Moving target imaging synthetic aperture radar

Abstract

A method and apparatus of imaging moving targets with an aircraft mounted complex radar system has a plurality of independent, but synchronized synthetic aperture radars (SARs) positioned on the aircraft at equal separation distance .DELTA.x along the flight velocity vector V.sub.p of the aircraft. Frequency modulated (or otherwise coded) pulses are transmitted therefrom with an interpulse period T.sub.p, where 1/T.sub.p is an integral multiple of V.sub.p /.DELTA.x. The pulse repetition frequency, platform velocity, and spacing between adjacent SARs are all chosen to create the effect of a stationary radar momentarily fixed in space. A two dimensional complex IF (intermediate frequency) output signal is recovered by the first SAR on the aircraft. This signal is identical to that recovered by a conventional SAR. The two dimensions are fast time and downtract position. If only one point target is present, the IF output signal is the point target's phase history. Typically, many point targets are simultaneously present, and the IF output signal is the sum of point target phase histories. Each additional SAR on the platform recovers a different two dimensional complex IF output signal. These signals are sequentially stacked to form a three dimensional complex data set. The stacking dimension is called subaperture time, and is unique to this invention. A two dimensional cut through the three dimensional complex data set and normal to the downtract position dimension, say at downtrack position x, contains the data which would be collected by a stationary radar at downtrack position x. This radar would transmit a total of M frequency modulated (or otherwise coded) pulses, where M is the number of SARs on the aircraft, at a rate of one pulse every .DELTA.x/V.sub.p seconds, as it observes all targets. For each target present, a slow doppler fluctuation, whose frequency is directly proportional to target slow relative velocity, appears along the subaperture time axis. Target slow relative velocity is the target velocity with respect to the fixed radar in space. Subaperture time and slow relative velocity are a Fourier transform pair. Consequently, targets, in terms of phase history