  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 useful post-processing 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 (APE). As the resolution becomes finer,
residual range cell migration (RCM), which makes the defocus inherently
two-dimensional, 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 two-dimensional (2-D) autofocus method is proposed.
Experimental results indicate the effectiveness of the proposed method.
