OCEAN WAVE SPECTRAL DISTORTION IN AIRBORNE SYNTHETIC APERTURE RADAR IMAGERY DURING THE NORWEGIAN CONTINENTAL-SHELF EXPERIMENT OF 1988

C band radar images of ocean gravity waves off the Norwegian coast were processed into one-dimensional azimuth spectra. These spectra were used to measure the azimuth spectral (width) cutoff on the basis of a least squares fit to a Gaussian spectral shape. The widths were calculated for a range of wave heights (2-5 m) and wind speeds (2-18 m/s) during 3 days in March, 1988. Velocity smearing (sigma-v) estimates were extracted, independent of R/V and incidence angle, based on an imaging model and the measured azimuth cutoffs with sigma-v values varying from 0.4 to 0.7 m/s. Quantitative velocity smearing estimates are important as input to models describing the distortion in wave imagery. We propose a first-order model which neglects velocity bunching for ocean swell with peak wavelengths longer than about 250 m. This model is offered as a first estimate of when ocean wave swell will be detected by the C band SAR on board the ERS 1 spacecraft. The model predicts that this swell will be imaged under light winds of the order of 2-4 m/s. Higher wind speeds cause larger smearing, which may result in significant distortion of the imaged swell provided that the swell is traveling near the direction of the spacecraft ground track.