AIRBORNE AND SPACEBORNE SYNTHETIC-APERTURE RADAR OBSERVATIONS OF OCEAN WAVES

The Grand Banks ERS-1 synthetic aperture radar (SAR) wave spectra vali dation experiment took place over a study site in which intensive in s itu wind and wave measurements were being taken. The unique aspect of the program was the nearly simultaneous acquisition (in space and time ) of spaceborne (ESA ERS-1) and airborne (CCRS CV-580) SAR imagery of the same ocean wave field. Although both SARs were operating at C-band with VV polarization, the geometry of acquisition was quite different . For example, the range-to-velocity ratio parameter was large for ERS -1 (RN approximately 115 s) and relatively small for the CV-580 (RN < 50 s). Thus, the SAR image spectra derived from ERS-1 are significantl y more susceptible to velocity bunching non-linearity and azimuth spec tral cut-off, which are well-known limitations of SAR in accurately me asuring azimuth-travelling ocean waves. Airborne and spaceborne SAR me asurements of ocean waves are presented and compared with directional wave buoy measurements. A technique is developed to estimate the azimu th spectral width based upon a quasi-linear ocean-to-SAR transform, an d is applied to all of the SAR spectra. This quasi-linear transform is qualitatively assessed by forward-mapping directional wave buoy spect ra into SAR image spectra. The width measurements are correlated with observed values for the significant wave height or the azimuth shift a nd the local wind speed. This allows definition of a quasi-linear tran sform that includes both velocity bunching decorrelation effects and w ind speed-dependent coherence time effects. Finally, a new SAR spectra l inversion scheme, based upon the quasi-linear transform, is demonstr ated. Wave model spectra are used as the starting point and validation is against directional wave buoy spectra.