Azimuth dependence of Doppler spectra of sea clutter at low grazing angle

The effect of varying the azimuth angle of a cliff-top radar on the Doppler spectra of low grazing angle sea clutter has been investigated. The peak of the horizontally polarised Doppler spectra was found to depend on the component of the phase velocity of the breaking waves in the line of sight of the radar. The analysis was presented in terms of two main scattering components, one slow and one fast, which could explain the Doppler spectra as well as the higher moments of the spectra. The connection to the amplitude statistics for the two components was also discussed. The slow component was present all the time and had amplitude statistics that were Rayleigh-distributed. The fast component was present only intermittently and had amplitude statistics that were spikier than the Rayleigh distribution. The width of the spectra varied with azimuth in a way unrelated to the direction relative to the wind. A wave tank experiment with long wavelength mechanical waves and wind-generated waves illustrated that the cause of the broadening of the spectra was probably due to the effect of swell waves.