A new modification in a practical method for directional spectrum parameters by HF radar is presented. This new method is a further modification of Bayesian method reported by Hashimoto and Tokuda (1998). In this paper, a formulation of directional spectrum is characterized by an exponential function having the power expressed by a Fourier series over the directional range and assumed to be a piecewise-constant function over the frequency range. The applicability and accuracy of the proposed method are comprehensively examined using numerical simulation and field data of various wave conditions. The results suggest that modified method provides several advantages more powerful, potentially more accurate and faster solution techniques in practical method.
High frequency radar is a very useful tool to measure ocean surface current with real time observation, also for wave spectra and wind direction. The simultaneously observational technology for sea surface currents by HF radar sea-echo Doppler spectrum has already been established and applied into practical use. Nonetheless, the estimation of directional wave spectra remains unexplored, and limited to theoretical aspect the greatest barrier is to accurately estimate the Doppler spectra. Several methods have been developed to estimate directional wave spectrum (Lipa and Barick, 1986; Wyatt, 1990; Howell and Walsh, 1993; and Hisaki, 1996), with some advantages and limitations. Nevertheless, the majority of them are yet unable to reflect the practicality of the method. Wyatt (1990) employed Chahine-Twomey algorithm to estimate directional spectrum with efficient computational method, and already well established in Europe. A different approach was introduced by Hisaki (1996), who proposed a method to estimate directional wave spectra using HF radar and solve the nonlinear integral equation iteratively with additional conditions. He introduced a priori condition where the directional wave spectrum is assumed to be a smooth and continuous function. In addition, he also introduced other conditions that the directional wave spectrum has a value greater than zero, and it changes according to the known ratio in both frequency and directional angle.
