報告書番号

V05020

タイトル（和文）

DBF海洋レーダで得られる流速の精度検証

タイトル（英文）

Validation of surface currents mesured with digital beam forming VHF Ocean Radar

概要 （図表や脚注は「報告書全文」に掲載しております）

電中研では短時間で広範囲に表層流動計測が可能なDBFレーダを開発してきている．本研究ではこのレーダの計測精度について，観測による検討および理論的解析を実施した．レーダとADCPを用いて，明石海峡付近の海域観測を実施し，両者で得られた流速を比較した結果，両者は高い相関をもち，測定結果を除けば,２つの観測機器に違いがないことが分かった．レーダで得られるビーム方向流速から流速ベクトルに変換する際に生じる誤差について，変換を表す行列を特異値分解し，解析を行った.この解析結果により，特異値の逆数がビーム方向流速の誤差の伸縮を表すこと，左特異ベクトルの方向に誤差が伸縮することを示した．さらに，特異値がレーダのビーム方向の角度差による正弦，余弦となることから，角度差が0，180度に近づくと，誤差が拡大することが示された．

概要 （英文）

CRIEPI has been developing VHF( Very High Frequency) radars with a digital beam-forming system (DBF-radars); the DBF-radars system can provide surface currents over an area approaching 400 km^2 by short periods of 15 minutes. This study presents an experimental and a theoretical analysis of the measurement accuracy of the velocities with DBF-radars. Experimental analysis examines the accuracy of DBF-radars current measurements using in situ data obtained with DBF-radar and ADCP, conducted off Akashi strait in the west part of Osaka bay. The radial velocities toward the ocean from the radar estimated from doppler spectrum obtained with DBF-radar are compared with the velocities at the depth above 2m measured with ship-borne ADCP at different 5 periods and different 20 points. The two intercomparisons of the radial velocities estimated from the doppler spectrum measured directly with DBF-radar and measured with ADCP and of the time-interpolated radial velocities with DBF-radar and the radial velocities directly measured with ADCP result in the high correlations greater than 94% and the low standard deviations of residuals from 7cm/s to 14cm/s. Moreover the t-tests at 5% significance level in these two comparisons could not suggest that the surface velocities with two measuring instruments, DBF-radars and ADCP, was in difference, if the several measurements were removed. We analyses the theoretical values and errors of the current vectors calculated from the two radial velocities from dual-sited DBF-radars by the singular values decomposition of the transforming matrix (A) from the two radial velocities to the cartesian components of the current vector. Firstly, the current vector can be directly calculated from two radial velocities using the singular values decomposition of the transforming matrix (A) without computing the inverse matrix of A. Secondly,The singular values is sine or cosine of the half of the azimuthal difference of two radar beams and the inverse of these values represent the expansion ratios of the standard deviations of radial velocities to the major and minor axes of ellipse represented the standard deviations of current vector. The left singular vectors are the functions of two beam azimuths and represent the direction of the major and minor axes. Consequently, The errors in either direction of the two left singular vectors becomes infinity when the azimuthal difference approaches 0 or 180 degrees, because either singular values becomes 0.

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