Adaptive Beamforming for SAR Ambiguity Rejection

摘要

The Lincoln SPARTA program is developing next-generation phased arrays for enhanced radar performance and efficiency. One novel capability being explored is simultaneous operation of MTI (moving target indicator) and SAR (synthetic aperture radar) imaging two functions that are usually mutually exclusive. MTI scans large areas quickly using short integration ((caret)10 msec) and low bandwidth ((caret)10 MHz). SAR in contrast stares for seconds at each location and has high bandwidth ((caret)600 MHz). Simultaneous operation requires multiple beams, frequency-division multiplexing, and synchronized pulsing. Also, MTI requires a higher PRF (pulse-repetition frequency) than SAR, and multiple PRFs for ambiguity resolution. Hence, it is desirable for SAR to function with MTI pulse scheduling. This presentation explores the use of adaptive beamforming to extend SAR performance to unfavorable PRFs. A low PRF implies that Doppler- ambiguous clutter is near the main lobe, while a high PRF implies that range- ambiguous clutter is near the main lobe. An image-domain, minimum-variance beamformer is presented which attenuates ambiguous clutter, adapting the beam uniquely at each location (pixel) in the image. A simulated SAR collection at high PRF using a 3-beam combiner demonstrates a practical, limited degree-of- freedom implementation.