1. RADAR system References: Intoduction to Radar Systems Merill I. Skolnik, 3rd Edition, McGraw-Hill, 2001 Radar systems principles, Harold R. Raemer, CRC Press Inc., 1997

2. Introduction and definition The two most basic functions of radar are inherent in the word, whose letters stand for Radio Aid for Detection And Ranging. Radar is used to extend the sense of vision. The value of radar lies not in doing what the eye cannot do. However, radar can be designed to see through those conditions impervious to normal human vision, such as darkness, haze, fog, rain, and snow. In addition, radar has the advantage of being able to measure the distance or range to the object. It was first developed to satisfy the needs of military for surveillance and weapons control. However, radar has seen significant civil applications for the safe travel of aircraft, ships and spacecraft.. Although a well-designed modern radar can usually extract more information from the target signal than merely range, the measurement of range is still one of radar's most important functions.

3. Radar block diagram

5. Radar block diagram: Transmitter The equipment has been divided into six subsystems. The transmitter may be an oscillator, such as a magnetron, that is " pulsed” by the modulator to generate a repetitive train of pulses. The magnetron has been the most widely used of the various microwave generators for radar. A typical radar for the detection of aircraft and space objects at ranges more than 100 or 200 miles might employ a peak power of the order of a megawatt, an average power of several kilowatts, a pulse width of several microseconds, and a pulse repetition frequency of several hundred pulses per second. Transmitters most operate over a wide bandwidth, with high efficiency and with long trouble-free life.

6. Radar block diagram: Antenna A single antenna is generally used for both transmitting and receiving. The waveform generated by the transmitter travels via a transmission line to the antenna, where it is radiated into space. It is a directive antenna which will concentrate the energy in a narrow beam. Mechanically steered parabolic reflector antennas and planar phased arrays both find wide application in radars. The size of a radar antenna depends in parts on the frequency, whether the radar is located on the ground or on a moving vehicle, and the environment in which it must operates. The lower the frequency the easier it is to produce a physically large antenna since the mechanical tolerances are proportional to wavelength.

7. Radar block diagram: Duplexer Since we are using the same antenna for receiving and transmitting signals, so the receiver must be protected from damage caused by the high power of the transmitter. This is the function of the duplexer. The duplexer also serves to channel the returned echo signals to the receiver and not to the transmitter. It acts as a rapid switch. The duplexer might consist of two gas-discharge devices, one known as a TR (transmit-receive) and the other an ATR (anti-transmit-receive). The TR protects the receiver during transmission and the ATR directs the echo signal to the receiver during reception. Solid-state ferrite circulators and receiver protectors with gas-plasma TR devices and/or diode limiters are also employed as duplexer.

8. Radar block diagram: Receiver The signal collected by the antenna is sent to the receiver. The receiver is usually of the super-heterodyne type. The receiver serves to separate the desired signal from the ever present noise and other interfering signals and amplify the signal sufficiently to actuate a display such as cathode-ray tube or to allow automatic processing by some form of digital device. Usually, the first stage might be a low-noise RF amplifier, such as a parametric amplifier or a low-noise transistor. However, in military applications, it is not always desirable to employ a low-noise first stage in radar. The receiver input can simply be the mixer stage, especially in military radars that must operate in a noisy environment. Although a receiver with a low-noise front-end will be more sensitive, the mixer input can have greater dynamic range, less susceptibility to overload, and less vulnerability to electronic interference.

9. Radar block diagram: Mixer The mixer and local oscillator (LO) convert the RF signal to an intermediate frequency (IF). A " typical" IF amplifier for an air-surveillance radar might have a center frequency of 30 or 60 MHz and a bandwidth of the order of one megahertz. The IF amplifier should be designed as a matched filter; i.e., its frequency-response function H (f) should maximize the peak-signal-to-mean-noise-power ratio at the output. This occurs when the magnitude of the frequency-response function | H(f) | ( is equal to the magnitude of the echo signal spectrum | S( f ) |, and the phase spectrum of the matched filter is the negative of the phase spectrum of the echo signal. The second detector or the quadrature detector in the receiver is an envelope detector which eliminates the IF carrier and passes the modulation envelope. It can be replaced in some cases by a phase detector which extracts the Doppler frequency by comparison with a reference signal at the transmitted frequency.

10. Information available from the Radar A radar operates by radiating electromagnetic energy and detecting the echo returned from the reflecting objects which are called targets. The nature of the echo signal provides information about target: 1.The range or distance to the target is found from the time it takes for the radiated energy to travel to the target and back. 2.The shift in frequency of the received echo signal due to the Doppler effect caused by a moving target allows a radar to separate desired moving target from an undesired stationary targets even though the stationary echo signal may be many orders of magnitude greater than the moving target. This can also permit us to measure the target's relative (radial) velocity. 3.The angular location of the target is found with directive antenna (with a narrow beamwidth) to sense the angle of arrival of the echo signal. 4.If the target is moving, a radar can derive its track or trajectory, or predict its future location. 5.With sufficiently high resolution a radar can discern something about the nature of a target’s size and shape.

11. The range This is the most conventional characteristics of a radar. No other sensor can measure range to the accuracy possible with radar, at such long ranges and under adverse weather conditions. Radar has demonstrated its ability to measure interplanetary distances to an accuracy to which the velocity of propagation is known. At more modest distances, the measurement of range can be made with a precision of a few centimeters. The usual radar pulse for determining range is the short pulse. The shorter the pulse the more precise can be the range measurements.

12. Radial velocity From successive measurements of range, the rate of change of range which is called the radial velocity can be obtained. The Doppler frequency shift of the echo signal from a moving target also provides a measure of radial velocity, it is more widely employed as the basis for sorting moving targets from unwanted stationary clutter echoes. Any measurement of velocity, whether by the rate of change of range or by the Doppler frequency Shift, requires time. The longer the time of observation, the more accurate can be the measurement of velocity because this will increase the signal to noise ratio, another factor that results in increased accuracy.

13. Angular location or direction The direction of a target is determined by sensing the angle at which the returning wave-front arrives et the radar. The direction in which the antenna points when the received signal is a maximum indicates the direction of the target. This assumes that the atmosphere does not perturb the straight-line propagation of the electromagnetic wave. The accuracy of the angle of arrival depends on the extent of the antenna aperture. The wider the antenna, the narrower the beam-width and the better the accuracy

14. Radar Frequency Bands and Carrier Selection While radar techniques can be used at any frequency, from a few megahertz up into the optical and ultraviolet ( f > 3 x 10 15 Hz, λ< 10 - 7 m), most equipment has been built for microwave bands between 0.4 and 40 GHz. The IEEE has adopted as a standard the letter band system, which has been used in engineering literature since World War II. The 1984 revision is shown in the table below. Note that only a small portion of each band is allocated for radar usage. One reason for identifying these separate radar bands, rather than using the coarser International Telecommunications Union (ITU) designation of UHF, SHF, and EHF, is that the propagation characteristics and applications of radar tend to change quite rapidly in the microwave region.

15. Radar Frequency Bands and Carrier Selection Attenuation in rain (measured in decibels) varies about f 2.8 and backscatter from rain and other small particles varies as f 4, over most of the microwave region. Ionospheric effects vary inversely with frequency, and can be important at frequencies below about 3 GHz. Backscatter from the aurora is significant near the polar regions at frequencies below about 2 GHz. The dimensions of the radar resolution cell tend to vary inversely with frequency, unless antenna size and percentage bandwidth of the signal are changed. These factors lead to the following general preferences in use of the different bands as illustrated