Focusing Bistatic Forward-looking SAR Data Based on Second-order Keystone Transform

Bistatic synthetic aperture radar (BSAR) data is more complicated to focus than that of the traditional monostatic SAR due to its strong 2-D coupling of echo signal, especially for bistatic forward-looking SAR (BFSAR). Although several algorithms have been proposed, they failed to take the spatial variance of range cell migration (RCM) into account. Moreover, residual RCM caused by higher order coupling have limited the imaging block size. In this paper, an algorithm based on second-order keystone transform (KT) and a modified nonlinear chirp scaling ( NLCS) is derived to deal with the above problems. First, linear range walk correction (LRWC) is performed to remove both the linear RCM and Doppler centroid. Then, a second-order KT is used to compensate for the range curvature. Finally, a modified NLCS is utilized to equalize the azimuth-variant Doppler frequency modulation (FM) rates of different targets. Simulation results verify the proposed algorithm well.