TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 55 (No. 1), pp. 13-30, 1999

Measurement of Ocean Surface Currents by the CRL HF Ocean Surface Radar of FMCW Type. Part 2. Current Vector

Akitsugu Nadai1, Hiroshi Kuroiwa2, Masafumi Mizutori3 and Shin'ichi Sakai3

1Global Environment Division, Communications Research Laboratory, Ministry of Posts and Telecommunications, 4-2-1 Nukui-kitamachi, Koganei, Tokyo 184-8795, Japan
2Office of Earth Observation Systems, National Space Development Agency of Japan, World Trade Center Bldg., 2-4-1 Hamamatsu-cho, Minato-ku, Tokyo 105-6127, Japan
3Environmental Science Department, Abiko Research Laboratory, Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko, Chiba 270-1166, Japan

(Received 1 May 1998; in revised form 4 September 1998; accepted 17 September 1998)

Abstract: The Communications Research Laboratory (CRL) has been developing high-frequency ocean surface radars (HFOSRs). The CRL dual-site HFOSR system can clarify the distribution of surface currents with a nominal range of 50 km. This paper presents a theoretical and experimental analysis of the measurement error of the current vector obtained by the CRL HFOSR system, using a comparison of instantaneous current vectors acquired by the HFOSR system and current meters moored at a depth of 2 m, taking account of the vertical current shear. The theoretical analysis shows that the probability distribution of the measurement error of the current vector forms concentric ellipses at a spatial scale that depends on the RMS measurement error of radial current velocity and with an aspect ratio that depends only on the azimuthal difference of the radar beams. When the azimuthal difference is a right angle, the measurement error of the current vector is at a minimum. A comparison between instantaneous current vectors measured by the CRL HFOSR system and moored current meters shows that the distribution of the difference vector between the radar current and the meter current agrees well with the theoretical measurement error of the current vector and that the RMS of difference vector length is about 10 cm s-1 while the azimuthal difference between two radar beams is between 45 and 135 degrees. The accuracy of current measurement by the dual-site HFOSR system is therefore considered to be less than 10 cm s-1 in this range of azimuthal difference. The theoretical analysis will be applicable for a wider range of the azimuthal difference of the radar beams.

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