Variation of the NRCS of the sea with increasing roughness

Recently a new model for the normalized radar cross section (NRCS) of the sea has been developed that is based on the scattering from a composite surface, i.e., one in which a slightly rough surface in the sense of Rice has been superimposed upon a large wave structure. The model predicts an upper bound (saturation value) of the NRCS with increasing wind speed and in addition yields expressions for the variation of the NRCS with frequency, polarization, and grazing angle. To verify the validity of the model in the higher sea states where spray, shadowing, and nonlinear wave interactions would be expected to influence the return, extensive measurements were made over the North Atlantic Ocean in February 1969. The instrumentation used in the program was the Naval Research Laboratory four-frequency radar system which is an experimental radar installed in an EC-121 (Super Constellation) aircraft. Nine flights were made, and data acquired in sea states with winds that varied from Beaufort Force 0 to 8 with a maximum sea state condition of a 24-m/sec wind with 8.5-meter significant wave height were observed. The results of the measurement have verified the existence of the upper bound for the NRCS that is asymptotically approached in windspeeds in excess of 5 m/sec. In addition, the data have confirmed the variation of the cross section with frequency, polarization, and grazing angle with the proviso that the ‘tilting’ of the scattering surface by the large wave structure be assumed in the region of shallow grazing angles.