Laboratory study of polarized scattering by surface waves at grazing incidence. I. Wind waves

Laboratory measurements of Ku-band scattering at grazing incidence are presented. This study was motivated by the need to understand the processes which significantly contribute to scattering at grazing incidence. A dual polarized (VV, HH) coherent pulsed Ku-band scatterometer with good temporal resolution (3 ns) was used to obtain Doppler spectra and the absolute cross-section of scattered signals for grazing angles from 6-12°, and winds in the range 2-12 m/s. Wire wave gauges were used to measure the wind-wave field. Measurements of the first few moments of the Doppler spectra (cross-section, central frequency and bandwidth) showed that the data separated into two groups. The first grouping corresponded to HH scattering in the upwind look direction, and was clearly associated with scattering from the dominant gravity wind-waves. The second grouping corresponded to HH scattering in the downwind look direction, and all VV scattering, and was consistent with Bragg scattering from free higher frequency waves. This classification of the electromagnetic scattering was consistent with comparisons of direct and Doppler measurements of the kinematics of the surface wave field. The electromagnetic classification was also consistent with asymmetries in the wave field which increased with increasing wind speed