Two-dimensional Autofocus for Spotlight SAR Polar Format Imagery

Conventional two-dimensional (2-D) autofocus algorithms blindly estimate the phase error in the sense that they do not exploit any a priori information on the structure of the 2-D phase error. As such, they often suffer from the low computational efficiency and the lack of data redundancy to accurately estimate the 2-D phase error. In this paper, an efficient semi-blind 2-D autofocus algorithm which is based on exploiting the a priori knowledge about the 2-D phase error structure, is presented. First, as a prerequisite of the proposed method, the analytical structure of the residual 2-D phase error in SAR imagery is investigated in the polar format algorithm framework. Then, by incorporating this a priori information, a novel 2-D autofocus approach is proposed. The new method only requires an estimate of the azimuth phase error and/or residual range cell migration, while the 2-D phase error can then be computed directly from the estimated azimuth phase error or residual range cell migration. Experimental results clearly demonstrate the effectiveness and the robustness of the proposed method.