AZIMUTH SMEARING IN OCEAN SYNTHETIC-APERTURE RADAR IMAGE SPECTRA - A STUDY OF HASSELMANN CLOSED-FORM TRANSFORMATION BASED ON NORWEGIAN CONTINENTAL-SHELF EXPERIMENT 1988 SYNTHETIC-APERTURE RADAR DATA

The well-known along-track resolution loss in synthetic aperture radar (SAR) ocean wave image spectra is investigated comparing simulations based on Hasselmann's nonlinear integral transform and measurements fr om the Norwegian Continental Shelf Experiment 1988 experiment. In the literature the resolution loss has often been modeled as a low-pass fi lter process, described by the rms azimuth shift width sigma(x), actin g within a quasi-linear SAR transformation. Estimates from real data o f sigma(x), as a function the range-to-platform velocity (R/V) and inc idence angle, are compared to Hasselmann's new nonlinear spectral tran sformation and the widely used quasi-linear model. Simulations correla ted with real wave data show that the quasi-linear model, with contrib utions to sigma(x) from the entire spectrum, overestimates sigma(x) by roughly 30-40%. The conformity between Hasselmann's model and real da ta is excellent, however. The numerics also indicate that the degree o f nonlinearity in Hasselmann's transform is explicitly related to the sur-face truth parameters' significant wave height and peak wavelength . Furthermore, the spectral bandwidth, including the subresolution par t of the ocean wave spectrum, seems to be of minor importance. It is a lso shown that the resultant smearing (due to imaging nonlinearities) cannot explicitly be separated from the coherent linear velocity bunch ing part of the transform. This point is especially discussed since it has led previously to some dissension regarding which ocean spectral components are most essential in the smearing process.