real time kinematic dgps

PRECISE REAL-TIME KINEMATIC DIFFERENTIAL GPSUSING A CELLULAR RADIO MODEM

Bill Falkenberg, Tom Ford, Janet Neumann, and Pat FentonGPS Products Group

NovAtel Communications Ltd.Calgary, Alberta, Canada

M. Elizabeth Cannon & Gerard LachapelleDepartment Of Surveying Engineering

The University Of CalgaryCalgary, Alberta, Canada

ABSTRACT

A series of tests, involving NovAtel’s GPSsensor and a NovAtel CRM (CellularRadio Module) in various differentialstatic and kinematic settings, was con-ducted in early 1992 to assess the accura-c y of differential GPS positioning.NovAtel is a cellular systems and sub-scriber equipment manufacturer locatedin Calgary and has recently developed anew GPS receiver. This receiver, availableinitially in a PC card format, has tendedicated channels and is capable of ex-tremely accurate and high rate measure-ments of both the C/A code and the Llcarrier phase of the GPS system.

After a brief overview of the technologyused in the main GPS differential system,details and some of the design advantagesand capabilities of the various componentsare given. This is followed by an overviewof the various experiments performedand presentation of the results. Data col-lection and post mission data processingtasks, used for verification of the real-time results, were performed by NovAteland The University of Calgary’sDepartment of Surveying Engineering.

INTRODUCTION

The results from differential positioningexperiments obtained using GPS are thesubject of this paper. The main goal ofthese experiments was to evaluate the ef-fectiveness of using precise C/A (10 cm)’code measurements and a cellular modem

data link to position consistently and ac-curately. To assess the performance of thepositioning system, various techniqueswere used. These include using commer-cially available GPS post processing soft-ware (Semikin and C3NAW2 and pre-viously surveyed baseline calibration pil-lars. After a brief description of theequipment used, the testing methodologyis reviewed. Various results are then pre-sented which demonstrate the effective-ness of the NovAtel GPSCardW receiver.

DATA COLLECTION SYSTEM

Overview

The differential GPS positioning systemused for this test involved the integrationof a portable PC and a cellular modem (cf.Figure 1). An expansion chassis on the PChad the required 8 bit ISA connector forthe GPS receiver card. A standard DB9cable was used to connect the serial portof the PC to an external CRM modem.Magnetic adapters were used on both thecellular and GPS antennas for vehiclemounting. Coaxial cables were requiredfor each of the antennas. Power for theentire system was provided from the testvehicle’s 12 volt DC source.

Real-time software used on the PC wasdeveloped by NovAtel and consisted of agroup of six display screens used to inter-face with the GPS receiver. The softwarealso was required to: download the firm-ware to the GPSCardTM, initiate and store

Presented at the IEEE Position Location And Navigation SymposiumPLANS 92, Monterey, Ca. March 2421.1992

raw and computed data to the PC’s harddisk, and provide an operator interface forthe cellular modem. Differential data fromthe monitor station was broadcast over aceIlular link. This data was subsequentlyreceived at the remote station and passedfrom the PC serial port, through theprogram, directly to the GPSCardtmthrough the ISA bus.

CPS DATA COLLECTION EQUIPMENT

’ LIi \GRID 386 LAPTOP

Figure l-Field Data CollectionSystem

The NovAtel GPSCardTM receiver1 is ahigh-performance 10 channel receiver ca-pable of independently tracking the CIAcode and carrier phase of all the GPSsatellites in view. A wide bandwidth RFfront end, combined with high rate multi-bit sampling, narrow code correlatorspacing, and digital signal processing fea-tures, yields substantially higher codephase tracking accuracy (10 cm RMS) andhigh multipath rejection3. The high per-formance onboard 32 bit CPU (INMOST805), permits rapid raw data recording(100 msec) and high position update rates(200 msec). The dual serial data/commandports and assorted input/output strobesprovide support for integration with ex-ternal systems, real-time differential pos-itioning, remote receiver control, datalogging, and time transfer.

The NovAtel GPSAntennam is a custommicrostrip GPS Ll antenna with inte-grated LNA (low noise amplifier). The in-ternal element design provides signal re-ception and other advantages of a micro-strip antenna (i.e. narrow bandwidth, low

cost, and ease of assembly), without suf-fering some of the inherent disadvantagessuch as cross polarization signal receptionand poor low elevation angle gain. Thesubstrate material used in the antennaelement is low loss, with very consistentdielectric properties and excellent hightemperature stability. It has a measuredphase center variation of 2.66” RMS (1.4mm at an Ll frequency of 1575.42 MHz).In order to minimize the effects of multi-path, a specially designed aluminumchoke ring ground plane was used duringsome of the tests.

The Cellular Radio Module (CRM) is aNovAtel designed miniature cellular tele-phone with built in modem and fax sup-port. This unit allows portable data andvoice communications through a cellularradio link. This product is designed to beincluded within the IBM PC-Radio, an in-dustrial notebook computer. The CRMunit measures 21.5x8x2 cm, requires 5volts input voltage, draws 9 Watts ofpower when transmitting, and operatesfrom -10°C to +6O”C. Some special fea-tures which the CRM supports are: dualoutput power (0.6 and 1.2 watts), upload-able modem firmware, 32 digit dialing,and dual system registration.

For the CRM to operate, the user must beregistered with a cellular system provider.All specialized cellular and modem func-tions are accessible using Hayes extendedprotocol commands4. The CRM is compat-ible with standard modems and thehigher rate transmit capabilities (9600baud) are facilitated by various data errorcorrection and compression standards (i.e.CCITT V.32, V.42, V.42BIS, and MNP-4,MNP-5). The CBM also supports GroupIII Facsimile, and comes in both EAMPS(North American) and ETACS (British)formats. The CRM unit, which was usedin a stand alone configuration for thistest, was controlled by software on thefield testing computer using a built in PCserial communications port.

Presented at the IEEE Position Location And Navigation SymposiumPLANS 92, Monterey, Ca. March 24-27, 1992

Two GRID 1535 EXP 386 portables wereused for the test. An expansion chassis,capable of taking two full length ISAcards (8 and 16 bit), provided a slot for theGPSCardm receiver. The computer fea-tures include a 12.5 MHz 386D with mathcoprocessor, a 40 MB hard disk drive anda VGA LCD screen. The GRID 1535 ispowered using 12 volts DC. The computerand the GPSCardTM required a total of 28Watts of power during testing operations.

TESTING METHODOLOGY

.c o -

In order to access accuracies of the differ-ential positioning capabilities of theGPSCardTM Model 1001 card, a series ob-servation sets where taken. Data wascollected over a zero baseline (i.e. usingtwo receivers and one antenna) atNovAtel’s corporate head office inCalgary. Data was subsequently recordedon various short base lines west of Cal-gary. The 4 km and 8 km GPS data setswere measured on an Electronic DistanceMeasurement (EDM) calibration baselinein the Springbank area. The absolute GPSposition for these pillars were obtaineddirectly from the Canadian Government’sGeodetic Survey Division. Additionally,data was collected over a 12.5 km baselinein the Bearspaw area with absolutecoordinates which have been confirmedusing other GPS receivers. The semi-kinematic survey was also conductedusing the Springbank EDM calibrationpillars. A vehicle was used to transportthe antenna from pillar to pillar over amaximum distance of 1.3 km. No antennaground plane was used on the movingremote during this phase of the testing.

All data collected during the various testswas logged in ASCII format. The recordeddata fields included: raw range, satelliteephemeris and almanac messages, posi-tion, velocity, receiver clock offset, satel-lite elevation and azimuth, and DOPS

(dilution of precision values). Most logs,with the exception of ephemeris, almanac,and DOPS data, were logged at a rate ofonce per second. At the monitor station,the transmitted differential correctionswere logged.

SPRINGBANK DIFFERENTIAL CORRECTIONSI 4Monitor Sutio~ Red Time Code Phw 1s

1

Figure 2-GPS DifferentialCorrections

The NovAtel receiver specific short formASCII differential corrections log in-cluded: time, satellite prn, differentialcorrections and their associated rates, andIODE (issue of data of the ephemeris).These correction values were satellitespecific (cf. Figure 2) and derived using afixed position entered by the operator.Carrier phase data was used in the postprocessing to help assess the accuracy ofthe real-time differential results derivedfrom code phase.

RESULTS

Evaluation of the results from this testwas accomplished using a variety ofmethods, Zero baseline results were usedto assess measurement noise in the ab-sence of atmospheric and multipath ef-fects. Fixed baselines over various dis-tances, derived using independent meth-ods, were then used to assess static posi-tioning real-time accuracies.

Results derived from carrier phase usinga fixed ambiguity double differencesolution, were used for comparison pur-


poses for both kinematic and static ob-serving conditions. This analysis was per-formed using the software package,Semikin5, developed at the University ofCalgary. Finally, in order to compare real-time with post-processed results usingeither pure code or carrier smoothed code,the C3NAV software program2 (alsodeveloped at the University of Calgary)was used.

ROOFTOP ZERO BASELINE DGPS RESULTSReal Time Differential Update rATE I 0

1o __;:;:_...~_ _ .._.__._ - - ._.- ..-....... - .--.- - --

-.- Faniq .-. Norrnbl~ - height

Figure 3-Zero Baseline DifferentialResults

In order to the assess the accuracy of theGPSCard” and the cellular modem, GPSdata was collected over a zero lengthbaseline. This configuration removes allcommon site specific mutipath errors andatmospheric errors. Examination ofFigure 3 reveals the code phasemeasurement noise translates intoapproximately 20 cm. This is consistentwith 10 cm RMS range accuracy and aGDOP of 2. The zero baseline data wascollected at a rate of 1 second from theroof of NovAtel’s head office in Calgary onMarch 4, 1992.

A series of tests were conducted to collectGPS differential data over fixed baselinesof 4, 8 and 12.5 km. The results fromthese tests are summarized in Tables 1 to3 below. The Springbank data wascollected on February 17 and theBearspaw data was collected on March 10,1992.

The basis for the true values used in thecomparison were derived from both previ-ous GPS surveys using other manufac-turer’s equipment and from Semikin purephase, fixed ambiguity, double differencedata.

Table l-Static DifferentialResults 4 km

Generally C3NAV post processed (P/P)code phase only results are slightly betterthan real-time W’l’) results. This was asexpected since there are differential rateerrors in real-time which disappear inpost processing. The large rates exhibitedby the differential corrections in Figure 2are largely due to S/A effects. This isverified by Block 1 satellites (PRNS 6 and11) which have no second order trends.

Table S-Static DifferentialResults 8 km

These rate error effects are of course morepronounced if there are any delays inmodem transmission due to high noisecellular reception problems. This is an un-fortunate by-product of using error detec-


tion and compression techniques totransmit data at a higher rate (i.e. 9600baud). During some of the field testing,delays of up to 30 seconds resulted in dif-ferential range correction errors whichare magnified by the current satellitegeometry into position errors of a fewmetres.

Bears aw 12.5 km Static Baseline(Means cm>

Lat -0.10 -0.14 1 -0.15Lon -0.44 -0.42 1 -0.41Hgt 1 -0.12 ) -0.22 1 -0.27

Standard Deviations (m)Lat 0.60 0.59 0.26Lon 0.35 0.32 0.13Hgt 1.01 0.97 0.43

Table S-Static DifferentialResults 12.5 km

C3NAV code and carrier phase combinedresults are generally better than purecode. This is as expected since the noiselevel on the ranges are reduced. Standarddeviations from the mean, however, arenot as low as might be expected. This isdue to the small remaining biases in thecarrier phase smoothed pseudo rangeobservations (cf. Figure 7).

STATIC BASELINE DIFFERENTIAL RESULTSBearspaw Real lime Differential 12.5 km

A

Figure 4-Real-Time StaticDifferential Positioning Results

A semi-kinematic test was performed onMarch 17, 1992 using a series of pillars at

the Springbank EDM calibration baseline.The raw data was later post-processed, asin the previous experiments, to generatepost-processed code and code/carrier posi-tion differences. The baseline distance forthis experiment ranged from 700 metresto 2.3 km.

Semikin software also generates inter-mediate positions between the pillars us-ing a Kalman filter and previous knowl-edge of the integer ambiguities. If a cycleslip occurs on only one satellite, its inte-ger ambiguity can easily be recovered. Itis therefore important to collect datawhen six or more GPS satellites arevisible.The Semikin kinematic data was used toderive code generated real-time andC3NAV kinematic position differences,since it is accurate to within a few centi-metres. This is of course assuming thatcontinuous phase data without cycle slipsis available.

SPRINGBANK KINEMATIC GPS TESTPort-Processed SKfDYN VI Realtime

Figure 5-Semikin Dynamic Positionsvs Real-Time PositionsSPRINGBANK KINEMATIC GPS TEST

Post-ProcessedProsared Semikin Phase Velocity

Figure 6-Semikin Phase DerivedVelocity


Figure 5 illustrates positions collectedwhen moving the antenna off of a pillar,attaching it to the roof of a vehicle, anddriving 2 km to the next pillar. Maximumvelocities reached while transporting theantenna were 18 m/s (40 mph) as illus-trated in Fi,gure 6. The pillar sites are nottotally free f r o m multipath due to localobstruction: (e.g. trees, fences, and powerlines) which may explain some of the highlevel of code errors illustrated in Figure 5.Note that these errors also closely matchthe periods when the antenna was on thevehicle before leaving for the next pillar.

SPRJNGBANK KJNEMATJC GPS TESTPost-Processed SK/DYN VI C3NAV Phase

31) __ ,..__... -...- . .._... - ..__.. - . .._... -...-. .-.-..- ..__. - -.-... -...-.. -

^ 2 _ ._.. __. ..-. ..__ .___. ..- _. ._ -.. _ .-...-.. _.-._ -. _. ._-..._..

r ,J __._. .._.... -.. .- .._... _.....-......... -... -.. - ..-. -...-...-... - -.. __

Figure ‘I-Semikin Dynamic Positionsvs C3NAV Code/Phase Positions

To verify that Semikin and C3NAV resultswere consistent, a final graph was gener-ated to illustrate the difference betweenpure phase and phase smoothed code re-sults (Figure 7). Note that minor disconti-nuities at 90 and 290 seconds are due tothe C3NAV switching from one smoothingfilter to another to dissipate any biases.Again, as we saw in previous staticresults, the standard deviations indicate asmall amount of biasing (lo-20 cm> stilloccurs.

Several runs were made between the pil-lars on the Springbank EDM baseline andthe results are summarized in Table 4.Kinematic data periods ranged from 300to 400 seconds and Semikin phase derivedresults were used for comparison. A ma-jority of the data used in this paper wascollected during periods of 5 and 6 satel-

lite coverage with GDOP values rangingfrom 2 to 5.

Table 4-Real-Time KinematicPositioning Results

CONCLUSIONS

The equipment used to collect real-timedifferential GPS positions was over-viewed. The two central components ofthis positioning system, the NovAtelGPSCardTM 10 channel receiver and theNovAtel CRM modem, were discussed.This GPS field portable system used fordata collection will no doubt evolve into asingle product and in the future proveeconomical for various applications inareas of cellular coverage.

Positioning accuracies using code phasebased differential corrections were foundto range from 20 cm, in an ideal zerolength baseline scenario, and up to 1metre over longer static baselines of 12.5km. Kinematic results over baselines ofless than 3 km were found to exhibit thesame errors as static data, with meansranging from 30 to 70 cm and standarddeviations from 40 to 90 cm.

Exploitation of accurate code data to re-solve integer ambiguities, and in effectachieve real-time cm level double differ-

Prcscntcd at the IEEE Position Location And Navigation SymposiumPLANS 92, Monterey, Ca. March 24-27. 1992

ence positioning accuracies is almost areality.6 The future of these and othertechniques to enhance positioning accura-cies, will no doubt be found embedded inthe high end application equipment.There will, however, be increased compu-tation and data transmission demandswhich will require further advances insoftware and hardware and more researchand development.

ACKNOWLEDGMENTS

The authors would like to extend theirthanks to the following people who con-tributed their time to help make this pa-per possible. First credit goes to thegraduate students in the Surveying Engi-neering Department at The University OfCalgary for their help in the arduous taskof data collection. Also involved were thepeople from Excel Geophysical who helpedus collect data and provide us withindependent confirmation of the 4 and 8km baselines.

Thanks to Rob Miller from the CRM de-velopment team at NovAtel who helpedget the cellular modems programmed forcellular system authorization and set allthose complicated protocol defaults. Lastbut not least an acknowledgment for thededication of Henk Kroon from the GPSProducts Group at NovAtel for his pa-tience in developing a software interfacefor real-time.

REFERENCES

1. Pat Fenton, Bill Falkenberg, Tom Ford,Keith Ng and A.J. Van Dierendonck,“NovAtel’s GPS Receiver - The High Per-formance OEM Sensor of the Future”,Proceedings of the ION GPS-91 FourthInternational Technical Meeting of theSatellite Division of the Institute of Navi-gation, Albuquerque, NM, 11-13 Septem-ber 1991.

2. M.E. Cannon and G. Lachapelle,“Analysis of a High Performance C/A CodeGPS Receiver In Kinematic Mode”, Pro-ceedings of the National Technical Meet-

ing of the Satellite Division of the Insti-tute of Navigation, San Diego, California,27-29 January 1992.

3. AJ. Van Dierendonck, Pat Fenton, andTom Ford, “Theory and Performance ofNarrow Correlator Spacing in a GPSReceiver”, Proceedings of the NationalTechnical Meeting of the Satellite Divi-sion of the Institute of Navigation, SanDiego, California, 27-29 January 1992.

4. Cellular Radio Module - User InterfaceDescription, NovAtel Communications LtdInternal Document, GU-00110070Revision F, 10 January 1992

5. M. Elizabeth Cannon. “High-AccuracyGPS Semikinematic Positioning: Modelingand Results, Navigation, Vol. 37, No. 1,Spring 1990

6. G. Lachapelle, M.E. Cannon, C.Erickson, and W. Falkenberg, “HighPrecision C/A Code Technology For RapidStatic DGPS Surveys”, Presented at theSixth International Geodetic Symposiumon Satellite Positioning, Columbus Ohio17-20 March 1992

Prcscntcd at the IEEE Position Location And Navigation SymposiumPLANS 92, Monterey, Ca. March 24-27, 1992

real time kinematic dgps

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