RTK-GPS Positioning In Japan By GPS-Based Control Stations Via DMCA .
RTK-GPS Positioning in Japanby GPS-Based Control Stations via DMCAMobile Radio Communication SystemHiromune Namie, Naoto Tanaka and Akio Yasuda,Laboratory of Communication Engineering, Tokyo University of Mercantile Marine.E-mail: project@denshi.tosho-u.ac.jpTEL & FAX: 81-3-5245-7376BIOGRAPHYThe authors belong to the Laboratory ofCommunication Engineering, Tokyo University ofMercantile Marine, 2-1-6 Etchujima, Koto-ku, Tokyo135-8533, Japan.Mr. Hiromune Namie is a graduate student in thedoctoral course of the university. His research subjectsare related to the application of RTK-GPS and DGPS.He is the administrator of Internet electric Mailing Listof GPS-USER in Japan.Mr. Naoto Tanaka is a student in the masters’ course ofthe university. His research subjects are also related tothe application of DGPS and RTK-GPS.Mr. Akio Yasuda is professor at the university. Hereceived a doctorate in engineering at NagoyaUniversity. He is head of GPS Society, Japan Institute ofNavigation (JIN-GPS). His major subjects are satellitecommunication and positioning system, including GPSapplication and the development of the instruments formarine researches.ABSTRACTRTK-GPS is a real time positioning system of theaccuracy of one cm order with carrier phasemeasurement. Many applications are now underexamination in Japan. The most indispensable for thepositioning procedure are carrier phase data fromreference stations. There is at present a mobile radiocommunication system operating in Japan, calledDMCA (Digital Multi-channel Access) system. It can beused as a medium of dissemination of the carrier phasedata. It is an 8 kbps radio system for many users onTDM/TDMA mode. Geographical Survey Institute(GSI) of Japan has established the network of as manyas 947 GPS-based control stations all over the JapaneseIslands for seismological researches. They are availablefor the system of RTK-GPS positioning through themedium of DMCA.Experiments have been operated since October 1997 forRTK-GPS data dissemination from several GPS-basedstations via DMCA both around Tokyo metropolitanregion and in the Kansai district. The Trimble CMR(Compact Measurement Record) data for RTK-GPSpositioning are transmitted all day long through MCAbase stations which have the service area of 20-30 km inradius.The authors tried RTK-GPS positioning first at a fixedpoint in Tokyo University of Mercantile Marine. Thedata were received from Nerima GPS-based ControlStation (base line length: 17 km) by three Trimble4000SSi GPS receivers with a single GPS antenna. It isconfirmed that their performance is within thespecification described by the manufacturer. Eachpattern of distribution of the positions calculated by thethree receivers is quite similar to each other. The ‘Fix’solution rate, or the rate of fixed positions with the flagof ‘Fix’, is 87.7 % for 145,259 fixed positions which arecalculated every second for 40 hours. The datatransmission delay measures normally 1.6-1.7 s. Thecommunication line of DMCA has originally beendesigned to quit sending the data every 1.5-5.0 minutesfor several, often several dozen seconds, depending onthe areas and the periods of time. It is widely desiredthat the system should be improved for continuouspositioning.Another experiment was carried out comparing thepositioning by two GPS receivers and a single fixedantenna which receive the data for 3.3 hours from twodifferent GPS-based control stations, at Nerima andKawasaki, respectively 17 and 16 km away from theuser GPS antenna. The rate of ‘Fix’ solution is 93.9 %and 52.4 %, respectively. The difference between their
average positions is 3.28 cm in latitude, 1.44 cm inlongitude and 0.91 cm in altitude. 2drms against theaverage points is 2.63 cm and 3.60 cm, and S.D. inaltitude is 2.08 cm and 2.83 cm, respectively. The lowrate of ‘Fix’ solution by Kawasaki GPS-based ControlStation is supposed to be due to the excess of datatransmission delay over 2.0 s, which is observedapproximately every other second. It seems that thetransmission delay should be within 2 s in order to get‘Fix’ solution by Trimble 4000SSi’s. There must besome technical problems to be solved in thecommunication system from Kawasaki GPS-based CSthrough Shinyokohama MCA Base Station.2 SYSTEM OUTLINENext, they tried the positioning on board a shipnavigating in Tokyo Bay. They confirmed that it workswell.Figure 1 shows the allocation of the GPS-based controlstations. They have been operated by GSI. The institutehas established the network of 947 GPS-based stations,called GEONET (GPS Earth Observation Network), forsurveying and seismological researches, which coversalmost all of the Japanese archipelago with thegeometrical interval of about 30 km.1 INTRODUCTIONRTK-GPS (Real Time Kinematic GPS) is a real timesatellite positioning system. The range to the satellitefrom a user GPS antenna determined by the phasemeasurement of the carrier waves from the GPSsatellites is of the order of mm in S.D. Thus theaccuracy of a few cm order can be easily obtained in2drms. The system is easier to operate than such atraditional system as ‘Total Station’. Many applicationshave been investigated in Japan[1][2].2.1 GPS-BASED CONTROL STATIONThe carrier phase data for RTK-GPS positioning havebeen transmitted experimentally from severalGPS-based control stations, which work as the referencestations of RTK-GPS via DMCA. The service isavailable at the moment in the following districts: theKanto (Tokyo and the surrounding prefectures), theKansai (Osaka and Nara, Hyogo prefectures), the Tokai(Aichi, Gifu, Mie and Shizuoka prefectures) and a partof Hokkaido prefecture.RTK-GPS needs a data communication line for carrierphase data transmission from a reference station settledat a known position to a user receiver. In Japan theyhave mainly used a specific low electric power radiocommunication device, which does not require license.Accordingly, the data transmission area is limited to justseveral hundred meters in radius from the referencestation. It is hard for them to bear the burden ofpreparing expensive GPS receivers. Furthermore it istechnically difficult to settle and survey a referencestation, whose position is to be obtained by less thanone cm order. So RTK-GPS users in Japan have desiredthe development of a wider and less expensivedissemination service.RTK-GPS carrier phase data dissemination service hasbeen operated experimentally since October 1997.There are several GPS-based control stationstransmitting the data on a mobile radio communicationsystem, called DMCA (Digital Multi-channelAccess[3]), supported by the National Mobile RadioCenters Council of Japan, similar to Trunk RadioSystem in European countries. They are among 947GPS-based stations established by Geographical SurveyInstitute (GSI) of Japan in order to observe crustmovement for seismic prediction.The authors tried RTK-GPS positioning using thepresent experimental DMCA system and evaluated itsvalidity. First they acquired the basic positioning data ata fixed point in their university and then those on boarda ship under navigation.Figure 1 Allocation of GPS-Based Control Stations inJapanPhoto 1 shows an appearance of a GPS-based controlstation. The station is equipped with a GPS receiver,meteorological observation instruments and datacommunication apparatus. They are contained in a5-meter stainless-steel tower. The GPS antenna is set upon the top.
Compared with CMR, the format is inefficient intransmitting data. It is expected RTCM will publishsoon a more efficient format, version 3.0.Most of RTK-GPS receivers accept data only if they areprocessed in the format developed by their ownmanufacturers. The publication of the format by such aprivate company as Trimble is to be highly evaluated. Itwill naturally lead to the standardization of the carrierphase transmission format in the not too distant future.2.2 DMCA COMMUNICATION SYSTEMThe service of MCA system in Japan for businessmobile radio communication started in 1982, and itsdigitalized system (DMCA) was introduced in 1994.There are now about 600 thousand mobile and portableterminals. The system has been operated experimentallyfor the transmission of CMR data since October 1997.Table 1 shows the main specifications of DMCA. Thecommunication is carried out between office stationsand mobile terminals, or between the terminals, via aMCA base station with the service area of about 30 kmradius for many users. A specific frequency used to beshared by only a pair of users to hold radio carried outbetween many users on a single frequency. The systemworks efficiently. The data transmission rate is wholly64 kbps with 6 channels of 8 kbps on TDM/TDMAmode. The system is designed to quit communicationevery 1.5-5.0 minutes, depending on the area and thetime zone so that a single frequency can be shared by asmany users as possible.Photo 1 GPS-Based Control StationThe format of the carrier phase data is of CMR(Compact Measurement Record), defined and publishedby Trimble Navigation Limited at ION GPS-96. Thevolume of CMR data is about 1.0 kbit/set, which is theaverage of about 30 minutes’ data transmitted everysecond by a Trimble 4000SSi. On the other hand,RTCM SC-104 defines the Message Type 18, 19 for thecarrier phase data transmission format by the documentof the version 2.1. The format requires that you shouldtransmit the volume of about 3.9 kbit/set every second.Table 1 Main Specification of DMCAItemValueFrequency1.5 GHzData Rate (Whole System)64 kbpsData Rate (Per Channel)8 kbpsMultiplex ModeTDM/TDMAMultiplex Channels6Figure 2 shows the schematic figure of datatransmission on DMCA. First, at a GPS-based controlFigure 2 Schematic Figure of Data Transmission by DMCA
station after receiving CMR data transmitted from theGPS receiver (B), the MCA transceiver (A) requests theMCA base station (C) to assign a channel, and then ittransmits the CMR data every second through thespecified channel to the MCA base station (C). In themeantime, the MCA transceiver (D) at the user stationrequests the MCA station (C) to assign a channelthrough which to receive the CMR data. The MCAtransceiver (D) checks the data by CRC (CyclicRedundancy Check) and further by BCC (Block CheckCharacter) added by the MCA transceiver (A) to detecttransmission errors. If it detects no error, the transceiver(D) transmit the CMR data to the GPS receiver (E) atthe user station. If any error is detected, the data areabandoned.Photo 2 shows the MCA transceiver and the antenna.Figure 3 Format of the Data Transmitted by DMCA3 BASIC EXPERIMENTBefore running RTK-GPS positioning experiment onboard a navigating ship, they tried to measure the datatransmission delay and evaluate the accuracy ofpositioning for a fixed point on land in TokyoUniversity of Mercantile Marine (TUMM).Figure 4 shows where the GPS-based control stationsand the MCA base stations used in the experiment areallocated. CMR data from Nerima GPS-based ControlStation are transmitted through Shinjuku MCA BaseStation and those from Kawasaki Control Stationthrough Shinyokohama MCA BS.Photo 2 MCA Transceiver and Antenna.Figure 3 shows the format for the data transmitted onDMCA. The transmission rate is 64 kbps for everyfrequency. One frequency is multiplexed into 6 channelson TDM/TDMA mode, each of which repeats every 90ms for a period of 15 ms, so the transmission rate forevery channel is reduced to 8 kbps. As it includes a"Preamble" for synchronization, CRC and BCC, thepractical rate seems to fall to much less than 8 kbps. Inthe near future you can use several channels at once sothat you may get higher rate.Figure 4 Allocation of GPS-Based Control Stations and
MCA Base Stations Used in the Experiment3.1 DATA TRANSMISSION DELAYRTK-GPS is requested to give a real time performance.The GPS receiver used here is a Trimble 4000SSi. Itcalculates its own position for about 2 s on asynchronized mode, by comparing the carrier phase datameasured at the mobile station with CMR datatransmitted from the GPS-based control station. Thecalculated result is outputted with the delay of 2.0-2.5 s,therefore if the data transmission delay exceeds 2 s, noresult is outputted.The authors tried the measurement of the delay byDMCA first. Delays were measured between the 1PPS(pulse per second) from the GPS receiver and the headof CMR data inputted to user’s 4000SSi, using auniversal counter 5313A (resolution: 50 ns) byHewlett-Packard Co., Ltd. As well known, the 1PPS isoutputted every second sharp, and the carrier phase datafrom satellites are measured at the GPS-based stationsynchronously. Although the present measurementcannot help producing ambiguity of seconds, you areable to measure the fractional second of the delay veryaccurately.induced by the asynchronous operation of DMCA datatransmission packet with CMR data. The figureindicates that the data transmission delay varies within0.2 s, while the data flows continuously.Figure 6 shows an example of the time series of themeasured delay of data transmission from NerimaGPS-based CS via Shinjuku MCA BS and those ofKawasaki CS via Shinyokohama MCA BS. The timeaxis is enlarged in comparison with figure 5. In thepresent measurement, the duration of continuous datatransmission on DMCA is 2.0 minutes. Figure 6 as wellas figure 5 shows that the delay becomes longer, everytime the data transmission is suspended. But in the caseof Kawasaki GPS-based CS, there is a large amplitudevariation. The delays of 1.6 and 2.1 seconds arise almostalternatively. The authors conceive there are someproblems to be solved in the transmission line fromKawasaki GPS-based CS, comparing with the result ofthe small variation of the Nerima GPS-based CS. Whatand where they are is now under examination.Figure 6 An Example of the Time Series of DataTransmission Delay Measurement by DMCA3.2 RTK-GPS AT A FIXED POINT ON LANDFigure 5 An Example of the Time Series of DataTransmission Delay Measurement by DMCAFigure 5 shows an example of the time series of datatransmission delay measurement. The long vertical linesindicate the transmission is suspended every 5.0 minutes.The communication line of DMCA is designed to quitsending data so as to provide the link to as many usersas possible. It takes several, often several dozen,seconds to recover. Since no ‘Fix’ solution can beobtained during the period of the deficit, the ratio of‘Fix’ solution to the total positioning falls to less than100 %. The figure also shows a part of datatransmission delay on an enlarged scale. The periodicalvariation shown in the figure seems to have beenFigure 7 shows the experimental configuration ofRTK-GPS positioning. First the authors triedpositioning at a fixed point on land in Tokyo Universityof Mercantile Marine. The GPS antenna is about 20 mhigh above the mean sea level on the roof of a 5-storiedbuilding. Simultaneous positioning was tried withthree 4000SSi’s and a single GPS antenna, using CMRdata transmission from Nerima GPS-based CS (baseline length: 17 km).
Figure 7 Experimental Configuration at a Fixed PointFigure 9 Temporal Series of Variation in Altitude UsingCMR Data Transmission from Nerima GPS-BasedControl StationFigure 8 Horizontal Distribution Using CMR DataTransmission from Nerima GPS-Based Control StationFigure 8 shows the horizontal distribution of positioning.In order to show the coincidence clearly, the positionscalculated by the three receivers are displayed with 5cm differences of average points in both latitudinal andlongitudinal directions. Figure 9 shows three temporalseries of variation in altitude with 30 cm offset ofaverage values. The positioning accuracy wererespectively 3.09 cm, 3.10 cm and 3.08 cm in 2drms,and 3.88 cm, 3.89 cm and 3.88 cm in S.D. of altitude.The three patterns of positioning distribution both in2drms and S.D. are coincident with each other. Thus thethree receivers are confirmed to show remarkableresemblance in their performance. Their ‘Fix’ solutionrate is 87.7 % every second for more than 20 hours’positioning.The present system uses GPS-based stations as thereference. Next, the authors investigated how the twogroups of fixed positions agree, by using CMR datatransmitted from the different two GPS-based stations.They tried RTK-GPS positioning simultaneously withtwo receivers and a single antenna, using CMR datatransmitted from Nerima and Kawasaki GPS-based CS.The base line length is 17 and 16 km, respectively.Figure 10 shows the horizontal distribution of the fixedpositions during about 3.3 hours' positioning. The linesconnect the points which are fixed by reference toNerima GPS-based CS, and the dots are those given byreference to Kawasaki GPS-based CS. Their 2drms is2.63 cm and 3.60 cm, respectively. Figure 11 shows atemporal variation of altitude. S.D. is 2.08 cm and 2.83cm, respectively. Although both receivers share onesingle GPS antenna, the average values of the fixedpoints show some discrepancy. The difference is
respectively 3.28 cm, 1.44 and 0.91 in latitudinal,longitudinal and altitudinal directions. ‘Fix’ solutionrate is 92.8 % and 57.2 %, respectively for the twoGPS-based stations. The authors are astonished by thesmall discrepancies, considering that the length of thebase lines is far beyond the distance specified by themanufacturer.Figure 11 Temporal Series of Altitude Fixed byReference to Nerima and Kawasaki GPS-Based ControlStationsFigure 10 Horizontal Distribution of Positions Fixed byReference to Nerima and Kawasaki GPS-Based ControlStationTable 2 shows the positioning result at a fixed point onland in TUMM. The authors tried RTK-GPS positioningfor more than 10 hours, making reference to bothGPS-based stations. With Kawasaki GPS-based CS, the‘Fix’ solution rate is much lower than with NerimaGPS-based CS. It implies that the data transmissiondelay from Kawasaki GPS-based CS often exceeds 2 s.Table 2 Positioning Result at a Fixed Pointwith the Data from GPS-Based CSReferenceNerimaKawasakiStationGPS-Based CS GPS-Based CSBase Line16,988 m15,700 mTime12.9 hour11.7 hour‘Fix’ Rate93.9 %52.4 %2drms3.29 cm3.05 cmAltitude S.D.3.58 cm2.20 cmDMCA is designed to quit connection every 1.5-5.0minutes and it takes several to several dozen seconds torecover. So the ‘Fix’ solution rate is always below100 %. In order to be practically usable, the systemmust be improved to keep data transmission withoutinterruption.4 RTK-GPS POSITIONING UNDER NAVIGATIONThe authors tried RTK-GPS positioning and collectedthe basic data on board a ship under navigation.Figure 12PositioningExperimentalDiagramofOnBoard
4.1 EXPERIMENTAL OUTLINEFigure 12 shows the experimental diagram. They triedon board RTK-GPS positioning in Tokyo Bay on thetraining ship Shiojimaru (photo 3), which belongs toTUMM.had already been determined by positioning everysecond for 3 hours with a 4000SSi, by using CMR datafrom Kawasaki GPS-based CS. Since the 2drmsmeasures 2.68 cm, it is confirmed the reference stationin TUMM has the accuracy of cm order.As shown in photo 5, the mobile phone was settled atthe cabin window of the training ship navigating inTokyo Bay, so the ideal data were not obtained. Datatransmission rate by the mobile phone is 9,600 bps.Photo 3 Training Ship Shiojimaru Belonging TUMMPhoto 4 shows the GPS antenna set up on the top of theflag post on her bow. The positioning was tried everysecond with a 4000SSi mobile receiver and a singleGPS antenna, using CMR data from KawasakiGPS-based CS.Photo 5 Mobile Phone at the Cabin Window4.2 RESULT AND DISCUSSIONFigure 13 shows the result of on board RTK-GPSpositioning. The experimental ship Shiojimaru steamedthrough Uraga-channel to the offshore of the Port ofKawasaki, and after taking 2 U-turns there, entered thePort of Tokyo.Photo 4 GPS Antenna on the Top of Flag Post ofShiojimaruIf you cannot find another convenient reference near by,it will be extremely hard to evaluate the accuracy of onboard positioning. The authors used another 4000SSimobile receiver with the same GPS antenna to calculateby the CMR data transmitted via mobile phone fromanother reference station settled in Tokyo University ofMercantile Marine. The reference position at TUMMFigure 13 On Board Positioning Result UnderNavigation
The line shows the route on which GPS positioning wastried using CMR data transmitted on DMCA fromKawasaki GPS-based Control Station, about 5-30 kmaway from their experimental area. The white parts ofthe line indicate where positions were calculated bothby mobile phone data transmission from the reference inTUMM station and by DMCA. The base line from thereference in TUMM was very long, more than 20 km.But it is confirmed that RTK-GPS positioning ispracticable in so wide an area as Tokyo Bay. The lower‘Fix’ rate of mobile phone positioning may have beendue to a weak electric field at the cabin window wherethe mobile phone was settled.Table 3 shows S.D. and the averaged differencesbetween positions fixed simultaneously by DMCA andmobile phone, on the 2 parts of the route, denoted by Aand B in figure 11. The positioning time was about 2.5hours by DMCA and 1.5 hours by mobile phone. ‘Fix’solution rate is 54.3 % and 79.7 %, respectively. Thelow ‘Fix’ rate with DMCA system is not unreasonable,for a low rate is also observed at the fixed point in ouruniversity, which seems due to a periodical enlargementof data transmission delay from Kawasaki GPS-basedCS. On the other hand, in the case of positioning bymobile phone data transmission, the position of themobile phone antenna is supposed to have beeninappropriate.Table 3 S.D. and Averaged difference between positionssimultaneously Fixed, Using CMR Data Transmitted byDMCA and Mobile Phone, Under NavigationHorizontalAltitudePlaceABABS.D.2.82 cm3.28 cm5.13 cm7.00 cmDiscrepancy99.9 cm26.7 cm62.3 cm112.7 cmS.D. of the difference between the positions fixed byDMCA and mobile phone is both 2-3 cm in thehorizontal distribution. But the offset is of as many asseveral dozen cm. The ambiguity may have been due tothe excessively long base line of the mobile phonepositioning.5 CONCLUSIONThe authors tried RTK-GPS positioning with tworeceivers and a single GPS antenna, using twoGPS-based control stations of Geographical SurveyInstitute of Japan to refer to RTK-GPS carrier phasedatatransmittedonDMCAmobileradiocommunication system. The positions fixed by twoGPS-based stations for 3.3 hours were compared witheach other to get the average values and S.D. The resultshows that 2drms is a few cm and that the averagedpositions are discrepancy by a few cm. DMCA isdesigned to quit connection every 1.5-5.0 minutes and ittakes several seconds to recover. This prevents thereceivers from obtaining the ‘Fix’ solution rate of100 %. Improvement is desired so that continuouslinkage may be made feasible.Next, they tried on board positioning in Tokyo Bay.Although there was no absolute reference to evaluatethe accuracy, it is confirmed that RTK-GPS positioningis practicable in so wide an area as Tokyo Bay. Thisexperiment was organized for the basic research ofRTK-GPS positioning. But they evaluate that thispositioning system can aid satisfactorily in real timeanchor watching, automatic berthing and entry into port,near the Ports of Tokyo and Yokohama.It is expected that the service areas will grow wider infuture. They hope that this experiment will help topromote practical use of RTK-GPS positioning systemin Japan.ACKNOWLEDGMENTSThe authors wish to thank Mr.Shigeru Yama of TrimbleJapan for his valuable suggestion.REFERENCE[1] Osamu Okamoto, Hiromi Tsuboi, Hiromune Namieand Akio Yasuda, "Application of RTK-GPS toImmersion of Caisson", International SymposiumNo.117 on GraGeoMar96, pp.704-711, 1996.[2] Osamu Okamoto, Akio Yasuda and HiromuneNamie, "Experimental Study on the Observation ofLandslide Movement by RTK-GPS", Proceedings ofApplied Survey Technology, Jun. 1998.[3] GPS Society, Japan Institute of Navigation,, "GPSSymposium '97", pp.145-159, Nov. 1997.
a ship under navigation. 2 SYSTEM OUTLINE 2.1 GPS-BASED CONTROL STATION The carrier phase data for RTK-GPS positioning have been transmitted experimentally from several GPS-based control stations, which work as the reference stations of RTK-GPS via DMCA. The service is available at the moment in the following districts: the
15.RTK:Marcasdereplanteopendiente 65 16.RTK:Ajustesdereplanteo 66 17.RTK:Seguimiento 66 18.RTK:Avanzado 68 19.RTK:Miscelánea 69 RedDGPS 69 1.RedDGPS:Configuración 69
As shown in Figure 1, positioning systems can be classified into two categories: 1) Global Positioning 2) Local Positioning Global Positioning Systems (GPS) allow each mobile to find its own position on the globe. Local Positioning System (LPS) is a relative positioning system and can be classified into Self and Remote Positioning.
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