An Improved Approach to Channel Mismatches Estimation for Bistatic Multichannel SAR

Bistatic synthetic aperture radar (SAR) is characterized by different locations for transmitter and receiver, which brings additional benefits with respect to monostatic SAR. The spaceborne bistatic multichannel SAR system can lift the restrictions between high-resolution and wide-swath. Multichannel SAR system utilize pulse-repetition frequencies less than Doppler bandwidth at the cost of ambiguous Doppler spectrum. This represents that the radar echoes received by each channel are under-sampling. Unambiguously reconstruct the Doppler spectrum is necessary for SAR image focusing. In practice, the unavoidable channel mismatches in phase badly degrade the reconstruction of the Doppler spectrum. However, Conventional azimuth channel mismatches in phase estimation method based on time-domain correlation for monostatic multichannel SAR result in failure because of limitation of conventional cross-correlation function coefficient of the radar echoes. To address this problem, an improved approach to channel mismatches in phase estimation for bistatic multichannel SAR is proposed in this letter. Application of spatial cross-correlation method in signal bandwidth can realize a robust estimation of constant phase between channels which have good coherence with each other. Theoretic analysis reveals that the constant phase includes the azimuth channel mismatches in phase and the phase caused by the Doppler centroid. The Doppler centroid frequency is as well as related to SAR focusing. Using modified spatial cross-correlation approach, the Doppler centroid frequency can be derived from the constant phase. After removing azimuth time delay phase, channel mismatches in phase could be calculated according to residual phase. The effectiveness of the proposed approach in this letter is demonstrated with experimental results.