Measuring sea surface currents by oceanic radar has reached a practical level, and in fact, many observations of the currents in costal zones have provided quite suitable results. In contrast, measuring ocean waves by oceanic radars is still at a theoretical level. An estimation method of the directional wave spectra derived from the Doppler spectra, which was proposed by Hashimoto and Tokuda (1998), is considered theoretically applicable, but it is problematic due to noise-contamination of the measured Doppler spectra which is significantly distorted from the theoretical one. This led to the present study in which we review the theoretical background of the principles used for measuring ocean waves by oceanic radar, subsequenly proposing a new theoretical approach to estimate the directional wave spectra form the noise-contaminated Doppler spectra.
　The original method only employs second-order backscattering components, yet accurately estimating the directional wave spectra from the observed Doppler spectra requires proper separation of Doppler spectra first- and second-order backscattering components; a difficult task due to the observed Doppler spectra exhibiting major variations. Accordingly, a method for signal treatment of the observed Doppler spectra is discussed using wavelet multiple resolution analysis for separating the baskscattering components. In addition, a new method applying the first- and second-order baskscattering components is described which improves the resolution of the directional wave spectra.
　Finally, two high-frequency (HF) oceanic radars located along the Kashima coast were used to conduct a series of field measurements of the Doppler spectra from which the directional spectra was then estimated. Simultaneous observations were also conducted using a wave gage array, with subsequent comparison showing good agreement with HF oceanic radar results.