Two-Stage Correction for Wavefront Curvature Effects of PFA in Focusing Nonideal Circular SAR Data

Although the polar-format algorithm (PFA) provides high computational efficiency in spotlight synthetic aperture radar imaging, it is still constrained to applications with large scene size requirements due to the unconsidered wavefront curvature effects. To solve this problem, a two-stage correction algorithm is proposed in this article to mitigate the image deterioration caused by planar wavefront approximation. Through analyzing the distribution characteristics of the quadratic wavefront curvature phase error (QWCPE) for a nonideal circular flight path with different elevation angles, the 2-D space-variant compensation problem is separated into two 1-D compensation. Based on the aforementioned analysis, the QWCPE is first assumed to be associated only with the distorted coordinates of the target, and the first-stage correction is applied to every range gate before the azimuth fast Fourier transform operation of the PFA. Then, the residual QWCPE after the first-stage correction (RQWCPE-1) is assumed to be associated only with the distorted azimuth coordinates of the target; to correct this error, the image after the first-stage compensation is separated into multiple subblocks along the azimuth direction; for each azimuth subblock, the RQWCPE-1 can be regarded as space invariant and corrected by constructing a filter according to the subblock center. Finally, all the subblocks are mosaicked together to generate the final refocused image. Simulated and real data experiments are performed to verify the performance of the proposed algorithm.