Radiometric slope correction of synthetic-aperture radar images

The brightness in a SAR image is affected by topographic height variations due to (1) the projection between ground and image coordinates, and (2) variations in backscattering coefficient with the local scattering geometry. This paper derives a new equation for (1), i.e. the radiometric slope correction, based on a calibration equation which is invariant under a coordinate transformation. An algorithm is described to obtain the slope correction from a SAR interferogram, which also enables retrieval of the full scattering geometry. Since the SAR image and interferogram are derived from the same data set, there is no need to match the image with the calibration data. There is also no need for phase unwrapping since the algorithm only uses the fringe frequencies. A maximum-likelihood estimator for the fringe frequency is analyzed and the algorithm is illustrated by processing ERS-1 SAR data. The example demonstrates that the spatial resolution and calibration error are adequate for most applications.