A modified Phase Gradient Autofocus (PGA) algorithm for Airborne/Stationary Bistatic Synthetic bistatic SAR imaging

In Airborne/Stationary Bistatic Synthetic Aperture Radar (A/S-BiSAR), the assumption of translational invariance is no longer available. After the Range Cell Migration Correction (RCMC), the range compressed signal in the same range gate exhibits azimuth-variant Doppler rates which make the phase error difficult to be estimated from the echoes by using the available Phase Gradient Autofocus (PGA) algorithms. To deal with this problem, a modified PGA algorithm is proposed. Comparing with the traditional PGA, the Residual Quadratic Phase (RQP) caused by the azimuth-variant Doppler rates is additionally estimated and compensated. As the influence of the azimuth-variant Doppler rates is greatly reduced, a phase gradient estimator is subsequently applied to accurate phase error retrieval. Furthermore, the range-variance of the phase error is also concerned in the phase error estimation and correction, thus the proposed algorithm is suitable for A/S-BiSAR with wide swath in range. Raw data experiments are presented to verify the advantages of the proposed algorithm.