4-8 Development of a 400MHz-band Wind Profiler Radar with RASS

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1 Introduction

The Okinawa Islands belong to the West-ern Pacific Subtropical Zone, close to the east-ern coast of the Asian Continent. Althoughthey are located in the Subtropical High-Pres-sure Zone, their climate is humid and warm,due to the influence of the nearby Kuroshio.In this region, a southeastern wind in summerand a northwesterly wind in winter dominate.The islands are strongly influenced by theAsian monsoon and also experience character-istic meteorological phenomena such as Baiu,the rainy season, and typhoons. The Commu-nications Research Laboratory (CRL) hasdeveloped three types of new remote-sensorsto take measurements of the atmosphere andocean dynamics in the Okinawa SubtropicalRegion, as part of a research and developmentproject on measuring techniques for the sub-tropical environment begun in 1997. In thispaper, we will describe one such remote sen-sor, a 400 MHz-band Wind Profiler Radar(hereafter referred to as 400 MHz-WPR). Atype of ground-based Doppler radar, the 400MHz-WPR measures the height profile of

three orthogonal components of the windvelocity vector by measuring the speed ofatmospheric turbulence and precipitation par-ticles moving with the background windfields, which are derived from the Dopplerfrequency shift in transmitting and receivingradio waves. By combining acoustic transmit-ters with the WPR, the Radio Acoustic Sound-ing System (RASS) is formed, which derivesthe height profile of the virtual temperaturefrom the Doppler frequency shift in the radiowaves scattered by propagating acousticwaves in the atmosphere. In RASS observa-tions, it is necessary to obtain in advance theradial wind velocity, which is the projectedcomponent of the background wind velocity inthe direction of the antenna beam, to find trueacoustic speed. This is because the propaga-tion speed of the acoustic wave obtained fromthe WPR’s Doppler velocity of the acousticwaves is an apparent, not a true, speed causedby the superimposition of the true acousticvelocity and the radial wind velocity. TheWPR that composes the RASS (hereafterreferred to as WPR/RASS) is characterized bya time resolution of a few minutes, far better

ADACHI Tatsuhiro

4-8 Development of a 400MHz-band WindProfiler Radar with RASS

ADACHI Tatsuhiro

Okinawa Subtropical Environment Remote-Sensing Center of the CommunicationsResearch Laboratory has developed a new 400 MHz-band Wind Profiler Radar (400M-WPR)since FY 1997 in a research and development project of remote-sensors for measuring sub-tropical environment. After construction of the Ogimi Wind Profiler Facility at Ogimi villagein Okinawa, the 400M-WPR has continued wind velocity profiling with an altitude range fromapproximately 400 m to typically 13 km, and with an interval of a few minutes. We were suc-cessful for the wind profiling up to 16 km near the center of a typhoon Nari approached inSeptember 2001. Besides, a RASS attached to the 400M-WPR could obtain virtual tempera-ture profiles up to about 3km.

Keywords Wind profiler, Remote-sensing, Wind velocity, Temperature, Troposphere

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than those of conventional sensors designedfor use in balloons, aircraft, or satellites. It ishoped that this device will contribute to theelucidation of meteorological phenomena on amore detailed level (i.e., the meso-scale).

2 The Structure of The System

Table 1 gives the specifications of ourWPR. This WPR is a monostatic Dopplerradar that serves as both transmitter andreceiver. Its transmission frequency is 443MHz, and an Active-Phased Array Antennamakes it possible to scan electronically in thedirection of the beam. Peak transmissionpower is 20 kW, and maximum average trans-mission power is 2 kW, since the maximumduty ratio of the transmitter is 10 percent. Theantenna is square-shaped with sides of 10.4 m.The width of the antenna beam is 3.3˚. Toimprove the signal-to-noise ratio of thereceived signals, our WPR uses pulse com-pression techniques that apply phase modula-tion codes devised by Spano et al.[1] (hereafterreferred to as Spano Codes) to transmitted andreceived pulse trains. There are three pairsconsisting of the width of each transmitted

pulse (sub-pulse) and the number of sub-puls-es, which constitute the transmitted pulsetrains. We can select one from those threepairs, namely: width 1.33μs with 4 bits, width2.00μs with 8 bits, and width 4.00μs with 4bits. It is also possible to select Complemen-tary Codes instead of Spano Codes for pulsecompression, or to forego pulse compressionand select a single pulse with width 1.33μs.Because the antenna consists of two sets oflinear arrays, each with 24 elements along twoorthogonal directions and because each ele-ment is connected to the independent trans-mission-and-reception switching module, theantenna beam can electronically scan in twovertical orthogonal planes.

The key feature of our WPR/RASS is thatthe azimuth of the antenna beam can point inany direction continuously, not just in fourdirections in 90˚-intervals. This significantlyimproves the altitude range for temperaturemeasurements by RASS. In RASS observa-tions using a monostatic WPR, the wave num-ber vector of the transmitted radio wave mustbe 1/2 that of the transmitted acoustic wave,according to the Bragg condition. In theregion to be observed, the antenna beam of the

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

Specifications for the CRL's 400 MHz-WPRTable 1

Radar type

Transmission frequency

Transmission power

Antenna type

Antenna dimensions

Antenna beam half powerwidthAntenna beam scanning range

Range resolution

Observation altitude range

Monostatic pulse Doppler radar

443 MHz

20 kW (peak); 2 kW (average)

Electromagnetic coupling type coaxial linear array (24 elements×2 orthogonal columns)width: 10.4 m; depth: 10.4 m; height: 1.2 m

3.3˚

azimuth 0˚-360˚ (electronic and mechanical scan-ning); zenith angle 0˚-15˚ (electronic scanning)variable within 200 m-600 m

wind velocity: 350 m-16 km; temperature: 350 m-6 km

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transmitted radio wave must be orthogonal tothe acoustic wavefront. However, the spacethat satisfies the Bragg condition is not alwaysat its zenith, but mostly limited to the wind-ward direction, because the shape of theacoustic wavefront is affected by the back-ground wind velocity profile. Therefore, wecalculate in advance the propagation path ofthe acoustic wave, based on the backgroundwind velocity and the temperature field. Wethen extend the altitude of the observation bypredicting the space region in which the Braggcondition is satisfied and by scanning theantenna beam in that direction. Although thismethod basically came into practice with MUradar-RASS, we have also improved our WPRto introduce automatic control. To realize thisRASS observation method, our WPR is capa-ble of beam scanning in all directions ofazimuth, by mounting the whole antenna planeonto a turntable and rotating it horizontally.Although a complete two-dimensional arrayantenna that electronically performs antennabeam scanning in all directions of azimuthwould be ideal, the method of rotating theorthogonal linear array antenna appears to be areasonable compromise, given constructioncosts.

As described in the following section,because our Wind Profiler is installed in an

observation facility some distance from ourresearch facility, we need to develop sophisti-cated software to control the radar and toobtain data without requiring a specializedoperator onsite. Particularly for RASS obser-vations, the key feature of our system is soft-ware that conducts automatic observations byestimating the propagation path of the acousticwave transmitted from the transmitter by theray-tracing technique, using the wind velocityand temperature profile obtained by onlineprocessing, and by calculating the appropriateobserved parameters. The system is designedfor use at the remotely located observationfacility, is controlled by a Web browser inter-face and features a “quick-look” displayallowing easy access to observed data.

3 Results of Initial Observations

(1) Ogimi Wind Profiler FacilityCRL built the Ogimi Wind Profiler Facili-

ty shown in Fig.1 in the north of OkinawaMain Island at long. 128˚09'32"E, lat.26˚40'41"N, and 225 m above sea level as abase for operating various atmosphere-obser-vation devices. Apart from a 400 MHz-WPR,this Facility is equipped with a 1.3 GHz-WPR,a Doppler Sodar, a GPS radiosonde, an ultra-sonic anemometer, an optical rain gauge, a

ADACHI Tatsuhiro

A view of the Okinawa Subtropical Environment Remote-Sensing Center (long. 128˚09'32"E,lat. 26˚40'41"N, and 225m above sea level)The square antenna in front of the observation building is the 400 MHz-WPR. The reception antennafor the GPS radiosonde, the optical rain gauge, the disdrometer, and the ultrasonic anemometer areinstalled on the rooftop of the observation building. On the left of the observation building is theDoppler Sodar, and on the right are the ground-based weather observation system and 1.3 GHz-WPR.

Fig.1

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disdrometer, and a ground-based weatherobservation system that measures atmosphericpressure, wind velocity vector, temperature,relative humidity, solar radiation, and rainfallnear the surface. The observation building hasan electric power generator that can operatecontinuously for up to 72 hours, which makescontinuous observation possible despite fre-quent power failures, particularly when atyphoon is approaching. The data obtained ateach observation device is collected at theserver computer and transmitted to the dataarchive device at the main CRL building viathe Okinawa Subtropical EnvironmentRemote-Sensing Center of the CRL.(2) Wind velocity profile for Typhoon No.16(Nari) in 2001

Next, we will describe the observationresults on the typhoon, Nari, whichapproached the Okinawa Main Island fromSeptember 7 to September 8, 2001. As shownin Fig. 2, the typhoon approached the OgimiWind Profiler Facility very slowly from the

southwest. At its closest approach to the tip ofthe observation site at 18:00 on September 7,it was 40 km south. It subsequently turnedtoward the west. Fig.3 shows the results ofcontinuous observation of the wind velocityprofile obtained by our WPR during that peri-od. The maximum observation altitude of thewind velocity profile increased as the typhoonapproached, falling after reaching over 16 kmnear the center of the typhoon. This appears

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

The path of the typhoon Nari in 2001Fig.2

The time-altitude distribution of the horizontal wind velocity vectors averaged over 30-minute intervals gathered by the 400MHz-WPRThe observation period, from September 7 through 8, 2001, coincides with the time during which thetyphoon Nari approached.

Fig.3

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to be due to radio scattering by precipitationparticles in the cumulonimbus clouds thatdominated at high altitude, close to the centerof the typhoon. We detected vertical down-ward Doppler velocity during this period. Themaximum observation altitude by our WPRwas typically around 13 km. Since this islower than estimates based on the radar equa-tion, we are currently reviewing the operatingconditions of the radar device.(3) The RASS observation

Fig.4 shows the results of initial observa-tions by RASS. The altitude distributionsshow a decrease in apparent acoustic velocityat high altitude, due to the fact that the RASSecho lay in the range between a Dopplervelocity of 347 m/s and 340 m/s, and that tem-perature decreased with increasing altitude.The maximum observation altitude of theRASS echo varied greatly, depending on thedirection in which the beams faced: 2.7 km forthe beam facing south, and 1.6 km for thebeam facing west, with the zenith angle 15˚.This indicates that the range of the observationaltitude of RASS is extremely sensitive to thedirection of the antenna beam.

Fig.5 is the altitude profile of the virtualtemperature obtained by 10-minute RASS

observations. The temperature fluctuationsare confined roughly in the range of 1 K.

4 Summary

The Okinawa Subtropical EnvironmentRemote-Sensing Center of the CRL has devel-oped a 400 MHz-WPR with RASS, which ithas installed at the Ogimi Wind Profiler Facil-ity on the Okinawa Main Island. We haveimproved the range of the observation altitudeof RASS by mounting this radar antenna, theorthogonal linear array antenna, on aturntable, enabling the antenna beam to scan

ADACHI Tatsuhiro

The range-velocity cross-sectional view of the Doppler spectrum obtained by RASS observationsThe difference of colors reflects the intensity of received signals. The directions of the radar beamswere: (azimuth, zenith angle) = (180˚, 14.8˚) for the left; (270˚, 14.8˚) for the right.

Fig.4

The altitude distribution of virtual tem-perature obtained by 10-minute RASSobservation

Fig.5

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in all directions of azimuth. Measurements ofwind velocity profile exceeded 16 km near the

center of the typhoon Nari in 2001.

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

References1 Spano, et al., "Sequences of complementary codes for the optimum decoding of truncated ranges

and high sidelobe suppression factors for ST/MST radar systems ", IEEE Geosci. Remote. Sens.,

34, pp. 330-345, 1996.

ADACHI Tatsuhiro, Ph. D.

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

Rader Remote-Sensing