GPS Compendium(GPS X 02007) - [PDF Document] (2024)

your position is our focus

Theory and Principles

Systems and Applications Overview

u-blox AG Zürcherstrasse688800ThalwilSwitzerlandwww.u-blox.comPhone+41447227444Fax+41447227447[emailprotected]

Essentials of Satellite Navigation Compendium

end

ium

your position is our focus

Title EssentialsofSatelliteNavigation

Subtitle

Doc Type Compendium

Doc Id GPS-X-02007-C

Revision Index Date Name Status / Comments

InitialVersion 11.Oct.2001 Jean-MarieZogg

A 1.Dec.2006 Jean-MarieZogg UpdateofSectionss:

• SBAS(WAAS,EGNOS)• UpdatingGPS• GALILEO• HighSensitivityGPS• AGPSErrorsandDOP• UTM-Projection• DGPS-Services• ProprietaryDataInterfaces• GPSReceivers

B 27.Feb.2007 TG UpdateofChapters:

• IntroductiontoSatelliteNavigation• SatelliteNavigationmadesimple

C 26.April2007 TG UpdateofSections:

• SpaceSegment• UserSegment• TheGPSMessage• Calculatingaposition(equations)• DGPSServicesforreal-timecorrection• WideAreaDGPS• HardwareInterfaces• GNSSReceiverModules

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Performancecharacteristicsshowninthisdocumentareestimatesonlyanddonotconstituteawarrantyorguaranteeofproductperformance.u-bloxdoesnotsupportanyapplicationsinconnectionwithweaponsystems.Sinceu-blox’productsarenotdesignedforuseinlife-supportandcommercialaviationapplicationstheyshallnotbeusedinsuchproducts.Indevicesorsystemswherebymalfunctionoftheseproductscanbeexpectedtoresultinpersonalinjuryandcasualties,u-bloxcustomersusingorsellingtheseproductsdosoattheirownriskandagreetokeepu-bloxharmlessfromanyconsequences.u-bloxreservestherighttomakechangestothisproduct,includingitscircuitsandsoftware,inordertoimproveitsdesignand/orperformance,withoutpriornotice.u-bloxmakes nowarranties, neither expressed nor implied, regarding the information and specifications contained in this document. u-blox assumes noresponsibility foranyclaimsordamagesarising from informationcontained in thisdocument,orfrom theuseofproductsandservicesdetailed therein.Thisincludes,butisnotlimitedto,claimsordamagesbasedontheinfringementofpatents,copyrights,maskworkand/orotherintellectualpropertyrights.u-bloxintegratedcircuits,softwareanddesignsareprotectedbyintellectualpropertylawsinSwitzerlandandabroad.u-blox,theu-bloxlogo,theTIM-typeGPSmodule,Antaris,SuperSense,"yourpositionisourfocus",NavLox,u-center,AssistNow,AlmanacPlus,FixNowandEKFare(registered)trademarksofu-bloxAG.Thisproductmayinwholeorinpartbesubjecttointellectualpropertyrightsprotection.Pleasecontactu-bloxforanyadditionalinformation.Copyright©2007,u-bloxAG.

EssentialsofSatelliteNavigation

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your position is our focus

CONTACT Formoreinformationpleasecontactus.

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EssentialsofSatelliteNavigation Contact

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mailto:[emailprotected]

your position is our focus

Foreword Where on Earth am I? The answer to this seemingly simplequestion can sometimes be amatter oflifeanddeath.Consideranaviator tryingto finda safedestination to land,or thecrew of a ship in distress seekingassistance, or a hiker in the mountainsdisoriented by poor weather conditions.Your position on Earth is of vitalimportance and can have an immensevarietyofimplicationsandapplications.

These needn’t be as dramatic as thecirc*mstances above, but they can besituations that also have a significantimpact on our daily lives.How do I findthataddress that I’vebeen searching for,or when or where should the publictransitvehicletriggerthenexttrafficlight?The potential applications and uses ofposition information are seeminglylimitless.Ourpositionon thisblueplanethas always been vitally important tohuman beings and today our exactposition is something thatwecanobtainwithastonishingease.

Amongthemoststunningtechnologicaldevelopments inrecentyearshavebeenthe immenseadvances intherealmof satellitenavigationorGlobalNavigationSatelliteSystems (GNSS) technologies. Inamatterofa fewyears,satellitenavigationhasevolvedfromthelevelofsciencefictiontosciencefactwithadynamicandrapidlygrowing industry providing customers around theworldwith technology devoted to the rapid, reliable andreadilyavailabledeterminationoftheirposition.

As global leaders in this fascinating and rapidly changing industry, u-bloxAG adds a Swiss accent and ourobsessionwith precision and quality shows through. Themen andwomen of this company are dedicatedsatellitenavigationenthusiasts,andasourmottosays,yourpositionisourfocus.Aspartofourcommitmenttocustomerservice,u-bloxAGispleasedtobeabletoprovideyouwiththiscompendiumtohelpleadyouintotheremarkableworldofsatellitenavigation.

Theaimof thisbook is toprovideacomprehensiveoverviewof theway inwhichsatellitenavigationsystemsfunctionandtheapplicationstowhichtheycanbeused.Thecurrent levelofdevelopmentaswellaschangesand innovations will be examined. It is written for users who are interested in the technology as well asspecialistsinvolvedinsatellitenavigationapplications.Thedocumentisstructuredinsuchawaythatthereadercan graduate from simple facts tomore complex concepts. The basic theory of satellite navigationwill beintroducedandsupplementedbyother importantfacets.Thiscompendium is intendedtoadditionallyserveasanaidinunderstandingthetechnologythatgoesspecificallyintocurrentsatellitenavigationreceivers,modulesandICs.Importantnewdevelopmentswillbedealtwithinseparatesections.Acquiringanunderstandingofthevarious current co-ordinate systems involved in usingGNSS equipment can be a difficult task. Therefore, aseparatechapterisdevotedtointroducecartography.

Wehopethatthisdocumentwillbeofassistancetoyouandthatyouwillbeasenthusiasticasweareaboutthetechnology involved in determining position. It is indeed an immensely fascinatingworld and industry thatanswersthequestion“whereonearthamI?”

EssentialsofSatelliteNavigation Foreword

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your position is our focus

Author’s Preface In1990,IwastravelingbytrainfromChurtoBrigintheSwisscantonofValais.Inordertopassthetimeduringthe journey, I had brought along a few trade journals with me. While thumbing through an Americanpublication, Icameacrossa technicalarticle thatdescribedanewpositioningandnavigationsystem involvingsatellites.Thenewsystem,knownasGlobalPositioningSystemorGPS,employedanumberofUSsatellitestodetermineone’spositionanywhereintheworldtowithinanaccuracyofabout100m1.

Asanavidsportsmanandmountainhiker,IhadonmanyoccasionsendedupinprecarioussituationsduetoalackofknowledgeoftheareaIwasin.Therefore,IwasfascinatedbytherevolutionaryprospectofbeingabletodeterminemypositioneveninfogoratnightbyusingaGPSreceiver.

Ibeganto intensivelyoccupymyselfwithGPS,arousingagreatdealofenthusiasmforthistechnologyamongstudentsatmyuniversity,whichresultedinseveralresearchsemestersandgraduatethesesonthesubject.Withtime I felt that Ihadbecomea trueexperton the subjectandwrote technicalarticlesaboutGPS forvariouspublications.

Why read this book? The development of themany new and fascinating potential applications of satellite navigation requires anappreciationofthewayinwhichthesesystemsfunction.Ifyouarefamiliarwiththetechnicalbackgroundofthesystem, it becomes possible to develop and use new positioning and navigation equipment.Aswell as thepossibilities,thisbookalsolooksatsomeofthelimitationsofthesysteminordertoprotectyoufromunrealisticexpectations.

How did this book come about? In2000IdecidedtoreducetheamountoftimeIspentlecturingatmyuniversityinordertogainanoverviewofthecommercialsatellitenavigationindustry.Mydesirewastoworkforacompanydirectlyinvolvedwithsatellitenavigation and just such a companywasu-bloxAG,who receivedmewithopen arms.u-blox askedme toproduce a brochure that they could give to their customers, and this compendium is the result and is asummationofearlierarticlesandnewlycompiledchapters.

A heartfelt wish Iwishyoueverysuccessasyouembarkonyourjourneythroughthewide-rangingworldofsatellitenavigationandtrustthatyouwillsuccessfullynavigateyourwaythroughthisfascinatingtechnicalfield.Enjoyyourread!

Jean-MarieZogg

October2001

July2006

1Thatwasin1990,positionaldataisnowaccuratetowithin5to10m!

EssentialsofSatelliteNavigation Author’sPreface

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Contents

Contact................................................................................................................................3

Foreword ............................................................................................................................4

Author’s Preface.................................................................................................................5

Contents..............................................................................................................................6

Introduction......................................................................................................................10

1 Satellite Navigation Made Simple.............................................................................12 1.1 Theprincipleofmeasuringsignaltransittime ..................................................................................... 12

1.1.1 BasicPrinciplesofSatelliteNavigation ......................................................................................... 13 1.1.2 Signaltraveltime......................................................................................................................... 15 1.1.3 Determiningposition................................................................................................................... 16 1.1.4 Theeffectandcorrectionoftimeerror........................................................................................ 17

2 GNSS Technology: The GPS example ........................................................................18 2.1 Descriptionoftheentiresystem.......................................................................................................... 18 2.2 Spacesegment ................................................................................................................................... 19

2.2.1 Satellitedistributionandmovement ............................................................................................ 19 2.2.2 TheGPSsatellites ........................................................................................................................ 22 2.2.3 Generatingthesatellitesignal ..................................................................................................... 24

2.3 Controlsegment ................................................................................................................................ 27 2.4 Usersegment ..................................................................................................................................... 27 2.5 TheGPSMessage ............................................................................................................................... 31

2.5.1 Introduction ................................................................................................................................ 31 2.5.2 Structureofthenavigationmessage ........................................................................................... 31 2.5.3 Informationcontainedinthesubframes ...................................................................................... 32 2.5.4 TLMandHOW ............................................................................................................................ 32 2.5.5 Subdivisionofthe25pages......................................................................................................... 32 2.5.6 Comparisonbetweenephemerisandalmanacdata..................................................................... 32

2.6 UpgradingGPS................................................................................................................................... 34 2.6.1 NewModulationProcedure,BOC................................................................................................ 34 2.6.2 GPSModernization ..................................................................................................................... 36

3 GLONASS and GALILEO..............................................................................................37 3.1 Introduction........................................................................................................................................ 37

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3.2 TheRussianSystem:GLONASS ........................................................................................................... 38 3.2.1 CompletionofGLONASS............................................................................................................. 38

3.3 GALILEO............................................................................................................................................. 39 3.3.1 Overview..................................................................................................................................... 39 3.3.2 ProjectedGALILEOServices ......................................................................................................... 40 3.3.3 Accuracy ..................................................................................................................................... 42 3.3.4 GALILEOTechnology ................................................................................................................... 43 3.3.5 MostImportantPropertiesofthethreeGNSSSystems ................................................................ 47

4 Calculating Position....................................................................................................48 4.1 Introduction........................................................................................................................................ 48 4.2 Calculatingaposition ......................................................................................................................... 48

4.2.1 Theprincipleofmeasuringsignaltraveltime(evaluationofpseudorange)................................... 48 4.2.2 Linearizationoftheequation....................................................................................................... 50 4.2.3 Solvingtheequation ................................................................................................................... 52 4.2.4 Summary..................................................................................................................................... 52 4.2.5 ErroranalysisandDOP ................................................................................................................ 53

5 Coordinate systems....................................................................................................57 5.1 Introduction........................................................................................................................................ 57 5.2 Geoids................................................................................................................................................ 57 5.3 Ellipsoidanddatum ............................................................................................................................ 58

5.3.1 Ellipsoid....................................................................................................................................... 58 5.3.2 Customizedlocalreferenceellipsoidsanddatum......................................................................... 59 5.3.3 NationalReferenceSystems......................................................................................................... 60 5.3.4 WorldwidereferenceellipsoidWGS-84 ....................................................................................... 60 5.3.5 Transformationfromlocaltoworldwidereferenceellipsoid......................................................... 61 5.3.6 ConvertingCo-ordinateSystems ................................................................................................. 62

5.4 Planarregionalcoordinates,projection ............................................................................................... 63 5.4.1 Gauss-Krügerprojection(TransversalMercatorProjection) .......................................................... 64 5.4.2 UTMprojection ........................................................................................................................... 64 5.4.3 Swissprojectionsystem(ConformalDoubleProjection) ............................................................... 66 5.4.4 Worldwideconversionofcoordinates.......................................................................................... 67

6 Improved GPS: DGPS, SBAS, A-GPS and HSGPS .......................................................68 6.1 Introduction........................................................................................................................................ 68 6.2 SourcesofGPSError........................................................................................................................... 68 6.3 Possibilitiesforreducingmeasurementerror....................................................................................... 70

6.3.1 DGPSbasedonSignalTravelTimeDelaymeasurement ............................................................... 71 6.3.2 DGPSbasedonCarrierPhasemeasurement ................................................................................ 73

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6.3.3 DGPSpost-processing(SignalTravelTimeandPhaseMeasurement)............................................ 73 6.3.4 Transmittingthecorrectiondata.................................................................................................. 74 6.3.5 DGPSclassificationaccordingtothebroadcastrange.................................................................. 75 6.3.6 Standardsforthetransmissionofcorrectionsignals .................................................................... 75 6.3.7 Overviewofthedifferentcorrectionservices ............................................................................... 76

6.4 DGPSservicesforreal-timecorrection................................................................................................. 77 6.4.1 GBASServices ............................................................................................................................. 77 6.4.2 EuropeanGBASServices.............................................................................................................. 77

6.5 WideAreaDGPS(WADGPS) ............................................................................................................... 78 6.5.1 SatelliteBasedAugmentationSystems,SBAS(WAAS,EGNOS) .................................................... 78 6.5.2 SatelliteDGPSservicesusingRTCMSC-104................................................................................. 81

6.6 AchievableaccuracywithDGPSandSBAS .......................................................................................... 82 6.7 Assisted-GPS(A-GPS).......................................................................................................................... 82

6.7.1 TheprincipleofA-GPS ................................................................................................................ 82 6.7.2 A-GPSwithOnlineAidingData(Real-timeA-GPS)....................................................................... 84 6.7.3 A-GPSwithOfflineAidingData(PredictedOrbits) ....................................................................... 84 6.7.4 ReferenceNetwork...................................................................................................................... 84

6.8 HighSensitivityGPS(HSGPS) .............................................................................................................. 85 6.8.1 ImprovedOscillatorStability ........................................................................................................ 85 6.8.2 Antennas..................................................................................................................................... 85 6.8.3 NoiseFigureConsiderations ........................................................................................................ 85 6.8.4 CorrelatorsandCorrelationTime ................................................................................................ 86

6.9 GNSS-RepeaterorReradiationAntenna.............................................................................................. 87 6.10 Pseudolitesforindoorapplications.................................................................................................. 87

7 Data Formats and Hardware Interfaces....................................................................88 7.1 Introduction........................................................................................................................................ 88 7.2 Datainterfaces ................................................................................................................................... 89

7.2.1 TheNMEA-0183datainterface ................................................................................................... 89 7.2.2 TheDGPScorrectiondata(RTCMSC-104)................................................................................... 99 7.2.3 Proprietarydatainterfaces......................................................................................................... 102

7.3 HardwareInterfaces ......................................................................................................................... 105 7.3.1 Antennas................................................................................................................................... 105 7.3.2 Supply ....................................................................................................................................... 109 7.3.3 Timepulse:1PPSandtimesystems............................................................................................ 110 7.3.4 ConvertingtheTTLleveltoRS-232............................................................................................ 112

8 GNSS RECEIVERS.......................................................................................................115 8.1 BasicsofGNSShandheldreceivers.................................................................................................... 115

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8.2 GNSSReceiverModules.................................................................................................................... 116 8.2.1 Basicdesignofa*gNSSmodule ................................................................................................. 116 8.2.2 Example:u-blox 5...................................................................................................................... 117

9 GNSS Applications....................................................................................................119 9.1 Introduction...................................................................................................................................... 119 9.2 Descriptionofthevariousapplications.............................................................................................. 120

9.2.1 LocationBasedServices(LBS)..................................................................................................... 120 9.2.2 CommerceandIndustry ............................................................................................................ 120 9.2.3 CommunicationsTechnology .................................................................................................... 121 9.2.4 AgricultureandForestry ............................................................................................................ 122 9.2.5 ScienceandResearch ................................................................................................................ 122 9.2.6 Tourism/Sport.......................................................................................................................... 124 9.2.7 Military...................................................................................................................................... 124 9.2.8 TimeMeasurement ................................................................................................................... 124

A Resources in the World Wide Web..........................................................................125 A.1 Summaryreportsandlinks ............................................................................................................... 125 A.2 DifferentialGPS ................................................................................................................................ 125 A.3 GPSinstitutes ................................................................................................................................... 125 A.4 GNSSantennas................................................................................................................................. 126 A.5 GNSSnewsgroupandGNSStechnicaljournal................................................................................... 126

B Index .........................................................................................................................127 B.1 ListofFigures ................................................................................................................................... 127 B.2 ListofTables..................................................................................................................................... 129 B.3 Sources............................................................................................................................................. 131

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EssentialsofSatelliteNavigation-Compendium Introduction

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Introduction SatelliteNavigationisamethodemployingaGlobalNavigationSatelliteSystem(GNSS)toaccuratelydetermineposition and time anywhere on the Earth. Satellite Navigation receivers are currently used by both privateindividualsandbusinessesforpositioning,locating,navigating,surveying,anddeterminingtheexacttimeinanever-growinglistofpersonal,leisureandcommercialapplications.

UsingaGNSSsystemthefollowingvaluescanaccuratelybedeterminedanywhereontheglobe(Figure1):

1. Exactposition(longitude,latitudeandaltitudeco-ordinates)accuratetowithin20mtoapprox.1mm.

2. Exacttime(UniversalTimeCoordinated,UTC)accuratetowithin60nstoapprox.5ns.

Speed and direction of travel (course) can be derived from these values,which are obtained from satellitesorbitingtheEarth.

Longitude: 9°24'23.43''Latitude: 46°48'37.20''Altitude: 709.1mTime: 12h33'07''

Figure 1: The basic function of satellite navigation

Asof2007,theGlobalPositioningSystem (GPS)developedandoperatedbytheUnitedStatesDepartmentofDefense(DoD)wastheonlyfullyoperationalGNSSsystem.TherapidlydevelopingSatelliteNavigation industryhassprunguparoundtheGPSsystem,andforthisreasonthetermsGPSandSatelliteNavigationaresometimesused interchangeably.ThisdocumentwillplaceanemphasisonGPS,althoughotheremergingGNSS systemswillbeintroducedanddiscussed.

GPS (thefullnameofthesystem is:NAVigationSystemwithTimingAndRangingGlobalPositioningSystem,NAVSTAR-GPS)isintendedforbothcivilianandmilitaryuse.TheciviliansignalSPS(Standard PositioningService)canbeusedfreelybythegeneralpublic,whilethemilitarysignalPPS(PrecisePositioningService)isavailableonlytoauthorizedgovernmentagencies.ThefirstsatellitewasplacedinorbitonFebruary22,1978,anditisplannedtohaveup to32operational satellitesorbiting the Earth at an altitudeof20,180 kmon6differentorbitalplanes.Theorbitsareinclinedat55°totheequator,ensuringthataleast4satellitesareinradiocommunicationwithanypointontheplanet.EachsatelliteorbitstheEarthinapproximately12hoursandhasfouratomicclocksonboard.

DuringthedevelopmentoftheGPSsystem,particularemphasiswasplacedonthefollowingthreeaspects:

1. Ithadtoprovideuserswiththecapabilityofdeterminingposition,speedandtime,whetherinmotionoratrest.

2. Ithadtohaveacontinuous,global,all-weather3-dimensionalpositioningcapabilitywithahighdegreeofaccuracy.

3. Ithadtoofferpotentialforcivilianuse.

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your position is our focus

Withinthenextfiveorsixyearstherewilllikelybe3fullyindependentGNSSsystemsavailable.TheUnitedStateswillcontinuetoprovideGPSandRussiaandtheEuropeanUnionshouldrespectivelybringtheirGLONASSandGALILEOsystemsintofulloperation.Allofthesesystemswillundergomodernizationandimprovements,whichshouldimprovetheirreliabilityandmakenewpotentialservicesandapplicationsavailable2.

This compendiumwill examine the essential principles of Satellite Navigation andmove beyond these intospecificapplicationsandtechnologies.GPSwillreceiveparticularfocusbecauseofitsimportanceasforerunnerand industrystandard,and importantdevelopmentssuchasDifferential-GPS (DGPS),Assisted-GPS (AGPS)andDeviceInterfaceswillbetreatedinseparatesections.Thisisallwiththegoalofprovidingthereaderwithasolidfoundationandunderstandingofthisfascinatingandincreasinglyimportantfield.

Figure 2: Launch of GPS Satellite

2Amongthesewillbeimportantadvancesforaviation,whereinapproachesandlandingsusingsatellitenavigationshouldbecomepossible.

EssentialsofSatelliteNavigation Introduction

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1 Satellite Navigation Made Simple

Do you want to . . .

o understand,howthedistanceoflightningcanbesimplydetermined?

o understand,howSatelliteNavigationessentiallyfunctions?

o know,howmanyatomicclocksareonboardaGPSsatellite?

o know,howtodetermineapositiononaplane?

o understand,whySatelliteNavigationrequiresfoursatellitestodetermineaposition?

Then you should read this chapter!

1.1 The principle of measuring signal transit time

Atsometimeorotherduringathunderstormyouhavealmostcertainlyattemptedtoworkouthowfarawayyouarefromaboltoflightning.Thedistancecanbeestablishedquiteeasily(Figure3):distance=thetimethelightningflash isperceived (starttime)until thethunder isheard (stop time)multipliedby thespeedofsound(approx.330m/s).Thedifferencebetweenthestartandstoptimeisreferredtoasthesignaltraveltime.Inthiscasethesignalissoundwavestravelingthroughtheair.

Eye determines the start time

Ear determines the stop time

Travel time

Figure 3: Determining the distance of a lightning flash

soundofspeedtimetraveldistance •=

SatelliteNavigationfunctionsbythesameprinciple.Onecalculatespositionbyestablishingthedistancerelativetoreferencesatelliteswithaknownposition.Inthiscasethedistanceiscalculatedfromthetraveltimeofradiowavestransmittedfromthesatellites.

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1.1.1 Basic Principles of Satellite Navigation

SatelliteNavigationSystemsallusethesamebasicprinciplestodeterminecoordinates:

• Satelliteswithaknownpositiontransmitaregulartimesignal.

• Basedonthemeasuredtraveltimeoftheradiowaves(electromagneticsignalstravelthroughspaceatthespeedoflightc=300’000km/s)thepositionofthereceiveriscalculated.

Wecanseetheprinciplemoreclearlyusingasimplemodel.Imaginethatweareinacarandneedtodetermineourpositionona longandstraightstreet.Attheendofthestreet isaradiotransmittersendingatimesignalpulse every second. Onboard the car we are carrying a clock, which is synchronized to the clock at thetransmitter.Bymeasuringtheelapsedtraveltimefromthetransmittertothecarwecancalculateourpositiononthestreet(Figure4).

DistanceD

TravelTime∆τ

τ τ

TransmittedSignal ReceivedSignal

∆τ

CalculatedPositiondueto1µsTimeError

300m

Street

TimeSignalTransmitter

Figure 4: In the simplest case Distance is determined by measuring the Travel Time

ThedistanceDiscalculatedbymultiplyingthetraveltime∆τbythevelocityoflightc.

D=∆τ•c

Becausethetimeoftheclockonboardourcarmaynotbeexactlysynchronizedwiththeclockatthetransmitter,there can be a discrepancy between the calculated and actual distance traveled. In navigation this incorrectdistance is referred to as pseudorange. In our example a time error of onemicrosecond (1µs) generates apseudorangeof300m.

Wecouldsolvethisproblembyoutfittingourcarwithanexactatomicclock,butthiswouldprobablyexceedourbudget.Anothersolutioninvolvesusingasecondsynchronizedtimesignaltransmitter,forwhichtheseparation(A)tothefirsttransmitterisknown.Bymeasuringbothtraveltimesitispossibletoexactlyestablishthedistance(D)despitehavinganimpreciseonboardclock.

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DistanceD

TravelTime∆τ1

τ1 τ

TransmittedSignal1 ReceivedSignals

∆τ1

SeparationA

τ2

TransmittedSignal2

TravelTime∆τ2

∆τ2

TimeSignalTransmitter1

TimeSignalTransmitter2

Street

Figure 5: With two transmitters it is possible to calculate the exact position despite Time Errors.

( )2

Ac∆∆D 21 +•−=

ττ

Aswehaveseen,inordertoexactlycalculatethepositionandtimealongaline(bydefinitionalineexpandsinonedimension)werequiretwotimesignaltransmitters.Fromthiswecandrawthefollowingconclusion:Whenanunsynchronizedonboardclock isemployed incalculatingposition, it isnecessary that thenumberof timesignaltransmittersexceedthenumberofunknowndimensionsbyavalueofone.

ForExample:

• Onaplane(Expansionintwodimensions)weneedthreetimesignaltransmitters.

• Inthree-dimensionalspaceweneedfourtimesignaltransmitters.

SatelliteNavigationSystemsusesatellitesastimesignaltransmitters.Contacttoatleastfoursatellites(Figure6)isnecessary inorder todetermine the threedesiredcoordinates (Longitude,Latitude,Altitude)aswellas theexacttime.Weexplainthisinmoredetailinfollowingsections.

Sat. 2

Sat. 1

Sat. 3Sat. 4

SatelliteSignal

Transmission Reception

TravelTime

Figure 6: Four satellites are needed to determine Longitude, Latitude, Altitude and Time

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1.1.2 Signal travel time

SatelliteNavigationSystemsemploysatellitesorbitinghighabovetheEarthanddistributed insuchawaythatfromanypointonthegroundthereislineofsightcontacttoatleast4satellites.

Eachoneof thesesatellites isequippedwithonboardatomicclocks.Atomicclocksare themostprecise timemeasurement instrumentsknown, losingamaximumofonesecondevery30,000to1,000,000years. Inordertomakethemevenmoreaccurate,theyareregularlyadjustedorsynchronizedfromvariouscontrolpointsonEarth.GNSSsatellitestransmittheirexactpositionandonboardclocktimetoEarth.Thesesignalsaretransmittedatthespeedof light (300,000km/s)andthereforerequireapprox.67.3mstoreachapositionontheEarth’ssurfacedirectlybelowthesatellite.Thesignalsrequireafurther3.33µsforeachaddtionalkilometeroftravel.Toestablishposition,all that is required isa receiverandanaccurateclock.Bycomparingthearrivaltimeofthesatellitesignalwiththeonboardclocktimethemomentthesignalwastransmitted, it ispossibletodeterminethesignaltraveltime(Figure7).

0ms

25ms

50ms

75ms

0ms

25ms

50ms

75ms

Signal transmission (start time)

Satellite andreceiver clockdisplay: 0ms

Satellite andreceiver clockdisplay: 67.3ms

Signal reception (stop time)

Signal

Figure 7: Determining the signal travel time

Aswiththeexampleofthecar,thedistanceDtothesatellitecanbedeterminedfromtheknownsignaltraveltime∆τ:

c•D

:lightofspeed•timetraveldistance

τ∆==

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1.1.3 Determining position

Imaginethatyouarewanderingacrossavastplateauandwouldliketoknowwhereyouare.Twosatellitesareorbitingfaraboveyoutransmitting theironboardclock timesandpositions.Byusing thesignal travel time tobothsatellitesyoucandrawtwocircleswiththeradiiD1andD2aroundthesatellites.Eachradiuscorrespondstothedistancecalculatedtothesatellite.Allpossiblepositionsrelativetothesatellitesarelocatedonthesecircles.Ifthepositionabovethesatellites isexcluded,the locationofthereceiver isattheexactpointwherethetwocirclesintersectbeneaththesatellites(Figure8),therefore,twosatellitesaresufficienttodetermineapositionontheX/Yplane.

Y - coordinates

X - coordinates

Circles

D1= τ1 • c

D2= τ2 • c

Sat. 1

Sat. 2

the receiverPosition of

(XP, YP)

Figure 8: The position of the receiver at the intersection of the two circles

In the realworld,apositionhas tobedetermined in three-dimensionalspace rather thanonaplane.As thedifferencebetweenaplaneandthree-dimensionalspaceconsistsofanextradimension(heightZ),anadditionalthirdsatellitemustbeavailabletodeterminethetrueposition.Ifthedistancetothethreesatellitesisknown,allpossiblepositionsarelocatedonthesurfaceofthreesphereswhoseradiicorrespondtothedistancecalculated.Thepositionisthepointwhereallthreeofthespheresintersect(Figure9).

Position

Figure 9: The position is determined at the point where all three spheres intersect

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1.1.4 The effect and correction of time error

Theconclusionsintheprevioussectionareonlyvalid,iftheclockatthereceiverandtheatomicclocksonboardthesatellitesaresynchronized,i.e.thesignaltraveltimecanbepreciselydetermined.Ifthemeasuredtraveltimebetweenthesatellitesandanearthboundnavigationalreceiverisincorrectbyjust1µs,thisproducesapositionerrororpseudorangeof300m.AstheclocksonboardalltheGNSSsatellitesaresynchronized,thesignaltraveltime inthecaseofallthreemeasurements is inaccuratebythesameamount.Mathematicscanhelpus inthissituation.

We are remindedwhen performingmathematical calculations that ifN variables are unknown,we needNindependentequationstoidentifythem.Ifthetimemeasurementisaccompaniedbyaconstantunknownerror(∆t),in3-Dimensionalspacewewillhavefourunknownvariables:

• longitude(X)

• latitude(Y)

• height(Z)

• timeerror(∆t)

Thesefourvariablesrequirefourequations,whichcanbederivedfromfourseparatesatellites.

SatelliteNavigationsystemsaredeliberatelyconstructed insuchawaythatfromanypointonEarth,at least4satellitesare“visible” (Figure10).Thusdespite inaccuracyonthepartofthereceiverclockandresultingtimeerrors,apositioncanbecalculatedtowithinanaccuracyofapprox.5–10m.

Sat. 2

Sat. 1

Sat. 3

Sat. 4

Signal

Figure 10: Four satellites are required to determine a position in 3-D space.

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2 GNSS Technology: The GPS example

If you would like to . . .

o understandwhythreedifferentGPSsegmentsareneeded

o knowwhatfunctioneachindividualsegmenthas

o knowhowaGPSsatelliteisbasicallyconstructed

o knowwhatsortofinformationistransmittedtoEarth

o understandhowasatellitesignalisgenerated

o understandhowSatelliteNavigationsignaltraveltimeisdetermined

o understandwhatcorrelationmeans

o understandwhyaminimumperiodoftimeisrequiredfortheGPSsystemtocomeonline

o knowwhatframesandsubframesare

then this chapter is for you!

2.1 Description of the entire system

InthefollowingsectionswewillexplorethedifferentsegmentsofGNSStechnologybyspecificallylookingattheGPSsystem.

TheGPSsystemiscomprisedofthreefunctionalsegments(Figure11):

• Thespacesegment(alloperatingsatellites)

• Thecontrolsegment(allgroundstations involved inthemonitoringofthesystem:mastercontrolstations,monitorstations,andgroundcontrolstations)

• Theusersegment(allcivilianandmilitaryusers)

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Space segment

Control segment User segment

- established ephemeris- calculated almanacs- satellite health- time corrections

- time pulses- ephemeris- almanac- satellite health- date, time

From satellitesL1 carrier signals

From the groundstation

Figure 11: The three GNSS segments

Ascanbe seen inFigure11 there isunidirectionalcommunicationbetween the space segmentand theusersegment.Thegroundcontrolstationshavebidirectionalcommunicationwiththesatellites.

2.2 Space segment

2.2.1 Satellite distribution and movement

ThespacesegmentoftheGPSsystemconsistsofupto32operationalsatellites(Figure12)orbitingtheEarthon6differentorbitalplanes(fourtofivesatellitesperplane).Theyorbitataheightof20,180kmabovetheEarth’ssurfaceandareinclinedat55°totheequator.Anyonesatellitecompletesitsorbitinaround12hours.DuetotherotationoftheEarth,asatellitewillbeatit*initialstartingpositionabovetheearth’ssurface(Figure13)afterapprox.24hours(23hours56minutestobeprecise).

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Figure 12: GPS satellites orbit the Earth on 6 orbital planes

Satellitesignalscanbereceivedanywherewithinasatellite’seffectiverange.Figure13showstheeffectiverange(shadedarea)ofasatellitelocateddirectlyabovetheequator/zeromeridianintersection.

Longitude

60°0° 120° 180°-60°-120°-180°

Latit

ude

0°

90°

12h

15h

18h

21h

12h

Figure 13: 24 hour tracking of a GPS satellite with its effective range

ThedistributionofthesatellitesataspecifictimecanbeseeninFigure14.Itisduetothisingeniouspatternofdistributionandtothehighorbitalaltitudesthatcommunicationwithatleast4satellitesisensuredatalltimesanywhereintheworld.

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Longitude

60°0° 120° 180°-60°-120°-180°

Latit

ude

0°

90°

Figure 14: Position of the GPS satellites at 12:00 hrs UTC on 14th April 2001

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2.2.2 The GPS satellites

2.2.2.1 Satellite Construction

Allofthesatellitesuseonboardatomicclockstomaintainsynchronizedsignals,whicharetransmittedoverthesame frequency (1575.42 MHz). The minimum signal strength received on Earth is approx. -158dBW to-160dBW[i].Accordingtothespecifications,themaximumstrengthisapprox.-153dBW.

Figure 15: A GPS satellite

2.2.2.2 The communication link budget analysis

Thelinkbudgetanalysis(Table1)betweenasatelliteandauserissuitableforestablishingtherequiredlevelofsatellitetransmissionpower.Accordingtothespecifications,theminimumamountofpowerreceivedmustnotfallbelow–160dBW (-130dBm). Inordertoensurethis level ismaintained,thesatelliteL1carriertransmissionpower,modulatedwiththeC/Acode,mustbe21.9W.

Gain(+)/loss(-) Absolutevalue

Poweratthesatellitetransmitter 13.4dBW(43.4dBm=21.9W)

Satelliteantennagain (due to concentrationofthesignalat14.3°)

+13.4dB

RadiatepowerEIRP

(EffectiveIntegratedRadiatePower)

26.8dBW(56.8dBm)

Lossduetopolarizationmismatch -3.4dB

Signalattenuationinspace -184.4dB

Signalattenuationintheatmosphere -2.0dB

Gainfromthereceptionantenna +3.0dB

Poweratreceiverinput -160dBW(-130dBm=100.0*10-18W)

Table 1: L1 carrier link budget analysis modulated with the C/A code

Accordingtothespecifications,thepowerofthereceivedGPSsignalinopenskyisatleast-160dBW(-130dBm).Themaximumof the spectralpowerdensityof the received signal isgivenas -190dBm/Hz (Figure16).The

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spectralpowerdensityofthethermalbackgroundnoiseisabout–174dBm/Hz(atatemperatureof290K).Thusthemaximumreceivedsignalpowerisapproximately16dBbelowthethermalbackgroundnoiselevel.

Thermal Noise

Received Signal

-2MHz -1MHz 0 1MHz 2MHzDeviation from median frequency

ectr

al P

er D

ensi

ty (

m/H

-180

-170

-190

-200

-210

-220

16dB

Figure 16: Spectral Power Density of received signal and thermal noise

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2.2.2.3 Satellite signals

Thefollowinginformation(thenavigationmessage)istransmittedbythesatelliteatarateof50bitspersecond[ii]:• Satellitetimeandsynchronizationsignals

• Preciseorbitaldata(ephemeris)

• Timecorrectioninformationtodeterminetheexactsatellitetime

• Approximateorbitaldataforallsatellites(almanac)

• Correctionsignalstocalculatesignaltransittime

• Dataontheionosphere

• Informationontheoperatingstatus(health)ofthesatellite

Thetimerequiredtotransmitallthisinformationis12.5minutes.Byusingthenavigationmessagethereceiverisabletodeterminethetransmissiontimeofeachsatellitesignalandtheexactpositionofthesatelliteatthetimeoftransmission.

EachGPSsatellitetransmitsauniquesignatureassignedtoit.ThissignatureconsistsofaPseudoRandomNoise(PRN)Codeof1023zerosandones,broadcastwithadurationof1msandcontinuallyrepeated(Figure17).

1 ms

1 ms/1023

Figure 17: Pseudo Random Noise

Thesignaturecodeservesthefollowingtwopurposesforthereceiver:

• Identification:theuniquesignaturepatternidentifiesthesatellitefromwhichthesignaloriginated.

• Signaltraveltimemeasurement

2.2.3 Generating the satellite signal

2.2.3.1 Simplified block diagram

Onboardeachofthesatellitesarefourhighlyaccurateatomicclocks.Theresonancefrequencyofoneoftheseclocksgeneratesthefollowingtimepulsesandfrequenciesrequiredforoperations(Figs.13and14):

• The50Hzdatapulse

• TheC/A(Coarse/Acquisition)code(aPRN-Codebroadcastat1.023MHz),whichmodulatesthedatausinganexclusive-oroperation(EXOR)3spreadingthedataovera2MHzbandwidth.

• ThefrequencyofthecivilL1carrier(1575.42MHz)

ThedatamodulatedbytheC/AcodemodulatestheL1carrierinturnbyusingBinary-Phase-Shift-Keying(BPSK)4.Witheverychangeinthemodulateddatathereisa180°changeintheL1carrierphase.

3Alogicaloperationontwooperandsthatresultsinalogicalvalueoftrueifandonlyifexactlyoneoftheoperandshasavalueoftrue.4Amethodofmodulatingacarrierwavesothatdataistranslatedinto90°phaseshiftsofthecarrier.

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Carrier frequencygenerator1575.42 MHz

PRN codegenerator1.023 MHz

Data generator50 Bit/sec

exclusive-or

Multiplier

Transmittedsatellite signal(BPSK)

Data

C/A code

Data

L1 carrier

Figure 18: Simplified satellite block diagram

Data,50 bit/s

C/A code(PRN-18)1.023 MBit/s

Data modulated by C/A code

L1 carrier,1575.42 MHz

BPSKmodulated L1 carrier

0 1 0 0 1 0 1 1

Figure 19: Data structure of a GPS signal

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2.2.3.2 Detailed block diagram

Satellitenavigation signalsaregeneratedusingaprocessknownasDSSS (DirectSequenceSpreadSpectrum)modulation [iii].This isaprocedure inwhichanominalorbaseband (not tobeconfusedwith thebasebandchipinthereceiver)frequencyisdeliberatelyspreadoutoverawiderbandwidththroughsuperimposingahigherfrequency signal. The principle of spread-spectrummodulationwas first devised in the 1940s in theUnitedStates,byscreenactressHedyLamarrandpianistGeorgeAnthell[iv].Thisprocessallowsforsecureradiolinksevenindifficultenvironments.

GPSsatellitesareeachequippedwithfourextremelystableatomicclocks(possessingastabilityofgreaterthan20·10

-12

[v]).Thenominalorbasebandfrequencyof10.23MHzisproducedfromtheresonantfrequencyofoneoftheseonboardclocks. Inturn,thecarrierfrequency,datapulsefrequencyandC/A (coarse/acquisition)codeareallderivedfromthisfrequency(Figure20).SincealltheGPSsatellitestransmiton1575.42MHz,aprocessknownasaCDMA(CodeDivisionMultipleAccess)Multiplex5isused.

TheC/Acodeplaysanimportantroleinthemultiplexingandmodulation.Itisaconstantlyrepeatedsequenceof1023bitsknownasapseudorandomnoise (PRN)code.Thiscode isuniquetoeachsatelliteandservesas itsidentifyingsignature.TheC/Acodeisgeneratedusingafeedbackshiftregister6.Thegeneratorhasafrequencyof1.023MHzandaperiodof1023chips7,whichcorrespondsto1ms.TheC/AcodeisaGoldCode8,whichhasadvantageous correlationproperties.Thishas important implications lateron in thenavigationprocess in thecalculationofposition.

Antenna

BPSK modulator

exclusive-or

C/A codegenerator

1 period = 1ms= 1023 Chips

Carrier freq.generator

1575.42MHz

Time pulse forC/A generator

1.023MHz

Data pulsegenerator

50Hz

1575.42MHz

Data

Atomic clockBaseband Frequency10.23MHz

10.23MHz

Dataprocessing

1 Bit = 20ms

1.023MHz

50Hz

x 154

: 10

: 204'600

1.023MHz

1575.42MHz

0/1

C/A code

Data

L1 carrier

BPSK

Figure 20: Detailed block diagram of a GPS satellite

5Aformofmultiplexingthatdividesuparadiochannelbyusingdifferentpseudo-randomcodesequencesforeachuser.CDMAisaformof"spread-spectrum"signalling,sincethemodulatedcodesignalhasamuchhigherbandwidththanthedatabeingcommunicated.6Ashiftregisterwhoseinputbitisalinearfunctionofitspreviousstate.7 transitiontimeforindividualbitsinthepseudo-randomsequence.The8 AGoldcodeisasetofbinarysequences.Picktwom-sequencesofthesamelengthn,suchthattheircross-correlationtakesjustthreevalues.Thesetofthenexclusive-orsofthetwosequencesintheirvariousphases(i.e.translatedintoallrelativepositions),togetherwiththetwon-sequencesthemselves,isasetofGoldcodes.TheexclusiveoroftwoGoldcodesisanotherGoldcodeinsomephase.

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2.3 Control segment

TheGPScontrolsegment(OperationalControlSystemOCS)consistsofaMasterControlStationlocatedinthestateofColorado,fivemonitorstations(eachequippedwithatomicclocksanddistributedaroundtheglobeinthevicinityoftheequator),andthreegroundcontrolstationstransmittinginformationtothesatellites.

Themostimportanttasksofthecontrolsegmentare:

• Observingthemovementofthesatellitesandcomputingorbitaldata(ephemeris)

• Monitoringthesatelliteclocksandpredictingtheirbehavior

• Synchronizingonboardsatellitetime

• Relayingpreciseorbitaldatareceivedfromsatellites

• Relayingtheapproximateorbitaldataofallsatellites(almanac)

• Relayingfurtherinformation,includingsatellitehealth,clockerrorsetc.

The control segment also oversees the artificial distortion of signals (SA, Selective Availability), in order todegrade the system’s positional accuracy for civil use. Until May 2000 the U.S.DoD (the GPS operators)intentionally degraded system accuracy for political and strategic reasons. This can be resumed, if deemednecessary,eitheronaglobalorregionalbasis.

2.4 User segment

TheradiosignalstransmittedbytheGPSsatellitestakeapprox.67millisecondstoreachareceiveronEarth.Asthe signals travel at a constant speed (the speed of light c), their travel time determines the exact distancebetweenthesatellitesandtheuser.

Fourdifferentsignalsaregeneratedinthereceiver,eachhavingthesamestructureasthesignalsreceivedfromthe4satellites.Bysynchronizingthesignalsgenerated inthereceiverwiththosefromthesatellites,thesignaltimeshifts∆tofthefoursatellitesaremeasuredasatimemark(Figure21).Themeasuredtimeshifts∆tofall4satellite signals are then used to determine the exact signal travel time. These time shifts are calledpseudoranges.

1 ms

∆t

Receiversignal (synchronised)

Satellitesignal

Receivertime mark

Synchronisation

Figure 21: Measuring signal travel time

Inordertodeterminethepositionofauser,radiocommunicationwithfourdifferentsatellites isrequired.Thedistancetothesatellites isdeterminedbythetraveltimeofthesignals.Thereceiverthencalculatestheuser’slatitudeϕ, longitudeλ,altitudehandtimetfromthepseudorangesandknownpositionofthefoursatellites.Expressedinmathematicalterms,thismeansthatthefourunknownvariablesϕ, λ, handtaredeterminedfromthedistanceandknownpositionofthesefoursatellites,althoughafairlycomplexlevelofiterationisrequired,whichwillbedealtwithingreaterdetailatalaterstage.

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Asmentioned earlier, all theGPS satellites transmit on the same frequency, butwith a differentC/A code.Identificationofthesatellitesandsignalrecoverytakeplacebymeansofacorrelation.AsthereceiverisabletorecognizeallC/A codes currently inuse,by systematically shiftingand comparingeveryknown codewithallincomingsatellitesignals,acompletematchwilleventuallyoccur(thatistosaythecorrelationfactorCFisone),andacorrelationpointwillbeattained (Figure22).Thecorrelationpoint isusedtomeasuretheactualsignaltraveltimeandtoidentifythesatellite.

Incoming signal from PRN-18bit 11 to 40, reference

Reference signal from PRN-18bit 1 to 30, leading

Reference signal from PRN-18bit 11 to 40, in phase

Reference signal from PRN-18bit 21 to 50, trailing

Reference signal from PRN-5Bit 11 to 40, in phase

CF = 0.07

Correlationpoint:CF = 1.00

CF = 0.00

CF = 0.33

Figure 22: Demonstration of the correction process across 30 bits

The quality of the correlation is expressed here as aCF (correlation factor). The value range of theCF liesbetweenminusoneandplusoneand isonlyplusonewhen thesignalscompletelymatch (bitsequenceandphase).

( ) ([ ]∑=

−•=N

iNCF

uBmB1 )

mB: numberofallmatchedbits

uB: numberofallunmatchedbits

N: numberofobservedbits.

AsaresultoftheDopplerEffect(satellitesandreceiversare inrelativemotiontooneanother)thetransmittedsignalscanbeshiftedbyupto±6000Hzatthepointofreception.Thedeterminationofthesignaltraveltimeanddatarecoverythereforerequiresnotonlycorrelationwithallpossiblecodesatallpossiblephaseshifts,butalsoidentificationofthecorrectphasecarrierfrequency.Throughsystematicshiftingandcomparisonofallthecodes(Figure22)andthecarrierfrequencywiththeincomingsatellitesignalstherecomesapointthatproducesacompleteagreement (i.e.thecorrelationfactor isone) (Figure23).Asearchposition inthecarrierfrequencylevelisknownasabin.

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511

1023

255

767

CodeSh

ift

FrequencyShift

0-6KHz +6KHz0

Correlatio

nFactor

MaximumLevel bin

Figure 23: Search for the maximum correlation in the code and carrier frequency domains

The spectralpowerdensityof the receivedGPS signal laysatapproximately16dBbelow the spectralpowerdensityofthethermalorbackgroundnoise(seeFigure16).ThedemodulationanddespreadingofthereceivedGPSsignalcausesasystemgainGGof:

43dB20,50050bps

bps1023signalninformatioofrateDataCode-C/AofrateModulation

GG ====

Afterdespreading, thepowerdensityof theusable signal isgreater than thatof the thermalorbackgroundsignalnoise(Figure24).

Thermal Noise

Correlated Signal

-100Hz -50Hz 0 50Hz 100HzDeviation from Median Frequency

Spec

tral

wer

Den

sity

/Hz)

-150

-140

-160

-170

-180

-190

Figure 24: Spectral Power Density of the correlated signal and Thermal Signal Noise

The sensitivityof aGPSReceiver canbe improved through increasing the correlation time (Dwell Time). Thelongeracorrelatorremainsataspecificpoint intheCodeFrequencyLevel,the lowerwillbetherequiredGPS

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signal strength at the antenna.When the correlation time is increased by a factor of k, therewill be animprovementGRinthedifferencebetweentheSignalandtheThermalBackgroundNoiseof:

GR=log10(k)

Doubling theDwellTime increasesthedifferencebetween theSignaland theThermalBackgroundNoise (thesensitivityofthereceiver)by3dB.Inpracticeitisnotaproblemtoincreasethecorrelationtimeupto20ms.Ifthevalueofthetransmitteddataisknown,thenthistimecanbeincreasedevenmore.

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2.5 The GPS Message

2.5.1 Introduction

TheGPSmessage[vi]isacontinuousstreamofdatatransmittedat50bitspersecond.EachsatelliterelaysthefollowinginformationtoEarth:

• Systemtimeandclockcorrectionvalues

• Itsownhighlyaccurateorbitaldata(ephemeris)

• Approximateorbitaldataforallothersatellites(almanac)

• Systemhealth,etc.

Thenavigationmessageisneededtocalculatethecurrentpositionofthesatellitesandtodeterminesignaltraveltimes.

Thedatastream ismodulatedtotheHFcarrierwaveofeach individualsatellite.Data istransmitted in logicallygroupedunitsknownasframesorpages.Eachframe is1500bits longandtakes30secondstotransmit.Theframesaredividedinto5subframes.Eachsubframeis300bitslongandtakes6secondstotransmit.Inordertotransmit a complete almanac, 25 different frames are required. Transmission time for the entire almanac istherefore 12.5minutes.Unless equippedwithGPS enhancement (see chapter 6) aGPS receivermust havecollectedthecompletealmanacatleastonceinordertocalculateitsinitialposition.

2.5.2 Structure of the navigation message

Aframeis1500bitslongandtakes30secondstotransmit.The1500bitsaredividedintofivesubframeseachof300bits(durationoftransmission6seconds).Eachsubframeisinturndividedinto10wordseachcontaining30 bits. Each subframe beginswith a telemetryword and a handoverword (HOW).A complete navigationmessageconsistsof25 frames (pages).The structureof thenavigationmessage is illustrated inadiagram inFigure25.

Frame(page)

1500 bits30s

1 2 3 4 5 6 7 8 9 10

TLM

W DataSubpage300 Bits

6s Word content

Word No.

Satellite clockand health data

1 2 3 4 5 6 7 8 9 10

TLM

1 2 3 4 5 6 7 8 9 10

TLM

1 2 3 4 5 6 7 8 9 10

TLM

1 2 3 4 5 6 7 8 9 10

TLM

W Almanac

1 2 3 4 5 6 7 8 9 10

TLM

WEphemeris Ephemeris Partial almanacother data

Telemetry word(TLM)30 bits0.6s

Handover word(HOW)30 bits0.6s

8Bitspre-

amble

6Bits16Bits

reserved pa-rity

6Bitspa-rity

17Bits 7BitsTime of Week

(TOW)div.,ID

Sub-frame 1 Sub-frame 2 Sub-frame 3 Sub-frame 4 Sub-frame 5

Navigationmessage

25 pages/frames37500 bits12.5 min

251 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Figure 25: Structure of the entire navigation message

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2.5.3 Information contained in the subframes

Aframeisdividedintofivesubframes,eachsubframetransmittingdifferentinformation.

• Subframe1containsthetimevaluesofthetransmittingsatellite,includingtheparametersforcorrectingsignaltransitdelayandonboardclocktime,aswellasinformationonsatellitehealthandanestimateofthepositionalaccuracyofthesatellite.Subframe1alsotransmitstheso-called10-bitweeknumber(arangeofvaluesfrom0to1023canberepresentedby10bits).GPStimebeganonSunday,6thJanuary1980at00:00:00hours.Every1024weekstheweeknumberrestartsat0.Thiseventiscalleda“weekrollover”.

• Subframes2and3containtheephemerisdataofthetransmittingsatellite.Thisdataprovidesextremelyaccurateinformationonthesatellite’sorbit.

• Subframe4containsthealmanacdataonsatellitenumbers25to32(N.B.eachsubframecantransmitdatafromonesatelliteonly), thedifferencebetweenGPSandUTC time (leapsecondsorUTCoffset)andinformationregardinganymeasurementerrorscausedbytheionosphere.

• Subframe5containsthealmanacdataonsatellitenumbers1to24 (N.B.eachsubframecantransmitdatafromonesatelliteonly).All25pagesaretransmittedtogetherwith informationonthehealthofsatellitenumbers1to24.

2.5.4 TLM and HOW

Thefirstwordofeverysingleframe,thetelemetryword (TLM),containsapreamblesequence8bits in length(10001011)used for synchronizationpurposes, followedby16bits reserved forauthorizedusers.Aswithallwords,thefinal6bitsofthetelemetrywordareparitybits.

Thehandoverword(HOW)immediatelyfollowsthetelemetrywordineachsubframe.Thehandoverwordis17bits in length (a rangeofvalues from0 to131071canbe representedusing17bits)andcontainswithin itsstructurethestarttimeforthenextsubframe,whichistransmittedastimeoftheweek(TOW).TheTOWcountbeginswiththevalue0atthebeginningoftheGPSweek (transitionperiodfromSaturday23:59:59hourstoSunday00:00:00hours)and is increasedbyavalueof1every6seconds.As thereare604,800seconds inaweek, thecount runs from0 to100,799,before returning to0.Amarker is introduced into thedatastreamevery6secondsandtheHOWtransmitted,inordertoallowsynchronizationwiththePcode.BitNos.20to22areusedinthehandoverwordtoidentifythesubframejusttransmitted.

2.5.5 Subdivision of the 25 pages

Acompletenavigationmessagerequires25pagesandlasts12.5minutes.Apageoraframeisdividedintofivesubframes.Inthecaseofsubframes1to3,theinformationcontentisthesameforall25pages.Thismeansthatareceiverhasthecompleteclockvaluesandephemerisdatafromthetransmittingsatelliteevery30seconds.

Theonlydifferenceinthecaseofsubframes4and5ishowtheinformationtransmittedisorganized.

• Inthecaseofsubframe4,pages2,3,4,5,7,8,9and10relaythealmanacdataonsatellitenumbers25to32.Ineachcase,thealmanacdataforonesatelliteonlyistransferredperpage.Page18transmitsthevaluesforcorrectionmeasurementsasaresultofionosphericscintillation,aswellasthedifferencebetweenUTCandGPStime.Page25containsinformationontheconfigurationofall32satellites(i.e.blockaffiliation)andthehealthofsatellitenumbers25to32.

• Inthecaseofsubframe5,pages1to24relaythealmanacdataonsatellitenumbers1to24. Ineachcase,thealmanacdataforonesatelliteonly istransferredperpage.Page25transfers informationonthehealthofsatellitenumbers1to24andtheoriginalalmanactime.

2.5.6 Comparison between ephemeris and almanac data

Usingbothephemerisandalmanacdata,thesatelliteorbitsandthereforetherelevantco-ordinatesofaspecificsatellitecanbedeterminedatadefinedpointintime.Thedifferencebetweenthevaluestransmittedliesmainlyintheaccuracyofthefigures. Inthefollowingtable(Table2),acomparison ismadebetweenthetwosetsoffigures.

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Information Ephemeris

No.ofbits

Almanac

No.ofbits

Squarerootofthesemimajoraxisoforbitalellipsea

32 16

Eccentricityoforbitalellipsee 32 16

Table 2: Comparison between ephemeris and almanac data

Theorbitofasatellitefollowsanellipse.ForanexplanationofthetermsusedinTable2,seeFigure26.

Figure 26: Ephemeris terms

Semi-majoraxisoforbitalellipse:a

Semi-minoraxisoforbitalellipse:b

Eccentricityoftheorbitalellipse: 2

abae −

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2.6 Upgrading GPS

2.6.1 New Modulation Procedure, BOC

Inorderforallsatellitestotransmitonthesamefrequency,theGPSsignalsarespreadout(modulated)withaspecialcode.ThiscodeconsistsofaPseudoRandomNoiseCode(PRN)of1023zeroesoronesandisknownasthe C/A-Code. The code,with a period of 1millisecond, has a chiprate of 1.023Mbit/s. It is continuouslyrepeated and due to its unique structure enables the receiver to identify from which satellite the signaloriginates.

Thespreading(ormodulation)ofthedatasignal isachievedwithanexclusive-or(EXOR)operation(Figure27).The result is referred toasBinaryPhaseShiftKeying (BPSK(1)).Thenominalorbaseband frequency signal isgeneratedbyoneoftheatomicclocksandallsatellitesignalsarederivedfromthis.ThenominalorbasebandfrequencyisthenspreadormodulatedbytheC/ACodeat1•1.023Mbit/s.

PRN-CodeGenerator1.023 Mbit/s

Data Generator(C/A-Code)50 Bit/sec

EXOR

NavigationData

C/A-Code

Data

1 ms 1 ms/1023

BPSK(1)

BasebandFrequency1.023MHz

x 1

Figure 27: With BPSK the Navigation Data Signal is first spread by a code

Inthefuture,GPSandtheEuropeanGALILEOsystemswilluseanewmodulationprocesscalledBinaryOffsetCodeModulation (BOC).With BOC the BPSK signal undergoes a furthermodulation [vii]. TheModulationFrequency isalwaysamultipleoftheBasebandFrequencyof1.023MHz.Thepropertiesofthismodulationarecommunicatedinaspecificway.ForexampleBOC(10,5)meansthatthemodulationfrequencyisafactorof10timestheNominalorBasebandFrequency(10•1.023MHz)andthechiprateoftheC/ACodeis5timesthebase(5•1,023Mbit/s)(Figure28).

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PRN-CodeGenerator5.115 Mbit/s

Data Generator(C/A-Code)50 Bit/s

EXOR

NavigationData

C/A-Code

Data

0.2 ms

BasebandFrequency1.023MHz

ModulationGenerator10.23 MHz

x 10

x 5

EXOR

BOC(10,5)

10.23MHz

Figure 28: Modulation for the Future: BOC(10,5)

Thanks toBOC the signalwillbebetterdistributedover thebandwidthand the influenceofopposing signalreflection (Multipath)onthereceptionoftheNavigationSignalwillbereduced incomparisontoBPSK.WhenBPSK(1)undBOC(1,1)aresimultaneouslyusedtheirmutualinfluenceisaminimumbecausethemaximaofthepowerdensitiesareseparated(Figure29).

Spec

tral

wer

Den

sity

/Hz)

Figure 29: With BPSK(1) and BOC(1,1) the

EssentialsofSatelliteNavigation

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Deviation from Median Frequency

signal maxima are separated (signal strength normalized at 1 W per signal)

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2.6.2 GPS Modernization

SincetheactivationoftheGPSsystemin1978allthesatellitestransmitthefollowingthreesignalstotheEarth:

• OntheL1-Frequency(1575.42MHz):oneciviliansignal(SPS-ServicewiththeC/A-Signal,BPSK(1))andonemilitarysignal(PPS-servicewiththeP(Y)-Signal,BPSK(10))

• OntheL2-Frequency(1227.60MHz):asecondmilitarysignal.

The U.S.DoD has planned incremental improvements to the GPS signal structure (Figure 30). For civilianapplicationstheintroductionofasecondandthirdfrequencyisveryimportant;whenmorefrequenciescanbeused for establishing position, then the influence of the ionosphere on the signal travel time can becompensated or even eliminated. This compensation is possible because the transmission velocity c in theionosphere isdependantonthefrequency. Inadditiontothetwonewsignals,themodernizationofGPSwillprovide an increase in the signal strength for civilian users as well as additional capabilities for militaryapplications.

OnSeptember25,2005thefirstofeightnewsatellitesofthetypeIIR-M(Block2,ReplenishmentandMilitary)was sent into orbit. On December 16, 2005 the satellitewas ready for transmission. The launches of theremainingsevensatellitesbeganinearly2006.Thesenewsatellitestransmitadditionally:

• Anewciviliansignalat1227.60MHz,theso-calledL2CFrequency.

• Supplementarymilitarysignalsat1575.42MHzand1227.60MHz:theMSignals,usingBOC(10,5)modulation.

A new generation of satellites is planned towards the end of this decade. This new series will have thedesignation IIF(Block2,Follow-ON)and III(Block3).Themost importantcharacteristicsofthesenewsatelliteswillbe:

• Newciviliansignalat1176.45MHz(L5Frequency).Thissignalshouldbemorerobustthanpreviousciviliansignalsandcanbeusedinaviationduringcriticalapproaches.

• IncreaseinthesignalstrengthoftheMSignals(=M+)throughtheuseofconcentratingbeamantennas.

• ImprovementoftheC/ASignalStructureforthecivilianL1Frequency.(TobedesignatedL1C).

FrequencyBand

L11575.42MHz

L21227.60MHz

L51176.45MHz

Untilearly2005 Frommid2005BlockIIR-M

From2010BlockIIF&III

CivilianSignal MilitarySignal

C/AP(Y)

P(Y)

C/A

P(Y)L2C

P(Y)L1C

P(Y)M+

L2C

MP(Y)

M+

Date

Figure 30: With Modernization the availability of GPS frequencies will be increased

TheGPSgroundstationswillalsoberenewed.Theentiredevelopmentshouldbecompleteandoperationalbythemiddleofthenextdecade.Thenewsignalswillthenbecompletelyavailabletousers.

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3 GLONASS and GALILEO

Do you want to . . .

o know,howtheRussianNavigationSystemGLONASSfunctions

o understand,whyGLONASSwillbebuiltup

o know,whichsystemEuropewillbeactivating

o understand,whyGALILEOwillprovidedifferentservices

o know,whatSARcanmeanforsailors

o know,howthenewmodulationprocessBOCfunctions

then this chapter is for you!

3.1 Introduction

OnDecember28,2005 the firstGALILEO satellitewasbrought intoorbit.The satellite,with thedesignationGIOVE-Abegananewepoch.ForthefirsttimeEuropeisalsoactivelyinvolvedinsatellitenavigation.GPSshouldreceive some competition: Probablywithin the next five to six years therewill be three independentGNSSsystems available. The USA will continue to provide GPS, and Russia and the European Union (EU) willrespectivelyofferfunctionalGLONASSandGALILEOsystems.WiththreefunctioningGNSSsystemswewillnotonlybeabletoachievemoreaccuratepositioningbutwillalsohavedifferentfunctionsavailable.

GPSwillalsobemodernizedintheforeseeablefutureandwillthereforebecomemorereliable(see2.6).

ThischaptergivesanoverviewofthenotyetcompletelyoperationalGLONASSsystem,andthefutureEuropeanGALILEOsystem.

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See Also

Become a Carrier | C.H. Robinson

⎛ −+

=ϕ

−

−3

2222

byxaztancosa

abayx

byxaztansinb

bbaz

tan (17a)

⎟⎠⎞

⎜⎝⎛= −

xytanλ 1 (18a)

( )( )[ ]2

sina

ba1

acos

yxh

ϕ⋅⎟⎟⎠

⎞⎜⎜⎝

⎛ −−

−ϕ

(19a)

5.3.6.2 Converting ellipsoidal to Cartesian co-ordinates

Ellipsoidalco-ordinatescanbeconvertedintoCartesianco-ordinates.

( )[ ]( ) ( )λcoscosh

sina

ba1

ax2

22⋅ϕ⋅

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

ϕ⋅⎟⎟⎠

⎞⎜⎜⎝

⎛ −−

= (20a)

( )[ ]( ) ( )λsincosh

sina

ba1

ay2

22⋅ϕ⋅

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

ϕ⋅⎟⎟⎠

⎞⎜⎜⎝

⎛ −−

= (21a)

( )[ ]( )ϕ⋅

⎥⎥⎥⎥⎥

⎦

⎤

⎢⎢⎢⎢⎢

⎣

⎡

+⎥⎦

⎤⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛ −−⋅

ϕ⋅⎟⎟⎠

⎞⎜⎜⎝

⎛ −−

= sinha

ba1

sina

ba1

az 2

22 (22a)

5.4 Planar regional coordinates, projection

UsuallytheordnancesurveydepictsthepositionofapointPonthesurfaceoftheearththroughtheellipsoidcoordinates’ latitude ϕ and longitude λ (in relation to the reference ellipsoid) and height (in relation to theellipsoidorgeoid).

Giventhatgeoidcalculations(e.g.thedistancebetweentwobuildings)onanellipsoidarenumericallyawkward,generalsurveytechnicalpracticesprojecttheellipsoidontoaplane.This leadstoplanar,right-angledXandYregionalcoordinates.Mostmapsfeatureagrid,whichenablesfindingapointintheopeneasily.Inthecaseofplanar regional coordinates there aremappings (projections) of ellipsoid coordinates of the survey referenceellipsoidinacalculationplane.Theprojectionoftheellipsoidinaplaneisnotpossiblewithoutdistortions.Itispossible,however,tochoosetheprojection insuchaswaythatthedistortionsarekepttoaminimum.Usual

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projection processes are cylindrical orMercator projection or theGauss-Krüger andUTM projection. Shouldpositioninformationbeusedinconjunctionwithmapmaterial,itmustberememberedwhichreferencesystemandwhichprojectionconfigurationisgoingtobeusedformakingthemaps.

5.4.1 Gauss-Krüger projection (Transversal Mercator Projection)

TheGauss-KrügerprojectionisatangentialconformaltransverseMercatorprojectionandisonlyapplicabletoalimited areaor region.Anelliptical cylinder is laid around theearth’s rotationellipsoid (e.g.Besselellipsoid),wherebythecylindersurfacetouchestheellipsoidinthecentralmeridian(animportantmeridianfortheregiontobe illustrated,e.g.9°)along itswhole longitudeand in thepoles.Thecylinderpositionwith regard to theellipsoidistransversal,e.g.rotatedby90°(Figure56)).Inordertokeepthelongitudinalandsurfacedistortionsto aminimum,3°wide zonesof the rotation ellipsoid areused. The zonewidth is fixedaround the centralmeridian.Differentcentralmeridiansareuseddependingontheregion(e.g.6°,9°,12°,15°,....).

Local spheroid

(Bessel ellipsoid)

1st step:projection

onto cylinder

Processing the cylinder:map with country

co-ordinates

Greenwich meridian

Equator Mapping of the equator

Mapping of the Greenwich meridians

Cylinder

N N

Figure 56: Gauss-Krüger projection

Thevaluesinthenorth-southdirectionarecountedasthedistancefromtheequator.Inordertoavoidnegativevaluesinthewest-eastdirectionthevalueof+500000m(Offset)isacceptedforthecentralmeridian.Thecentralmeridian’snumberofdegreesisdividedby3andplacedinfrontofthisvalue.

Exampleofaposition:

Ellipsoidcoordinates: N:46.86154° E9.51280°

Gauss-Krüger(Centralmeridian:9°): N-S:5191454 W-E:3539097

Thepositionisatadistanceof5191454mfromtheequatorand399097mfromthecentralmeridian(9°).

5.4.2 UTM projection

IncontrasttotheGauss-KrügerprojectiontheUTM(UniversalTransversalMercator)systemprojectsalmosttheentire surface of the earth on 60∗20 = 1200 planes. The actual projection of the rotation ellipsoid on thetransversalcylinderiscarriedoutinaccordancewiththesameprocessasintheGauss-Krügerprojection.

The UTM system is often based on theWGS84 ellipsoid. However, it only defines the projection and thecoordinatesystemandnotthereferenceellipsoidandthegeodesicdatum.

TheUTMsystemdividesthewholeworldinto6°widelongitudinalzones(Figure57).Thesearenumberedfrom1to60beginningwith180°W,andendingwith180°E.If,forexamplezone1stretchesfrom180°Wto174°W,thecentralmeridianofthiszone1issituatedat177°W,zone2stretchesfrom174°Wto168°,thecentralmeridianofthiszone2issituatedat171°W,etc.

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Thecentralmeridiansforeachprojectionzoneare3°,9°,15°,21°,27°,33°,39°,45°,51°,57°,63°,69°,75°,81°,87°,93°,99°,105°,111°,117°,123°,129°,135°,141°,147°,153°,159°,165°,171°,177°east(E)andwest(W)(longitude)(Figure58).

Inthenorth-southdirection(tothepoles)thezonesaresubdivided,withanexceptioninthe8°beltoflatitude,andareidentifiedwithlettersbeginningwithC.Onlytheareabetween80°southto84°northisadmitted.Thelinefrom80°southto72°southisdesignatedasSectionC,thelinefrom72°southto64°southSectionD,etc.AnexceptiontothisisbeltknownaslatitudeXbetween72°northand84°north.Itis12°wide.

Figure 57: Principle of projecting one zone (of sixty)

Figure 58: Designation of the zones using UTM, with examples

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AsisthecasewithGauss-KrügerProjection,thenorth-southvalueismeasuredinkilometersasthedistanceofthe point from the equator. In order to avoid negative values in the southern hemisphere, the equator isarbitrarilyassignedthevalueof10,000,000m.

Thewest-eastvaluesare thedistanceof thepoint from thecentralmeridian,which (alsoaswith theGauss-KrügerProjection)isgiventhevalueof500,000m.

AnexampleofUTMcoordinatesincomparisontoWGS84wouldbe:

WGS84: N46,86074° E9,51173°

UTM:32T 5189816(N-S) 0539006(W-E)

5.4.3 Swiss projection system (Conformal Double Projection)

TheBesselellipsoid is conformallyprojectedontoaplane in two steps, i.e.anglepreserving. Initially there isconformalprojectionoftheellipsoidonasphere,thenthesphereisconformallyprojectedontoaplaneusinganobliquecylindricalprojection.Thisprocess iscalleddoubleprojection (Figure59).Amainpoint is fixed in theplaneontheellipsoid(oldobservatoryfromBern)intheprojectionoftheorigin(withOffset:YOst =600,000mandXNord=200,000m)ofthecoordinatesystem.

OnSwitzerland’smap(e.g.scale1:25000)therearetwodifferentpiecesofcoordinateinformation:

• Theregionalcoordinatesprojectedintheplane(XandYinkilometers)withtheaccompanyinggridand

• Thegeographicalcoordinates(Longitudeandlatitudeindegreesandseconds)relatedtotheBesselellipsoid

200'000

600'000

BERN

Local reference ellipsoid(Bessel ellipsoid)

1st step:projection

onto sphere

2nd step:projection

onto sphere

Processing the cylinder:map with country

co-ordinates

Figure 59: The principle of double projection

Thesignaltransittimefrom4satellitesmustbeknownbythetimethepositionalco-ordinatesareissued.Onlythen, after considerable calculation and conversion, is the position issued in Swiss land survey co-ordinates(Figure60).

Known signaltransit time

from 4 satellites

Calculation of WGS-84Cartesian

co-ordinaten

Conversion into CH-1903

Cartesian co-ordinaten

Projectiononto sphere

Projectiononto

oblique-angled cylinder

Figure 60: From satellite to position

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5.4.4 Worldwide conversion of coordinates

Internetoffersvariouspossibilitiesforconvertingcoordinatesfromonesystemintoanother[xxii].

5.4.4.1 Example: conversion of WGS-84 coordinates to CH-1903 coordinates

(Fromreferencesystemsinpractice,UrsMarti,DieterEgger,SwissFederalOfficeofTopography)

! Note:accuracyiswithin1meter!

1. Conversion of latitude and longitude:

ThelatitudeandlongitudeoftheWGS-84datahavetobeconvertedintosexagesimalseconds[´´].

Example:

1. The latitude(WGS-84)of46°2´38,87´´onceconverted is165758.87´´.This integer isdescribedasB:B=165758.87´´.

2. Thelongitude(WGS-84)of8°43´49,79´´onceconvertedis31429.79´´.ThisintegerisdescribedasL:L=31429.79´´.

2. Calculation of auxiliary integers:

1000066.169028 ′′−

=ΦB

10000

5.26782 ′′−=Λ

Example: Φ = − 0.326979

Λ = 0.464729

3. Calculation of the abscissa (W---E): y

)54.44()36.0()51.10938()93.211455(37.600072][ 32 Λ∗−Φ∗Λ∗−Φ∗Λ∗−Λ∗+=my

Example: y=700000.0m

4. Calculation of the ordinate (S---N): x

)79.119()56.194()63.76()25.3745()95.308807(07.200147][ 3222 Φ∗+Φ∗Λ∗−Φ∗+Λ∗+Φ∗+=mx

Example: x=100000.0m

5. Calculation of the height H:

)94.6()73.2()55.49(][ 84 Φ∗+Λ∗+−= −WGSHeightmH

Example:

HeightWGS-84=650.60mresultsfromtheconversion:H=600m

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6 Improved GPS: DGPS, SBAS, A-GPS and HSGPS

If you would like to . . .

o Knowwhichkindsoferrorsinfluencetheaccuracyofdeterminingposition

o KnowwhatDGPSmeans

o Knowhowcorrectionvaluesaredeterminedandrelayed

o UnderstandhowtheD-signalcorrectserroneouspositionalmeasurements

o KnowwhatDGPSservicesareavailableinCentralEurope

o KnowwhatEGNOSandWAASmean

o KnowhowA-GPSfunctions

Then this chapter is for you!

6.1 Introduction

The forerunnerofallGNSSsystems isGPS. Infactthis issomuch thecase thatGPS isoftenusedtorefertosatellite navigation in general. In its development GPS has shown some limitations, which have requiredrefinements and improvements in the technology. This chapter examines some of these technologicalenhancementstoGPS,whichhavebecomestandardstoGNSS.

Althoughoriginally intendedformilitarypurposes,theGPSsystem isusedtodayprimarilyforcivilapplications,suchassurveying,navigation,positioning,measuringvelocity,determining time,monitoringetc,etc,etc.GPSwasnot initiallyconceivedforapplicationsdemandinghighprecision,securitymeasures,orutilization inclosedrooms.Forthisreasonimprovementshavebeenimplemented.

• Toincreasetheaccuracyofpositioning,Differential-GPS(D-GPS)wasintroduced.

• Toimprovetheaccuracyofpositioningandtheintegrity(reliability,importantforsecurityapplications)SBAS(SatelliteBasedAugmentationSystem)suchasEGNOSandWAASwasimplemented.

• To improvethesensitivity inclosedrooms,orrespectivelytoreducetheacquisitiontime,Assisted-GPS (A-GPS)serviceswereoffered.

• ThereceptionpropertiesofGPSreceiversarecontinuallybeingimprovedandincreasethesensitivityofthereceiverswithHighSensitivity-GPS(HSGPS).

6.2 Sources of GPS Error

Thepositioningaccuracyofapprox.13mfor95%ofallmeasurements(withHDOPtheaccuracyiswithin1.3m)discussed in thepreviouschapter isnotsufficient forallapplications. Inorder toachieveaccuracy towithinameter or better, extra efforts are necessary. Different sources can contribute to the total error in GPSmeasurements.ThesecausesandthetotalerrorarelistedinTable11.Thesevaluesshouldbeviewedastypicalaveragesandcanvaryfromreceivertoreceiver.

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Error Source Error

Ephemerisdata 2.1m

Satelliteclocks 2.1m

Effectoftheionosphere 4.0m

Effectofthetroposphere 0.7m

Multipathreception 1.4m

Effectofthereceiver 0.5m

TotalRMSvalue 5.3m

Total RMS value (filtered, i.e. slightly averaged) 5.0m

Table 11:Error Source and total error

Theerrorcausesarestudiedinmoredetailbelow:

• Ephemeris data:thesatellitepositionatthetimeofthesignalemissionis,asageneralrule,onlyknowntobeaccurateuptoapprox.1...5m.

• Satellite clocks:althougheachsatelliteincludesfouratomicclocks,thetimebasecontainsdefects.Atimeerrorof10nsisreachedatanoscillatorstabilityofapprox.10-13perday.Atimeerrorof10nsimmediatelyresultsinadistanceerrorofabout3m.

• Effect of the ionosphere:theionosphereisanatmosphericlayersituatedbetween60to1000kmabovetheEarth’ssurface.Thegasmoleculesintheionosphereareheavilyionized.Theionizationismainlycausedbysolarradiation(onlyduringtheday!).Signalsfromthesatellitestravelthroughavacuumatthespeedoflight.Intheionospherethevelocityofthesesignalsslowsdownandthereforecannolongerbeviewedasconstant.Thelevelofionizationvariesdependingontimeandlocation,andisstrongestduringthedayandattheequator.Iftheionizationstrengthisknownthiseffectcan,toacertainextent,becompensatedwithgeophysical correction models. Furthermore, given that the change in the signal velocity is frequencydependent,thiscanadditionallybecorrectedbytheuseofdualfrequencyGPSreceivers.

• Effect of the troposphere: thetroposphereistheatmosphericlayerlocatedbetween0...15kmabovetheEarth’ssurface.Thecauseoftheerrorhereisthevaryingdensityofthegasmoleculesandtheairhumidity.Thedensitydecreasesastheheight increases.The increase indensityorhumidityretardsthespeedofthesatellite signals. In order to correct this effect, a simplemodel is usedwhich is based on the standardatmosphere(P)andtemperature(T):

o H=Height[m]

o T=288.15K–6.5∗10-3∗h[K]

o P=1013.25mbar(T/288.15K)5,256[mbar]

• Multipath: GPS signalscanbe reflected frombuildings, trees,mountainsetc.andmakeadetourbeforearrivingatthereceiver.Thesignal isdistorteddueto interference.Theeffectofmultipathcanbepartiallycompensated by the selection of themeasuring location (free of reflections), a good antenna and themeasuringtime(Figure41)).

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Figure 61: Effect of the ti

e ectiver

• Effect of the recinthereceiver.Ad

• Effect of the satchapter4.2.5.2.

6.3 Possibilitie

Reducingtheeffectoareusedforreducing

• Dual frequency Such receiversmeionosphere, it isdsignals,thedelay

• Geophysical corionosphereandtr

• Differential GPSThe evaluation oprocessingor inRbasestationandt

o RTDG

o Post-

• Choice of locontacttothe

EssentialsofSatelliteNaviga

GPS-X-02007-C

me of measuring on the reflections

eflectionineffective

reflection

eiver:furthererrorsareproducedduetoGPSreceivermeasurementnoiseandtimedelaysvancedtechnologiescanbeusedtoreducethiseffect.

ellite constellation, includingshadowing (DOP):thiseffectwasdiscussed indetail in

s for reducing measurement error

fmeasurementerrorscanconsiderablyincreasepositioningaccuracy.Differentapproachesthemeasurementerrorandareoftencombined.Theprocessesmostfrequentlyusedare:

measurement: L1/L2 signalsareused to compensate for theeffectof the ionosphere.asure theGPS L1andL2 frequency signals. Ifa radio signal is transmitted through theeceleratedreverselyproportionalto itsfrequency.Bycomparingthearrivaltimesofbothcanbedeterminedandthustheeffectoftheionization.

rection models. This is used primarily for the compensation of the effect of theoposphere.Correctionfactorsareonlyuseful,ifappliedtoaspecifiedandlimitedarea.

(DGPS):bycomparingwithoneorseveralbasestations,variouserrorscanbecorrected.f the correction data available from these stations can take place either during postealTime (RT).RealTime solutions (RTDGPS) requiredata communicationbetween thehemobilereceiver.DGPSemploysavarietyofdifferentprocesses:

PS,normallybasedontheRTCMSC104standard

DGPS derived from signal travel time delaymeasurement (Pseudorange corrections,achievableaccuracyapprox.1m)

DGPSderived from thephasemeasurementof thecarriersignal (achievableaccuracyapprox.1cm)

processing(subsequentcorrectionandprocessingofthedata).

cation and of the measurement time for improving the ”visibility” or line of sightsatellites(SeeexplanationonDOP4.2.5).

tion ImprovedGPS:DGPS,SBAS,A-GPSandHSGPS

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6.3.1 DGPS based on Signal Travel Time Delay measurement

TheprincipleofDGPSbasedonsignaltraveltimemeasurement(pseudorangeorC/Acodemeasurement)isverysimple.AGPSreferencestationislocatedataknownandaccuratelysurveyedpoint.TheGPSreferencestationdeterminesitsGPSpositionusingfourormoresatellites.GiventhatthepositionoftheGPSreferencestationisexactlyknown,thedeviationofthemeasuredpositiontotheactualpositionandmoreimportantlythemeasuredpseudorange to each of the individual satellites canbe calculated. These variations are valid for all theGPSreceiversaroundtheGPSreferencestationinarangeofupto200km.Thesatellitepseudorangescantherebybeused for the correctionof themeasuredpositionsofotherGPS receivers (Figure62). Thedifferencesareeither transmitted immediatelyby radioorusedafterwards for correction (Seepost-processing, section6.3.3)aftercarryingoutthemeasurements.

ItisimportantthatthecorrectionbebasedonthesatellitepseudorangevaluesandnotthespecificdeviationinpositionoftheGPSreferencestation.Deviationsarebasedonthepseudorangestothespecificsatellites,andthesevarydependingonpositionaswellaswhichsatellitesareused.Acorrectionbasedsimplyonthepositionaldeviationofthereferencebasestationfailstotakethisintoaccountandwillleadtofalseresults.

GPS receiver

Berne

Geneva

ZurichBasel

ChurGPS reference station

Sat. 1

Sat. 2 Sat. 3

Sat. 4

Figure 62: Principle of DGPS with a GPS base station

6.3.1.1 Detailed description of how it runs

Theerrorcompensationiscarriedoutinthreephases:

1. Determinationofthecorrectionvaluesatthereferencestation

2. TransmissionofthecorrectionvaluesfromthereferencestationtotheGPSuser

3. CompensationforthedeterminedpseudorangestocorrectthecalculatedpositionoftheGPSuser

6.3.1.2 Definition of the correction factors

A referencestationwithexactlyknownpositionmeasures theL1signal travel time toallvisibleGPSsatellites(Figure63)anduses thesevalues tocalculate itsposition relative to thesatellites.Thesemeasuredvalueswilltypically includeerrors.Since the realpositionof the referencestation isknown, theactualdistance (nominalvalue)toeachGPSsatellitecanbecalculated.Thedifferencebetweenthenominalandthemeasureddistances

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canbecalculatedbyasimplesubtractionandcorresponds toacorrection factor.Thesecorrection factorsaredifferentforallGPSsatellitesandarealsoapplicabletoGPSuserswithinaradiusofseveralhundredkilometers.

9°24'26"46°48'41" GPS

Decoder

RTCM SC-104

RF receivingantenna

GPS user

Reference station

RF transmitantenna

Satelliteantenna

GPS satellite

Figure 63: Determination of the correction factors

6.3.1.3 Transmission of the correction values

Given that the correction values canbeusedbyotherGPSuserswithina large area to compensate for themeasuredpseudoranges, theyare immediately transmittedbyusingasuitablemedium (telephone, radio,etc)(Figure64).

9°24'26"46°48'41" GPS

Decoder

RTCM SC-104

RF receivingantenna

GPS user

Reference station

RF transmittingantenna

Satelliteantenna

GPS satellite

Figure 64: Transmission of the correction factors

6.3.1.4 Correction of the measured pseudo ranges

Afterreceivingthecorrectionvalues,theGPSusercancompensateforthepseudorangesinordertodeterminetheactualdistancetothesatellites (Figure65).Theseactualdistancescanthenbeusedtocalculatetheexactpositionoftheuser.Allerrors,whicharenotcausedbyreceivernoiseandmultipathreception,canbeovercomeinthisway.

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9°24'26"46°48'41" GPS

Decoder

RTCM SC-104

RF receiving antenna

GPS user

Reference station

RF transmittingantenna

Satellite antenna

GPS satellite

Figure 65: Correction of the measured pseudoranges

6.3.2 DGPS based on Carrier Phase measurement

TheDGPSaccuracyof1meterachievedbymeasuringsignaltraveltime isnotenoughforsomerequirementssuch as solving survey problems. In order to obtain a precisionwithinmillimeters, the carrier-phase of thesatellitesignalmustbeevaluated.

Thewavelengthλ ofthecarrierwaveisapprox.19cm.Thedistancetoasatellitecanbedeterminedasshownbelow(Figure66).

Wave lengthλ

Phaseϕ

Number of complete cycles NDistance D

D = (N . λ) + (ϕ . λ)

UserSatellite

Figure 66: Principle of the phase measurement

SinceNisunknownthephasemeasurementisambiguous.Byobservingseveralsatellitesatdifferenttimesandcontinuallycomparingresultsfromuserandreferencestationreceivers (duringorafterthemeasurement),theposition can be calculated using an extensive series of mathematical equations to an accuracy of a fewmillimeters.

6.3.3 DGPS post-processing (Signal Travel Time and Phase Measurement)

DGPSpost-processingimplementsthedeterminedcorrectionfactorsbyusingappropriatesoftwareafter carryingout fieldmeasurements. Reference data is either obtained from private reference stations or from publiclyaccessible server systems.Thedisadvantage is thatproblemswith the fielddata (e.g.poor satellite reception,damagedfilesetc.)aresometimesnotdetecteduntilafterthecorrectionfactorsarecalculatedandbroadcast,necessitatingarepetitionofthewholeprocess.

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6.3.4 Transmitting the correction data

DGPSservicescollectdatafromreferencestationsandtransmit itby radio to themobile receiver.Thereareavarietyofchannelsavailableoverwhichtobroadcast thiscorrectiondata.Eachof thesebroadcastingsystemspossesses individual radio-technical properties and frequency ranges which have specific advantages anddisadvantagesforDGPS(Table12).

Broadcasting system Frequency

range Advantages Disadvantages Transmission

of correction data

Longandmediumwavebroadcasters(LW,MW)

100-600KHz Extensiverangeoftransmission(1000km)

Lowbitrates RTCMSC104

Maritimeradiobeacon 283-315KHz Extensiverangeoftransmission(1000km)

Lowbitrates RTCMSC104

Aviationradiobeacon 255-415KHz Extensiverangeoftransmission(1000km)

Lowbitrates RTCMSC104

Shortwavebroadcaster(KW)

3–30MHz Extensiverangeoftransmission

Lowbitrates,qualitydependsonthetimeandfrequency

RTCMSC104

VHFandUKW 30-300MHz Highbitrates,jointuseoftheexistinginfrastructure

Rangeoftransmissionlimitedbythequasi-opticalconditions

RTCMSC104

Mobilecommunication/telephonenetworks(GSM,GPRS)

450,900,1800MHz

Jointuseofexistingnetworks

Limitedrangeoftransmission,synchronizationproblem

RTCMSC104

GEOsatellitesystem 1.2–1.5GHz Extensiveareacoverage

Highinvestmentcost RTCMSC104(forMSAT,Omnistar,Landstar,Starfire)

RTCADO-229C(forSBASsystemssuchasWAAS,EGNOS,MSAS)

Table 12: Transmission process of the differential signal (for code and phase measurement)

Manycountriesprovidetheirownsystemsfortransmittingcorrectiondata.Acomprehensivedescriptionofallthesesystemsisbeyondthescopeofthiscompendium.Someindividualsystemswillbedescribedbelow.

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6.3.5 DGPS classification according to the broadcast range

ThevariousDGPSservicesavailablearecategorizedaccordingtothebroadcastrangeofthecorrectionsignals:

• Local DGPS: Local Area Augmentation System (LAAS). These are sometimes called Ground BasedAugmentationSystems(GBAS).

• RegionalDGPS

• Wide Area DGPS (WADGPS) or Satellite Based Augmentation Systems (SBAS): Employ satellites totransmitDGPScorrectiondata.Inthesecasesnotjustsinglereferencestations,butwholenetworksofreferencestationsareused.

6.3.6 Standards for the transmission of correction signals

DGPS broadcasters transmit the signal travel time and carrier phase corrections. FormostGBAS and somesatellite basedWADGPS systems (LandStar-DGPS,MSAT, Omnistar or Starfire) the DGPS correction data istransmittedaccording to theRTCMSC-104 standard. Typically the receivermustbe equippedwith a servicespecificdecoderinordertoreceiveandprocessthedata.

SatelliteBasedAugmentationSystemssuchasWAAS,EGNOSandMSASusetheRTCADO-229standard.SinceRTCA frequenciesanddata formatsare compatiblewith thoseofGPS,modernGNSS receivers can calculateRTCAdatawithouttheuseofadditionalhardware,incontrasttoRTCM(Figure67).

Table13 liststhestandardsusedforDGPScorrectionsignalsaswellasthereferencespertainingspecificallytoGNSS.

Standard References pertaining to GNSS

RTCMSC104: RadioTechnicalCommissionforMaritimeServices,SpecialCommittee104

•RTCMRecommendedStandardsforDifferentialNavstarGPSService,Version2.0and2.1

•RecommendedStandardsforDifferentialGNSSService,Version2.2and2.3

RTCA: RadioTechnicalCommissionforAeronautics

•DO-229C,MinimumOperationalPerformanceStandardsforGlobalPositioningSystem/WideAreaAugmentationSystemAirborneEquipment.

Table 13: Standards for DGPS correction signals

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Figure 67: Comparison of DGPS systems based on RTCM and RTCA standards

RTCM

Decoder

6.3.7 Overview of the different correction services

Uncorrected Corrected (DGPS)

TwoFrequency(L1/L2)

RTCMSC-104(Code+Phase)

PhaseMeasurement

RTCADO-229C(SBASoverGEO-

Satellites)

ProprietaryFormats

(Code+Phase)

PostProcessing(Code+Phase)

GBAS+LAAStransmissionover

LandStation

WADGPStransmissionover

GEO-Sat.

WAAS

EGNOS

MSASOmnistar

Landstar

StarFire

MeasurementbasedonCode

GSM,etc

LW/MW/KW

UKW/VHF

GPS

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6.4 DGPS services for real-time correction

Allcorrectiondata istransmitted intheuserreceiverreceptionareaviaasuitablebroadcaster(LW,KW,UKW,radio,GSM, internet, satellitecommunication,etc). InNorthAmericaandEurope, thecorrection signals frommultiplepublicDGPSservicescanbereceived.Dependingontheservice,anannuallicensefeemayberequiredoraone-timefeeischargedwhenpurchasingtheDGPSreceiver.

In the following sectiona few selectedEuropeanGBAS serviceswillbedescribed.Subsequently the satellite-basedDGPSserviceswillbediscussedindetail.

6.4.1 GBAS Services

Worldwide thereare far toomanyground-basedDGPS services,alsoknownasGroundBasedAugmentationServices (GBAS), todescribe themall indetailhere. Inmanycountries therearemultiplesystemsoffered.ThefollowinglistdescribesafewGBASservicesavailableinEurope.

6.4.2 European GBAS Services

• SAPOS:(GermanSurveyingandMappingAdministrationSatellitePositioningService)isaDGPSserviceinpermanentoperation.ThisserviceisavailableinallofGermany.ThebasisofthesystemisanetworkofGPS reference stations. For real-time correction values the data is transmitted usingUKW radio,longwave,GSMandtheirown2-meterband(VHF)frequencies.UKWradiotransmittersbroadcastthecorrection data signals in RASANT (Radio Aided Satellite Navigation Technique) format. This is aconversionofRTCM2.0fordatatransmission intotheRadioDataSystem (RDS)formatusedbyUKWsoundbroadcasting.SAPOSincludesfourserviceswithdifferentfeaturesandaccuracies:

o SAPOSEPS: Real-TimePositioningService

o SAPOSHEPS: High-PrecisionReal-TimePositioningService

o SAPOSGPPS: GeodeticPrecisionPositioningService

o SAPOSGHPS: GeodeticHigh-PrecisionPositioningService

• ALF:(AccuratePositioningbyLowFrequency)broadcaststhecorrectionvalueswithanoutputof50kWfrom Mainflingen, Germany (near Frankfurt). The longwave broadcaster DCF42 (LW, 123.7 kHz)transmits the correction values over an area of 600–1000 km. This upper sideband (USB) is phase-modulated (Bi-Phase-Shift-KeyingBPSK).TheGermanFederalOffice forCartographyandGeodesy, incooperationwiththeGermanTelecomservice(DTAG),providestheservice.Whenbuyingtherequireddecoder,theuserpaysaone-timefee.Dueto longwavepropagationpatternsthecorrectiondatacanbereceiveddespiteshadowing.

• AMDS:(AmplitudeModulatedDataSystem)isusedfordigitaltransmissionovermediumandlongwavefrequenciesusingexistingradiobroadcasters.Thedataisphase-modulatedandtransmittedoveranareaof600–1000km.

• Swipos-NAV: (SwissPositioningService)distributescorrectiondatausingFM-RDSorGSM.TheRadioDataSystemRDS isaEuropeanstandardforthedistributionofdigitaldataviatheUKWbroadcastingnetwork (FM,87-108MHz).RDSwasdeveloped inorder toprovide travelerswith traffic informationoverUKW.TheRDSdataismodulatedatafrequencyof57kHzontheFMcarrier.TheuserrequiresanRDSdecoderinordertoextracttheDGPScorrectionvalues.Toguaranteegoodreception,thereshouldgenerallybeline-of-sightcontactwithaUKWbroadcaster.Usersofthisservicecaneitherpayanannualsubscriptionoraone-timefee.

• Radio Beacons: radiobeacons arenavigation installationsdistributedworldwideprimarily along thecoasts.DGPScorrectionsignalsareusuallytransmittedalongafrequencyofapproximately300kHz.Thesignalbitratevariesdependingonthebroadcasterbetween100and200bitpersecond.

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6.5 Wide Area DGPS (WADGPS)

6.5.1 Satellite Based Augmentation Systems, SBAS (WAAS, EGNOS)

6.5.1.1 Introduction

SatelliteBasedAugmentationSystems(SBAS)areusedtoenhancetheGPS,GLONASSandGALILEO(once it isoperational) functions. Correction and integrity data for GPS or GLONASS is broadcast from geostationarysatellitesovertheGNSSfrequency.

6.5.1.2 The most important SBAS functions

SBASisaconsiderableimprovementcomparedtoGPSbecausethepositioningaccuracyandthereliabilityofthepositioninginformationisgreater.SBAS,incontrasttoGPS,deliversadditionalsignalsbroadcastfromdifferentgeostationarysatellites.

• Increased positioning accuracy using correction data:SBASprovidesdifferentialcorrectiondatawithwhichtheGNSSpositioningaccuracyisimproved.Firstofalltheionosphericerror,whicharisesduetothesignaldelaysintheionosphere,hastobecorrected.Theionosphericerrorvarieswiththetimeofdayandisdifferent from region to region.Toensure that thedata iscontinentallyvalid, it isnecessary tooperateacomplicatednetworkofearthstations inordertobeabletocalculatethe ionosphericerror. Inadditiontothe ionospheric values, SBAS passes on correction information concerning the satellite position location(Ephemeris)andtimemeasurement.

• Increased integrity and security:SBASmonitorseachGNSS satelliteandnotifies theuserofa satelliteerrororbreakdowninaquickadvancewarningtimeof6s.Thisyes/noinformationisonlytransmittedifthequalityofthereceivedsignalsremainsbelowspecificlimits.

• Increased availability through the broadcasting of navigation information: SBAS geostationarysatellitesemit signals,which are similar to theGNSS signals althoughmissing the accurate timedata.AGNSSreceivercaninterpretpositionfromthesesignalsusingaprocedureknownas“pseudoranging”.

6.5.1.3 Overview of existing and planned systems

AlthoughallSatelliteBasedAugmentationSystems(SBAS)includelargerregions(e.g.Europe)itmustbeensuredthat they are compatiblewitheachother (interoperability)and that theSBASproviders cooperatewith eachotherandagreeontheirapproach.Compatibility isguaranteedbyusingtheRTCADO-229Cstandard.Atthecurrenttime,theSBASsystemsidentifiedfortheareasbelowarecurrentlyinoperationordevelopmentandare(orwillbe)compatible(Figure68):

• North America (WAAS, Wide Area Augmentation System):theUSFederalAviationAdministration(FAA) is leadingthedevelopmentoftheWideAreaAugmentationSystem(WAAS),whichcoversmostof the continental United States and large parts of Alaska and Canada.WAAS operates over thesatellitesPORandAOR-W.Thesesatellitesshouldbecomeactiveduring2007/2008.Theuninterruptedcontinuationofthisservicewillbeachievedthroughtwonewsatellitessituatedat133°Wand107°W.Itisplanned to extend the service intoCanada through the augmentationofWAASwith aCanadian“CWAAS”system.

• Europe (EGNOS, European Geostationary Overlay Service): the European group of threecomprising ESA, the European Union and EUROCONTROL, is developing EGNOS, the EuropeanGeostationary Navigation Overlay Service. EGNOS is intended for the region of the European CivilAviationConference(ECAC).AsofJune2006EGNOSwasnotyetfullyapprovedforoperationforhighsecurityapplications(e.g.aviation).Thedefinitivereleaseofthesystemisscheduledfor2007/2008.ThecurrenttransmissionstatusoftheEGNOSsatellitescanbeviewedunder[xxiii].

• Japan (MSAS, Multifunctional Satellite Based Augmentation System): the JapaneseOffice forCivilAviation isdeveloping theMTSATbasedAugmentationSystem (MSAS) that is intended tocovertheAirTrafficControlAirspaceassociatedwithJapan.

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• India (GAGAN, GPS and GEO Augmented Navigation): the Indian Space ResearchOrganization(ISRO) istryingtodevelopasystem,which iscompatiblewiththeotherSBASsystems.This istobeginwith the launchoftheGSAT-4satellite,planned for2007.This isplannedtobeapreparationforanindependentGNSSsystem for India tobeknownas the IndianRegionalNavigationalSatelliteSystem(IRNSS).

• China (Beidou): Beidou involvesthreegeostationarysatellites(140°E,110.5°Eand80°E)belongingtotheChinese government and is foreseen as a regional expansion system for the proposedChinesesatellitenavigationsystemCOMPASS.Thedefinitivetimeframefortheactivationofthissystemremainsunclear.

Figure 68: Position and provision of WAAS, EGNOS, GAGAN and MSAS

Thegeostationarysatellites(Table14)broadcasttheirsignalsfromanaltitudeofapprox.36,000kmabovetheequator in the direction of the area of use. The Pseudo RandomNumber (PRN) for each satellite has beenallocated.ThebroadcastingfrequencyofthesignalsisthesameasGPS(L1,1575.42MHz).

Anik-F1R 107.3°W PRN 138 GSAT- 4

111.5°E PRN 127

Galaxy XV 133°W PRN 135

Service Satellite description Position PRN

WAAS Inmarsat3F3POR(PacificOceanRegion) 178°E 134

WAAS Inmarsat3F4AOR-W(AtlanticOceanRegionWest) 54°W 122

WAAS IntelsatGalaxyXV 133°W 135

WAAS TeleSatAnikF1R 107.3°W 138

EGNOS Inmarsat3F2AOR-E(AtlanticOceanRegionEast) 15.5°W 120

EGNOS Artemis 21.5°E 124

EGNOS Inmarsat3F5IOR-W(IndianOceanRegionWest) 25°E 126

GAGAN GSAT-4 111.5°E 127

MSAS MTSAT-1R 140°E 129

MSAS MTSAT-2 145°E 137

Table 14: The GEO satellites used (or to be used) with WAAS, EGNOS and MSAS

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6.5.1.4 System description

Thecomplexgroundsegmentiscomposedofseveralreferencebase-stations,groundcontrolcentersand2to3satellite earth stations (Figure 69). Each system uses its own designation for its stations. Table 15 belowcomparesthedesignations.

General description EGNOS designation WAAS designation MSAS designation

ReferenceBaseStation RIMS:ReferenceandIntegrityMonitoringStation

WRS:WideAreaBasestation

GMS:GroundMonitorStation

ControlCenter MCC:MissionControlCenter

WMS:WAASMasterStation

MCS:MasterControlStation

SatelliteGroundStation NLES:NavigationLandEarthStation

GES:GroundEarthStation

NES/GES:NavigationEarthStation/GroundEarthStation

Table 15: Designation of the SBAS stations

Figure 69: Principle of all Satellite Based Augmentation Systems SBAS

• Reference Station:intheSBASareathereareseveralreferencebasestations,whicharenetworkedtoeachother.ThebasestationsreceivetheGNSSsignals.Theyareexactlysurveyedwithregardtotheirposition.Eachbasestationdeterminesthedeviationbetweentheactualandcalculatedpositionsrelativetothesatellites(thepseudorange).Thisdataisthentransmittedtoacontrolcenter.

• Control Center:thecontrolcenterscarryouttheevaluationofthecorrectiondatafromthereferencebase stations, determine the accuracy of all GNSS signals received by each base station, detectinaccuracies,possiblycausedby turbulence in the ionosphere,andmonitor the integrityof theGNSSsystem.Dataconcerningthevariationsarethenintegratedintoasignalandtransmittedviadistributedsatelliteearthstations.

• Satellite Ground Station:thesestationsbroadcastsignalstothedifferentgeostationarysatellites.

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• GEO satellites: the SBASGEO (geostationary) satellites receive the signals from the satellitegroundstationsandbroadcastthemtotheGNSSusers.UnliketheGNSSsatellites,theseGEOsatellitesdonothave onboard signal generators but rather are equippedwith transponders,which relay the signalsprocessedonthegroundandtransmittedtothem.ThesignalsaretransmittedtoearthontheGNSS-L1-frequency (1575.42MHz). The SBAS signals are received and processed by suitably equipped GNSSreceivers.

6.5.2 Satellite DGPS services using RTCM SC-104

Severalgeostationarysatellitescontinuouslybroadcastcorrectiondataworldwide.Belowarelistedsomeoftheseservices.TheseservicesusetheRTCMSC-104standardandrequireaspecialdecoder.

• MSAT: developedbytheNationalResearchCouncilofCanada,thisservicebroadcaststheCanada-WideDGPS(CDGPS)signalsusingtwogeostationarysatellites.

• Omnistar (FugroGroup)andLandStar-DGPS, (ThalesCompany), independentlybroadcastcorrectiondatavia6GEOsatellites(Figure70).Theservicesmustbepaidforandtheusermusthaveaccesstoaspecialreceiver/decoderforusingtheservice.OmnistarandLandstarbroadcasttheirinformationinL-band(1-2GHz)toearth.Basestationsaredistributedworldwide.Thegeostationarysatellitesarelocatedinthecentral latitudedeepoverthehorizon (10° ...30°).Line-of-sightcontact isrequired inordertoestablishradiocontact.

Figure 70: LandStar-DGPS and Omnistar illumination zone

• StarfirePropertyofNavComTechnology,Inc.,broadcastscorrectiondatavia3InmarsatGEOsatellites.Theservicehastobepaidforandtheusermusthaveaccesstoaspecialreceiver/decoderinordertouse the service.Starfirebroadcasts its information in L-band (1-2GHz) toearth.The respectivebasestationsaredistributedthroughoutthewholeworld.Theservice isavailableworldwideovertherangeof±76°latitude.

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6.6 Achievable accuracy with DGPS and SBAS

Table16showstypicallyachievablepositioningaccuracywithandwithoutDGPS/SBAS.

Error cause and type Error without DGPS/SBAS

Error with DGPS/SBAS

Ephemerisdata 2.1m 0.1m

Satelliteclocks 2.1m 0.1m

Effectoftheionosphere 4.0m 0.2m

Effectofthetroposphere 0.7m 0.2m

Multipathreception 1.4m 1.4m

Effectofthereceiver 0.5m 0.5m

TotalRMSvalue 5.3m 1.5m

TotalRMSvalue(filteredi.e.slightlyaveraged) 5.0m 1.3m

Horizontalerror(1-Sigma(68%)HDOP=1.3) 6.5m 1.7m

Horizontal error (2-Sigma (95%) HDOP=1.3) 13.0m 3.4m

Table 16: Positioning accuracy without and with DGPS/SBAS

6.7 Assisted-GPS (A-GPS)

6.7.1 The principle of A-GPS

Itcanbeassumed thatdevices forLocationBasedServices (LBS, see9.2.1)aren’talways inoperation.This isespeciallyso incaseswhere localization isachievedwithGNSSbecausebatteryoperation ispreventedduringlongerstationaryperiodsinordertominimizepowerconsumption.BecausetheGNSSdeviceisonlyinfrequentlyinoperationitisprobablethatnoinformationisavailableregardingsatelliteposition.Afterbeinginactivefor2ormorehourstheorbitaldataofthesatellitesmustfirstbedownloaded inordertostartup.AGNSSreceivernormallyrequiresatleast18-36secondsinordertoobtaintheorbitaldataandcalculatethefirstposition.Underdifficult reception conditions (e.g. in urban areas where high buildings block direct sight to the sky) thecalculationofthefirstpositioncanrequireminutesforcompletion(ifatall).

IntheabsenceoftheorbitaldatatheGNSSreceiversmustcarryoutacompletesearchprocedure inordertofindtheavailablesatellites,downloadthedataandcalculatetheposition.SearchingfortheGPSsatellites(forexample) intheCode-Frequency-Level isverytimeconsuming.Thecorrelationtimenormallyrequiresat least1ms(1C/ACodePeriod)perpositionintheCode-Frequency-Level.Shouldthefrequencyrangebebrokeninto50steps(i.e.thefrequencyintervalamountsto(2x6000/50Hz=240Hz)thentherecanbeasmanyas1023x50=51,150positions(bins)tobesearchedfor(thisrepresents51seconds).Seealsosection6.8.

ThisproblemcanberemediedbymakingthesatelliteorbitaldataandotherGNSSinformationavailablethroughothercommunicationschannels,forexampleviaGSM,GPRS,CDMAorUMTS.ThisisreferredtoasAidingandisemployedbyAssisted-GPS.Assisted-GPS(orA-GPS) isafunctionorservicethatusesAiding-Data inordertoexpeditethepositioncalculation.TheGNSSreceiverobtainsAiding-Dataoveramobilecommunicationsnetwork(perhapsdirectlyovertheinternet).TheAiding-Dataincludesinformationoversuchthingsas:

• SatelliteConstellation(Almanac)

• PreciseOrbitalData(Ephemeris,Orbits)

• TimeInformation

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Withtheavailabilityofthishelp informationtheGNSSreceivercanveryquicklycalculateposition,evenunderpoorconditions.Dependingonthecomplexityandcompletenessofthehelp informationthereductionofthestart-uptimecanbesignificant.Thestart-uptimeremainsdependantonthestrengthoftheGNSS-Signal. It isgenerallytrue,however,thatthemorehelpinformationavailable,thefasterthestart-uptime.

Amobile transmitter stationwith integratedGNSSdevice still requires sight toat least four satellites.TouseA-GPStheGNSSreceiversrequireaninterfacethroughwhichtoreceivetheAiding-Data.

Thegreatesttimesavingoccursthrougheliminatingthereceptiontimefortheorbitaldata. Inadditiontothis,thesearchareacanbelimitedwhentheDopplerFrequencyandFrequencyOffsetoftheGNSSreceiverisknown(Figure71).ThiscausestheSignalAcquisitiontobeacceleratedwhichsavesadditionaltime.

511

1023

255

767

CodeSh

ift

FrequencyShift

0-6KHz +6KHz0

Correlatio

nFactor

LocationofMaximum

Figure 71: Acceleration of the search procedure with A-GPS by reducing the search area

TwodifferenttechniquesareemployedtousetheHelpInformation:

• WiththeOnline PrincipletheAiding-Dataaredirectlydownloadedfromaserverasneededinreal-time.Thisinformationisonlyvalidforalimitedtime.(e.g.AssistNow® Onlinebyu-bloxAG)

• WiththeOffline PrincipletheAiding-Data(generallypredeterminedEphemerisorAlmanacinformation)isdownloadedfromaserverandstoredintheGNSSdevicepriortotheapplication.Thedatacanremainvalidforuptoseveraldays.Asneededthestoreddatacanbeutilizedinordertoacceleratethestart-up.(e.g.AssistNow® Offlinebyu-bloxAG)

The help information is collected from a network of GNSS-Reference Stations (GNSS Reference Network)distributedworldwide.

A typicalA-GPS system, as illustrated in thebelowblockdiagram (Figure72), consistsof aglobal referencenetworkofGNSSreceivers,acentralserverthatdistributes�Aiding-Data,andA-GPScapablereceivers(theGNSSenddevices).TheGNSSreceiversoftheglobalreferencenetworkreceivetherelevantsatellite informationandforward ittotheserver.TheservercalculatestheAiding-Dataandtransmits it (overamobilecommunicationsnetworkoroverthe Internet)uponrequesttotheGNSSenddevices,which inturncanmorequicklycalculatetheirfirstposition.

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Mobile Comm.Network (GSM, GPRS, UMTS, CDMA)

GPS Satellites

GPS-Receiverwith A-GPSinterface

MobileStation

Aiding Data(over mobile comm. network)

ReferenceNetworkCentral Server

Internet

Aiding Data(directly overinternet)

Figure 72: Assisted-GPS system

6.7.2 A-GPS with Online Aiding Data (Real-time A-GPS)

WiththeOnlineorRealtimePrincipletheAidingDataaredirectlydownloadedfromtheserverasneededandareonlyvalid forashorttime.Thedisadvantageofthisprinciple istherelativelyslowconnection time (GPRS, forexample,requiresupto30seconds)orinadequateavailabilityofInternetAccessPoints.

6.7.3 A-GPS with Offline Aiding Data (Predicted Orbits)

A-GPS with Offline Aiding Data is a system providing the GNSS receiver with predetermined orbital data(PredictedOrbits).Thereceiverstoresthisinformation,andtheconnectiontotheserveristerminated.ThenexttimetheGNSSreceiverstartsupthestoredinformationisusedtodeterminethecurrentorbitalinformationfornavigation.Consequently it isno longernecessary towaituntilallof this informationhasbeendownloadedfrom the satellitesand the receiver can immediatelybeginnavigating.Dependingon theprovider theAidingData canbe valid forup to10days,although it shouldbe considered that the resultingpositionalaccuracydecreaseswithtime.

6.7.4 Reference Network

Predetermining theorbitalswithwhich to supplyReal-timeA-GPSData requiresanextensiveandworldwidenetwork of monitoring stations, which continually and accurately monitor satellite movements. A highperformance server uses this data to predict satellitemovements over the nextdays.An example of such anetworkistheonedevelopedbytheInternationalGNSS-Service(IGS,orInternationalGPS-Service[xxiv]),whichworldwideoperatesapermanentnetwork(Figure73).

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Figure 73: IGS reference stations (as of October 2006) with approx. 340 active stations

6.8 High Sensitivity GPS (HSGPS)

Whilecertainapplications,suchascallingemergencynumbersorLocationBasedServices,requireclearreceptioninbuildingsor inurbancanyons, the receptionperformanceofGNSS-Receivers isbeingcontinually improved.Theprimaryfocusesoftheseeffortsare:

• IncreasedSignalSensitivity• Quickeracquisitionuponactivationofthereceiver(timetofirstfix,TTFF)• Reducedsensitivitytointerference(e.g.multipathinterference,orelectromagneticinterferenceEMC)Variousstrategiesarebeingemployedbydifferentmanufacturers inordertoachieve improvements.Themostimportantofthesearediscussedinthischapterincluding:

• ImprovedOscillatorStability• Antennas• NoiseFigureconsiderations• Increasingthecorrelatorsandthecorrelationtime

6.8.1 Improved Oscillator Stability

The development and use of increasingly stable oscillators is an attempt to reduce or compensate for thetemperaturedependenceofquartzinordertodecreasesignalacquisitiontimeintherequiredfrequencyareas.Thismostlyinvolvestheemploymentoftemperaturecompensatedcrystaloscillators(TCXO).

Additionally,studieshaveshown[xxv]thatnormalquartzoscillatorscanproducemicrovariationsinfrequency(several10-9Hz).Thecausesofthesefrequencychangesaregenerallystructuralimpurityofthequartzcrystal.OnthebasisofthesesuddenfrequencyshiftstheacquisitiontimeisincreasedbecausethesearchintheFrequency-Code-Levelduringthecorrelationprocessisdisrupted.Developingquartzoscillatorswithreducedtendenciestomicrovariationscanreducethisdisturbance.

6.8.2 Antennas

Antennas can bemade tobe less sensitive to disturbances and to selectively receiveGNSS frequencies. Thedisadvantageofthisperformanceimprovementisanincreaseinsize.Thiscontradictsthegeneraltrendtowardsminiaturizationofmobilestations.

6.8.3 Noise Figure Considerations

TheNoiseFigure(NF)isameasurethatindicatestowhatextentthesignaltonoiseratioofanincomingsignalisdecreasedbytheadditionalnoiseofthereceiver.Minimizingthenoiseandmaximizingtheamplificationatthe

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firstamplificationstage(LNA),minimally improvesthereceiversensitivity.As isthecasewitheveryreceiverthefirststageamplificationdeterminesthenoisecharacteristicsfortheentirereceiver.

This isdemonstrated inthebelowequationaswellasinthesimplifiedblockdiagram(Figure74)withtheLNAandthecombinedsubsequentstages(SS):

LNA

SSLNATotal

GNFNFNF +=

NF:NoiseFigure(dB)oftheStage

G:GainoftheStage

LNA SS

NFLNA(dB)GLNA

NFSS(dB)

ReceivedGPS-Signal

OutputSignalforfurtherprocessing

Figure 74: Block Diagram of input stages

With typicalnoise figures for the firstandsubsequentamplificationstages-of20dBand1.6dB respectively,onlymarginalimprovementsarepossiblewithnewLNAdevelopments[xxvi].Furtheradvancementinthisareaappearstobeunlikely.

6.8.4 Correlators and Correlation Time

ThespectralpowerdensityofthereceivedGNSSsignalsisapprox.16dBbelowthepowerdensityofthethermalbackgroundnoise (seeFigure16).Thedemodulationandde-spreadingofthe receivedGNSSsignalscreatesasystemgainGGof43dB(seeFigure24).

Increasingthecorrelationtime(IntegrationTimeorDwell-Time)improvesthesensitivityofa*gNSSmodule.Thelongeracorrelatorremainsataspecificcode-frequencylevel,thelowertherequiredstrengthoftheGNSSsignalat theantenna. If the correlation time is increasedbya factorofk, then therewillbean increaseGR in theseparationtothethermalbackgroundnoiseof:

GR=log10(k)

Doubling the Correlation time results in an increase of the signal-background noise separation of 3 dB. Inpracticean increase inthecorrelationtimeof20msisnotaproblem.Whenthevalueofthetransmitteddatabitsisknownthistimecanbeadditionallyincreased.Otherwiseitispossiblethroughanon-coherentintegrationto increasethecorrelationtimetoover1second,however,thisprocedureresults inaone-time lossofseveraldB.

Inordertoincreasetheacquisitionsensitivitythenumberofimplementedcorrelatorsissignificantlyincreased.

ModernGNSSreceiverstypicallypossessasensitivityofapproximately–160dBm.GiventhattheGPSoperator(USDepartmentofDefense)guaranteessignalstrengthof–130dBm,GNSSreceiverscanthereforefunction inbuildingsthatweakenthesignalbyupto30dB.

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6.9 GNSS-Repeater or Reradiation Antenna

A GNSS-Repeater (also known as a Reradiation Antenna or Transceiver) receives GNSS-Signals from visiblesatellites throughanexternallysituatedantenna,amplifies thesignalsand transmits them toanother location(e.g. intoabuilding).TheyrequirenodirectconnectiontotheGNSSdevice.Thereceptionantenna is installedoutdoorsinalocationfavorableforreceivingsatellitesignals.TheGNSS-Repeater(Figure75)consistsof:

• ExternalAntenna(ReceptionAntenna)• InternalAntenna(TransmissionAntenna)• Electricaladapter• Amplifier• Cable

Figure 75: GNSS Repeater (external antenna, electrical adapter and power cord, amplifier and internal antenna)

6.10 Pseudolites for indoor applications

APseudolite(shortformforpseudosatellite)isaground-basedtransmitter,whichfunctionslikeaGNSSsatellite.Pseudolitesareoftenused inaviationtosupportaircraft landingapproaches.Thisprocedure isnotcommonlyusedforindoorapplicationsbecausethenecessarycomponentsarerelativelyexpensive.

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7 Data Formats and Hardware Interfaces

If you would like to . . .

o knowwhatNMEAandRTCMmean

o knowwhataproprietarydatasetis

o knowwhatdatasetisavailableinthecaseofallGNSSreceivers

o knowwhatanactiveantennais

o knowwhetherGNSSreceivershaveasynchronizedtimingpulse

then this chapter is for you!

7.1 Introduction

GNSS receivers require different signals in order to function (Figure 76). These variables are broadcast afterpositionandtimehavebeensuccessfullycalculated.Toensurethatthedifferenttypesofappliancesareportablethere are either international standards fordata exchange (NMEA andRTCM),or themanufacturerprovidesdefined(proprietary)formatsandprotocols.

Antenna

Power supply

DGPS signal(RTCM SC-104)

Data interface(NMEA-Format)

Data interface(Proprietary format)

Timing mark(1PPS)

GNSSreceiver

Figure 76: Block diagram of a GNSS receiver with interfaces

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7.2 Data interfaces

7.2.1 The NMEA-0183 data interface

InordertorelaycomputedGNSSvariablessuchasposition,velocity,courseetc.toaperipheral(e.g.computer,screen,transceiver),GNSSmoduleshaveaserialinterface(TTLorRS-232level).Themostimportantelementsofreceiverinformationarebroadcastviathisinterfaceinaspecialdataformat.ThisformatisstandardizedbytheNationalMarineElectronicsAssociation(NMEA)toensurethatdataexchangetakesplacewithoutanyproblems.Nowadays,dataisrelayedaccordingtotheNMEA-0183specification.NMEAhasspecifieddatasetsforvariousapplications e.g.GNSS (GlobalNavigation Satellite System),GPS, Loran,Omega, Transit and also for variousmanufacturers.The following sevendata setsarewidelyusedwithGNSSmodules to relayGNSS information[xxvii]:

1. GGA(GPSFixData,fixeddatafortheGlobalPositioningSystem)

2. GGL(GeographicPosition–Latitude/Longitude)

3. GSA(GPSDOPandActiveSatellites,degradationofaccuracyandthenumberofactivesatellitesintheGlobalSatelliteNavigationSystem)

4. GSV(GNSSSatellitesinView,satellitesinviewintheGlobalSatelliteNavigationSystem)

5. RMC(RecommendedMinimumSpecificGNSSData)

6. VTG(CourseoverGroundandGroundSpeed,horizontalcourseandhorizontalvelocity)

7. ZDA(Time&Date)

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7.2.1.1 Structure of the NMEA protocol

InthecaseofNMEA,therateatwhichdataistransmittedis4800Baudusingprintable8-bitASCIIcharacters.Transmissionbeginswithastartbit(logicalzero),followedbyeightdatabitsandastopbit(logicalone)addedattheend.Noparitybitsareused.

Data Bits

D2 D3 D4 D5 D6 D7D1

StartBit

StopBit

Data Bits

D2 D3 D4 D5 D6 D7D1

StartBit

StopBit

1 ( ca. Vcc)

0 ( ca. 0V)TTL-Level

0 ( U>0V)

1 ( U<0V)

RS-232-Level

Figure 77: NMEA format (TTL and RS-232 level)

ThedifferentlevelsmustbetakenintoconsiderationdependingonwhethertheGNSSreceiverusedhasaTTLorRS-232interface(Figure77):

• InthecaseofaTTLlevelinterface,alogicalzerocorrespondstoapprox.0Vandalogicaloneroughlytotheoperatingvoltageofthesystem(+3.3V...+5V)

• InthecaseofanRS-232interfacealogicalzerocorrespondstoapositivevoltage(+3V...+15V)andalogicaloneanegativevoltage(-3V...–15V).

If aGNSSmodulewith a TTL level interface is connected to an appliancewith an RS-232 interface, a levelconversionmustbeeffected(see7.3.4).

MostGNSSreceiversallowthebaudratetobeincreased(upto115200bitspersecond).

EachGNSSdatasetisformattedinthesamewayandhasthefollowingstructure:

$GPDTS,Inf_1,Inf_2,Inf_3,Inf_4,Inf_5,Inf_6,Inf_n*CS<CR><LF>

Table17explainsthefunctionsofindividualcharactersandcharactergroups.

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Field Description

$ Startofthedataset

GP InformationoriginatingfromaGNSSappliance

DTS Datasetidentifier(e.g.RMC)

Inf_1toInf_n Informationwithnumber1...n(e.g.175.4forcoursedata)

, Commausedasaseparatorfordifferentitemsofinformation

* Asteriskusedasaseparatorforthechecksum

CS Checksum(controlword)forcheckingtheentiredataset

<CR><LF> Endofthedataset:carriagereturn(<CR>)andlinefeed,(<LF>)

Table 17: Description of the individual NMEA DATA SET blocks

Themaximumnumberofcharactersusedmustnotexceed79.Forthepurposesofdeterminingthisnumber,thestartsign$andendsigns<CR><LF>arenotcounted.

ThefollowingNMEAprotocolwasrecordedusingaGNSSreceiver(Table18):

$GPRMC,130303.0,A,4717.115,N,00833.912,E,000.03,043.4,200601,01.3,W*7D<CR><LF>

$GPZDA,130304.2,20,06,2001,,*56<CR><LF>

$GPGGA,130304.0,4717.115,N,00833.912,E,1,08,0.94,00499,M,047,M,,*59<CR><LF>

$GPGLL,4717.115,N,00833.912,E,130304.0,A*33<CR><LF>

$GPVTG,205.5,T,206.8,M,000.04,N,000.08,K*4C<CR><LF>

$GPGSA,A,3,13,20,11,29,01,25,07,04,,,,,1.63,0.94,1.33*04<CR><LF>

$GPGSV,2,1,8,13,15,208,36,20,80,358,39,11,52,139,43,29,13,044,36*42<CR><LF>

$GPGSV,2,2,8,01,52,187,43,25,25,074,39,07,37,286,40,04,09,306,33*44<CR><LF>

$GPRMC,130304.0,A,4717.115,N,00833.912,E,000.04,205.5,200601,01.3,W*7C<CR><LF>

$GPZDA,130305.2,20,06,2001,,*57<CR><LF>

$GPGGA,130305.0,4717.115,N,00833.912,E,1,08,0.94,00499,M,047,M,,*58<CR><LF>

$GPGLL,4717.115,N,00833.912,E,130305.0,A*32<CR><LF>

$GPVTG,014.2,T,015.4,M,000.03,N,000.05,K*4F<CR><LF>

$GPGSA,A,3,13,20,11,29,01,25,07,04,,,,,1.63,0.94,1.33*04<CR><LF>

$GPGSV,2,1,8,13,15,208,36,20,80,358,39,11,52,139,43,29,13,044,36*42<CR><LF>

$GPGSV,2,2,8,01,52,187,43,25,25,074,39,07,37,286,40,04,09,306,33*44<CR><LF>

Table 18: Recording of an NMEA protocol

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7.2.1.2 GGA data set

TheGGAdataset(GPSFixData)containsinformationontime,longitudeandlatitude,thequalityofthesystem,thenumberofsatellitesusedandtheheight.

Anexampleofa*gGAdataset:

$GPGGA,130305.0,4717.115,N,00833.912,E,1,08,0.94,00499,M,047,M,,*58<CR><LF>

ThefunctionoftheindividualcharactersorcharactersetsisexplainedinTable19.

Field Description

$ Startofthedataset

GP InformationoriginatingfromaGNSSappliance

GGA Datasetidentifier

130305.0 UTCpositionaltime:13h03min05.0sec

4717.115 Latitude:47°17.115min

N Northerlylatitude(N=north,S=south)

00833.912 Latitude:8°33.912min

E Easterlylongitude(E=east,W=west)

1 GPSqualitydetails(0=noGPS,1=GPS,2=DGPS)

08 Numberofsatellitesusedinthecalculation

0.94 HorizontalDilutionofPrecision(HDOP)

00499 Antennaheightdata(geoidheight)

M Unitofheight(M=meter)

047 Heightdifferentialbetweenanellipsoidandgeoid

M Unitofdifferentialheight(M=meter)

,, AgeoftheDGPSdata(inthiscasenoDGPSisused)

0000 IdentificationoftheDGPSreferencestation

* Separatorforthechecksum

58 Checksumforverifyingtheentiredataset

<CR><LF> Endofthedataset

Table 19: Description of the individual GGA data set blocks

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7.2.1.3 GLL data set

TheGLLdataset(geographicposition–latitude/longitude)containsinformationonlatitudeandlongitude,timeandhealth.

Exampleofa*gLLdataset:

$GPGLL,4717.115,N,00833.912,E,130305.0,A*32<CR><LF>

ThefunctionoftheindividualcharactersorcharactersetsisexplainedinTable20.

Field Description

$ Startofthedataset

GP InformationoriginatingfromaGNSSappliance

GLL Datasetidentifier

4717.115 Latitude:47°17.115min

N Northerlylatitude(N=north,S=south)

00833.912 Longitude:8°33.912min

E Easterlylongitude(E=east,W=west)

130305.0 UTCpositionaltime:13h03min05.0sec

A Datasetquality:Ameansvalid(V=invalid)

* Separatorforthechecksum

32 Checksumforverifyingtheentiredataset

<CR><LF> Endofthedataset

Table 20: Description of the individual GGL data set blocks

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7.2.1.4 GSA data set

TheGSAdataset(GNSSDOPandActiveSatellites)containsinformationonthemeasuringmode(2Dor3D),thenumberof satellitesused todetermine thepositionand theaccuracyof themeasurements (DOP:DilutionofPrecision).

Exampleofa*gSAdataset:

$GPGSA,A,3,13,20,11,29,01,25,07,04,,,,,1.63,0.94,1.33*04<CR><LF>

ThefunctionoftheindividualcharactersorsetsofcharactersisdescribedinTable21.

Field Description

$ Startofthedataset

GP InformationoriginatingfromaGNSSappliance

GSA Datasetidentifier

A Calculatingmode(A=automaticselectionbetween2D/3Dmode,M=manualselectionbetween2D/3Dmode)

3 Calculatingmode(1=none,2=2D,3=3D)

13 IDnumberofthesatellitesusedtocalculateposition

20 IDnumberofthesatellitesusedtocalculateposition

11 IDnumberofthesatellitesusedtocalculateposition

29 IDnumberofthesatellitesusedtocalculateposition

01 IDnumberofthesatellitesusedtocalculateposition

25 IDnumberofthesatellitesusedtocalculateposition

07 IDnumberofthesatellitesusedtocalculateposition

04 IDnumberofthesatellitesusedtocalculateposition

,,,,, DummyforadditionalIDnumbers(currentlynotused)

1.63 PDOP(PositionDilutionofPrecision)

0.94 HDOP(HorizontalDilutionofPrecision)

1.33 VDOP(VerticalDilutionofPrecision)

* Separatorforthechecksum

04 Checksumforverifyingtheentiredataset

<CR><LF> Endofthedataset

Table 21: Description of the individual GSA data set blocks

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7.2.1.5 GSV data set

TheGSV data set (GNSS Satellites in View) contains information on the number of satellites in view, theiridentification,theirelevationandazimuth,andthesignal-to-noiseratio.

Anexampleofa*gSVdataset:

$GPGSV,2,2,8,01,52,187,43,25,25,074,39,07,37,286,40,04,09,306,33*44<CR><LF>

ThefunctionoftheindividualcharactersorcharactersetsisexplainedinTable22.

Field Description

$ Startofthedataset

GP InformationoriginatingfromaGNSSappliance

GSV Datasetidentifier

2 TotalnumberofGVSdatasetstransmitted(upto1...9)

2 CurrentnumberofthisGVSdataset(1...9)

09 Totalnumberofsatellitesinview

01 Identificationnumberofthefirstsatellite

52 Elevation(0°....90°)

187 Azimuth(0°...360°)

Signal-to-noiseratioindb-Hz(1...99,nullwhennottracking)

25 Identificationnumberofthesecondsatellite

25 Elevation(0°....90°)

074 Azimuth(0°...360°)

Signal-to-noiseratioindB-Hz(1...99,nullwhennottracking)

07 Identificationnumberofthethirdsatellite

37 Elevation(0°....90°)

286 Azimuth(0°...360°)

Signal-to-noiseratioindb-Hz(1...99,nullwhennottracking)

04 Identificationnumberofthefourthsatellite

09 Elevation(0°....90°)

306 Azimuth(0°...360°)

Signal-to-noiseratioindb-Hz(1...99,nullwhennottracking)

* Separatorforthechecksum

44 Checksumforverifyingtheentiredataset

<CR><LF> Endofthedataset

Table 22: Description of the individual GSV data set blocks

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7.2.1.6 RMC data set

TheRMCdata set (RecommendedMinimumSpecificGNSS) contains informationon time, latitude, longitudeandheight,systemstatus,speed,courseanddate.AllGNSSreceiversrelaythisdataset.

AnexampleofanRMCdataset:

$GPRMC,130304.0,A,4717.115,N,00833.912,E,000.04,205.5,200601,01.3,W*7C<CR><LF>

ThefunctionoftheindividualcharactersorcharactersetsisexplainedinTable23.

Field Description

$ Startofthedataset

GP InformationoriginatingfromaGNSSappliance

RMC Datasetidentifier

130304.0 Timeofreception(worldtimeUTC):13h03min04.0sec

A Datasetquality:Asignifiesvalid(V=invalid)

4717.115 Latitude:47°17.115min

N Northerlylatitude(N=north,S=south)

00833.912 Longitude:8°33.912min

E Easterlylongitude(E=east,W=west)

000.04 Speed:0.04knots

205.5 Course:205.5°

200601 Date:20thJune2001

01.3 Adjusteddeclination:1.3°

W Westerlydirectionofdeclination(E=east)

* Separatorforthechecksum

7C Checksumforverifyingtheentiredataset

<CR><LF> Endofthedataset

Table 23: Description of the individual RMC data set blocks

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7.2.1.7 VTG data set

TheVGTdataset(CourseoverGroundandGroundSpeed)containsinformationoncourseandspeed.

AnexampleofaVTGdataset:

$GPVTG,014.2,T,015.4,M,000.03,N,000.05,K*4F<CR><LF>

ThefunctionoftheindividualcharactersorcharactersetsisexplainedinTable24.

Field Description

$ Startofthedataset

GP InformationoriginatingfromaGNSSappliance

VTG Datasetidentifier

014.2 Course14.2°(T)withregardtothehorizontalplane

T Angularcoursedatarelativetothemap

015.4 Course15.4°(M)withregardtothehorizontalplane

M Angularcoursedatarelativetomagneticnorth

000.03 Horizontalspeed(N)

N Speedinknots

000.05 Horizontalspeed(Km/h)

K Speedinkm/h

* Separatorforthechecksum

4F Checksumforverifyingtheentiredataset

<CR><LF> Endofthedataset

Table 24: Description of the individual VTG data set blocks

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7.2.1.8 ZDA data set

TheZDAdataset(timeanddate)containsinformationonUTCtime,thedateandlocaltime.

AnexampleofaZDAdataset:

$GPZDA,130305.2,20,06,2001,,*57<CR><LF>

ThefunctionoftheindividualcharactersorcharactersetsisexplainedinTable25.

Field Description

$ Startofthedataset

GP InformationoriginatingfromaGNSSappliance

ZDA Datasetidentifier

130305.2 UTCtime:13h03min05.2sec

20 Day(00…31)

06 Month(1…12)

2001 Year

Reservedfordataonlocaltime(h),notspecifiedhere

Reservedfordataonlocaltime(min),notspecifiedhere

* Separatorforthechecksum

57 Checksumforverifyingtheentiredataset

<CR><LF> Endofthedataset

Table 25: Description of the individual ZDA data set blocks

7.2.1.9 Calculating the checksum

Thechecksumisdeterminedbyanexclusive-oroperationinvolvingall8databits(excludingstartandstopbits)fromalltransmittedcharacters,includingseparators.Theexclusive-oroperationcommencesafterthestartofthedataset($sign)andendsbeforethechecksumseparator(asterisk:*).

The 8-bit result is divided into 2 sets of 4 bits (nibbles) and each nibble is converted into the appropriatehexadecimalvalue (0 ...9,A ...F).Thechecksumconsistsofthetwohexadecimalvaluesconverted intoASCIIcharacters.

Theprincipleofchecksumcalculationcanbeexplainedwiththehelpofabriefexample:

ThefollowingNMEAdatasethasbeenreceivedandthechecksum(CS)mustbeverifiedforitscorrectness.

$GPRTE,1,1,c,0*07 (07 isthechecksum)

Procedure:

1. Onlythecharactersbetween$and*areincludedintheanalysis:GPRTE,1,1,c,0

2. These13ASCIIcharactersareconvertedinto8bitvalues(seeTable26)

3. Eachindividualbitofthe13ASCIIcharactersislinkedtoanexclusive-oroperation(N.B.Ifthenumberofonesisuneven,theexclusive-orvalueisone)

4. Theresultisdividedintotwonibbles

5. Thehexadecimalvalueofeachnibbleisdetermined

6. BothhexadecimalcharactersaretransmittedasASCIIcharacterstoformthechecksum

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Character ASCII (8 bit value)

G 0 1 0 0 0 1 1 1

P 0 1 0 1 0 0 0 0

R 0 1 0 1 0 0 1 0

T 0 1 0 1 0 1 0 0

E 0 1 0 0 0 1 0 1

, 0 0 1 0 1 1 0 0

1 0 0 1 1 0 0 0 1

, 0 0 1 0 1 1 0 0

1 0 0 1 1 0 0 0 1

, 0 0 1 0 1 1 0 0

C 0 1 1 0 0 0 1 1

, 0 0 1

Directionto

proceed

0 1 1 0 0

0 0 0 1 1 0 0 0 0

Exclusive-or value 0 0 0 0 0 1 1 1

Nibble 0000 0111

Hexadecimalvalue 0 7

ASCIICScharacters

(meetsrequirements!)

0 7

Table 26: Determining the checksum in the case of NMEA data sets

7.2.2 The DGPS correction data (RTCM SC-104)

TheRTCMSC-104 standard isused to transmit correction values.RTCMSC-104 stands for“RadioTechnicalCommissionforMaritimeServicesSpecialCommittee104“andiscurrentlyrecognizedaroundtheworldastheindustry standard [xxviii]. There are two versions of the RTCM Recommended Standards for DifferentialNAVSTARGPSService

• Version2.0(issuedinJanuary1990)

• Version2.1(issuedinJanuary1994)

Version2.1isareworkedversionof2.0andisdistinguished,inparticular,bythefactthatitprovidesadditionalinformationforrealtimenavigation(RealTimeKinematic,RTK).

Both versionsaredivided into63message types,numbers1,2,3and9beingusedprimarily for correctionsbasedoncodemeasurements.

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7.2.2.1 The RTCM message header

Eachmessage type is divided intowords of 30 bits and, in each instance, beginswith a uniform headercomprising twowords (WORD1andWORD2). From the information contained in theheader it isapparentwhichmessage type follows [xxix] andwhich reference station has determined the correction data (Figure78from[xxx]).

Figure 78: Construction of the RTCM message header

Contents Name Description

PREAMBLE Preamble Preamble

MESSAGETYPE: Messagetype Messagetypeidentifier

STATIONID ReferencestationIDNo. Referencestationidentification

PARITY Errorcorrectioncode Parity

MODIFIEDZ-COUNT ModifiedZ-count Modified Z-Count, incrementaltimecounter

SEQUENCENO. FramesequenceNo. Sequentialnumber

LENGTHOFFRAME Framelength Lengthofframe

STATIONHEALTH Referencestationhealth Technicalstatusofthereferencestation

Table 27: Contents of the RTCM message header

Thespecificdatacontentforthemessagetype(WORD3...WORDn)followstheheader,ineachcase.

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7.2.2.2 RTCM message type 1

Message type 1 transmits pseudorange correction data (PSR correction data, range correction) for allGNSSsatellites visible to the reference station, based on the most up-to-date orbital data (ephemeris). Type 1additionallycontainstherate-of-changecorrectionvalue(Figure79,extractfrom[xxxi],onlyWORD3toWORD6areshown).

Figure 79: Construction of RTCM message type 1

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Contents Name Description

SCALEFACTOR Pseudorangecorrectionvaluescalefactor PSRscalefactor

UDRE Userdifferentialrangeerrorindex Userdifferentialrangeerrorindex

SATELLITEID SatelliteIDNo. Satelliteidentification

PSEUDORANGECORRECTION

Pseudorangecorrectionvalue Effectiverangecorrection

RANGE-RATECORRECTION

Pseudorangerate-of-changecorrectionvalue Rate-of-changeofthecorrectiondata

ISSUEOFDATA DataissueNo. Issueofdata

PARITY Errorcorrectioncode Checkbits

Table 28: Contents of RTCM message type 1

7.2.2.3 RTCM message type 2 to 9

Messagetypes2to9aredistinguishedprimarilybytheirdatacontent:

• Message type 2 transmitsdeltaPSR correctiondata,basedonpreviousorbitaldata.This information isrequiredwhenever theGNSSuserhasbeenunable toupdatehissatelliteorbital information. Inmessagetype 2, the difference between correction values based on the previous and updated ephemeris istransmitted.

• Message type 3 transmitsthethreedimensionalco-ordinatesofthereferencestation.

• Message type 9relaysthesameinformationasmessagetype1,butonlyforalimitednumberofsatellites(max.3).Dataisonlytransmittedfromthosesatelliteswhosecorrectionvalueschangerapidly.

Inorderfortheretobeanoticeable improvement inaccuracyusingDGPS,thecorrectiondatarelayedshouldnotbeolderthanapprox.10to60seconds(differentvaluesaresupplieddependingontheserviceoperator,theexactvaluealsodependsontheaccuracyrequired,seealso[xxxii]).Accuracydecreasesasthedistancebetweenthereferenceanduserstation increases.TrialmeasurementsusingthecorrectionsignalsbroadcastbytheLWtransmitterinMainflingen,Germany,(seesectionA1.3)producedanerrorrateof0.5–1.5mwithinaradiusof250km,and1–3mwithinaradiusof600km[xxxiii].

7.2.3 Proprietary data interfaces

Themajorityofmanufacturersofferproprietarydata interfaces for theirGNSS receivers. Incomparison to theNMEAstandard,proprietarydatainterfaceshavethefollowingadvantages:

• Emissionofanaugmenteddatascope;e.g.informationwhichisnotsupportedbytheNMEAProtocol.

• Higherdatadensity:mostproprietaryprotocolsusebinarydataformatswithwhichnumericalandBooleaninformation can be transmitted in amore consolidated way. Data intensive notifications e.g. satelliteephemeris,canbecontained inanotification.Withhigherdatadensity,ahigheremission intervalwithaconstantdatatransmissionspeedcanbecarriedout.

• ExtensiveconfigurationpossibilitiesfortheGNSSreceiver.

• Optimal linking tomanufacturer-specificevaluation and visualization toolsenablesprecise analysisof thereceptionbehavior.

• Possibility of downloads from the current versions of the manufacturer-specific GNSS firmware. ThisfunctionisonlysupportedinGNSSreceiverswiththesuitableFlashmemory.

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• From theGNSSmanufacturer’spointof view,an improveddistributionofGNSS information todifferentdatasetswiththeobjectiveofavoidingredundancyandthetransmissionofdatawhicharenotrequiredfortheapplication.

• Verygoodintegritysecurityprovidedbychecksums.

• Minimum work for the host computer in reading and accepting the received data. The conversion ofnumericaldataintoASCIIformatinaninternalbinaryformatisnotrequired.

Threedifferenttypesofproprietarydatainterfacesaretypicallyused:

• AdditionalNMEAdatasets:theinformationiscodedintousualNMEAdataformat(textbased,separationofthe datawith commas etc.).However, immediately after the initial symbol (Dollar sign) amanufacturer-specificaddressdatafollows.ManyGNSSmanufacturersusetheadditionalnotificationstoconveyfurtherfrequently used information. The NMEA format is, however, not suitable for efficiently sending largeamountsofinformationduetoinadequatedatadensityandtheintensiveconversionofbinarydataintotextformat.

• Binaryformat(e.g.u-bloxUBX).

• Textbasedformat.

7.2.3.1 Example: UBX protocol for u-blox 5 GNSS receivers by u-blox AG

Apart fromNMEAandRTCM, theANTARIS®andu-blox5GNSS receiversbyu-blox support thebinaryUBXprotocol.AswiththeNMEAformat,aframeworkformatisgivenasfollows:

Symbol SYNCCHAR1,2

CLASS ID LENGTH PAYLOAD CHECKSUM

Explanation Synchronizationcharacter

Messageclass

Messageidentification

Length of thedatablock

Structureddatacontent

Checksum

Length(Bytes) 2 1 1 2 LENGTH 2 Checksumcoveragearea

Figure 80: Structure of the UBX data sets

Eachdatasetbeginswithtwoconstantsynchronizationcharacters (Hexadecimalvalues:alwaysB5,62).Thesecharactersareusedforrecognizingthestartofanewdataset.Thefollowingtwofields,CLASSandID,identifythedatasettype.Thistwo-tier identificationallowsacleanstructuringofthedifferentdatasetsaccordingtoclasses. The overview is obtained also after adding new data sets. Symbolic concepts, which are easy tounderstand such as “NAV-POSLLH” (CLASS 01, ID 02), are used for the documentation. Following this, thelength information and the actual data content are given. u-blox stipulates specific data types for the datacontent.Finally,eachdatasetendswitha2-bytechecksum.Adatasetisonlyvalidifthecorrectsynchronizationcharactersareavailableandthecalculatedandpredeterminedchecksumcoincide.

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Messageclass Description Content(Extract)

NAV(01) Navigationinformation Position,speed,time,DGPSandSBASinformation

RXM(02) ReceiverManagement:AmplifiedGNSSreceptiondata

GNSSrawdata,e.g.pseudo-ranges,ephemeris,yearbook,satellitestatus

CFG(06) Configurationnotifications(Configureandrequest)

Serialinterfaces,emissioninterval,receptionandnavigationparameters,energysavingmethods

ACK(05) Receptionconfirmationoftheconfigurationnotifications

Acceptedorrejected

MON(0A) OperationalstatusoftheGNSSreceiver CPUcapacityutilization,conditionoftheoperatingsystem,useofsystemresources,antennamonitoring

AID(0B) Feedingofauxiliaryinformationtoacceleratethestartup.

Ephemeris,yearbook,coldstart,lastposition,time,satellitestatus

INF(04) Issuingoftextbasedinformationnotifications

TIM(0D) Configurationtimepulseandtimemeasurementofinputsignals

UPD(09) Downloadofnewsoftware

USR(4*) Userspecificnotifications

Table 29: Message classes (Hexadecimal values in brackets)

With the aid of customer specific software additional data sets can be integrated to existing protocols oradditionaluser-specificprotocols.Furthermore,ANTARIS®and u-blox-5supportsseveralprotocolsonthesameinterface,e.g.nestedNMEAandUBXdatasetsinbothdirectionssothattheadvantagesofseveralprotocolscanbemadeuseof.

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7.3 Hardware Interfaces

7.3.1 Antennas

GNSSsignalsare right-handcircularpolarized (RHCP).Thisrequiresadifferenttypeofantenna than thewell-knownwhipantennastypicallyusedforlinearpolarizedsignals.

GNSSmodulesoperatewitheitherapassiveoractiveantenna.Anactiveantennacontainsabuilt-inLNA(LowNoiseAmplifier)preamplifier.TheGNSSreceiverprovidespowertotheactiveantennaovertheRFsignalline.Formobilenavigationpurposesacombinedantenna(e.g.GSM/FMandGNSS)issupplied.

Asmallerantennawillpresentasmalleraperturetocollectthesignalenergyfromtheskyresulting ina loweroverall gain. There is noway to get around this problem.Amplifying the signal after the antennawill notimprovethesignaltonoiseratio.

ThetwomostcommontypesofGNSSantennaavailableonthemarketarethePatchandtheHelixantenna.ThissectionwilldescribeavarietyofdifferentkindsofantennasusedinGNSStechnology.

7.3.1.1 Patch Antenna

ThemostcommonantennatypeforGNSSapplicationsisthepatchantenna.

Patchantennasareflat,generallyhaveaceramicandmetalbodyandaremountedonametalbaseplate.Theyareoftencastinahousing.

Patchantennasare idealforapplicationswheretheantenna ismountedonaflatsurface,e.g.therooforthedashboardofacar.Patchantennascandemonstrateaveryhighgain,especiallyiftheyaremountedontopofalargegroundplane(70x70mm).Ceramicpatchantennasareverypopularbecauseoflowcostsandthehugevarietyofavailablesizes.

Figure 81: Patch Antennas

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7.3.1.2 Helix Antenna

AnothertypeofantennausedinGNSSapplicationsisthehelixantenna.

Helixantennasarecylindrical inshapeandaretypicallyusedwheremultipleantennaorientationsarepossible.Theyarerobustandshowgoodnavigationperformance.

Theactualgeometricsizedependsonthedielectricthatisusedtofillthespacebetweentheactivepartsoftheantenna. If the antenna is only filled with air it needs to be comparatively large (60mm length x 45mmdiameter).Usingmaterialswithahighdielectricconstantresultsinamuchsmallerformfactor.Sizesintheorderof18mmlengthx10mmdiameterareavailableonthemarket.Thesmallerthedimensionsoftheantenna,thegreatertheinfluencetightmanufacturingtoleranceshaveonperformance.

Figure 82: Helix Antennas

7.3.1.3 Chip Antenna

Chipantennasaresmallerthanpatchorhelicalantennas.Theyofferawiderangeofsizesdownto(11.0x1.6x1.6mm).Sincecurrenttrendsareforincreasingminiaturization,theyarebecomingmorepopular.Theavailableground plane has a significant impact on their performance. Chip antennas are not recommended forapplicationswherenavigationprecisionisacorefeature.

Figure 83: Chip Antenna

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7.3.1.4 Fractal Element Antennas (FEA)

Afractalantennaisanantennathatusesaself-similardesigntomaximizethelength,orincreasetheperimeter(oninsidesectionsortheouterstructure),ofmaterialthatcanreceiveortransmitelectromagneticsignalswithinagiventotalsurfacearea.Forthisreason,fractalantennasareverycompact.

Fractalantennashavea3dBlosscomparedtohelicalorpatchantennasduetothelinearpolarization.Andtheyshowastrongdependencyonthesizeofthegroundplane.

Figure 84: Fractal Chip Antenna top and bottom view

7.3.1.5 Dipole Antenna

Dipoleantennascanbeaverycosteffectivesolution,especiallywhenprintedonPCB.Theyshowanacceptableperformanceinindoorenvironments.Thefielddoesnotdependonthegroundplane.

Dipoleantennasarelinear,notcircularpolarized.Thisresultsina3dBlossinopenspaceforGPSbuthassomeadvantageforthebacklobe,whichishelpfulforindoorreception.

Figure 85: Dipole and Printed PCB Dipole Antenna

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7.3.1.6 Loop Antenna

Loop antennas are typicallyprintedon a sticker,which can for examplebe attached to awindshield.Whenmountedthiswayloopantennasdemonstrategoodnavigationperformance.Sincethefieldisnotdependantonagroundplane,theimpedanceandcenterfrequencyarenotverysensitivetoobjectsnearthefield.

Figure 86: Loop Antenna, Laser Antenna 775

7.3.1.7 Planar Inverted F Antenna (PIFA)

ThePIFAantenna literally looks likethe letter 'F' lyingon itssidewiththetwoshortersectionsprovidingfeedandgroundpoints and the 'tail' (or toppatch)providing the radiating surface. PIFAsmakegoodembeddedantennasinthattheyexhibitasomewhatomnidirectionalpatternandcanbemadetoradiateinmorethanonefrequencyband.Theyarelinearpolarizedwithonlymoderateefficiency.PIFAareusedincellularphones(E-911)butitisnotrecommendedtousetheminapplicationswherenavigationprecisionisacorefeature.

Figure 87: Planar inverted-f-antenna (PIFA)

Figure 88: Ceramic Planar inverted-f-antenna (PIFA) and PIFA for a cellular phone

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7.3.1.8 High Precision GNSS Antennas

Forapplications requiringhighaccuracysuchassurveyingor timing,someverypreciseantennasystemsexist.Commontothesedesignsare largesize,highpowerconsumptionandhighprice.Highprecisionantennasarenotgenerallyusedformass-marketGNSSapplications.

Theseantnennadesignsarehighlyoptimized tosuppressmulti-pathsignals reflected from theground (chokeringantennas,multi-pathlimitingantennas,MLA).Anotherareaofoptimizationisaccuratedeterminationofthephasecenteroftheantenna.ForprecisionGNSSapplicationswithpositionresolutioninthemillimeterrangeitisimportantthatsignalsfromsatellitesatallelevationsvirtuallymeetatexactlythesamepointinsidetheantenna.Forthistypeofapplicationreceiverswithmultipleantennainputsareoftenrequired.

7.3.1.8.1 Choke Ring and PinwheelTM technology (Novatel) antennas

Choke Ring antennas are high performanceGPS antennas. The co-central rings around are suppressing thereflectedsignalsfromthegroundandthereforeit’smulti-pathsensitivity.

PinwheelTechnologyoffersexcellentmultipathsuppression,withthesuppressionringsbeingprintedonPCB.

Figure 89: Leica Choke Antenna AT504 and PinwheelTM Antenna (Novatel)

7.3.2 Supply

GNSSmodulesmustbepoweredfromanexternalvoltagesourceof3.3Vto6Volts. Ineachcase,thepowerdrawisverydifferent.

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7.3.3 Time pulse: 1PPS and time systems

MostGNSS receiversgeneratea timepulseevery second, referred toas1PPS (1pulseper second),which issynchronizedtoUTC.ThissignalusuallyhasaTTLlevel(Figure90).

1s±40ns ca. 200ms

Figure 90: 1PPS signal

Thetimepulsecanbeusedtosynchroniescommunicationnetworks(PrecisionTiming).

As timecanplaya fundamentalpartwhenGNSS isused todetermineaposition,adistinction isdrawnherebetweenfiveimportantGNSStimesystems:

7.3.3.1 Atomic time (TAI)

The International Atomic Time Scale (Temps Atomique International)was introduced in order to provide auniversal 'absolute' time scale thatwouldmeet various practical demands and at the same time also be ofsignificanceforGNSSpositioning.Since1967,thesecondhasbeendefinedbyanatomicconstant inphysics,thenon-radioactiveelementCaesium133Csbeingselectedasareference.Theresonantfrequencybetweentheselected energy statesof this atomhasbeendeterminedat9192631770Hz. Timedefined in thisway isthereforepartof theSIsystem (Système International).Thestartofatomic time tookplaceon01.01.1958at00.00hours.

7.3.3.2 Universal Time Coordinated (UTC)

UTC (Universal TimeCoordinated)was introduced, inorder tohave apractical time scale thatwasorientedtowardsuniversalatomictimeand,atthesametime,adjustedtouniversalcoordinatedtime.ItisdistinguishedfromTAI in theway the secondsare counted, i.e.UTC=TAI -n,wheren= complete seconds that canbealteredon1stJanuaryor1stJuneofanygivenyear(leapseconds).

7.3.3.3 GPS time

GPSsystemtimeisspecifiedbyaweeknumberandthenumberofsecondswithinthatweek.ThestartdatewasSunday,6thJanuary1980at0.00hours(UTC).EachGPSweekstartsinthenightfromSaturdaytoSunday,thecontinuoustimescalebeingsetbythemainclockattheMasterControlStation.ThetimedifferencethatarisesbetweenGPSandUTCtime isconstantlybeingcalculatedandappendedtothenavigationmessage(thesearetheleapsecondsorUTCoffset).

7.3.3.4 Satellite time

Because of constant, irregular frequency errors in the atomic clocks onboard theGNSS satellites, individualsatellitetimeisatvariancewithGPSsystemtime.Controlstationsmonitorsatelliteclocksandanyapparenttimedisparity is relayed to Earth.Any time differencesmust be taken into accountwhen conducting localGNSSmeasurements.

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7.3.3.5 Local time

Local time is the time referred towithinacertainarea.The relationshipbetween local timeandUTC time isdeterminedbythetimezoneandregulationsgoverningthechangeoverfromnormaltimetosummertime.

Exampleofatimeframe(Table30)onJune21st,2001(Zurich)

Timebasis Timedisplayed(hh:min:sec) DifferencentoUTC(sec)

Localtime 08:31:26 7200(=2h)

UTC 06:31:26 0

GPS 06:31:39 +13

TAI 06:31:58 +32

Table 30: Time systems

The interrelationship of time systems (valid for 2006):

TAI–UTC=+33sec

GPS–UTC=+14sec

TAI–GPS=+19sec

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7.3.4 Converting the TTL level to RS-232

7.3.4.1 Basics of serial communication

ThepurposeoftheRS-232interfaceismainly

• tolinkcomputerstoeachother(mostlybidirectional)

• tocontrolserialprinters

• toconnectPCstoexternalequipment,suchasGSMmodems,GNSSreceivers,etc.

Theserialports inPCsaredesigned forasynchronous transfer.Personsengaged in transmittingand receivingoperationsmustadheretoacompatibletransferprotocol, i.e.anagreementonhowdata istobetransferred.Bothpartnersmustworkwiththesameinterfaceconfiguration,andthiswillaffecttherateoftransfermeasuredinbaud.Thebaudrate isthenumberofbitspersecondtobetransferred.Typicalbaudratesare4800,9600,19200,38400,57600and115200baud, i.e.bitspersecond.Theseparametersare laiddown inthe transferprotocol. In addition, agreementmust be reached by both sides on what checks should be implementedregardingthereadytotransmitandreceivestatus.

During transmission,7 to8databitsare condensed intoadataword inorder to relay theASCII codes.Thelengthofadatawordislaiddowninthetransferprotocol.

Astartbitidentifiesthebeginningofadataword,andattheendofeveryword1or2stopbitsareappended.

Acheckcanbecarriedoutusingaparitybit.Inthecaseofevenparity,theparitybitisselectedinsuchawaythatthetotalnumberoftransferreddataword»1bits«iseven(inthecaseofunevenparitythereisanunevennumber).Checkingparityisimportant,becauseinterferenceinthelinkcancausetransmissionerrors.Evenifonebitofadatawordisaltered,theerrorcanbeidentifiedusingtheparitybit.

7.3.4.2 Determining the level and its logical allocation

DataistransmittedininvertedlogicontheTxDandRxDlines.TstandsfortransmitterandRforreceiver.

Inaccordancewithstandards,thelevelsare:

• Logical0=positivevoltage,transmitmode:+5..+15V,receivemode:+3..+15V

• Logical1=negativevoltage,transmitmode:-5..-15V,receivemode-3..-15V

Thedifferencebetween theminimumpermissible voltageduring transmission and receptionmeans that lineinterferencedoesnotaffectthefunctionoftheinterface,providedthenoiseamplitudeisbelow2V.

Converting theTTL levelof the interfacecontroller (UART,universalasynchronous receiver/ transmitter) to therequiredRS-232levelandviceversaiscarriedoutbyalevelconverter(e.g.MAX3221andmanymorebesides).The following figure (Figure91) illustrates thedifferencebetweenTTLandRS-232 levels. Level inversion canclearlybeseen.

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Data-Bits

D2 D3 D4 D5 D6 D7D1

StartBit

StopBit

Data-Bits

D2 D3 D4 D5 D6 D7D1

StartBit

StopBit

1: ( ca. Vcc)

0: ( ca. 0V)TTL-Level

0: ( U>0V)

1: ( U<0V)

RS-232-Level

Figure 91: Difference between TTL and RS-232 levels

7.3.4.3 Converting the TTL level to RS-232

ManyGNSSreceiversandGNSSmodulesonlymakeserialNMEAandproprietarydataavailableusingTTLlevels(approx.0Vorapprox.Vcc=+3.3Vor+5V).ItisnotalwayspossibletoevaluatethisdatadirectlythroughaPC,asaPCinputrequiresRS232levelvalues.

Asacircuitisneededtocarryoutthenecessaryleveladjustment,theindustryhasdevelopedintegratedcircuitsspecificallydesignedtodealwithconversionbetweenthetwolevelranges,toundertakesignalinversion,andtoaccommodate the necessary equipment to generate negative supply voltage (by means of built-in chargepumps).

Acompletebidirectional levelconverterthatusesa"MaximMAX3221" [xxxiv] is illustratedonthefollowingcircuitdiagram(Figure92).Thecircuithasanoperationalvoltageof3V...5Vand isprotectedagainstvoltagepeaks(ESD)of±15kV.ThefunctionoftheC1...C4capacitorsistoincreaseorinvertthevoltage.

Figure 92: Block diagram pin assignment of

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TTL-Level

the MAX32121 level converter

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Thefollowingtestcircuit(Figure93)clearlyillustratesthewayinwhichthemodulesfunction.Inthecaseofthisconfiguration,aTTLsignal(0V...3.3V)isappliedtolineT_IN.Theinversionandvoltageincreaseto±5VcanbeseenonlinesT_OUTandR_INoftheRS-232output.

TTL-Level

Figure 93: Functional test on the MAX32

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21 level converter

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8 GNSS RECEIVERS

If you would like to . . .

o knowhowaGNSSreceiverisconstructed

o understandwhyseveralstagesarenecessarytoreconstructGNSSsignals

o knowhowanRFstagefunctions

o knowhowthesignalprocessorfunctions

o understandhowbothstagesinteract

o knowhowareceivermodulefunctions

then this chapter is for you!

8.1 Basics of GNSS handheld receivers

AGNSSreceivercanbedividedintothefollowingmainstages(Figure61).

Antenna1575.42MHz

LNA Mixer AGC

IF filter

ADC

LocalOscillator

TimingReferenceOscillator

RF Stage

Crysta l

Correlator 1

Spreadsignal

processor(SSP)

C/A-Codegenerator

DGPS(RTCM)

Controller

Power Supply

Data

Control

Microcontroller

1 2 3 45 6 7 89 0 . +- * # =Keyboard

Lat.: 12°14'15''

Long.: 07°32'28''

Altitude: 655,00m

Display

Synchronisation

AGC Control

LNA1

RF filter

Digita l IF

Signal Processor

Time base(RTC)

Crysta l

Mem ory(RAM /ROM)

InterfaceControl

Figure 94: Simplified block diagram of a GNSS receiver

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• Antenna: The antenna receives extremelyweak satellite signals on a frequency of 1572.42MHz. Signaloutputisaround–163dBW.Some(passive)antennaehavea3dBgain.

• LNA 1:Thislownoiseamplifier(LNA)amplifiesthesignalbyapprox.15...20dB.

• RF filter: The GNSS signal bandwidth is approx. 2MHZ. The RF filter reduces the affects of signalinterference.TheRFstageandsignalprocessoractuallyrepresentthespecialcircuitsinaGNSSreceiverandareadjustedtoeachother.

• RF stage:TheamplifiedGNSSsignalismixedwiththefrequencyofthelocaloscillator.ThefilteredIFsignalismaintainedataconstantlevelinrespectofitsamplitudeanddigitalizedviaAmplitudeGainControl(AGC)

• IF filter: The intermediate frequency is filtered out using a bandwidth of several MHz. The imagefrequenciesarisingatthemixingstagearereducedtoapermissiblelevel.

• Signal processor:Up to16different satellite signals canbe correlated anddecoded at the same time.Correlation takesplacebyconstantcomparisonwiththeC/Acode.TheRFstageandsignalprocessoraresimultaneously switched to synchronizewith the signal.The signalprocessorhas itsown timebase (RealTime Clock, RTC). All the data ascertained is broadcast (particularly signal transit time to the relevantsatellitesdeterminedbythecorrelator),and this is referredtoassourcedata.Thesignalprocessorcanbeprogrammedbythecontrollerviathecontrollinetofunctioninvariousoperatingmodes.

• Controller: Usingthesourcedata,thecontrollercalculatesposition,time,speedandcourseetc.Itcontrolsthe signal processor and relays the calculated values to the display. Important information (such asephemeris,themostrecentpositionetc.)aredecodedandsavedinRAM.TheprogramandthecalculationalgorithmsaresavedinROM.

• Keyboard:Usingthekeyboard,theusercanselect,whichco-ordinatesystemhewishestouseandwhichparameters(e.g.numberofvisiblesatellites)shouldbedisplayed.

• Display:Thepositioncalculated (longitude, latitudeandheight)mustbemadeavailable to theuser.Thiscan either be displayed using a 7-segment display or shown on a screen using a projectedmap. Thepositionsdeterminedcanbesaved,wholeroutesbeingrecorded.

• Power supply: Thepowersupplydeliversthenecessaryoperationalvoltage toallelectronic components.

8.2 GNSS Receiver Modules

8.2.1 Basic design of a GNSS module

GNSSmodules have to evaluateweak antenna signals from at least four satellites, in order to determine acorrect three-dimensional position.A time signal is also often emitted in addition to longitude, latitude andheight.This timesignal issynchronizedwithUTC (UniversalTimeCoordinated).From thepositiondeterminedand the exact time,additionalphysical variables, such as speedand acceleration canalsobe calculated.TheGNSSmoduleissuesinformationontheconstellation,satellitehealth,andthenumberofvisiblesatellitesetc.

Figure95showsatypicalblockdiagramofa*gNSSmodule.

Thesignalsreceived(1575.42MHz)arepre-amplifiedandtransformedtoa lower intermediatefrequency.Thereferenceoscillatorprovidesthenecessarycarrierwaveforfrequencyconversion,alongwiththenecessaryclockfrequency for the processor and correlator. The analogue intermediate frequency is converted into a digitalsignalbymeansofanADC.

SignaltraveltimefromthesatellitestotheGNSSreceiverisdeterminedbycorrelatingPRNpulsesequences.ThesatellitePRNsequencemustbeusedtoestablishthistime,otherwisethere isnocorrelationmaximum.Data isrecoveredbymixingitwiththecorrectPRNsequence.Atthesametime,theusefulsignalisamplifiedabovetheinterference level[xxxv].Upto16satellitesignalsareprocessedsimultaneously.Asignalprocessorcarriesoutthe controlandgenerationofPRN sequencesand the recoveryofdata.Calculatingand saving theposition,includingthevariablesderivedfromthis,iscarriedoutbyaprocessorwithamemoryfacility.

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Passiveantenna

Activeantenna

LNA Signal

Supply

ReferenceOscillator Processor

RAM

ROM

Interface

NMEA Proprietary

DGPS InputRTCM

CorrelatorsSignal processorPRN generator

Time mark1 PPS

RF amplifierMixerA/D converter

Power supply(3,3V ... 5V)

Figure 95: Typical block diagram of a GNSS module

8.2.2 Example: u-blox 5

u-blox 5chipshavebeenspecificallydesignedforapplicationswithtightcost,sizeandpowerconstraintsthatrequire ultra-fast acquisition and high-precision tracking. The highly integrated architecture brings fullpositioningfunctionality,fromantenna inputtopositiondataoutput, inaself-containedsolutionthatrequiresfewexternalcomponents.Moreover, innovativepowerhardwareand software featuresenable theengine tooperateonaslittleas50mW.Thisensureslongbatterylifetimes,acriticalfeatureforportableapplications.

u-blox 5chipscomputepositions instantlyandaccurately.Adedicatedacquisitionenginewithover1millioneffectivecorrelatorsiscapableofmassiveparallelsearchesacrossthetime/frequencyspace.Thismakessatelliteacquisitionpossibleinlessthan1secondwhilelongintegrationtimesenablea–160dBmacquisitionsensitivity.Acquired satellites are then passed on to a tracking engine. This setup allows for the GNSS engine tosimultaneouslytrackupto16satellitesandsearchfornewones.

Theon-chipPowerManagementUnit(PMU)enablesasinglesupplyvoltagesourceandfeaturesaswitch-modeDC/DCconverterthatoptimizespowerefficiencyandextendsthesupplyvoltagerange.AllrequiredcoreandI/OvoltagesaregeneratedinternallybymeansofLDOs(Low-Drop-Out).

WhenGALILEO-L1signalsbecomeavailable,u-blox 5receiverswillbecapableofreceivingandprocessingthemviaa simpleupgrade.Theability to receiveand trackGALILEO satellite signalswill result inhigher coverage,improved reliability and better accuracy. The chip’s advanced jamming suppressionmechanism automaticallyfilterssignalsfrominterferingsources,thusmaintaininghighGNSSperformance.

Theu-blox5singlechipconsistsoftwoICsassembledintoasinglepackage,oftenreferredtoas'SiP'or’Systemin Package’. This enables the independent selection of the optimal technology for the RF-IC and for thebaseband-IC. The RF-IC is diffused on 0.18 µm RF-CMOS technologywhile the baseband-IC is on 0.13 µmCMOS.Alternatively,thetwo ICscanbeassembled intotwoseparatepackages.Thischipsetsolutionprovidesanexternalbusinterfacetoconnectanexternalmemory.ForasimplifiedblockdiagramseeFigure96.

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Front-End withIntegrated LNA

BasebandProcessor

Crystalor TCXO RTC

UBX-G5010

SAWFilter

Clock

PowerControl

Figure 96: Block diagram of u-blox 5 chipset

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9 GNSS Applications

If you would like to . . .

o knowwhatvariablescanbedeterminedusingGNSS

o knowwhatapplicationsarepossiblewithGNSS

o knowhowtimeispreciselydetermined

then this chapter is for you!

9.1 Introduction

UsingGNSSthefollowingtwovaluescanbedeterminedanywhereonEarth:

• Exactposition (longitude, latitudeandheightco-ordinates)accuratetowithinarangeof20mtoapprox.1mm

• Precisetime(UniversalTimeCoordinated,UTC)accuratetowithinarangeof60nstoapprox.1ns.

InAddition,othervaluescanalsobedetermined,suchas:

• speed

• acceleration

• course

• localtime

• rangemeasurements

The established fields forGNSS usage are surveying, shipping and aviation. However, satellite navigation iscurrentlyenjoyingasurgeindemandforLocationBasedServices(LBS)andsystemsfortheautomobileindustry.Applications,suchasAutomaticVehicleLocation(AVL)andthemanagementofvehiclefleetsalsoappeartobeon the rise. In addition,GNSS is increasinglybeingutilized in communications technology. For example, thepreciseGNSStimesignalisusedtosynchronizetelecommunicationsnetworksaroundtheworld.Since2001,theUSFederalCommunicationsCommission (FCC)has required that,whenAmericanscall911 inanemergency,theirpositionbeautomaticallydeterminedtowithinapprox.125m.This law,knownasE-911(Enhanced911),necessitatesthatmobiletelephonesbeupgradedwiththisnewtechnology.

In the leisure industry,GNSS is becoming increasinglywidespread and important.Whether hiking, hunting,mountain biking, or windsurfing across Lake Constance in Southern Germany, a GNSS receiver providesinvaluableinformationforagreatvarietyofsituations.

GNSScanessentiallybeusedanywhereonEarthwheresatellitesignalreception ispossibleandknowledgeofpositionisofbenefit.

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9.2 Description of the various applications

GNSS aided navigation and positioning is used in many sectors of the economy, as well as in science,technology,tourism,researchandsurveying.GNSScanbeutilizedwhereverprecisethree-dimensionalpositionaldatahasasignificantroletoplay.Afewimportantsectorsaredetailedbelow.

9.2.1 Location Based Services (LBS)

LocationBasedServices(LBS)areservicesbasedonthecurrentpositionofauser(e.g.MobileCommunicationsNetworkusersequippedwithacell-phone).Normallythemobilestation(e.g.cell-phone)mustbeloggedonanditspositiongiveninordertorequestorobtainspecificinformation/servicesfromtheprovider.Anexampleofthisisthedistributionoflocalinformation,suchasthelocationofthenearestrestaurantorautomaticallyprovidingthecallerpositiontoemergencynumberservices(E-911orE-112).

Theprerequisite for LBS is thedeterminationof accurateposition information. Location isdetermined eitherthroughsignalsfromthecell-phonenetworkorthroughusingsatellitesignals.

Thelocationoftheuseriseithergivenwithabsolutegeographiccoordinates(longitudeandlatitude)orrelativeto the position of a given reference point (e.g. “the user is located within a radius of 500m to themonument…”).Therearebasicallytwokindsofservicesprovided,knownas“pushservices”or“pullservices”.Apushservicesendstheuser informationonthebasisofhisorherpositionwithouttheirhavingtorequest it(e.g.“Inthevicinity is…”).Apullservicerequiresthattheuserfirstrequestthe informationfromtheservice(e.g.callinganemergencynumberE-911orE-112).

Knowinglocationisofcriticalimportanceforsurvivingemergencies.However,publicsecurityandrescueserviceshave shown in a study that 60% of thosemaking emergency callswithmobile telephoneswere unable tocommunicatetheirexactposition(incomparisonto2%ofcallersfromfixed-nettelephones).EveryyearwithintheEuropeanUnionthereare80millionemergencycallsmade,ofthese50%aremadewithmobiletelephones.

The determination of the user’s position can either be obtainedwithin themobile station or by themobilenetwork.Fordeterminingthepositionthemobilestationreferstoinformationfromthemobilecommunicationnetworkorsatellitesignals.

Countlesstechnologiesforpositioninghavealreadybeenintroducedandhavebeenstandardized.Fewoftheseare currentlybeingusedand it remains tobe seen ifall the ideaswilleverbe realized. InEurope, themostcommonapplicationscurrentlybeingusedare:

• Positiondeterminationthroughtheidentificationofactivecellsinthecell-phonenetwork(Cell-ID).ThisprocedureisalsoknownasCellofOrigin(COO)orCellGlobalIdentity(CGI).

• PositiondeterminationbythetimedelayofGSM-SignalsTA(TimingAdvance).TAisaparameterinGSM-Networksthroughwhichthedistancetothebasestationcanbedetermined.

• SatellitePositioningthroughSatelliteNavigation:e.g.GNSS

9.2.2 Commerce and Industry

Forthetimebeing,roadtransportationcontinuestobethebiggestmarketforGNSS.Ofatotalmarketvalueestimated at60billionUS-$ in2005,21.6billion alonewas accounted forby road transportation and10.6billionbytelecommunicationstechnology[xxxvi].Vehicleswillbeequippedwithacomputerandascreen,sothatasuitablemapshowingpositioncanbedisplayedatalltimes.Thiswillenableselectingthebestroutetothedestination.Duringtraffic jamsalternativeroutescanbeeasilydeterminedandthecomputerwillcalculatethejourneytimeandtheamountoffuelneededtogetthere.

Vehiclenavigationsystemswilldirectthedrivertohisorherdestinationwithvisualandaudibledirectionsandrecommendations.Using thenecessarymaps storedonCD-ROMandpositionestimatesbasedonGNSS, thesystemwilldeterminethemostfavorableroutes.

GNSS isalreadyusedasamatterofcourse inconventionalnavigation(aviationandshipping).ManytrainsareequippedwithGNSSreceiversthatrelaythetrain’spositiontostationsdowntheline.Thisenablespersonneltoinformpassengersofthearrivaltimeofatrain.

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GNSScanbeusedforlocatingvehiclesorasananti-theftdevice.Armoredcars,limousinesandtruckscarryingvaluable or hazardous cargowill be fittedwithGNSS.An alarmwill automatically be set off if the vehicledeviatesfrom itsprescribedroute.Withthepressofabuttonthedrivercanalsooperatethealarm.Anti-theftdeviceswillbeequippedwithGNSSreceivers,allowingthevehicletobeelectronically immobilizedassoonasmonitoringcentersreceiveasignal.

GNSS can assist in emergency calls. This concept has already been developed to the marketing level. Anautomobile is equippedwith an onboardGNSS receiver connected to a crash detector. In the event of anaccident thissignalsanemergencycallcenterprovidingprecise informationaboutwhichdirection thevehiclewastravelinganditscurrentlocation.Asaresult,theconsequencesofanaccidentcanbemadelesssevereandotherdriverscanbegivenadvancedwarning.

Railways are other highly critical transportation applications,where human life is dependent on technologyfunctioningcorrectly.Precautionsneedtobetakenhereagainstsystemfailure.Thisistypicallyachievedthroughbackupsystems,wherethesametask isperformed inparallelbyredundantequipment.During idealoperatingsituations, independent sourcesprovide identical information.Well-devised systems indicate (in addition to astandardwarningmessage)iftheavailabledataisinsufficientlyreliable.Ifthisisthecase,thesystemcanswitchtoanothersensorasitsprimarydatasource,providingself-monitoringandcorrection.GNSScanprovideavitalrolehereinimprovingsystemreliabilityandsafety.

OtherpossibleusesforGNSSinclude:

• Navigationsystems

• Fleetmanagement

• Geographicaltachographs

• Railways

• Transportcompanies,logisticsingeneral(aircraft,water-bornecraftandroadvehicles)

• Automaticcontainermovements

• Extensivestoragesites

• Layingpipelines(geodesyingeneral)

• Positioningofdrillplatforms

• Developmentofopen-pitmining

• Reclamationoflandfillsites

• Explorationofgeologicaldeposits

9.2.3 Communications Technology

Synchronizingcomputerclocksisvitalinsituationswithseparatedprocessors.ThefoundationofthisisahighlyaccuratereferenceclockusedtoreceiveGNSSsatellitesignalsalongwithNetworkTimeProtocol(NTP),specifiedinRFC1305.

OtherpossibleusesforGNSSinclude:

• Synchronizationofsystemtime-staggeredmessagetransfer

• Synchronizationincommonfrequencyradionetworks

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9.2.4 Agriculture and Forestry

GNSScontributestoprecisionfarming intheformofareaandusemanagement,andthemappingofsites interms of yield potential. In a precision farming system, combined harvest yields are recorded byGNSS andprocessedinitiallyintospecificplotsondigitalmaps.SoilsamplesarelocatedwiththehelpofGNSSandthedataaddedtothesystem.Analysisoftheseentriesthenservestoestablishtheamountoffertilizerthatneedstobeappliedtoeachpoint.Theapplicationmapsareconvertedintoaformthatonboardcomputerscanprocessandaretransferredtothesecomputerusingmemorycards.Inthisway,optimalpracticescanbedevisedoveralongtermthatcanprovidehightime/resourcesavingsandenvironmentalconservation.

OtherpossibleusesforGNSSinclude:

• Useandplanningofareas

• Monitoringoffallowland

• Planningandmanagingofcroprotation

• Useofharvestingequipment

• Seedingandspreadingfertilizer

• Optimizingloggingoperations

• Pestmanagement

• Mappingdiseasedandinfestedareas

For the forest industry as well, there are many conceivable GNSS applications. The USDA (United StatesDepartment of Agriculture) Forest Service GPS Steering Committee 1992, has identified over 130 possibleapplicationsinthisfield.

Examplesofsometheseapplicationsarebrieflydetailedbelow:

• Optimizing logtransportation:ByequippingcommercialvehiclefleetswithonboardcomputersandGNSS,andusingremotedatatransferfacilities,transportvehiclescanbeefficientlydirectedfromcentraloperationsunits.

• InventoryManagement:Manual identification prior to timber harvesting ismade redundant by satellitenavigation.Fortheworkersonsite,GNSScanbeusedasatoolforcarryingoutspecificinstructions.

• SoilConservation:ByusingGNSS,remoteroadsandtracksused inharvestingwoodcanbe identifiedandtheirfrequencyofuseestablished.

• Managementofprivatewoodlots: Inwoodedareasdividedup intosmallparcels,cost-effectiveandhighlymechanized harvesting processes can be employed using GNSS, allowing the transport of increasedquantitiesoftimber.

9.2.5 Science and Research

With the advent of the use of aerial and satellite imaging in archaeology, GNSS has also become firmlyestablished in this field. By combining GIS (Geographic Information Systems) with satellite and aerialphotography,aswellasGNSSand3Dmodeling,ithasbeenpossibletoanswersomeofthefollowingquestions:

• Whatconclusionsregardingthedistributionofculturescanbemadebasedonthelocationofthefinds?

• Isthereacorrelationbetweenareasfavoringthegrowthofcertainarableplantsandthespreadofcertaincultures?

• Whatdidthelandscapelooklikeinthisvicinity2000yearsago?

Surveyorsuse(D)GPS,inordertocarryoutsurveys(satellitegeodesy)quicklyandefficientlytowithinanaccuracyofamillimeter.Forsurveyors,theintroductionofsatellite-basedsurveyingrepresentsaprogresscomparabletothatbetween theabacusand thecomputer.Theapplicationsareendless.Theserangefrom land registryand

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property surveys to surveying roads, railway lines, rivers and the ocean depths. Geological variations anddeformationscanbemeasuredandlandslidesandotherpotentialcatastrophescanbemonitored,etc.

In land surveying, GNSS has virtually become the exclusive method for pinpointing sites in basic grids.Everywherearoundtheworld,continentalandnationalGNSSnetworksaredevelopingthat,inconjunctionwiththe global ITRF, provide consistent and highly accurate networks of points for density and point-to-pointmeasurements. At a regional level, the number of tenders to set up GNSS networks as a basis for geo-informationsystemsandcadastrallandsurveysisgrowing.

GNSS alreadyhas an establishedplace inphotogrammetry.Apart fromdetermining co-ordinates forgroundreferencepoints,GNSSisregularlyusedtodetermineaerialsurveynavigationandcameraco-ordinatesforaero-triangulation. Using this method, over 90% of ground reference points can be dispensed with. FuturereconnaissancesatelliteswillbeequippedwithGNSSreceiverstoaidtheevaluationofdataforproducingandupdatingmapsinunderdevelopedcountries.

In hydrography, GNSS can be used to determine the exact height of a survey boat. This can simplify theestablishmentofclearlydefined referencepoints.Theexpectation is thatusableGNSSprocedures in this fieldwillbeoperationalinthenearfuture.

OtherpossibleareasofapplicationforGNSSare:

• Archaeology

• Seismology(geophysics)

• Glaciology(geophysics)

• Geology(mapping)

• Surveyingdeposits(mineralogy,geology)

• Physics(flowmeasurements,timestandardizationmeasurement)

• Scientificexpeditions

• Engineeringsciences(e.g.shipbuilding,generalconstructionindustry)

• Cartography

• Geography

• Geo-informationtechnology

• Forestryandagriculturalsciences

• Landscapeecology

• Geodesy

• Aerospacesciences

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9.2.6 Tourism / Sport

InsailplaneandhangglidercompetitionsGNSSreceiversareoftenused tomaintainprotocolswithno riskofbribery.

GNSS can be used to locate personswho have found themselves in amaritime or alpine emergency. (SAR:SearchandRescue)

OtherpossibleusesforGNSSinclude:

• Routeplanningand selectingpointsofparticular significance (naturaland culturally/historically significantmonuments)

• Orienteering(trainingroutes)

• Outdooractivitiesandtrekking

• Sportingactivities

9.2.7 Military

GNSS is used anywherewhere combatants, vehicles, aircraft and guidedmissiles are deployed in unfamiliarterrain.GNSS isalso suitable formarking thepositionofminefieldsandundergrounddepots,as itenablesalocationtobedeterminedandfoundagainwithoutanygreatdifficulty.Asarule,themoreaccurate,encryptedGNSSsignal(PPS)isusedformilitaryapplications,andcanonlybeusedbyauthorizedagencies.

9.2.8 Time Measurement

GNSS provides the opportunity to exactlymeasure time on a global basis. Around theworld “time” (UTCUniversalTimeCoordinated)canbeaccuratelydeterminedtowithin1 ...60ns.MeasuringtimewithGNSS ismuchmoreaccurate thanwithso-called radioclocks,whichareunable tocompensate forsignal travel timesbetween the transmitter and the receiver. If, for example, the receiver is 300 km from the radio clocktransmitter, the signal travel time already accounts for 1ms,which is 10,000 times less accurate than timemeasuredbyaGNSSreceiver.Globallyprecisetimemeasurementsarenecessaryforsynchronizingcontrolandcommunicationsfacilities,forexample.

Currently,themostcommonmethodformakingprecisiontimecomparisonsbetweenclocksindifferentplacesis a “common-view“ comparison with the help of GNSS satellites. Institutes that wish to compare clocksmeasurethesameGNSSsatellitesignalsatthesametimeandcalculatethetimedifferencebetweenthe localclocksandGNSSsystemtime.Asaresultofthedifferencesinmeasurement,thedifferencebetweentheclocksat the two institutes can be determined. Because this involves a differential process, GNSS clock status isirrelevant.TimecomparisonsbetweenthePTBandtime institutesaremade inthiswaythroughouttheworld.ThePTBatomicclockstatus,determinedwiththehelpofGNSS, isalsorelayedtothe InternationalBureauforWeightsandMeasures(BIPM)inParisforcalculatingtheinternationalatomictimescalesTAIandUTC.

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A Resources in the World Wide Web If you would like to...

o know,whereyoucangetmoreinformationaboutGNSS

o know,wheretheGPSsystemisdocumented

o becomeaGNSSexpert

then this chapter is for you!

A.1 Summary reports and links GlobalPositioningSystemOverviewbyPeterH.Dana,UniversityofColorado

http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

GlobalPositioningSystem(GPS)ResourcesbySamWormley,

http://www.edu-observatory.org/gps/gps.html

NMEA-0183andGPSInformationbyPeterBennett,

http://vancouver-webpages.com/peter/

JoeMehaffey,YeazelandDaleDePriest’sGPSInformation

http://gpsinformation.net

TheGlobalPositioningSystems(GPS)ResourceLibrary

http://www.gpsy.com/gpsinfo/

GPSSPSSignalSpecification,2ndEdition(June2,1995),USCGNavigationCenter

http://www.navcen.uscg.gov/pubs/gps/sigspec/default.htm

A.2 Differential GPS DifferentialGPS(DGPS)bySamWormley,

http://www.edu-observatory.org/gps/dgps.html

DGPScorrectionsovertheInternet

http://www.wsrcc.com/wolfgang/gps/dgps-ip.html

EGNOSOperationsManager

http://www.essp.be/

WideAreaDifferentialGPS(WADGPS),StanfordUniversity

http://waas.stanford.edu/

A.3 GPS institutes InstituteforappliedGeodesy:GPSinformationandobservingsystem

http://gibs.leipzig.ifa*g.de/cgi-bin/Info_hom.cgi?de

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http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

http://www.edu-observatory.org/gps/gps.html

http://vancouver-webpages.com/peter/

http://gpsinformation.net/

http://www.gpsy.com/gpsinfo/

http://www.navcen.uscg.gov/pubs/gps/sigspec/default.htm

http://www.edu-observatory.org/gps/dgps.html

http://www.wsrcc.com/wolfgang/gps/dgps-ip.html

http://www.essp.be/

http://waas.stanford.edu/

http://gibs.leipzig.ifa*g.de/cgi-bin/Info_hom.cgi?de

your position is our focus

GPSPRIMER:AerospaceCorporation

http://www.aero.org/publications/GPSPRIMER/index.html

U.S.CoastGuard(USCG)NavigationCenter

http://www.navcen.uscg.gov/

U.S.NavalObservatory

http://tycho.usno.navy.mil/gps.html

RoyalInstituteofNavigation,London

http://www.rin.org.uk/

TheInstituteofNavigation

http://www.ion.org/

UniversityNAVSTARConsortium(UNAVCO)

http://www.unavco.org

A.4 GNSS antennas REELReinheimerElectronicLtd.

http://www.reinheimer-elektronik.de/

WISI,WILHELMSIHNJR.KG

http://www.wisi.de/

Matsush*taElectricWorks(Europe)AG

http://www.mew-europe.com/gps/en/news.html

KyoceraIndustrialCeramicCorporation

http://www.kyocera.com/kicc/industrial/products/dielectric.htm

M/A-COM

http://www.macom.com/

EMTACTechnologyCorp.

http://www.emtac.com.tw/

AllisCommunicationsCompany,Ltd.

http://www.alliscom.com.tw/

A.5 GNSS newsgroup and GNSS technical journal Newsgroup:sci.geo.satellite-nav

http://groups.google.com/groups?oi=djq&as_ugroup=sci.geo.satellite-nav

Technicaljournal:GPSWorld(appearsmonthly)

http://www.gpsworld.com

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http://www.aero.org/publications/GPSPRIMER/index.html

http://www.navcen.uscg.gov/

http://tycho.usno.navy.mil/gps.html

http://www.rin.org.uk/

http://www.ion.org/

http://www.unavco.org/

http://www.wisi.de/

http://www.mew-europe.com/gps/en/news.html

http://www.kyocera.com/kicc/industrial/products/dielectric.htm

http://www.macom.com/

http://www.emtac.com.tw/

http://www.alliscom.com.tw/

http://groups.google.com/groups?oi=djq&as_ugroup=sci.geo.satellite-nav

http://www.gpsworld.com/

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B Index

B.1 List of Figures Figure1:Thebasicfunctionofsatellitenavigation........................................................................................................................................10 Figure2:LaunchofGPSSatellite .................................................................................................................................................................11 Figure3:Determiningthedistanceofalightningflash .................................................................................................................................12 Figure4:InthesimplestcaseDistanceisdeterminedbymeasuringtheTravelTime......................................................................................13 Figure5:WithtwotransmittersitispossibletocalculatetheexactpositiondespiteTimeErrors. .................................................................14 Figure6:FoursatellitesareneededtodetermineLongitude,Latitude,AltitudeandTime .............................................................................14 Figure7:Determiningthesignaltraveltime..................................................................................................................................................15 Figure8:Thepositionofthereceiverattheintersectionofthetwocircles ...................................................................................................16 Figure9:Thepositionisdeterminedatthepointwhereallthreespheresintersect .......................................................................................16 Figure10:Foursatellitesarerequiredtodetermineapositionin3-Dspace. .................................................................................................17 Figure11:ThethreeGNSSsegments............................................................................................................................................................19 Figure12:GPSsatellitesorbittheEarthon6orbitalplanes ..........................................................................................................................20 Figure13:24hourtrackingofa*gPSsatellitewithitseffectiverange ...........................................................................................................20 Figure14:PositionoftheGPSsatellitesat12:00hrsUTCon14thApril2001 .............................................................................................21 Figure15:AGPSsatellite .............................................................................................................................................................................22 Figure16:SpectralPowerDensityofreceivedsignalandthermalnoise ........................................................................................................23 Figure17:PseudoRandomNoise .................................................................................................................................................................24 Figure18:Simplifiedsatelliteblockdiagram .................................................................................................................................................25 Figure19:Datastructureofa*gPSsatellite ...................................................................................................................................................25 Figure20:Detailedblockdiagramofa*gPSsatellite .....................................................................................................................................26 Figure21:Measuringsignaltraveltime ........................................................................................................................................................27 Figure22:Demonstrationofthecorrectionprocessacross30bits ................................................................................................................28 Figure23:SearchforthemaximumcorrelationintheCodefrequencylevel .................................................................................................29 Figure24:SpectralPowerDensityofthecorrelatedsignalandThermalSignalNoise ....................................................................................29 Figure25:Structureoftheentirenavigationmessage ..................................................................................................................................31 Figure26:Ephemeristerms ..........................................................................................................................................................................33 Figure27:WithBPSKtheNavigationDataSignalisfirstspreadbyacode ....................................................................................................34 Figure28:ModulationfortheFuture:BOC(10,5) .........................................................................................................................................35 Figure29:WithBPSK(1)andBOC(1,1)thesignalmaximaareseparated(signalstrengthnormalizedat1Wpersignal) ...............................35 Figure30:WithModernizationtheavailabilityofGPSfrequencieswillbeincreased .....................................................................................36 Figure31:GLONASS-MSatellite(SourceESA)..............................................................................................................................................38 Figure33:UnlikeSARSAT-COSPAS,GALILEO'sSearchAndRescueservicealsoprovidesareplytothedistresssignal ...................................41 Figure34:ConstellationoftheGALILEOsatellites(picture:ESA-J.Huart) .......................................................................................................43 Figure35:GALILEOsatellite(Picture:ESA-J.Huart) ........................................................................................................................................43 Figure36:Ariane5Rocketdelivering8GALILEOsatellitesintospace(GALILEO-industries.net) .....................................................................44 Figure37:FrequencyPlanforGALILEO.........................................................................................................................................................45 Figure38:TheL1bandwillbeintensivelyusedbyGALILEOandGPS(PowerDensitystandardizedat1Wpersignal)...................................45 Figure39:GIOVE-AanditslaunchonDecember28,2005(PictureESA) .......................................................................................................46

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Figure40:Foursatellitesignalsmustbereceived..........................................................................................................................................48 Figure41:Three-dimensionalco-ordinatesystem .........................................................................................................................................49 Figure42:ConversionoftheTaylorseries.....................................................................................................................................................50 Figure43:Estimatingaposition....................................................................................................................................................................51 Figure44:SatellitegeometryandPDOP.........................................................................................................................................................54 Figure45:EffectofthesatelliteconstellationontheDOPvalue....................................................................................................................54 Figure46:GDOPvalueandthequantityofvisiblesatellitesaccordingtothetime ........................................................................................55 Figure47:HDOPvalueovera24hperiod,withoutshadowing(max.valueis1.9).........................................................................................55 Figure48:HDOPvalueovera24hperiod,withshadowing(max.valueisgreaterthan20)...........................................................................56 Figure49:AgeoidisanapproximationoftheEarth’ssurface.......................................................................................................................58 Figure50:Producingaspheroid ...................................................................................................................................................................58 Figure51:Customizedlocalreferenceellipsoid ............................................................................................................................................59 Figure52:Differencebetweengeoidandellipsoid .......................................................................................................................................59 Figure53:IllustrationoftheCartesianco-ordinates ......................................................................................................................................60 Figure54:Illustrationoftheellipsoidalco-ordinates ....................................................................................................................................61 Figure55:Geodeticdatum...........................................................................................................................................................................62 Figure56:Gauss-Krügerprojection ..............................................................................................................................................................64 Figure57:Principleofprojectingonezone(ofsixty) .....................................................................................................................................65 Figure58:DesignationofthezonesusingUTM,withexamples....................................................................................................................65 Figure59:Theprincipleofdoubleprojection................................................................................................................................................66 Figure60:Fromsatellitetoposition..............................................................................................................................................................66 Figure61:Effectofthetimeofmeasuringonthereflections........................................................................................................................70 Figure62:PrincipleofDGPSwithaGPSbasestation....................................................................................................................................71 Figure63:Determinationofthecorrectionfactors .......................................................................................................................................72 Figure64:Transmissionofthecorrectionfactors ..........................................................................................................................................72 Figure65:Correctionofthemeasuredpseudoranges...................................................................................................................................73 Figure66:Principleofthephasemeasurement ...........................................................................................................................................73 Figure67:ComparisonofDGPSsystemsbasedonRTCMandRTCAstandards.............................................................................................76 Figure68:PositionandprovisionofWAAS,EGNOS,GAGANandMSAS ......................................................................................................79 Figure69:PrincipleofallSatelliteBasedAugmentationSystemsSBAS..........................................................................................................80 Figure70:LandStar-DGPSandOmnistarilluminationzone ...........................................................................................................................81 Figure71:AccelerationofthesearchprocedurewithA-GPSbyreducingthesearcharea.............................................................................83 Figure72:Assisted-GPSsystem ....................................................................................................................................................................84 Figure73:IGSreferencestations(asofOctober2006)withapprox.340activestations ...............................................................................85 Figure74:BlockDiagramofinputstages .....................................................................................................................................................86 Figure75:GNSSRepeater(externalantenna,electricaladapterandpowercord,amplifierandinternalantenna) .........................................87 Figure76:Blockdiagramofa*gNSSreceiverwithinterfaces.........................................................................................................................88 Figure77:NMEAformat(TTLandRS-232level) ...........................................................................................................................................90 Figure78:ConstructionoftheRTCMmessageheader ...............................................................................................................................100 Figure79:ConstructionofRTCMmessagetype1 ......................................................................................................................................101 Figure80:StructureoftheUBXdatasets ...................................................................................................................................................103 Figure81:PatchAntennas..........................................................................................................................................................................105 Figure82:HelixAntennas...........................................................................................................................................................................106

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Figure83:ChipAntenna ............................................................................................................................................................................106 Figure84:FractalChipAntennatopandbottomview................................................................................................................................107 Figure85:DipoleandPrintedPCBDipoleAntenna.....................................................................................................................................107 Figure86:LoopAntenna,LaserAntenna775.............................................................................................................................................108 Figure87:Planarinverted-f-antenna(PIFA) .................................................................................................................................................108 Figure88:CeramicPlanarinverted-f-antenna(PIFA)andPIFAforacellularphone ......................................................................................108 Figure89:LeicaChokeAntennaAT504andPinwheelTMAntenna(Novatel)................................................................................................109 Figure90:1PPSsignal ................................................................................................................................................................................110 Figure91:DifferencebetweenTTLandRS-232levels .................................................................................................................................113 Figure92:BlockdiagrampinassignmentoftheMAX32121levelconverter ...............................................................................................113 Figure93:FunctionaltestontheMAX3221levelconverter ........................................................................................................................114 Figure94:Simplifiedblockdiagramofa*gNSSreceiver ..............................................................................................................................115 Figure95:Typicalblockdiagramofa*gNSSmodule ...................................................................................................................................117 Figure96:Blockdiagramofu-blox5chipset ..............................................................................................................................................118

B.2 List of Tables Table1:L1carrierlinkbudgetanalysismodulatedwiththeC/Acode...........................................................................................................22 Table2:Comparisonbetweenephemerisandalmanacdata ........................................................................................................................33 Table3:PlannedpositioningaccuraciesforGALILEO ....................................................................................................................................42 Table4:FrequencyplanofGALILEOanddistributionofservices...................................................................................................................44 Table5:ComparisonofthemostimportantpropertiesofGPS,GLONASSandGALILEO...............................................................................47 Table6:Errorcauses(typicalranges) ...........................................................................................................................................................53 Table7:TotalerrorinHDOP=1.3................................................................................................................................................................56 Table8:Nationalreferencesystems..............................................................................................................................................................60 Table9:TheWGS-84ellipsoid......................................................................................................................................................................61 Table10:Datumparameters ........................................................................................................................................................................62 Table11:ErrorSourceandtotalerror............................................................................................................................................................69 Table12:Transmissionprocessofthedifferentialsignal(forcodeandphasemeasurement) ........................................................................74 Table13:StandardsforDGPScorrectionsignals ...........................................................................................................................................75 Table14:TheGEOsatellitesused(ortobeused)withWAAS,EGNOSandMSAS ........................................................................................79 Table15:DesignationoftheSBASstations ..................................................................................................................................................80 Table16:PositioningaccuracywithoutandwithDGPS/SBAS .......................................................................................................................82 Table17:DescriptionoftheindividualNMEADATASETblocks....................................................................................................................91 Table18:RecordingofanNMEAprotocol....................................................................................................................................................91 Table19:DescriptionoftheindividualGGAdatasetblocks .........................................................................................................................92 Table20:DescriptionoftheindividualGGLdatasetblocks ..........................................................................................................................93 Table21:DescriptionoftheindividualGSAdatasetblocks ..........................................................................................................................94 Table22:DescriptionoftheindividualGSVdatasetblocks ..........................................................................................................................95 Table23:DescriptionoftheindividualRMCdatasetblocks .........................................................................................................................96 Table24:DescriptionoftheindividualVTGdatasetblocks ..........................................................................................................................97 Table25:DescriptionoftheindividualZDAdatasetblocks ..........................................................................................................................98 Table26:DeterminingthechecksuminthecaseofNMEAdatasets ............................................................................................................99

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Table27:ContentsoftheRTCMmessageheader ......................................................................................................................................100 Table28:ContentsofRTCMmessagetype1 .............................................................................................................................................102 Table29:Messageclasses(Hexadecimalvaluesinbrackets)........................................................................................................................104 Table30:Timesystems...............................................................................................................................................................................111

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B.3 Sources

[i] GlobalPositioningSystem,StandardPositioningSystemService,SignalSpecification,2ndEdition,1995,page18,http://www.navcen.uscg.gov/pubs/gps/sigspec/gpssps1.pdf

[ii] NAVCEN:GPSSPSSignalSpecifications,2ndEdition,1995,http://www.navcen.uscg.gov/pubs/gps/sigspec/gpssps1.pdf

[iii] LemmeH.:SchnellesSpread-Spectrum-ModemaufeinemChip,Elektronik1996,H.15p.38top.45

[iv] http://www.maxim-ic.com/appnotes.cfm/appnote_number/1890

[v] ParkinsonB.,SpilkerJ.:GlobalPositioningSystem,Volume1,AIAA-Inc.

[vi] GPSStandardPositioningServiceSignalSpecification,2ndEdition,June2,1995

[vii] JournaloftheInstituteofNavigation,2002,Vol.48,No.4,pp227-246,Author:JohnW.Betz

[viii] http://www.glonass-center.ru/nagu.txt

[ix] http://www.dlr.de/dlr/News/pi_191004.htm

[x] http://www.cospas-sarsat.org/Status/spaceSegmentStatus.htm

[xi] http://europa.eu.int/comm/dgs/energy_transport/galileo/documents/brochure_en.htm

[xii] http://www.esa.int/esaCP/SEMT498A9HE_Austria_0.html

[xiii] http://europa.eu.int/scadplus/leg/de/lvb/l24004.htm

[xiv] http://www.spiegel.de/wissenschaft/weltraum/0,1518,392467,00.html

[xv] http://www.esa.int/esaCP/SEMQ36MZCIE_Improving_0.html

[xvi] http://www.esa.int/esaCP/SEM0198A9HE_Germany_0.html

[xvii] ManfredBauer:VermessungundOrtungmitSatelliten,Wichman-Verlag,Heidelberg,1997,ISBN3-87907-309-0

[xviii] http://www.geocities.com/mapref/mapref.html

[xix] B.Hofmann-Wellenhof:GPSinderPraxis,Springer-Verlag,Wien1994,ISBN3-211-82609-2

[xx] BundesamtfürLandestopographie:http://www.swisstopo.ch

[xxi] ElliottD.Kaplan:UnderstandingGPS,ArtechHouse,Boston1996,ISBN0-89006-793-7

[xxii] http://www.tandt.be/wis

[xxiii] http://www.egnos-pro.esa.int/IMAGEtech/imagetech_realtime.html

[xxiv] http://igscb.jpl.nasa.gov/[xxv] GPS-World,November2003:VittoriniundRobinson:OptimizingIndoorGPSPerformance,page40

[xxvi] www.maxim-ic.com/quick_view2.cfmDatenblattMAX2640,MAX2641

EssentialsofSatelliteNavigation Index

GPS-X-02007-C page131

http://www.navcen.uscg.gov/pubs/gps/sigspec/gpssps1.pdf

http://www.cnde.iastate.edu/staff/swormley/gps/gps_accuracy.html

http://www.geocities.com/mapref/mapref.html

http://www.swisstopo.ch/

http://www.tandt.be/wis

http://www.egnos-pro.esa.int/IMAGEtech/imagetech_realtime.html

http://www.maxim-ic.com/quick_view2.cfm

your position is our focus

[xxvii] NMEA0183,StandardForInterfacingMarineElectronicsDevices,Version2.30

[xxviii] http://www.navcen.uscg.gov/pubs/dgps/rctm104/Default.htm

[xxix] GlobalPositioningSystem:TheoryandApplications,VolumeII,BradfordW.Parkinson,page31

[xxx] UserManual:SonyGXB100016-channelGPSreceivermodule

[xxxi] UserManual:SonyGXB100016-channelGPSreceivermodule

[xxxii] swipos,PositionierungsdiensteaufderBasisvonDGPS,page6,BundesamtfürLandestopographie

[xxxiii] http://www.potsdam.ifa*g.de/potsdam/dgps/dgps_2.html

[xxxiv] http://www.maxim-ic.com

[xxxv] SatellitenortungundNavigation,WernerMansfield,page157,ViewegVerlag

[xxxvi] http://www.alliedworld.com

EssentialsofSatelliteNavigation Index

GPS-X-02007-C page132

http://www.navcen.uscg.gov/pubs/dgps/rctm104/Default.htm

http://www.potsdam.ifa*g.de/potsdam/dgps/dgps_2.html

http://www.maxim-ic.com/

http://www.alliedworld.com/