Comparisons between wave directional spectra from SAR and pressure sensor arrays

Abstract

Simultaneous directional wave measurements were made at Torrey Pines Beach, California, by a synthetic aperture radar (SAR) and a linear array of pressure sensors. The measurements were conducted during the West Coast Experiment in March 1977. Quantitative comparisons of the normalized directional spectra from the two systems were made for wave periods of 6.9-17.0 s. The comparison results were variable but generally showed good agreement of the primary mode of the normalized directional energy. An attempt was made to quantify the physical criteria for good wave imaging in the SAR. A frequency band analysis of wave parameters such as band energy, slope, and orbital velocity did not show good correlation with the directional comparisons. It is noted that absolute values of the wave height spectrum cannot be derived from the SAR images yet and, consequently, no comparisons of absolute energy levels with corresponding array measurements were intended.

Airborne microwave Doppler measurements of ocean wave directional spectra

“…In any case, resolution will be limited by antenna dimensions (Kenney et al 1979, Walsh et al 1985. The short pulse radar system and its Fourier transform, the two-or three-frequency microwave system, have been shown to be effective techniques for the measurement of ocean wave directional spectra (Plant 1977, Alpers and Hasselmann 1978, Jackson 1981, Johnson and Weissman 1984, Schuler et al 1985, Jackson et al 1985a, b, Weissman and Johnson 1986). Both systems offer good angular resolution and are capable of being flown at spacecraft altitudes.…”

“…In situ measurement of such spectra is difficult since it requires either area-extensive arrays of probes or point probes capable of measuring high-order derivatives of surface displacement. While large arrays allow good angular resolution to be achieved, the difficulties of operating extensive arrays of pressure gauges or wave staffs have limited their use primarily to coastal areas or lakes (Pawka et al 1980, Donelan et al 1985. Point probe methods measuring second and lower derivatives of surface displacement yield angular resolutions which are only about 60° (LonguetHiggins et al 1983, Mitsuyasu et al 1975.…”

Dominant wave directions and significant wave heights from synthetic aperture radar imagery of the ocean

“…Theoretically, the apparent direction of ocean wave propagation determined from synthetic aperture radar (SAR) imagery of the ocean can differ significantly from the true direction of propagation [Briining et al, 1990]. However, experimental results have indicated that the SAR directions lie rather close to the true propagation directions for many observation conditions [Pawka et at., 1980;Beat et at., 1986]. Obviously, the dominant wave direction observed in SAR imagery depends on the overall transfer function of the SAR, which is itself 1Now at Lincoln Laboratory, Massachusetts Institute of Technology, Lexington.…”

“…However, experimental results have indicated that the SAR directions lie rather close to the true propagation directions for many observation conditions [Pawka et at., 1980;Beat et at., 1986]. Obviously, the dominant wave direction observed in SAR imagery depends on the overall transfer function of the SAR, which is itself Germany based on the work of Alpers and Hasselmann [1982] that utilize signal-to-noise ratios to obtain significant wave heights from ERS 1 SAR imagery of the ocean [Briining et al, 1994].…”

The West Coast experiment: An overview

“…A detailed comparison of the directional wave proper ties derived from SAR wave images and from the Scripps Institution of Oceanography (SIO) linear array at Torrey Pines is given by Pawka et al [1980]. The results indicate that the SAR wave directional resolution compares favorably with the in situ measurements at Torrey Pines.…”