Measurement of the ocean wave‐radar modulation transfer function at 4.3 GHz

Measurements of the ocean wave‐radar modulation transfer function M at 4.3 GHz (C band) carried out in the North Sea are presented here. It was found that the values for M for C band lie within the same range as for X, L, and Ka bands. The measurements were made at incidence angles of 54° (HH and VV polarization) and 40° (VV polarization only). It was found that M decreases with increasing wind speed U and ocean wave frequency ƒ. The phase of M was such that maximum backscatter occurs at the forward (leeward) face of the long ocean waves. For small dimensionless frequencies, ƒ* = (ƒU)/g, the modulus of M reaches values of up to 30. This indicates that there is a source of strong hydrodynamic modulation, most probably wind induced, that is as yet unexplained by existing theories.