Airborne and spaceborne synthetic aperture radar observations of ocean waves

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Abstract
The Grand Banks ERS‐1 synthetic aperture radar (SAR) wave spectra validation experiment took place over a study site in which intensive in situ 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 W polarization, the geometry of acquisition was quite different. For example, the range‐to‐velocity ratio parameter was large for ERS‐1 (R/V ∼ 115 s) and relatively small for the CV‐580 (R/V < 50 s). Thus, the SAR image spectra derived from ERS‐1 are significantly more susceptible to velocity bunching non‐linearity and azimuth spectral cut‐off, which are well‐known limitations of SAR in accurately measuring azimuth‐travelling ocean waves. Airborne and spaceborne SAR measurements of ocean waves are presented and compared with directional wave buoy measurements. A technique is developed to estimate the azimuth spectral width based upon a quasi‐linear ocean‐to‐SAR transform, and is applied to all of the SAR spectra. This quasi‐linear transform is qualitatively assessed by forward‐mapping directional wave buoy spectra into SAR image spectra. The width measurements are correlated with observed values for the significant wave height or the azimuth shift and the local wind speed. This allows definition of a quasi‐linear transform that includes both velocity bunching decorrelation effects and wind speed‐dependent coherence time effects. Finally, a new SAR spectral inversion scheme, based upon the quasi‐linear transform, is demonstrated. Wave model spectra are used as the starting point and validation is against directional wave buoy spectra.