A spatially-variant autofocus technique for synthetic-aperture radar

A description is provided of a technique for automatically removing cross-range phase errors, left by the motion compensation system, from nominally focussed synthetic-aperture radar (SAR) images which have been stored in complex number format. These phase errors are assumed to be quadratic in form and spatially-variant. Attention is given to the derivation of the focussing function, the algorithm used to implement the considered autofocussing procedure, questions of computational efficiency, a defocussed Seasat SAR image, segments of the Seasat image which could be automatically focussed, and a defocussed L-band airborne SAR image, processed with a velocity error of 50 percent.