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1 Introduction

The high-frequency (HF) ocean surfaceradar is a kind of a Doppler radar that canmonitor ocean-surface condition, such as sur-face current vectors, ocean surface winds, andocean waves, by making a frequency analysisof backscattered echoes from the ocean sur-face. The HF ocean surface radar can continu-ously observe over a large area from land,while the conventional methods using shipsand mooring buoys can measure only at someparticular points on the ocean. Therefore, theimportance of HF ocean surface radar isincreasing in various subjects such asoceanography, coastal engineering, and fish-ery.

HF ocean surface radar has been devel-oped in many countries since the late 1970sand its effectiveness for the current fieldobservation has been rated highly. The Com-munications Research Laboratory (CRL)began to develop the HF ocean surface radarwith frequency of 24.5 MHz in 1986, and thefirst HF ocean surface radar in Japan wascompleted in 1988. Since then, a lot of exper-imental observations have been conducted inJapan in collaboration with various institutes

and universities and improvement of theobservation system and data analyzingprocesses have been performed. The 24.5MHz HF ocean surface radar is useful inmeasuring small-scale oceanographic phe-nomena. However, it is not suitable for large-scale phenomena such as the Kuroshio currentbecause maximum current observation rangeof the 24.5 MHz HF ocean surface radar is notlong enough. Therefore, the development ofnew HF ocean surface radar capable of long-range observations using the 9.2 MHz fre-quency was launched in 1999 and the systemwas completed in July 2001.

2 The Principles of HF Ocean Sur-face Radar

The ocean surface measurement by HFocean surface radar is based on the scatteringtheory of radio waves by ocean surfacewaves[1]～[4]. Fig.1 shows a schematic dia-gram of observation using HF ocean surfaceradar. The shore-based radar transmits HFradio waves toward the sea surface, and theradio waves are strongly backscattered byocean surface waves with half the radio wave-length (Bragg resonant scattering). Frequency

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4-12 Development of Long-Range OceanRadar System

SATO Kenji, MATSUOKA Takeshi, and FUJII Satoshi

Communications Research Laboratory developed a new high-frequency ocean surfaceradar system named Long-Range Ocean Radar (LROR) that is composed of two radarslocated on two of the Ryukyu Islands in the southwest part of Japan. LROR is designed toobserve surface currents up to 200 km from the radar sites with range resolution of 7 kmand will be used for the experimental observation of upper region of the Kuroshio Current inthe southern part of the East China Sea. We started performance evaluations and experi-mental observations in July, 2001.

Keywords HF radar, Surface current, Kuroshio current, Remote sensing

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analyzing this backscattered signal, there aretypically two prominent peaks called “first-order echoes” near the Bragg frequency,which is the frequency corresponding to thephase speed of the ocean wave contributing tothe Bragg resonant scattering. Fig.2 shows atypical scheme of a Doppler spectrum meas-ured with the HF ocean surface radar. Peakwith positive Doppler frequency is correspon-ding to approaching waves toward the radar,and that with negative frequency is correspon-ding to receding waves from the radar. InFig.2, fB represents the Bragg frequency,which can be expressed as:

Here, C0 is the phase speed of the ocean wavecontributing to the Bragg resonant scatteringand λ is wavelength of the transmitted radiowave. From the linear dispersion relationshipof ocean surface waves, C0 can be uniquelydetermined as:

Here, g represents the gravitational accelera-tion. The actually measured Doppler velocitycorresponding to the peak frequency of thefirst-order echo is the sum of the phase speedof the ocean wave of a stationary sea surfaceand the radial current component along theradar beam direction. Therefore, the radialcurrent component can be obtained by sub-tracting the phase speed of the ocean wavefrom the Doppler velocity corresponding tothe first-order echo. However, since it is onlythe radial current component that can be meas-ured for a single radar in order to calculatetwo-dimensional current vectors it must be

necessary to combine two radial current com-ponents at a point measured from differentdirection. Therefore, in general, two or moreradars must be needed for current vectorsobservation using HF ocean surface radar.The root-mean-square (RMS) error of thecombined current vector is dependent on thecrossing angleα of the two radar beams. Ifwe assume that both the radial current compo-nents and errors are independent and thatRMS values of the measuring errors of theradial current components are equal betweentwo radars, the RMS error of the combinedcurrent vector is 1 / |sin (α)| times the RMSerror of the radial current component[5].

According to the analysis of the motion ofwater particles in the vertical plane accompa-nying ocean surface waves, it is shown thatthe current velocity measured by HF oceansurface radar corresponds to that at depth ofλ/8πfrom the surface[4].

An ocean wave with a wavelength of sev-eral meters observed by the HF ocean surfaceradar is mainly induced by ocean surface windnear the observation point, and so the direc-tion of the ocean surface wind and the direc-

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Schematic diagram of the HF ocean surface radar observationFig.1

A typical scheme of the observedDoppler spectrum

Fig.2

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tion of maximum growth of the ocean waveusually coincide. The positive and negativefirst-order echoes observed by HF ocean sur-face radar correspond to ocean wavesapproaching and receding from the radar,respectively, and the peak intensity of thefirst-order echo is dependent on the growth ofthe ocean wave contributing to the Bragg scat-tering. Thus, it is possible to estimate thedirection of the ocean surface wind based onthe peak intensity ratio between the two first-order echoes. Furthermore, the ocean wavespectrum can be estimated by analyzing theshape of “the second-order echoes”, a shapewhich is a secondary continuum that appearsaround the first-order echoes. However, sincethe intensity of the second-order echoes aresignificantly lower than that of the first-orderechoes, Doppler spectrum with extremelygood signal-to-noise ratio is required forocean wave spectrum estimation. Therefore,the areas for which ocean wave spectrum canbe estimated is extremely restricted comparedto that of the current vectors.

3 The Long-Range Ocean Radar(LROR)

Characteristics of the long-range oceanradar (LROR) system are summarized in Table1. The maximum observable range of theLROR depends on various factors such as seaconditions, background noise level, receiversensitivity, and electromagnetic conditionssurrounding the radar site, but the most domi-nant factor is the propagation loss of groundwaves. Fig.3 shows ground wave propagationcurves for various frequencies on the sea sur-face taken from a CCIR (ITU-R) report[6].When compared at the same distance, it can beseen that the ground-wave propagation loss issmaller at lower frequencies. For example,the propagation loss of a 9.2 MHz is smallerby about 30 dB at 200 km compared to a 24.5MHz, which is the transmission frequency ofthe HF ocean surface radar already developedby CRL. With the LROR, current measure-ment up to 200 km offshore has been made

possible by transmitting radio waves at fre-quency of 9.2 MHz with maximum transmis-sion power of 1 kW. In Japan, it is almostimpossible to secure a sufficiently wide andcontinuous frequency bandwidth in the HFband, so the LROR adopts a frequency modu-lated interrupted continuous wave (FMICW)type radar that is able to use frequenciesresources more efficiently than the short-pulsetype radar. The sweep bandwidth is 22 kHz,which corresponds to the range resolution ofabout 7 km. The current velocity resolution,which is determined by the incoherent integra-tion time of received signal, is 2.5 cm/s.

Fig.4 shows the antenna system of theLROR. The LROR adopts a digital beamforming (DBF) technique in which the signalssimultaneously received by multiple-elementsantenna are digitally processed to obtain angu-lar information. The transmission antenna is a3-elements Yagi antenna that transmits a fanbeam with horizontal beam width of 120˚

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Radar Type FMICW

Frequency 9.2 MHz

Sweep Bandwidth 22 kHz

Transmission Power 1 kW (Max.), 500 W (Average)

Range Resolution 7.0 km

Current Velocity Resolution 2.5 cm/s

Beam Width 8˚

Transmission Antenna 3-element Yagi antenna

Receiving Antenna 16-elements linear array of 2-element Yagi antenna DBF (Digital Beam Forming)

Characteristics of the LROR systemTable 1

Ground wave propagation curves onthe sea surface[6]

Fig.3

toward the sea surface. The receiving antennais a 16-elements linear array antenna with anaperture of 208.2 m, consisting of 2-elementsYagi antennas, which are individuallyequipped with both a receiver and an A/D con-verter. The signals received by each elementantenna are digitally processed to form a virtu-al beam in an arbitrary direction within a +_60˚to antenna front direction. The horizontalbeam width is about 8˚ in antenna front direc-tion. The CRL 24.5 MHz HF ocean surfaceradar adopts a narrow-beam scanning type,and so it takes 2 hours to scan the wholeobservation area (90˚ width), resulting in amaximum lag time of 2 hours among theobservation points. With the LROR, whichuses the DBF technique, data can be acquiredsimultaneously at all observation points.Therefore, it is now possible to detect phe-nomena with extremely short-term variationsand to obtain the information that requiressimultaneous data acquisition in whole obser-vation area, both of them can not be observedwith the CRL 24.5 MHz HF ocean surfaceradar.

Fig.5 shows the observation area of sur-face current vector by the LROR. In general,to obtain current vectors with HF ocean sur-face radars, a multiple-radars observation isnecessary, and so the LROR system is consist-ing of two radars at Ishigaki Island and Yona-guni Island. Both radars can measure the radi-al current component along the radar beamdirection within a fan-shaped area with aradius of 200 km and a central angle of 120˚.

The current vector can be calculated in theoverlapping area of the two fan-shaped areas.In this area, the main flow axis of theKuroshio current exists approximately 100 kmaway from the Ryukyu Islands, and so theLROR has a sufficient ability to observe thecurrent field of the Kuroshio current. Theerror factor 1/|sin (α)|, which represents theRMS value of the error induced in combiningthe current vector, was equal to or less than2.0 in the whole observation area except forsome small part on its eastern, western, andsouthern edges.

Fig.6 is a simple block diagram of theLROR system. Each of the LROR sites atIshigaki Ocean Radar Facility (Fig.7) and theYonaguni Ocean Radar Facility (Fig.8) is con-nected to the Okinawa Subtropical Environ-ment Remote-Sensing Center (CRL Okinawa)by dedicated lines, and each radars can becontrolled from the CRL Okinawa. In the nor-mal operation mode, the raw data obtained byeach radar are converted at the radar site into

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The antenna system of the LRORFig.4 Surface current observation area ofthe LROR

Fig.5

A simple block diagram of the LRORsystem

Fig.6

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16 Doppler spectral data sets for every 8˚ inbeam direction. The total of 32 Doppler spec-tral data sets produced at the two sites are sentautomatically to the CRL Okinawa and theradial current components and current vectorsare automatically calculated in pseudo real-time. The observation times and points of theLROR on Ishigaki and Yonaguni islands donot necessarily coincide. Therefore, when cal-culating the current vector, it is necessary toprepare a set of radial-current components ofboth the same observation time and the sameobservation point by performing some tempo-ral and spatial interpolations. First, we make atemporal interpolation to obtain two data setwhose observation time are the same. A dataset consisting of radial current components ata particular observation time is produced foreach radar by linear interpolation using twodata sets of radial current components derived

directly from the Doppler spectra acquiredmost close to the required time. Next, wemake a spatial interpolation to obtain a pair ofradial current components at an observationpoint. For one of the components, we directlyuse the value belonging to the data set calcu-lated in the previous step. The other value atthe same observation point is calculated bylinear interpolation using 4 values, which areselected from the other data set obtained in theprevious step, at the nearest to the point inevery one point. Then, the paired componentsare combined to obtain current vector at eachobservation point. Finally, the current vectorsmay be converted into that on grid points at 7-km intervals if it is necessary for conveniencein actual application. In normal operationmode, radars at Ishigaki site and at Yonagunisite are operated alternately every 30 minutesto avoid interference between them, and so thecurrent vectors are calculated once an hour.

Fig.9 shows an offshore-observation buoy(COMPASS: CRL Ocean Monitoring Platformin Sakishima) moored in March 2001 at thepoint indicated by a star in Fig.5 (123˚26'54"

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Ishigaki Ocean Radar FacilityFig.7

Yonaguni Ocean Radar FacilityFig.8

Bird's eye view of COMPASSFig.9

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E, 24˚37'55''N). The buoy has a diameter ofabout 9 m, displacement of about 100 t, andthe height above the sea surface is about 10 m.Ocean surface wind is observed with theanemovane (Kona System KDC-S4) attachedto the top of the buoy, and the water tempera-ture and current direction and velocity at adepth of 4 m are measured with the acousticDoppler current meter with thermometer (Son-tek ARGONAUT-MD). Ocean wave meas-urement is taken using the motion sensor (Sea-tex MRU) installed inside the buoy. Themeasured data is automatically transferred tothe CRL Okinawa via the Orbcomm satellitecommunication service and Satellite MarinePhone. The COMPASS measurement datawill be used to verify the performance of theLROR. The error factor 1/|sin (α)| of LRORat the COMPASS is 1.8.

4 Preliminary Observation Resultsof Current Vectors

Fig.10 shows current vectors observed at

12:00 on July 24, 2001. It is clearly shownthat the Kuroshio current flowing into theobservation area from the southwest turns eastnear the continental shelf. The maximummeasured current velocity exceeds 200 cm/s.As in this example, the first-order echoes frommore than 200 km away can be detected byboth radars in the daytime, and so the currentvectors can be measured at sufficiently longranges. However, during the night it is diffi-cult to constantly detect the echoes at longrange and, in some cases, the detectable rangemay decrease to below 100 km. This charac-teristic decrease of the detectable range in thenighttime seems to be due to ionosphericbehavior, and further research must be neces-sary.

Fig.11 shows the current vectors observedat 09:30 on Sept.16, 2001. It is the very timewhen typhoon Nari (#16) of 2001 was passingthrough the observation area of the LROR.Typhoon Nari generated near Miyako Islandand took an extremely irregular course. It hadbeen staying for a record period in a region

Journal of the Communications Research Laboratory Vol.49 No.2 2002

Observed current vectors for 07/24/2001 1200JSTFig.10

west of Okinawa Island, causing great damageto the Okinawa Island and the surroundingislands. Then, it turned south and passedthrough the observation area of the LRORfrom the northeast to the southwest from thenight of Sept.15 to the evening of Sept.16.The position of the eye of the typhoon at09:00 on Sept.16 is indicated by an×in theupper-left flame of Fig.11. According to theJapan Meteorological Agency, the pressure atthis time was 970 hPa and the maximum windvelocity was 35 m/s. In the northwestern partof the observation area, there was a very fasteddy-like current seemed to be induced by thestrong winds of the typhoon. Considering thecurrent field of this area in a calm sea state,the net velocity of the eddy-like current wasapproximately 100-150 cm/s, which is 2.8-4.3% of the maximum wind velocity of thetyphoon. This value is considered to be rea-sonable because, in general, the maximumcurrent velocity induced by ocean surfacewind is thought to be about 3% of the wind

velocity. The LROR observation indicatedthat this eddy-like current was moving tosouthwest between 07:00 and 16:00 onSept.16.

5 Summary

The Communications Research Laboratory(CRL) has developed a long-range ocean radar(LROR) system, which can measure surfacecurrents up to 200 km offshore, and experi-mental observations of the upper region ofKuroshio current in the southern part of theEast China Sea has been performed since July2001.

In the future, radar performance evalua-tions of the LROR will be done using theobservation data from the offshore-observa-tion buoy COMPASS moored in the EastChina Sea as well as a long-term continuousobservation of the current field of theKuroshio current.

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Observed current vectors for 09/16/2001 0930JSTFig.11

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CRL HF ocean radar of FMCW type. Part 2. current vector", J. Oceanogr., 1999, Vol. 55, pp. 13-

30.

6 CCIR, "Propagation in non-ionized media. Recommendation 386-5: Ground-wave propagation

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1986, Vol. V.

MATSUOKA Takeshi, Ph.D.

Researcher, Subtropical EnvironmentGroup, Applied Research and Stand-ards Division

Radar Remote Sensing, Glaciology

SATO Kenji

Senior Researcher, Subtropical Envi-ronment Group, Applied Research andStandards Division

Radar Remote Sensing

FUJII Satoshi, Ph.D.

Leader, Subtropical EnvironmentGroup, Applied Research and Stand-ards Division

Radar Signal Processing