DIRECTIONAL WAVE SPECTRA BY INVERSION OF ERS-1 SYNTHETIC-APERTURE RADAR OCEAN IMAGERY

An algorithm that extracts the directional ocean wave spectrum from sy nthetic aperture radar (SAR) ocean image spectra is implemented and ap plied to spaceborne c-band SAR data obtained from the ERS-1 satellite. The nonlinear iterative algorithm is based on the Hasselmann's forwar d spectral transform extended to include the range bunching effect. An analytic expression for the wave spectral increment is derived based on the exact gradient of the quasi-linear ocean-to-SAR transform. Enha nced wave spectra have been obtained using first-guess wave spectra ei ther from the numerical wave model WINCH operated by the Norwegian Met eorological Institute or synthesized from nondirectional wave data and meteorological conditions. The inverted spectra are compared to in si tu directional wave data. It is concluded that the wave imagery from E RS-1 appears to be of excellent quality, and as soon as the backscatte r modulation transfer functions are properly understood, satellite SAR data will be an important tool for enhancing and extending convention al wave measurements and results from numerical wave models.