High order nonlinearities in HF radar backscatter from the ocean surface

HF radar measurements of ocean waves make use of a second-order theory for the hydrodynamic and electromagnetic processes that generate the received signal. The author presents evidence that the second order theory is not sufficient to explain the power spectrum of the received signal in all cases. This is not a new result; radars have been developed in both Britain and the USA that allow a reduction in radio frequency for operation in high sea-states. However, the data in the present paper were obtained at frequencies in the lower half of the HF band, and still show evidence of a significant departure from second order. The effect on ocean wave measurement appears to be small. The results reveal a directional dependence of this effect which may be compensating for the higher-order nonlinearities in the inversion of dual radar spectra. The use of the departure from second order theory to provide a crude significant waveheight estimate is suggested