating elements, that has some impacts also the mechanical and thermal designs of the whole system. B) RF subsystem, including passive and active units. C) Digital sub-system. D) Power sub-system E) Mechanical sub-system. F) Thermal sub-system. Moreover it is necessary to divide the antenna in electronics subassemblies to allow the production chain to guarantee the needed rate and to perform all the required acceptance test in short time. Now we will discuss all these sub-systems in detail. 1)

RF Technologies:- Central frequency, bandwidth, ohmic losses, cross-polarization

purity, but also mass and manufacturing cost, impacts on the selection of the technology used to realize the linear array. Typically two technologies have been used up to now for the RF radiator of SAR Antennas: 1.1)Slotted waveguide:- Slotted waveguide has a very good performance in terms of ohmic losses and cross purity, but it is applicable only for BW up to 2%. One more major drawback is strong mutual coupling phenomena, which implies accurate modelling of the antenna surface to control the antenna side-lobes and to avoid high active mismatch. Also when dual polarization is needed, the electromagnetic design becomes quite complex since different wave-guide design has to be considered for the vertical and horizontal polarization. 1.2)Linear patches of array:- Linear array of patches is simpler to be designed and manufactured, cheap and lightweight, presents larger bandwidths (up to 4/5% or more for accurate design). Cross-level can be improved by sequential rotation techniques, while the major drawback consists in higher ohmic losses due to the micro-strips used for the RF Beam-formers. Patch array can be used from lower frequency (P band) up to X band. EIRP and noise power strongly depend on the

1) a) b)

1)

amplifier performance. Distributed T/R modules are used to maximize the antenna aperture control to steer and shape the antenna beam. RF blind mate and solderless connections are used to allow a simple mounting and dismounting of the module. We use separate attenuator /phase shifter to control the amplitude and phase. Multifunctional chip(core chip) includes both control plus amplification and capabilities are used most of the times. Electronic front end(EFE):- Two additional conditioning circuits are used for the proper functioning of T/R module:a digital I/F able to receive/transmit the data from the Tile controller and to implement electronic protection of the hybrids; this is achieved by a dedicated ASIC(Application specific integrated circuit), which reduces the circuit complexity; a power section, which includes the capacitor bank to work in pulsed way, and the highcurrent switches, to switch on/off the RF Tx and Rx chains. These circuits can be mounted on dedicated PCB externally, or can be placed inside each the TR module to drive a group (4 or 8) TR modules. The whole assembly is named (Electronic Front End) EFE and includes also the RF combiners/dividers for the TR modules. True time delay lines(TTDL):- According to the equation given below, as the frequency changes, the beam pointing also changes which we need to be constant. sinθ =fof *sinθo

Where, Ѳ= beam pointing at frequency f; Ѳo= beam pointing at frequency fo and f0= central frequency If the variation is small with respect to the beam-width(5%-10%), it can be tolerated, but if it is more then we need TTDL in the beam forming network.

Formula of phase shift shown above includes frequency. If we somehow get rid of this frequency, there would be no beam variation.So we do it like this,

Here τ is the time delay provided by TTDL ab dwe can see that the final result we get id free from frequency. So thsi is how TTDL is used in the SAR antenna, to get rid of beam pointing variation.

2)

Digital subsystem:- Digital subsystem is used to control the settings of T/R module in

order to generated desired beam whcih are required to perform a certain function at thar instance of time. Central controller secondary controller TR module It consists of a central controller whcih knows all the desired settings of the beam to be produced. It communicates with secondary control and send them all the desired settings and secondary control further communicates and sends those settings to the T/R module. 3) Power sub-system:Number of power supplies on the antenna depends o number of T/R modules to be fed, their power consumption, duty cycle, reliability performance and total number of T/R modules in the antenna. 4) Mechanical design:- Due to large aperture of several square meters, it is divided into mechanical panels that are in stowed condition during the launch and deployed when the satellite has reached the final orbit. Either the electronics can be integrated in subassemblies frames(tiles), which are mounted on a panel supporting frame or the electronics can integrated in large and closed panel structures. Critical aspects of mechanical design is to guarantee the antenna planarity and therefore reduced thermal distortion during the antenna functioning, to guarantee patterns stable performance that is why we use materials with low coefficient of thermal expansion and with good thermal conductivity. 5) Thermal design:- It is of prime importance because it has to guarantee the limited thermal excursion as applicable to the electronic devices inside the antenna. Number of images per orbit, duration of being in image mode, single or dual looking operation which can determine a rapid grow of the temperature are the function requirements on which thermal design is made. Fast temperature increment is controlled mainly from the thermal capacity of antenna. The capability of satellite to work in front to sun and back to sun is considered to be the most important aspect of thermal design. The antenna should be able to work in both the conditions. In front to sun case, the heat is dissipated from the back and in back to sun configuration, the antenna can radiate with the radiating arrays, if there thermal conductivity and there surface is adequate.

IN FLIGHT ACTIVE PHASED ARRAY SAR ANTENNA COSMO SKY MED ANTENNA (CONSTELLATION OF SMALL SATELLITES FOR MEDITERRANEAN BASIN OBSERVATION) Funded by ASI(Agenzia spaziale Italiana) and Italian ministry of defence 1)Antenna in stowed and

deployed The overall objective of this program is the global earth observation We and relevant configuration. have data already exploitation for the needs of both military and civil. That is why it is a dual mission. It discussed about the mechanical design has four satellites and each satellite is equipped with and an SAR operating in X-band with about stowed and deployed multimode and multi-polarization capability. Some ofconfiguration. the figures ofInthe stowed SAR antenna the wings made up of columns of electrical panel parts are as follows:are closed and when it is deployed, the wings are opened.

2)Tile is the fundamental brick of antenna and includes linear arrays, TR modules and is characterized by the presence of the TTDL. This is the SAR antenna with all the tiles The tiles are mounted on an aluminum panel frame which supports also the hold down and release mechanism, the deployment mechanism, the antenna harness and two BFN networks. Tile is internally divided into 7 separated rows; 4 for TR modules, two for power supply units, 1 for Digital control and TTDL as shown below.

3)

The digital controller of the antenna is achieved by 5 rebounded 1553 digital buses, one for each column of 8 tiles.

3) The RF radiator consists of a dual polarized linear array of 12 stacked patches, electromagnetically coupled through slots to the distribution networks.

4)EFE and T/R module

.

5)TTDL:- Four bit device able to reach upto 15 wavelengths of slope compensation. It also amplify both Tx and Rx signals.

References 1) Mordern Radar System:- Hamish Meikle(2nd edition) 2) Radar Handbook:-Merrill Skolnik(3rd edition)

3) Review Article Active SAR Antennas: Design, Development, and Current Programs P. Capece Thales Alenia Space Italia, Via Saccomuro 24, 00131 Roma, Italy