Synthetic-aperture radar autofocus by maximizing sharpness

To focus a synthetic-aperture radar image that is suffering from phase errors, a phase-error estimate is found that, when it is applied, maximizes the sharpness of the image. Closed-form expressions are derived for the gradients of a sharpness metric with respect to phase-error parameters, including both a point-by-point (nonparametric) phase function and coefficients of a polynomial expansion. Use of these expressions allows for a highly efficient gradient-search algorithm for high-order phase errors. is demonstrated with an example. (C) 2000 Optical Society of America OCIS codes: 100.5070, 280.6730, 100.3010, 100.3020, 110.3000, 110.5100.