The early radars worked by sending out a continuous wave and the reflected energy showed the presence of an object, such as a ship or an aircraft. Separate transmitting and receiving antennas were used. Today, police radars, designed for speed traps, use this principle as do intruder alarms, and simple radars used as proximity fuses mounted in the noses of shells. Frequency modulated continuous wave radars, in addition to those used in radar altimeters and in the HAWK antiaircraft missile system, are starting to be used in vehicles. Current pulse radars switch the same antenna between the transmitter and receiver to allow transmission and reception (monostatic) and are capable of measuring the range of the reflecting or scattering object, its azimuth angle, elevation angle, and radial velocity.

1.1 Primary and Secondary Radar

A primary radar illuminates its surroundings, like a searchlight, and picks up part of the energy scattered by the objects it illuminates. Monostatic radars, the majority of all radars, use the same antenna for transmission, or illumination, and reception. Such a radar is shown in Figure 1.1.

Figure 1.1: A primary ground radar and its environment.

If the transmitter sends short pulses, the scattered energy from the objects that form the environment are copies of the transmitted pulses with a time delay proportional to the distance traveled, as with a sound echo. The frequency of the echoes, as with sound, depends on the rate of change of range or radial velocity. Thus the echoes from the objects in...