Autofocus Correction of APE and Residual RCM in Spotlight SAR Polar Format Imagery

Synthetic aperture radar (SAR) images are often blurred by phase perturbations induced by uncompensated sensor motion and/or unknown propagation effects caused by turbulent media. To get refocused images, autofocus proves to be a useful postprocessing technique applied to estimate and compensate the unknown phase errors. However, a severe drawback of the conventional autofocus algorithms is that they are only capable of removing one-dimensional azimuth phase errors (APEs). As the resolution becomes finer, residual range cell migration (RCM), which makes the defocus inherently two dimensional (2-D), becomes a new challenge. In this paper, correction of APE and residual RCM are presented in the framework of polar format algorithm (PFA). First, an insight into the underlying mathematical mechanism of polar reformatting is presented. Then, based on this new formulation, the effect of polar reformatting on the uncompensated APE and residual RCM is investigated in detail. By using the derived analytical relationship between APE and residual RCM, an efficient 2-D autofocus method is proposed. Experimental results indicate the effectiveness of the proposed method.