Imaging of Multi-Channel Sliding Spotlight SAR with Up- and Down-Chirp Modulation for Range Ambiguity Suppression

Range ambiguity level is one of the most important metrics of the spaceborne synthetic aperture radar (SAR) image quality. Alternating transmission of up- and down-chirp signals is an effective method to suppress the range ambiguity. However, this modulation method brings new unwanted azimuth ambiguity, since an intra-pulse Doppler effect causes opposite range shift for up- and down- chirp. In this paper, we propose a signal processing strategy for multi-channel sliding spotlight SAR with alternate up- and down-chirp modulation. The proposed algorithm is based on the sub-aperture method with azimuth digital beamforming, and a special care is taken for the correction of the intra-pulse Doppler effect to suppress the azimuth ambiguity. The performance of the intra-pulse Doppler effect correction process is evaluated through a point target simulation, and it is shown that the ambiguity-to-signal ratio (AASR) has been improved by 4.4dB.