Estimation of the sar system transfer function through processor defocus

It is generally accepted that in order to derive directional waveheight spectra from synthetic aperture radar (SAR) image data, the system transfer function (STF) must be removed from the raw SAR image spectrum. The STF equilvalent to the squared Fourier transform of the N-look perfect focus impulse response, may be estimated from actual data as the autocorrelation function of the speckle in a lowcontrast scene. In this paper, we outline a new approach for estimating the STF which is based on the observation that system defocus, to first order, does not impact speckle statistics. Therefore, a speckle pattern suitable for STF estimation may be produced from ocean wave SAR data by operating the processor out of focus, irrespective of the presence of waves in the image.