Focus Improvement of Squint Bistatic SAR Data Using Azimuth Nonlinear Chirp Scaling

High-resolution imaging for squint azimuth-variant bistatic synthetic aperture radar system is a challenging task due to the existence of the spatial variance of range cell migration (RCM) and Doppler frequency modulation (FM) rate. To address this problem, azimuth nonlinear chirp scaling (ANLCS) is investigated in this letter. First, linear range walk is removed and then ANLCS is applied in the range frequency azimuth time domain to correct the azimuth-variant RCMs and to equalize the different FM rates. Taking the 2-D variance caused by the azimuth-variant configuration into consideration, a new perturbation function is derived based on the bistatic geometry. Using method of series reversion, a close form of range-azimuth coupling is obtained and corrected in the range Doppler domain by an interpolation-free operation. Incorporated with the secondary range compression, this method leads to a more accurate focusing for azimuth-variant bistatic configurations, even with high squints. Simulation results validate the effectiveness of the method.