Limits to the inversion of HF radar backscatter for ocean wave measurement

The power spectrum of backscatter from the ocean surface at HF radio freque ncies is characterized by two large peaks called the first-order Bragg peak s. These are surrounded by a continuum due to second-order effects. The pow er spectrum can be described in low to moderate sea states by a nonlinear i ntegral equation relating it to the ocean wave directional spectrum. Invert ing this equation provides an estimate of the directional spectrum. A numbe r 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 secon d extension presented here tackles the nonlinear problem but points out the limitations of the approach and shows that improvements to the solution wi th this or any other method are likely to be minor. Of more importance is t he limitation of second-order theory in high sea states, particularly at th e higher radio frequencies used at the moment in operational systems. A the ory to describe the backscattered signal in these circumstances is a major challenge for the future.