Properties of marine radar imagery and directional spectra of the sea surface

In the last decade a number of researchers have published "directional wave spectra" calculated from marine radar images. The author´s group has continued this line of research using a unique radar system that digitizes and stores radar images eight bits deep directly related to the strength of the radar backscatter. The author collected in excess of 3000 sea surface backscatter images during the Grand Banks ERS-1 SAR Wave Spectrum Validation Experiment cruise in November, 1991. This image set demonstrates new features of microwave reflection at grazing incidence. The images show maximum backscatter intensity in the direction of the local wind, which is not necessarily in the direction of the dominant waves. The fall-off in intensity with distance appears to be relatively independent of both wind speed and wave conditions. From these images the author has produced a number of directional spectra. Spectral peaks show a strong azimuthal asymmetry unrelated to the wind direction. Both of these asymmetries must be considered in the creation of an unbiased spectrum. The spectra compare favorably with those calculated from directional wave buoy data, satellite synthetic aperture radars and other directional wave measurements and models.