On the polarization and the wavelength dependence of sea echo

Abstract

Data on radar cross section per unit area \sigma\deg of the sea at 6.3 Gc/s and 35.0 Gc/s and for grazing angles between 1.5\deg and 4.0\deg are reviewed. The data were obtained by simultaneously receiving both horizontally and vertically polarized echo components for transmitted polarizations which were sequentially changed between horizontal and vertical. Results indicate that sea echo is primarily caused by two scattering mechanisms: 1) a wind-dependent fine structure of the sea (presumably ripples) that partly depolarizes and has a scattering cross section which depends on wavelength in accordance with \lambda^{-1} , and 2) a gross structure of the sea (presumably smooth facets between the ripples) that does not depolarize and has a scattering cross section which is independent of wavelength. The cross sections for transmitting and receiving vertical polarization ( \sigma\deg VV ) and for transmitting and receiving horizontal polarization ( \sigma\deg HH ) are caused by the sum of the contributions from the two mechanisms. Therefore, although dependent on sea state and polarization, it seems that \sigma\deg VV and \sigma\deg HH will tend to be independent of wavelength at the lower frequency end of the microwave spectrum and will tend to depend on wavelength in accordance with \lambda^{-1} at the higher end of the spectrum. A comparison of data at 6.3 Gc/s and 35.0 Gc/s and for grazing angles between 1.5\deg and 4.0\deg indicates that the wavelength dependences of \sigma\deg VV and \sigma\deg HH are functions of sea state but are greater than \lambda\deg and are considerably less than \lambda^{-1} .