| Budget Amount *help |
¥3,100,000 (Direct Cost: ¥3,100,000)
Fiscal Year 2003: ¥800,000 (Direct Cost: ¥800,000)
Fiscal Year 2002: ¥2,300,000 (Direct Cost: ¥2,300,000)
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| Research Abstract |
X-band oceanic radars are a new type of observation apparatus that can evaluate the sea surface current distributions and directional wave spectra quasi-instantaneously. The settling and maintenance of these apparatuses are easier than those for ordinary observation systems. In spite of this advantage, the radars are not widely used, mainly, because of the relative low accuracy in its data. In this work, we proposed two modifications to a radar for observations with a suitable accuracy for practical uses.
First, we duplicated the rotating speed of the radar antenna from 20 rpm to 40 rpm ; therefore the Nyquist frequency of image sampling became 0.167 Hz from 0.333 Hz. This modification expanded the limitation of the observed short wave components from 6 s to 3 s in period, and also reduced the aliasing effect in data sampling. This contributes to the evaluation of sea surface currents and wave spectra with higher accuracy.
Next, we introduced the nonlinear least square method for the evaluation of the sea surface currents from the sea clatter image spectra. Since the target function, that is the wave dispersion relation equation, is nonlinear, the observation errors of all spectrum components were not evaluate evenly. Instead, the low-frequency components were emphasized with the ordinary linear least square method. Introduction of the nonlinear least square method in this study avoids this problem.
We applied the modified X-band oceanic radar to the field observations at Ogata in Niigata Pref. from December 2003 to February in the next year, and observed appropriate sea surface current distributions and directional wave spectra every hour in the nearshore region. In this way, we probed that the modifications proposed in this study improved the observation accuracy of the oceanic radar.
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