TERRAPUB Journal of Oceanography

Journal of Oceanography, Vol. 53 (No. 4), pp. 325-342, 1997

Measurement of Ocean Surface Currents by CRL HF Ocean Surface Radar of the FMCW Type. Part 1. Radial Current Velocity

Akitsugu Nadai1, Hiroshi Kuroiwa2, Masafumi Mizutori3 and Shin'ichi Sakai4

1Okinawa Radio Observatory, Communications Research Laboratory, Ministry of Posts and Telecommunications, 829-3 Daigusukubaru, Aza-kuba, Nakagusuku-son, Okinawa 901-24, Japan
2National Space Development Agency of Japan, Hamamatsu-cho Central Bldg.,1-29-6 Hamamatsu-cho, Minato-ku, Tokyo 105, Japan
3Environmental Science Department, Abiko Research Laboratory, Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko-shi, Chiba 270-11, Japan
4Marine Science Section, Environmental Science Department, Abiko Research Laboratory, Central Research Institute of Electric Power Industry, 1646 Abiko, Abiko-shi, Chiba 270-11, Japan

(Received 4 January 1995; in revised form 18 November 1996; accepted 17 December 1996)

Abstract: The Communications Research Laboratory (CRL) has been developing high-frequqency ocean surface radars (HFOSRs) of the FMCW type. This paper compares the radial current velocity measured by the CRL HFOSR with that from current meters moored 2 m below the ocean surface. Regression analysis shows that the HFOSR radial current velocity is about 1.3 times that from the moored current meters, the standard deviation of the residual being less than 7 cm s-1. The HFOSR measures the spatially averaged radial current velocity at a depth of 0.5 m, while the moored current meters make point measurements. Cross spectral analysis shows the poor correspondence between the two kinds of measurements for periods shorter than 10 hours, possibly due to the difference in measuring depths. To take account of this difference, the radial current velocities are processed through a low-pass filter with a cutoff frequency of 1/9.95 cph. The filtered radial current velocities show a good correlation; the standard deviation of the residual is less than 6 cm s-1. The range of the spatial variation of radial current velocity within a radar target cell is estimated from the width of the first-order echo in the Doppler spectrum. The estimated range of spatial variation is about 10 cm s-1. When taking account of the spatial variation of current, the RMS of the difference becomes less than 3 cm s-1. We thus conclude that the accuracy of radial current velocity measurement by the HFOSR is less than 6 cm s-1.


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