Focusing Nonparallel-Track Bistatic SAR Data Using Extended Nonlinear Chirp Scaling Algorithm Based on a Quadratic Ellipse Model

Image formation for bistatic synthetic aperture radar data with nonparallel tracks is a challenging task due to the existence of the spatial variance of range cell migration (RCM) and azimuth frequency modulation (FM) rate. In this letter, an extended nonlinear chirp scaling (ENLCS) algorithm based on a quadratic ellipse model with two motion platform parameters is proposed for this configuration. For range processing, a method combining linear range walk correction and keystone transform is adopted to remove linear RCM completely and mitigate range-azimuth cross coupling. For azimuth focusing, a quadratic ellipse model using the two motion platform parameters is established to depict the azimuth-variant properties of the azimuth FM rate, and its accurate high-order approximation is derived as well. Investigations of the quadratic phase error show that great improvement can be made by this model. Following which, coefficients of the ENLCS are derived to accomplish the azimuth equalization. Simulation results validate the effectiveness of the algorithm proposed by this letter.