Tower based measurements of the ocean wave radar modulation transfer function with a three-frequency scatterometer

Quasi-simultaneous measurements of the ocean wave-radar modulation transfer function (MTF) were performed from the German North Sea Research Platform (FPN) at 1.0 GHz (L-band), 53 GHz (C-band), and 10.0 GHz (X-band) at HH and VV polarization by using one antenna. MTFs obtained at different radar frequencies and polarizations are compared. The results show that the coherence between the radar backscatter and the long ocean wave field is a strongly decreasing function of radar frequency. A residual MTF, M_res is calculated which is obtained by subtracting the theoretical tilt and range MTFs from the measured total MTF. According to conventional theory, M_res should be identical to the hydrodynamic MTF and therefore be independent of polarization. However, the experimental data show a strong dependence of the magnitude and phase of M_res on polarization. The differences between M_res obtained at HR and VV polarization can partly be explained by a three-scale composite surface model