SIMULTANEOUS MEASUREMENTS OF THE OCEAN WAVE-RADAR MODULATION TRANSFER-FUNCTION AT L-BANDS, C-BANDS, AND X-BANDS FROM THE RESEARCH PLATFORM NORDSEE

Radar backscatter measurements were performed from the German Forschun gsplattform Nordsee (FPN) in the North Sea in order to determine the o cean wave-radar modulation transfer function (MTF), which relates the backscattered radar power to the long surface waves. The radar operate d quasi-simultaneously at 1.0 GHz (L band), 5.3 GHz (C band), and 10.0 GHz (X band) at HH and VV polarization by using a single antenna. MTF s obtained at these radar frequencies and polarizations are compared. Our measurements of the dependence of the MTF on wind speed and long w ave frequency are in agreement with earlier measurements. It is shown that the dependence of the coherence between the backscattered radar p ower and the long ocean wave height is a strongly decreasing function of radar frequency. This implies that a real aperture radar operating at a low radar frequency, e.g., at L band, is best suited for imaging ocean waves. Residual MTFs, M(res), are calculated by subtracting the theoretical tilt and range MTFs from the measured total MTFs. Accordin g to conventional ocean wave-radar modulation theory, M(res) should be identical to the hydrodynamic MTF and therefore be independent of pol arization. However, the experimental data show a strong dependence of the modulus and phase of M(res) on polarization. We find larger values of \M(res)\ for HH than for VV polarization at C and X bands. In prin ciple, a difference between M(res) for HH and VV polarization can be e xplained by a three-scale composite surface model which takes into acc ount also the modulation of the Bragg waves by intermediate-scale wave s (i.e., waves with wavelengths between the long waves and the Bragg w aves). However, the differences observed in this experiment are found to be much larger than expected from this theory.