Spotlight SAR is an effective way to obtain high resolution radar imagery.The effect of the wavefront curvature will defocus the point far away from the imagery center by the classical polar format algorithm.We analyse the mechanism of wavefront curvature and space variant phase error in polar format algorithm

then present an overlapped sub-aperture polar format algorithm of the wavefront curvature correction during the course of SAR imaging

which can increase the imaging area for PFA algorithm and can be used in P band imaging and ultra high resolution imaging.