StarFire and Real-Time GIPSY - NavCom Technology Inc.

StarFire and Real-Time GIPSY:A Global High-Accuracy Differential GPSSystemTenny Sharpe, Ron Hatch, NavCom Technology Inc.; Dr. Fred Nelson, John Deere & Co.BiographyMr. Tenny Sharpe is Director of Deere programs atNavCom Technology Inc. Mr. Sharpe received a B.S. inPhysics from Case Institute of Technology in 1969 and aM.S. in Computer Science from the University ofCalifornia, Los Angeles in 1976. Mr. Sharpe has over 30years experience in the development of aerospace andindustrial electronics. His specializations are software andsystems design for GPS navigation systems. He is thechief architect and program manager of the StarFireWADGPS.Mr. Ron Hatch is a principal in NavCom Technology, Inc.where he is the Director of Navigation Systems. Mr.Hatch received a B.S. degree in Math and Physics fromSeattle Pacific University in 1962. His primary research isin high-precision differential GPS navigation. He hasbeen awarded eleven patents in GPS technology. Mr.Hatch was the 1994 recipient of the Kepler Award fromthe Satellite Division of the Institute of Navigation andreceived the Thomas L. Thurlow Award from the Instituteof Navigation in 2001. In June he completed a one-yearterm as the president of the Institute of Navigation.Dr. Fred Nelson is Senior Staff Engineer in the PrecisionFarming Group of John Deere and Co. Dr. Nelsonreceived his B.S. (1962), M.S. (1978) and PhD (1980) inAgricultural Engineering from the University ofWisconsin. He has over 20 years of engineeringexperience with John Deere & Co. Dr. Nelson’s primaryfields of research include sensors, software and systemdesign for precision agricultural applications.AbstractNavCom Technology, Inc., a wholly owned subsidiary ofDeere & Company, together with the Ag ManagementSolutions Group of John Deere, have designed andimplemented a Wide Area Differential GPS (WADGPS)system, referred to as StarFire, which provides a newlevel of accuracy across continental distances. Thiscontinental system is being transitioned into a globalsystem with even higher accuracy using technologydeveloped by and licensed from the Jet Propulsion Lab(JPL). Several key developments have made this systempossible. They include:Low-cost, high-quality dual frequency receivers weredeveloped for use as both reference and mobilereceivers.Dual frequency extended smoothing was developedwhich removes the two largest error sources in aWAGPS, i.e. ionospheric refraction effects andmultipath effects.A new L-Band satellite communication module wasdeveloped which uses a single, multi-functionantenna to receive both the L1 and L2 GPSfrequencies and also the Inmarsat L-bandcommunication frequencies (1525-1565MHz.).The continental WADGPS Starfire system usescorrection algorithms which provided a single set ofcorrections for an entire continental area. This allows theuse of a much smaller bandwidth to provide thecorrections than that used by other WADGPS systems.NavCom Technology is in the process of transitioning thecontinental WADGPS solution to a truly global solutionwith an improved accuracy as well. The principal sourceof error, which limited the solution to continental sizeareas, was the inaccuracy in the GPS satellite orbits. Realtimeorbital corrections and clock corrections are nowbeing continuously computed for all the GPS satellites.The technology to compute orbital corrections and clockcorrections in real-time was licensed from JPL and usestheir real-time GIPSY software which had beendeveloped for NASA over many years. In additionNavCom has contracted with JPL to receive the data fromJPL’s extensive set of world-wide dual-frequencyreference stations. These reference stations together withNavCom’s reference stations provide the raw data fromwhich are computed two sets of corrections. Every few1

minutes a set of orbital corrections are computed for eachGPS satellite. At a more rapid rate, every few seconds, aset of clock corrections are computed for each GPSsatellite.Currently the Inmarsat L-Band communication channelprovides both the continental WADGPS corrections andthe global Real-Time GIPSY (RTG) corrections. Thesoftware within the StarFire equipment is beingtransitioned to use the new RTG corrections. These globalcorrections provide a new level of unmatched accuracyfor differential GPS.The Starfire system was developed by John Deerespecifically for agriculture applications. With theavailability of very accurate position information on aglobal basis, many other applications are now beingaddressed.IntroductionFigure 1 shows an overview of the StarFire WADGPSarchitecture. At a conceptual level, it is similar to otherwide-area dGPS systems such as the Federal AviationAdministration’s Wide Area Augmentation System(WAAS), Europe’s EGNOS, and Japan’s MSAS.However, a significant difference is the use of dualfrequencyreceivers rather than the single-frequencyreceivers. The use of dual-frequency receivers allows thedirect removal of ionospheric refraction effects and,therefore, only a few reference sites are needed to achievethe high accuracy results.GPSGPSL-bandCommunicationSatelliteGPSGPSdGPSCorrectionsLand EarthStation UplinkUser Equipment on Ag MachinesProcessing HubsNetwork of GPS Reference SitesFigure 1. Overview of the StarFire WADGPS System2

The seven reference sites used in the US network areshown in Figure 1. Separate sets of reference receivers aredeployed elsewhere which are used to generateindependent sets of WADGPS corrections around theworld. There are 4 reference stations in the StarFireEuropean Network, 5 in the Australian Network and 3 inthe South American Network. Each of the networks ofreference and monitor sites sends dual-frequency codeand carrier-phase measurements from all receivers for allsatellites in view, as well as system integrity information,to two redundant processing hubs via terrestrial andsatellite communication links. The processing hubscombine the measurements from all of the sites in eachregion and generates a single set of wide-area correctionsfor each region based upon the refraction correctedmeasurements. The corrections are sent, via land-lines, tothe land-earth station for the geostationary, L-bandcommunications satellite where they are uplinked forbroadcast to users throughout each region.These separate continental WADGPS systems are in theprocess of being transitioned into a single GlobalDifferential GPS (GDGPS) system which will provide anunprecedented level of accuracy worldwide. Thecommunications satellites now provide both the regionalcorrections and the more accurate global corrections.The StarFire user equipment receives the correctionsbroadcast from the communications satellite, applies themto its own observed, refraction corrected pseudorangesand performs a navigation solution. The resulting dGPSposition, velocity and time are output from the userequipment to other subsystems on the platform to supportmapping and control system requirements.Reference/Monitor Site Processing Hub Satellite UplinkFigure 2. StarFire CONUS Ground Reference NetworkStarFire Ground Reference NetworksFigure 2 shows the overall topology of the StarFireGround Reference Network (GRN) for CONUS. It iscomprised of seven reference/monitor sites, tworedundant processing hubs and an uplink facility for thegeostationary communications satellite.Each of the reference/monitor sites is configured with anidentical set of equipment including:3

a) two redundant NCT2000D GPS reference receiverswhich send a full set of dual frequency observables forall satellites in view to both of the redundantprocessing hubs,b) a fully packaged production StarFire user equipmentunit which serves as an independent monitor receiver,c) communications equipment (routers, ISDN modems),d) a remotely controlled power switch and UPS module.The main communication lines used to link the referencesites with the processing hubs are frame relay privatevirtual circuits (orange and blue lines in Figure 2). Eachframe relay circuit is backed up with an ISDN dial up linewhich is activated automatically from the processing hubin the event any frame relay connection fails. The sameimplementation is used for the communication lines toand from the hubs and the uplink facility.The StarFire user equipment units located at each of thereference sites, called monitor units, operateindependently. They receive the broadcast correctionstream from the communications satellite, performdifferential GPS navigation and report their positioningresults back to the processing hubs using the samecommunication lines as the reference receivers.In addition to the dGPS positioning results, the monitordata includes the received signal strength of the L-bandcommunications satellite, packet error statistics, age ofdifferential corrections, signal strengths for the receivedGPS satellites, PDOP and other operating parameters.This data, from all of the GRN sites, is continuouslymonitored by an Alert Service processor whichautomatically generates E-mail and pager messages toon-call network service engineers in the event of a dGPSservice failure.AmericasEuropeAsia PacificMolineRedondoN.A. WCTEurope WCTS.A. WCTAustralia WCTFigure 3 The Four Regional WADGPS NetworksThe three other regional networks are shown togetherwith the CONUS network in Figure 3. The structure andoperation of the three additional WADGPS regionalnetworks are almost exact copies of the structure andoperation of the CONUS network. However, thecommunication of the data between the reference and4

monitor receivers takes advantage of the Internet and, insome cases, satellite communication links as well. Theprocessing hubs for these regional networks is co-locatedThe network of global reference stations, used to provideboth orbit and clock corrections for all satellites at alltimes, does not currently share any of the regionalnetwork locations. Eventually, all of the regional sites willbe incorporated into the global network of referencestations. Currently only the global reference sites operatedby JPL for NASA, identified by the red flags in Figure 4,are used in computing the global corrections. TheInternet and satellite links are used to bring the dualfrequencymeasurements from these global referenceswith the CONUS processing hubs at the Redondo andMoline locations and shares the communication links andredundant structure of the CONUS network.back to the processing hubs at Redondo and Moline.Since the hubs receive data from more than 30 referencesites, the corrections are robust and the loss of data fromone or more sites would not affect the accuracysignificantly. Like the other networks, duplicateprocessing at both Redondo and Moline are used so thatan automatic switch of the data uploaded to thecommunication satellites occurs if a failure is detected inone of the hubs.Figure 4 The Global NASA Network of Reference StationsHub Processing SoftwareThe algorithm used at the processing hubs to compute theStarFire WADGPS corrections for the regional networksis called the Wide Area Correction Transform (WCT).The WCT uses the following inputs:a) dual frequency observables (CA code pseudoranges,L1 carrier phase, P2 code pseudoranges and L2 carrierphase) for all of the GPS satellites tracked at the GRNreference receivers, delivered at 1Hz in real time,b) broadcast ephemeris records from the GRN referencereceivers delivered in real time,c) a configuration file defining the precise location(±2cm) of each of the GRN reference receiverantennas as determined from network solutions basedon the IGS worldwide control stations.The dual frequency observables are used to formsmoothed, refraction corrected pseudoranges which arefree of ionosphere delay and, due to extended smoothingwith the carrier phase, virtually free of multipath. Theseare then normalized with respect to receiver clock offsetsand modeled site troposphere delays. Finally, thenormalized pseudoranges for each satellite are combinedin a weighted average to form a single, wide areapseudorange correction for that satellite. A similarprocess is performed using the finite difference of the5

carrier phase to generate pseudorange rate corrections.The ensemble of these corrections for all satellites in viewis formatted into a tightly packed, binary message andsent from the hub to the uplink facility for broadcast onthe geostationary communications satellite.Because the WCT uses refraction corrected pseudoranges,the resulting WADGPS corrections are free of the errorscaused by spatial decorrelation of ionosphere delayswhich are inherent in single frequency corrections. Whendual frequency mobile receivers are used which employthe same refraction corrected techniques, a single set ofcorrections can be used across the entire continentalservice area with uniform, high accuracy.Two major advantages result from having oneconsolidated set of corrections for the entire service area:a) Bandwidth requirements on the geostationarycommunications satellite are minimized. This resultsin a significant cost savings since the price of leasedsatellite channels is roughly proportional to thebroadcast power required which is directlyproportional to the bandwidth required.b) The correction computation algorithm, including thefinal weighting, is done at a centralized facility (atthe processing hubs) instead of being performed bythe user equipment based on location dependentmodels. This enables improvements and upgrades tothe WCT to be made, in most cases, withoutrequiring changes to the algorithms in the mobileuser equipment. This is a significant logistic benefitwhen, as is the case now with StarFire, thousands ofuser equipment units are deployed across thecontinental U.S.The processing software used to generate the globalcorrections requires the same set of inputs as is used forthe regional processing. However, the specific data comesfrom the global NASA network of reference receivers.Further, the real-time GIPSY (RTG) processing softwareused within the processing hubs at Redondo and Moline isthat developed by JPL and licensed to NavCom. Changesto the software to improve the efficiency of thetransmitted corrections have been made by JPL incoordination with NavCom. Orbit corrections for eachsatellite are generated and transmitted every five minutesand clock corrections for each satellite are generated andtransmitted ever two seconds. The correction data is sentvia the land-lines to the uplink facility and is broadcastfrom the communications satellites using the samechannels as the regional corrections. Because of theefficiency of the correction data both the regionalcorrections and the global corrections can be sent over thesame channel and the user equipment can select whichcorrection stream to use.StarFire User EquipmentFigure 5 shows the major components of the StarFire userequipment.a) A multi-function antenna assembly is used which iscapable of receiving the L1 and L2 GPS frequenciesas well as the Inmarsat receive frequency band. Thegain pattern of this antenna is designed to be relativelyconstant even at lower elevation angles. This allowsfor an efficient link budget when the unit is operated athigher latitudes where the elevation of thegeostationary communications satellite is low.b) An L-band receiver was developed to acquire, track,downconvert, sample and demodulate the StarFire datastream broadcast from the geostationarycommunications satellite. The receiver is frequencyagile across the Inmarsat receive band under softwarecontrol.c) A state-of –the-art, dual frequency GPS receivermodule, designed and produced by NavCom, providesthe most important enabling technology in the userequipment.Connections for the external interfaces of the StarFireuser equipment are provided through a sealed 10-pinconnector. Power connections include main, battery andprogramming voltage inputs. Data interfaces includeCAN Bus and RS232 serial.An alternate packaging of the StarFire equipment isshown in Figure 6. This package uses an external remotetri-band antenna to receive the two GPS frequencies andthe L-band communication satellite signal.The NCT2000D Dual Frequency GPS EngineThe NCT2000D is a compact, high-performance, dualfrequency GPS engine aimed at OEM applications. In theStarFire user equipment, it is mounted inside the lowerhousing and interfaces to the digital board of the L-bandreceiver via an RS232 serial port. The StarFire correctionsare input from the L-band receiver and 1, 5, or 10Hz PVTdata is output to the L-band receiver for transmission viathe external interfaces (RS-232 and CAN Bus).The NCT2000D has ten, full dual frequency channels andtwo WAAS channels. It produces GPS observables of thehighest quality suitable for use in the most demandingapplications including millimeter level static surveys.Key features of the NCT2000D include:a) A patented multipath reduction technique is built intothe digital signal processing ASICs of the receiver.6

This greatly reduces the magnitude of multipathdistortions on both the CA code and P2 codepseudorange measurements. When combined withextended, dual frequency code-carrier smoothing,MultifunctionantennaSealed package suitable forharsh environmentsL-bandcomm.receiverNCT2000DdualfrequencyGPS engineExternal Data Interfaces:CAN Bus, RS-232Figure 5. Major Components of the StarFire User Equipmentmultipath errors in the code pseudorangemeasurements are virtually eliminated.The measurement processing of the NCT2000D softwareversion in the StarFire user equipment is designed to beb) A patented technique is used to achieve near optimalrecovery of the P code from the anti-spoofing Y-coderesulting in more robust tracking of the P2/L2 signals.c) The compact size (4” x 3”x 1”) of the NCT2000Dallows it to be readily integrated into the StarFirepackage.d) A high resolution 1pps output signal, synchronized toGPS time, is provided by the NCT2000D. This signalis used by the L-band communications receiver tocalibrate its local oscillator, which aids the acquisitionof the StarFire correction signal. This technique hasalso been patented by NavCom.7

Figure 6. Alternate packaging of the StarFire UserEquipmentfully compatible with the both the StarFire regionalcorrection signal and the global correction signal. Eitherof the correction streams can be used depending upon thecontrol software within the StarFire unit. Dual frequencycode and carrier phase measurements are used to formsmoothed, refraction corrected code pseudoranges. Whenthe regional correction stream is used, the smoothed codemeasurements are adjusted with the StarFire regionalcorrections and used in a weighted least-squares fix togenerate PVT estimates.The global corrections are processed a bit differentlywithin the NCT2000D receiver. Specifically, a greaterreliance is put on the carrier phase measurements and afloating ambiguity estimate is made of the whole-cycleambiguities. All of the measurement data from allsatellites is used to make this ambiguity estimate asaccurate as possible. In addition, a constrained estimate ofthe tropospheric refraction is made using the data from allthe satellites. This constrained solution removes some ofthe unmodeled tropospheric refraction effects. Theresulting PVT estimates are output at either 1, 5, or 10Hzunder software control.StarFire Positioning Accuracy and Test ResultsFigures 7, 8 and 9 shows the position accuracy obtainedby using the global StarFire corrections for 24 hours atthree test sites around the world. The one-sigma accuracyper horizontal axis rarely exceeds 10 cm. As expected, theaccuracy is not dependent upon geographical location.The accuracy obtained by using the StarFire globalcorrections in within a StarFire receiver cannot be equaledby any other global system. Furthermore very few, if any,regional networks can match these results.Agricultural Applications of the StarFire SystemStarFire equipment is currently used in a number ofagricultural applications including yield mapping, fielddocumentation, operator assisted steering and automaticsteering.In terms of sheer numbers of units, yield mapping is thesingle largest agricultural application of StarFire. In thisRedondo Beach, California, StarFire Monitor Receiver24 Hour Positioning Results336Position Error (meters)210-1-2East North UpStd. Dev. (m.) 0.07 0.05 0.13302418126DGPS Status and Number of SVsDeDnNsatsQual-30 4 8 12 16 20 24GPS Time (seconds in week)0Figure 7 StarFire Results in California using Global Corrections8

Zweibrucken, Germany, StarFire Monitor Receiver24 Hour Positioning Results336Position Error (meters)210-1East North UpStd. Dev. (m.) 0.07 0.04 0.0830241812DGPS Status and Number of SVsDeDnNsatsQual-26-300 4 8 12 16 20 24GPS Time (seconds in week)Figure 8 StarFire Results in Germany using Global CorrectionsMelbourne, Australia, StarFire Monitor Receiver24 Hour Positioning Results336Position Error (meters)210-1-2East North UpStd. Dev. (m.) 0.09 0.06 0.17302418126DGPS Status and Number of SVsDeDnNsatsQual-300 4 8 12 16 20 24GPS Time (seconds in week)9

Figure 9 StarFire Results in Australia using Global Correctionsapplication, the precise real-time position of a harvestingmachine, typically a combine, is recorded simultaneouslywith data from yield sensors which measure the amountof crop being taken. Figure 10 shows a StarFire unitmounted on a John Deere combine.Table 1. Position Accuracies Needed for AutomaticSteering for Selected Field Operationsto the excellent short term accuracy. In automatic steeringapplications the machine is steered to follow a preplannedcourse by a control system comprised of positionsensors, a computerized control algorithm, electrohydraulicor electro-mechanical steering controls andfeedback sensors. The operator may take control of themachine manually to execute turns or unplannedmaneuvers but the repetitive, row-following operationsare done automatically. This leaves the operator free forother tasks such as monitoring the performance of atowed implement.Typical position accuracies needed for yield mapping,assisted and automatic steering are shown in Table 1Applications Beyond AgricultureFigure 10. StarFire Unit on a John Deere CombineAfter the field has been harvested, the recorded data ismoved from the combine to a personal computer andprocessed into a color coded map with statistics whichshow the yield distribution as a function of position.Operator assisted steering involves presentation of agraphic display, which shows a driver the currentdeviation of the machine from a planned course. Thedriver manually controls the machine to minimize thedisplayed deviation.Even before the implementation of the global correctionsand the resulting accuracy, the use of the StarFire systemin automatic steering systems was increasing rapidly dueField OperationFertilizer application (anhydrous)Fertilizer application (bulk)Heavy TillageFinish TillagePlantingSpraying (pre-emerge w/o markers)CultivatingHarvestingStalk ChoppingPositionAccuracy(2σ horiz.)12" (30 cm.)18" (46 cm.)12" (30 cm.)18" (46 cm.)10" (25 cm.)18" (46 cm.)2" (5 cm.)10" (25 cm.)10" (25 cm.)The enhanced accuracy of the StarFire system makes it anatural fit for many applications outside of agricultureincluding:Land survey and geographic information systems,Construction equipment guidance and control,Marine survey and resource exploration,Hydrographic mapping and dredging systems,Land transportation tracking applications, such asrailway monitoring, which require high levels ofposition accuracy.ConclusionThe StarFire system, including its major subsystems, hasbeen described in some detail. Key enabling technicaldevelopments have been identified and discussed.Position plots and statistics showing excellent accuracyperformance have been presented. The use of StarFireSystem in agriculture has been described. Its use in manyother fields is beginning to be exploited. There is no otherglobal positioning system which is as economical to useand as precise as the StarFire system using the globalcorrection stream broadcast over Inmarsat regionalsatellites.AcknowledgementsThe StarFire system would not have been developedwithout the sponsorship, vision and perseverance of theAdvanced Management Systems group of John Deere.The contributions of Mr. Terrence Pickett are especiallynoteworthy.10