Directional wave spectra by inversion of ERS-1 synthetic aperture radar ocean imagery

An algorithm that extracts the directional ocean wave spectrum from synthetic aperture radar (SAR) ocean image spectra is implemented and applied to spaceborne C-band SAR data obtained from the ERS-1 satellite. The nonlinear iterative algorithm is based on the Hasselmann´s forward 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 quasilinear ocean-to-SAR transform. Enhanced wave spectra have been obtained using first-guess wave spectra either from the numerical wave model WINCH operated by the Norwegian Meteorological Institute or synthesized from nondirectional wave data and meteorological conditions. The inverted spectra are compared to in situ directional wave data. It is concluded that the wave imagery from ERS-1 appears to be of excellent quality, and as soon as the backscatter modulation transfer functions are properly understood, satellite SAR data will be an important tool for enhancing and extending conventional wave measurements and results from numerical wave models