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Radar System

by Jntu Heroes
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Jntu Heroes
www.jntuworld.com 97.460 RADAR ENGINEERING NOTES R O D L W U T N J www.jntuworld.com
www.jntuworld.com D L R O W U T N J www.jntuworld.com
www.jntuworld.com RADAR ENGINEERING 1. Introduction - Radar is an electromagnetic system for the detection and location of objects (RAdio Detection And Ranging) - radar operates by transmitting a particular type of waveform and detecting the nature of the signals reflected back from objects - radar can not resolve detail or colour as well as the human eye (an optical frequency passive scatterometer) - radar can see in conditions which do not permit the eye to see such as darkness, haze, rain, smoke - radar can also measure the distances to objects - the elemental radar system consists of a transmitter unit, an antenna for emitting electromagnetic radiation and receiving the echo, an energy detecting receiver and a processor. D L - a portion of the transmitted signal is intercepted by a reflecting object (target) and is reradiated in all directions R O - the antenna collects the returned energy in the backscatter direction and delivers it to the receiver - the distance to the receiver is determined by measuring the time taken for the electromagnetic signal to travel to the target and back. W U - the direction of the target is determined by the angle of arrival (AOA) of the reflected signal. T N - also if there is relative motion between the radar and the target, there is a shift in frequency of the reflected signal (Doppler effect) which is a measure of the radial component of the relative velocity. This can be used to distinguish between moving targets and stationary ones. J -Radar was first developed to warn of the approach of hostile aircraft and for directing anti aircraft weapons. - modern radars can provide AOA, Doppler, MTI etc. - the simplest radar waveform is a train of narrow (0.1µs to 10µs) rectangular pulses modulating a sinusoidal carrier - the distance to the target is determined from the time TR taken by the pulse to travel to the target and return and from the knowledge that electromagnetic energy travels at the speed of light thus: cT R = ---------R2 or R(km)=0.15TR(µs) or R(nm)=0.081TR(µs) radarnotes_2006.mif 1/6/06 1 www.jntuworld.com
www.jntuworld.com 97.460 PART II RADAR - once the pulse is transmitted by the radar a sufficient length of time must elapse before the next pulse to allow echoes from targets at the maximum range to be detected. - thus the maximum rate at which pulses can be transmitted is determined by the maximum range at which targets are expected. This rate is called the pulse repetition rate (PRF) - if the PRF is too high echo signals from some targets may arrive after the transmission of the next pulse. This leads to ambiguous range measurements. Such pulses are called second time around pulses - the range beyond which second time around pulses occur is called the maximum unambiguous range c R UNAMBIG = ---------2fP where fP is the PRF in Hz. - more advanced signal waveforms then the above are often used D L -e.g. the carrier may be frequency modulated (FM or chirp) or phase modulated (pseudorandom biphase) too permit the echo signals to be compressed in time after reception. R O - this achieves high range resolution without the need for short pulses and hence allows the use of the higher energy of longer pulses - this technique is called pulse compression W U -also CW waveforms can be used by taking advantage of the Doppler shift to separate the received echo from the transmitted signal. Note: unmodulated CW waveforms do not permit the measurement of range. T N The Radar Range Equation - the radar range equation relates the range of the radar to the characteristics of the transmitter, receiver, antenna, target and the environment. J - it is used as a tool to help in specifying radar subsystem specifications in the design phase of a program. -If the transmitter delivers PT Watts into an isotropic antenna, then the power density R from the radar is (W/m2) at a distance PT ------------2 4πR - here the 4πR2 represents the surface area of the sphere at distance R - radars employ directional antennas to channel the radiated power Pt in a particular direction - the gain G of an antenna is the measure of the increased power radiated in the direction of the target, compared to the power that would have been radiated from an isotropic antenna 2 radarnotes_2006.mif 1/6/06 www.jntuworld.com

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