W-band radar backscattering at low grazing angles measured in a wave tank at various wind speeds

W-band backscatter experiments have been performed at the wind-wave tank of the University of Hamburg. The aim of the experiments is to study the backscatter mechanisms under shallow incidence angles and to compare the results with previous ones obtained with an X-band scatterometer. The measurements were performed at three grazing angles, 7.5°, 10°, and 20°, and at VV-, HH-, and VH-polarization. The wind speed ranged from 2 to 10 ms^-1 and was increased in steps of 1 ms^-1. The analysis of the acquired radar Doppler spectra shows that Bragg scattering from both bound and freely propagating Bragg waves is the dominant backscattering mechanism at all deployed wind speeds. In particular, at low wind speeds (up to 4 ms^-1), when the overall backscatter is small, bound waves axe the dominating scatterers, whereas at higher wind speeds (5 ms^-1 and above) the acquired signal is mainly caused by freely propagating Bragg waves. This is in qualitative agreement with similar findings, which have been made earlier with an X-band scatterometer working at moderate incidence angles.