Extending the phase gradient autofocus algorithm for low-altitude stripmap mode SAR

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. This algorithm is extended for application to low-altitude stripmap mode SAR. PGA assumes a narrow beam, which is valid for most SAR systems. However, lower altitude SARs have large range dependencies that cannot be ignored. A new phase estimator for PGA is introduced and extended to allow range dependence. Three SAR images with different characteristics are used in simulations comparing the new estimator to the widely used maximum likelihood approach and in demonstrating the range-dependent PGA algorithm. The PGA algorithm is also extended to stripmap mode SAR data through a new compression method.