Synthetic aperture radar imaging of ocean waves: Comparison with wave measurements

Synthetic aperture radar images of ocean waves were obtained in conjunction with reference wave data near Marineland, Florida, December 14, 1975. Each of the various types of measurements were processed into a form that allowed direct comparisons with the others. Maxima of radar spectra occurred at the same frequencies as the maxima of reference wave height spectra. In a comparison of a radar spectrum with observed spectra of wave height, wave orbital velocity, and surface slope the high-frequency portion of the radar spectrum lay near and between the wave height and the orbital velocity spectra but differed significantly from the surface slope spectrum. The radar-derived mean directions and model-fitted directional spreads of wave energy were close to the values from a directional wave buoy and indicated the accuracy of radar measurements of wave direction. However, a directional plot of a radar spectrum near shore at the frequency of the maximum showed a sharper peak than such a plot of a fitted spectrum derived from reference data. The high directional resolution of the radar, in addition to its making observations at different locations, allowed radar images to provide information about ocean waves not available from the other instruments. As a swell propagated across the continental shelf, it was scattered in direction, apparently by the irregularities of the bottom, and very little of its energy reached shore. The shorter sea waves had a narrow directional distribution when first observed offshore that may have been sharpened by interaction with the Gulf Stream. Radar images showed effects of bottom refraction on the sea waves as they moved into progressively shallower water.