Azimuthal dependence of the radar cross section and the spectral background noise of a nautical radar at grazing incidence

The radar signal backscattered from the sea surface is a composite of speckle and the modulation due to the surface gravity waves. In the spectral domain the speckle results in a background noise floor and the modulation component in the wave signal. Both components are wind dependent. The signal to noise ratio (SNR) of synthetic aperture radar (SAR) image spectra is used for the calibration of the spectra. This method is applicable to nautical radar image spectra as well. The SAR sensor detects the complex backscattered electromagnetic field. In contrast the nautical radar detects only the envelope, presented as grey levels. Furthermore the SAR operates at moderate incidence angles, while the nautical radar is scanning the water surface at grazing incidence. Nautical radar image sequences are transformed in the three dimensional frequency-wavenumber domain. There the two components are separated with the dispersion relation of surface gravity waves used as a spectral filter. The subject of the present study is to clarify, the azimuthal dependence of the variation of the background noise in the spectral domain and the radar backscatter in the spatial domain. The results obtained from the nautical radar are compared to the C-band model (CMOD.IFR2), which is used to determine wind fields over the ocean with the scatterometer and the SAR with vertical polarisation.