说明：  The Phase Gradient Autofocus (PGA) algorithm has been widely used in Spotlight Synthetic Aperture Radar (SAR) to remove motion-induced blurs in the images. The PGA algorithm has been proven to be a superior autofocus method. PGA assumes a narrow beam, which is valid for most SAR systems. However, lower altitude SA& have large range dependencies that cannot be ignored. A new phase estimator for PGA is introduced and extended to allow range dependence. An ERS-1 image of Death Valley is used in simulations comparing the new estimator to the widely used maximum likelihood approach and in demonstrating the range-dependent PGA algorithm.