LABORATORY STUDY OF POLARIZED SCATTERING BY SURFACE-WAVES AT GRAZING-INCIDENCE .1. WIND-WAVES

Laboratory measurements of Ku-band scattering at grazing incidence are presented. This study was motivated by the need to understand the pro cesses which significantly contribute to scattering at grazing inciden ce, A dual polarized (VV, HH) coherent pulsed Ku-band scatterometer wi th 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 degrees, and winds in the range 212 m/s. Wire wave gauges w ere 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 g rouping corresponded to HH scattering in the upwind look direction, an d was clearly associated with scattering from the dominant gravity win d-waves, The second grouping corresponded to HH scattering in the down wind look direction, and all VV scattering, and was consistent with Br agg scattering from free higher frequency waves, This classification o f the electromagnetic scattering was consistent with comparisons of di rect and Doppler measurements of the kinematics of the surface wave fi eld, The electromagnetic classification was also consistent with asymm etries in the wave field which increased with increasing wind speed.