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 th eory is not sufficient to explain the power spectrum of the received s ignal in all cases. This is not a new result, radars have been develop ed in both Britain and the USA that allow a reduction in radio frequen cy for operation in high sea-states. However, the data presented here were obtained at frequencies in the lower half of the HF band, and sti ll show evidence of a significant departure from second order. None th e less the effect on ocean wave measurement appears to be small. The r esults reveal a directional dependence of this effect which may be com pensating for the higher-order nonlinearities in the inversion of dual radar spectra. The use of the departure from second order theory to p rovide a crude significant waveheight estimate is suggested.