LABORATORY STUDY OF POLARIZED MICROWAVE-SCATTERING BY SURFACE-WAVES AT GRAZING-INCIDENCE - THE INFLUENCE OF LONG WAVES

Laboratory measurements of microwave scattering at grazing incidence f rom superposed wind and weakly nonlinear (AK<0.024) regular long waves are presented, This study is an extension of previous measurements wi th wind waves only, A dual polarized (VV, HH) coherent pulsed Ku-band (14 GHz) scatterometer with temporal resolution of 3 ns was used to ob tain Doppler spectra and the absolute cross section of scattered signa ls for grazing angles from 6 degrees to 25 degrees and winds in the ra nge 2-12 m/s, A wire wave-gauge array was used to measure the wind-wav e field. Measurements of the frequency and amplitude modulation of the scattered signal due to the long waves showed that the data separated into two groups, The first grouping corresponded to HH scattering in the upwind direction and was clearly associated with scattering from t he dominant gravity wind-waves on the crests of the long waves, In thi s case, the wind speed clearly influences the frequency modulation due to long waves. The second grouping corresponded to scattering in the downwind direction and was consistent with Bragg scattering from highe r frequency waves, In this case the frequency modulation due to orbita l velocity of the long waves was found to be weakly dependent on wind speed over the range of parameters studied. This classification of the electromagnetic scattering was consistent with comparisons of direct and Doppler measurements of the kinematics of the surface wave field.