Measuring the sea surface currents by HF radar has already reached the practical level. Many observations have been done in the costal zone, and it has gotten great results so far. On the other hand, measuring the ocean waves by HF radar is not in the practical level yet, is actually in the theoretical level. The directional wave spectrum can be recently estimated from Doppler spectrum by the method which Hashimoto and Tokuda (1998) proposed theoretically. The directional wave spectrum, however, cannot always be estimated from Doppler spectrum by this method because the noise is contained in the received signal and Doppler spectrum estimated from the received signal is different from it in the theory. This study numerically simulates the received signal containing the noise and makes it clear what kind of influence is in Doppler spectrum calculated from it. In addition, it is evaluated that the influence of the noise to the directional spectrum estimated from Doppler spectrum, too.
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