74 
Measurement of Complicatedness of Ocean Surface 
Current Field by Using HF Ocean Surface Radar 
Akitsugu Nadai 
Okinawa Radio Observatory, Communications research Laboratory, 
Ministry of Posts and Telecommunications 
829-3 Daigusukubaru Aza-kuba Nakagusuku-son, Okinawa 901-24, JAPAN 
Tel: +81-98-895-2045, Fax: +81-98-895-4010 
E-mail: nandai@crl.go.jp 
Abstract 
HF ocean surface radar (HFOSR) is an instrument to measure several ocean 
surface parameters. HFOSR measures radial current velocity using Doppler fre 
quency of strong peak called first-order echo on the Doppler spectra of received 
signal backscattered by the ocean surface waves. The extension of first-order echo 
represents variation of radial current velocity in one radar cell, and the complicat 
edness of ocean surface current field can be measured by using the extension of 
first-order echo. 
1 Introduction 
HF ocean surface radar (HFOSR) is an instrument for measuring several ocean 
surface parameters as radial current velocity, ocean wave parameters (Barrick et.al, 1974: 
Stewart and Joy, 1974). HFOSR transmits the radio waves, and receives and analyzes 
the signal backscattered by ocean surface waves. Because of Bragg resonance, the signal 
backscattered by the ocean surface waves which propagate along the line of sight of the 
radar and whose wavelength is half of the transmitted radio wave make strong peaks, 
called ’first-order echoes’, on Doppler spectra like Figure 1-a. The Doppler frequency of 
first-order echo represents phase velocity of the causal ocean surface waves to the HFOSR. 
The radial current velocity is the difference between the measured phase velocity and the 
theoretical phase velocity of the causal ocean surface waves (determined by the wave 
length through dispersion relation). 
2 Extent of first-order echo 
Okinawa Radio Observatory/Communications Research Laboratory observed the 
surface current field of the southern sea of Yakushima Island twice, from October to 
December of 1992, and from December 1993 to March 1994. Figure 2 shows the radar 
sites and antenna beam directions of 1992 observation. In this area the Kuroshio interacts 
with the coastal water, so a velocity front is made between them and the current field has 
quite complicated features. 
Within 1992 observation, first-order echo on Doppler spectra sometimes showed 
broadening shape like Figure 1-b. The extents of the first-order echoes in Figure 1-b are 
about 200cm/s in radial current velocity. There are many small peaks in first-order echo, 
and they have corresponding echoes in another first-order echo (indicated by characters). 
Figure 3 shows spatial change of the Doppler spectrum with radar range and antenna 
beam direction. There is few difference between the spatial changes of the shape of first- 
order echo without the area showing widely broadening first-order echo. The area showing 
widely broadening first-order echo is about 15kilometer diameter, and the shapes of the 
first-order echo change gradually. After now, for convenience, the velocity converted from