Estimation of the SAR system transfer function through processor defocus

It is generally accepted that in order to derive wave directional spectra from synthetic-aperture radar (SAR) image data, the system transfer function (STF) must be removed from the raw SAR image spectrum. The STF, very nearly equivalent to the magnitude of the Fourier transform of the N-look perfect-focus impulse response, can be estimated from actual SAR image data as the magnitude of the Fourier transform of the autocorrelation function of the speckle in a low-contrast scene. The authors outline and demonstrate a novel approach for estimating the STF based on the observation that system defocus, to the first order, does not impact speckle statistics.