Analysis of MARSEN X band SAR ocean wave data

Abstract

Analysis of X band SAR imagery collected during the MARSEN experiment indicates that the APD‐10 SAR system imaged both range‐ and azimuth‐traveling gravity waves. However, only the near‐edge portion of the APD‐10 imagery provided reliable spectral wave estimates. Numerous motion artifacts, which manifest themselves as azimuth‐oriented streaks, are visible on the data and are believed to be caused by breaking waves. Because of the large platform velocity, the APD‐10 SAR data are relatively insensitive to wave enhancement adjustments performed during the processing of SAR signal histories. A modulation transfer function to relate SAR‐derived spectra to in situ measurements has been developed. The transfer function is smaller and falls off more rapidly with wave number for azimuth‐traveling waves than for range‐traveling waves. This is a consequence of the smaller inherent modulation for azimuth‐traveling waves and the degraded resolution in the azimuth direction as a result of motion effects and agrees, at least qualitatively, with theoretical predictions

Specular point scattering contribution to the mean synthetic aperture radar image of the ocean surface

“…A second image of a region approximately 600 km away on the same Seasat orbit shows little or no streaking; thus the streaking appears not to be a system artifact. Azimuthal streaks have also been reported by Shuchman et al [1983] and Lyzenga et al [1985], but because of the large incidence angles in these studies, these streaks appear to have been caused by some scattering mechanism other than specular point scattering. Nonetheless, the effects of specular point scattering streaks on the image spectrum can be expected to be similar to those observed by these authors, namely, the introduction of noise along the SAR azimuth wave number axis.…”

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].…”