Overcoming polar-format issues in multichannel SAR Autofocus

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Abstract
Multichannel autofocus (MCA) for synthetic aperture radar (SAR) assumes one-dimensional defocusing and that the perfectly focused image has low return at the image edge, where the scene is attenuated by the antenna pattern. When the assumptions are satisfied, MCA yields better or even perfect solutions compared to other autofocus methods. However, the performance is very sensitive to the violation of the one-dimensional defocusing assumption; thus it should not be used with polar formatting. Fortunately, the problem can be solved if we reverse the order of two steps in the image formation process and apply MCA to a warped version of the image, which is obtained by inverse Fourier transforming the polar-grid data without interpolation. The warped versions of the image satisfy a low-return assumption in practice, but the region of low-return must be carefully specified for best performance.