Dominant wave effects in wavetank measurement of microwave Doppler spectra

Coherent microwave measurements of backscattering from a rough water surface were made in a wind wavetank at the Canada Centre for Inland Waters. Ku-band return was collected simultaneously at HH and VV polarizations at a 45 degree incidence angle looking up wind. Data were collected when the wind was steady and when it was suddenly started. The authors found that the Doppler spectra exhibited peak frequencies which correspond to those expected from a scatterer moving at the phase speed of the dominant wave in the tank. The ratio of VV and HH backscattering cross sections goes from about 6 dB at low wind speeds to about 2 dB at high. Finally, they find that when the wind is suddenly started, both HH and VV cross sections grow rapidly and that the polarization ratio is very nearly 8 dB during this growth. Shortly after the growth stops, however, the polarization ratio drops to a value corresponding to that in the steady wind state. They interpret these results to mean that the primary scatterers in this wind wavetank are not free Bragg waves but scatterers tied to the dominant waves in the tank. They suggest that a tilted Bragg wave bound to the dominant wave could explain these results