Limits to the Inversion of HF Radar Backscatter for Ocean Wave Measurement

Abstract

The power spectrum of backscatter from the ocean surface at HF radio frequencies is characterized by two large peaks called the first-order Bragg peaks. These are surrounded by a continuum due to second-order effects. The power spectrum can be described in low to moderate sea states by a nonlinear integral equation relating it to the ocean wave directional spectrum. Inverting this equation provides an estimate of the directional spectrum. A number of inversion methods have been published. In this paper, the Wyatt method is discussed. This method uses the part of the backscatter power spectrum that surrounds the larger Bragg peak. An extension of the method to include the spectrum around the weaker peak is discussed, and some improvements in the solution are demonstrated. Until recently, all published work in this area concerned solutions to a linearized version of the equation. The second extension presented here tackles the nonlinear problem but points out the limitations of the approach and shows that improvements to the solution with this or any other method are likely to be minor. Of more importance is the limitation of second-order theory in high sea states, particularly at the higher radio frequencies used at the moment in operational systems. A theory to describe the backscattered signal in these circumstances is a major challenge for the future.