Signal Processing Techniques to Reduce the Clutter Competition in Forward Looking Radar

Radar systems placed in the nose of fast moving jets have to detect moving targets, the radial velocity of which is close to that of the surrounding clutter, relative to the platform's speed. A widely used moving target detection strategy for moving platforms is that of space-time adaptive processing (STAP). STAP is now readily applied to the case of a sideways-looking array, where the majority of the clutter occurs along a narrow ridge, which crosses the angle-Doppler graph diagonally. When the array orientation is at an angle (termed the crab angle) to the direction of platform motion, the clutter at a given range no longer occupies the diagonal ridge, but an ellipse. The eccentricity of the ellipse decreases as the crab angle increases, so that when the array is forward-facing with respect to the platform motion, clutter forms a circle on the angle-Doppler plot. It is far more difficult for the STAP to compensate for this clutter because it is now range dependant. As such, ground clutter is range ambiguous, and the clutter arcs at different ranges and angles can interfere with the detection and tracking of targets. Thus the performance of the radar is reduced because of the increased clutter power competing with the target's signal. Current research has concentrated on altering the STAP architecture to cope with the range dependent returns. However, there is already a mechanism which can help mitigate the Doppler-range ambiguities, but which is not used in the adaptive part of the STAP architecture. This, of course, is the matched filter and its ambiguity function. There has been increasing interest in adaptive waveform design in radar research recently, and optimizing the transmitted waveform for the environment has shown to be effective in a number of areas. The aim of the adaptivity proposed for this STAP situation is to reduce the clutter power competing with any target signals which may be present. The waveform design can achieve this by ensuring that the matched filter response to the waveform with an applied Doppler shift is low at the range of the target of interest. This will have the effect of increasing the target power with respect to the competing clutter, and therefore the detection probability of moving targets. The research we propose here is to develop methods to efficiently and adaptively design the transmitted waveform based on the received signals. The study will encompass the use of the received signals, prior knowledge of target and clutter locations, and spatial beam pattern of the array on transmit and receive, in designing the signal.