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

Abstract

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 o! 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. the USSR [Bondarenko et al., 1972] since the early 1970's. Airborne synthetic aperture radar has been used to observe waves generated by hurricanes at L band [Ross et al., 1974], while X band systems have observed features of the Gulf Stream [ Wright et al., 1976]. The Seasat A satellite was the first attempt to obtain wave imagery from space. Preliminary results from its data [Gonzalez et al., 1979; Teleki et al., 1979] show that useful imagery can be obtained.concerned with wavelength and direction information. The results are encouraging, but it must be noted that the majority of waves imaged appear to have been swell and the limitations of the synthetic aperture approach in imaging local sea is not known.According to the theory of synthetic aperture systems, as discussed by Brown and Porcello [1969], the location of a reflecting element on the earth's surface is determined by tracking the Doppler phase history of the returned signal. For a fixed surface, there is a unique Doppler relationship for the received radar signal from a given earth location. If the surface is in motion, however, as in the case of ocean waves, the

The two‐scale radar wave probe and SAR imagery of the ocean

“…Since that time, several authors have addressed SAR imagery of the ocean using both point target and distributed scatterer models [Jain, 1978;Swift and Wilson, 1979;Alpers and Rufenach, 1979;Valenzuela, 1980;Harger, 1980;Shuchman et al, 1981;Rufenach and Alpers, 1981;Raney, 1981;Jain, 1981;Alpers, 1982]. The results of these efforts have not always been consistent; attempts to check their predictions against experiment are still underway [Gonzalez et al, 1979;Beal, 1979;McLeish et al, 1980;Shuchman, 1981;Alpers et al, 1981;Beal et al, 1983]. One convenient instrument with which SAR imagery of the ocean could be compared is the two-scale radar wave probe [Wright et al, 1980].…”

Analysis of MARSEN X band SAR ocean wave data

“…Previous experiments with aircraft SAR's have shown that it is possible to detect the dominant wavelength and direction of a surface wave field [Shemdin et al, 1978;Gonzalez et al, 1979;McLeish et al, 1980;Pawka et al, 1980]. The shape of in situ and SAR-derived surface wave spectra, however, show large differences.…”

SAR imaging of waves in water and ice: Evidence for velocity bunching

“…Imaging of gravity waves in the ocean by synthetic aperture radar (SAR) has been amply demonstrated by several investigators • Gonzalez et al, 1979;McLeish et al, 1980;Shuchman et al, 1983], although there is still disagreement as to the exact mechanisms responsible for this imaging. In particular, the relative importance of hydr0dynamic modulation,, tilt modulation, and velocity bunching is no.t well known [Alpers et al, 1981;Harger, 1981;Jain, 1981].…”