When viewed by a moving radar platform, fixed targets on the ground lie within a particular Doppler bandwidth. One could simply infer that targets detected outside this bandwidth were moving; however, this approach is generally far from adequate. Many moving targets on the ground may lie within the same bandwidth. A more reliable approach takes advantage of a technique used to suppress ground clutter, the fixed-target returns that interfere with the moving-target returns.
Moving-target-indicating (MTI) radars can suppress ground clutter by employing multiple phase centers—portions of the antenna that act as independent antennas to form a so-called displaced phase-center antenna. The basic concept is to keep pairs of phase centers motionless from pulse to pulse, simulating an antenna that stays motionless in space. This has the effect of driving the Doppler bandwidth of clutter to zero so it can be cancelled upon subtraction of the data from these pulse pairs. With the background "removed," all that remains are moving objects and noise. Moving targets will, however, suffer some amount of loss upon subtraction, depending upon their range rate and the difference in time between observations.
Still, after detection, the location of the target remains unknown. Multiple phase centers can, in an approximate sense, solve this problem by means of monopulse techniques. In classic airborne interceptor designs, the antenna is divided into portions along both azimuth and elevation. Amplitude or phase comparisons are made between returns from these subapertures to estimate the direction of arrival of the target signal. With a minimum of three phase centers both the displaced phase-center antenna technique of clutter cancellation and monopulse techniques for location of targets can be combined to detect and locate targets on the ground.