Multiple Local Autofocus Back-Projection Algorithm for Space-Variant Phase-Error Correction in Synthetic Aperture Radar

The back-projection (BP) algorithm is an ideal solution for large-swath airborne SAR imaging. However, space-variant phase errors induced by trajectory deviations dramatically degrade the BP-focusing performance in a large-swath mode. In this letter, we propose an autofocus method that is compatible with the BP imagery, in which the phase-error function is constructed for individual pixels. In the new method, multiple local areas at different illumination directions within the radar beam are synthesized to trace the local phase gradient. From these phase gradient estimates, the accurate pixel-wise phase-error correction for all the contaminated pulses is achievable. This approach is capable of correcting space-variant phase errors with high precision and efficiency for large-swath SAR imaging. Experiments based on the real data that are recorded by a highly squinted SAR system validates the effectiveness of the proposed autofocus method.