Waveform Diversity and Adaptive Signal Processing to Improve SBR GMTI Performance Degraded by MEO Antenna Mechanical Distortions

Abstract

For a moving platform lookdown radar with the antenna aligned to the platform's velocity vector, the clutter Doppler is a function of the azimuth angle. For space-based radar (SBR), the earth's rotation induces another component to the clutter motion, causing the clutter Doppler to be a function of the slant range and increasing the clutter spectral width. This significantly degrades the ability of space-time adaptive processing (STAP) algorithms to reject the interference. Because of the SBR wider elevation field of view and higher platform speed, the impact of this effect is much greater than for airborne radars. At medium-earth orbit (MEO), SBRs have very long slant range requirements, which result in very large antenna apertures to meet target detection. The large physical sizes of these antenna systems and the unequal thermal heating in the space environment results in mechanical distortion of the active electronically-scanned array (AESA) or reflector structures. This paper presents the MEO Ground Moving Target Indication (GMTI) performance for both a large AESA and a cylindrical reflector fed by a linear array. To compensate for the degrading effects of both clutter range ambiguities enhanced by the earth's rotation and mechanical distortions (on the reflector feed horizontal aperture), an integrated approach was investigated which consisted of: 1) Quadratic phase modulation waveforms; 2) Transmit RF phase shift compensation; and 3) Receive STAP true target steering vector correction. The GMTI signal-to-interference-plus-noise ratio (SINR) performance of a generic MEO SBR was greatly improved using combinations of these techniques.