Simultaneous measurements of the ocean wave—Radar modulation transfer function at L, C, and X bands from the research platform Nordsee

Abstract

Radar backscatter measurements were performed from the German Forschungsplattform Nordsee (FPN) in the North Sea in order to determine the ocean wave‐radar modulation transfer function (MTF), which relates the backseat t ered radar power to the long surface waves. The radar operated 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. MTFs obtained at these radar frequencies and polarizations are compared. Our measurements of the dependence of the MTF on wind speed and long wave frequency are in agreement with earlier measurements. It is shown that the dependence of the coherence between the backscattered radar power 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, Mres, are calculated by subtracting the theoretical tilt and range MTFs from the measured total MTFs. According to conventional ocean wave radar modulation theory, Mres should be identical to the hydrodynamic MTF and therefore be independent of polarization. However, the experimental data show a strong dependence of the modulus and phase of Mres on polarization. We find larger values of |Mres| for HH than for VV polarization at C and X bands. In principle, a difference between Mres for HH and VV polarization can be explained by a three‐scale composite surface model which takes into account also the modulation of the Bragg waves by intermediate‐scale waves (i.e., waves with wavelengths between the long waves and the Bragg waves). However, the differences observed in this experiment are found to be much larger than expected from this theory.