Focusing highly squinted Azimuth variant Bistatic SAR

Accurate focusing of high-resolution highly squinted azimuth variant bistatic synthetic aperture radar is a more challenge issue than its translational azimuth invariant bistatic counterpart due to the following two reasons: 1) higher order terms spatial variant feature in azimuth caused by removing relatively large range migration and 2) the azimuth-dependent range offset (RO) induced by inherent azimuth variant bistatic geometric configuration. In this paper, to solve those two issues, a geometry information-aided extended azimuth nonlinear chirp scaling algorithm to provide a better focus is proposed. First, the range compression containing two procedures, namely a linear range walk correction operation and a bulk secondary range compression, is utilized. Second, based on the geometry property of azimuth variant bistatic geometric configuration, the azimuth-dependent RO is obtained and then compensated to enhance focusing ability in the azimuth dimension. By evaluating the spatial variant characteristic of the frequency modulation (FM) rate and the higher order terms, a new azimuth scaling function is derived by adopting higher order approximation operation. With this new azimuth scaling function, not only the azimuth spatial variances of the higher order terms of FM rate and cubic phase terms are equalized, but also the azimuth-dependent RO is eliminated. Compared with existing azimuth nonlinear chirp scaling method, large azimuth depth of focusing can be realized without changing the overall imaging procedure. The experimental results with simulated data verify the effectiveness of the proposed algorithm.