مرکز منطقه ای اطلاع رساني علوم و فناوري - 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 $\\lambda ^{-1}$ .