Scientech Technologies ST2276 User manual

GPSTrainer
ST2276
OperatingManual
Ver1.1
AnISO9001:2000company
94-101,ElectronicComplexPardesipura,
Indore-452010,India
Tel:[phone]
Fax:[phone]
email : [e-mail]
Website: www.scientech.bz
Toll free :1800-103-5050
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.2
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.3
GPSTrainer
ST2276
TableofContents
1. Introduction4
2. Experimentstobeperformed:
•Experiment15
Understanding theprincipleofGPSTechnology
•Experiment212
a. UnderstandingtheprincipleofGPSSatellite
b. UnderstandingthegenerationofL1carrierfrequency.
c. UnderstandingtheoperationofGPSReceiver.
d. EstablishingthelinkbetweentheGPSSatelliteandGPSTrainer.
•Experiment315
a. UnderstandingtheshapeofEarth.
b. Measurementoflatitude, longitude.
•Experiment417
a. UnderstandingtheprincipleofPRNcodeinGPS.
b. UnderstandingtheprincipalofautocorrelationinGPS.
•Experiment521
a. UnderstandingtheprincipleofGeometryoftheSatellite.
b. Understandingtheimportance ofPDOP, HDOP,andVDOP.
•Experiment624
a. UnderstandingtheprincipleofNMEA0183protocol.
b. AnalysisofNMEA0183protocols
•Experiment728
StudyofotherNMEASentence
•Experiment847
StudyofthecompleteGPSEnvironment.
3. GPSQuiz49
4. GPSGlossary52
5. GPSAcronyms63
6. Warranty64
7. ListofAccessories64
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.4
RoHSCompliance
ScientechProducts areRoHSComplied.
RoHSDirectiveconcerns withtherestrictiveuseof
Hazardoussubstances (Pb,Cd, Cr, Hg,
Brcompounds)inelectricandelectronicequipments.
Scientechproducts are“LeadFree”and “EnvironmentFriendly”.
Itismandatorythatserviceengineersuseleadfree solderwireandusethesolderingirons
upto(2
5W)thatreachatemperatureof450°Catthetipasthemelting temperatureofthe
unleadedsolderis higherthantheleadedsolder.
Introduction
Haveyou everbeenlostandwishedtherewasaneasywayyouneededtogo?
Everfindthatperfectfishingorhunting spotandnotbeenabletorememberhowto
getbacktoiteasily?
Howaboutfinding yourselfouthikingandnotknowing whichdirectionyoushould
gotogetbacktoyourcamporcar?
Everbeenflyingalong andneededtolocatethenearestairspaceyouwerein?
Maybeyou'vebeenfacedwiththefactthatit'stimetopull overandasksomeonefor
directions.
GlobalPositioningSystem technologyisrapidlychanginghowpeoplefindtheirway
aroundtheearth.Whetheritisforfun,saving lives,getting therefaster,orwhatever
usesyoucandreamup,GPSnavigationisbecoming morecommoneveryday.
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.5
Experiment1
Objective:
UnderstandingtheprincipleofGPSTechnology
Theory:
TheGlobalPositioning System(GPS)isandearthorbiting-satellitebasednavigation
system.GPS isanoperationalsystem,providingusersworldwidewithtwenty-four
houradayprecisepositioninthreedimensionsandprecisetimetraceabletoglobal
timestandards. GPS isoperatedbytheUnitedStatesAirForceunderthedirectionof
theDepartmentofDefense(DoD)andwasdesignedfor,andremainsunderthe
controlof,theUnitedStatesmilitary.Whiletherearenowmanythousandsof
commercialandrecreationalciviluser’sworldwide,DoDcontrolstillimpactsmany
aspectsofGPSplanning,operation,anduse.Primarilydesignedasaland,marine,
andaviationnavigationsystem,GPSapplicationshaveexpandedtoinclude
surveying,space navigation,automaticvehiclemonitoring,emergencyservices
dispatching,mapping,andgeographicinformationsystemgeoreferencing.Because
thedissemination ofprecisetimeisanintegralpartofGPS,alargecommunityof
precisetime,timeinterval,andfrequencystandardusershascometodependonGPS
asaprimarysourceofcontroltraceablethroughtheUnitedStatesNavalObservatory
toglobaltimeandfrequencystandards.
HistoryofGPS:
Developedinthe1960s,theNavyTransitsatellitenavigationsystemstillprovides
someserviceasatwo-dimensional(horizontal)positioning system. Good (200meter)
Transitpositioningrequiresknowledgeoftheuseraltitudeaswell asamodelofuser
dynamicsduring thefix,aprocess ofintegratingsatellitesignalDopplershifts(the
changeinreceivedsignalfrequencycausedbythechangingrange)during thefly-over
ofthesatellite.AnotherNavysystem,basedontheTimationsatellitescarriedstable
clocks(quartz,rubidium, andcesium)overthecourseoftheprograminthe1960sand
70sandwastheprecursortotheprecisetimecapabilitiesofGPS(Easton1978).GPS
beganin1973 asatestprogramusingground-basedtransmittersattheU.S.Army
ProvingGroundatYuma,Arizona,lateraugmentedwithearlyversionsofGPS
satellitesfirstlaunchedin1978.During the1980s,GPS,althoughnotyetfully
operationalandrequiringcarefulplanning formissionsduringtimesofsatellite
availability,wasincreasinglyusedbybothmilitaryandcivilianagencies.Land,air,
andseanavigation,precisepositioning,carrierphasesurveytechniques,andprecise
timeandfrequencydisseminationwereallaccomplishedtoalimitedextentduring the
initialphasesofGPSdeployment(Klepczynski1983).By1989 tendevelopment
satellites,termedBlockIsatellites,hadbeensuccessfullylaunched.By1990,43
laboratoriesrequiringprecisetimewereusingGPStosynchronizetheiratomicclocks
(Clements1990).By1994,24BlockIIandIIAoperationalGPSspacevehicles
(SVs)hadbeenlaunched.TheBlockIIASVscanstoreup to14daysofuploaded
dataincasecontactislostwithgroundstationsandcanoperatefor180 dayswith
degradednavigationreceiverperformance.Thenextgenerationofspacevehicles,the
BlockIIRSVswillincorporatechangestoincludethecapabilityofmaintaining
precisetimekeeping withoutControlSegmentuploadsforperiodsofup to210days
byexchangingdatabetweenGPSSVs(Rawicz, Epstein, andRajan1992).
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.6
InDecemberof1993,GPS reachedInitialOperationalCapability,withaminimumof
24 satellitesinorbit.OnJuly17,1995 theAirForceannouncedthatGPShadmetall
requirementsforFull OperationalCapabilitywith24 BlockII SVsinorbit.Withover
50 companiessupplyingaselectionofover275 GPSreceiverstoaglobalmarket,the
wellestablishedusercommunityofnavigators, surveyors,geologists,geodesists,time
andfrequencyusers, andmanythousandsofrecreationaluserhascometoacceptGPS
asaviablemilitaryandciviliansystem.
Civil andMilitaryGPS:
WhilecontrolledandmaintainedbytheDoD,theGPSusercommunityhasalarge
civilcomponent.Inthe1977NationalPlanforNavigation,publishedbytheU.S.
DepartmentofTransportation(DoT),theNAVSTARGPS usercommunitywas
plannedtoinclude27,000militaryreceivers.Whilethepotentialforacivil-sectoruser
basewasrecognized,thedocumentdidnotincludeplansforacivilGPSservice
(U.S.DoT19773-14;3-15).AdecadelatertheFederalRadionavigationPlan(FRP)
(U.S.DoDandDoT1986)statedthatGPSwouldbeavailabletocivilusers,
worldwide,onacontinuousbasisbutwithaccuracylimitedto100 meters(95
percent).
Intheseradionavigationdocumentspositionaccuracyisusuallyspecifiedasatwo
standarddeviation(95 percent)radialerroror2drms(2distancerootmeansquared)
uncertaintyestimate.ForGPSthe95percentprobabilityand2drmsaccuracyare
equivalent(DoDandDoT1995,A-2).The1985 ComprehensiveGlobalPositioning
SystemUserPolicy definedbothamilitary,encrypted,PrecisePositioningService
anda"lowerlevelofaccuracy"StandardPositioningService(U.S.DoDandDoT
1986, B-32).
StandardPositioningService:
TheStandardPositioning Service(SPS)isdefinedinthemostrecentFRPas:the
standardspecifiedlevelofpositioningandtimingaccuracythatisavailable,without
restrictions, toanyuseronacontinuousworldwidebasis. Theaccuracyofthisservice
willbeestablishedbytheDODandDOTbasedonU.S.securityinterests.SPS
providesapredictablepositioningaccuracyof100 meters(95percent)horizontally
and156 meters(95 percent)verticallyandtimetransferaccuracytoUTCwithin340
nanoseconds(95percent).
PrecisePositioningService :
TheFRPdefinesthePrecisePositioning Service(PPS)as:themostaccuratedirect
positioning,velocity,andtiming informationcontinuouslyavailable, worldwide, from
thebasicGPS.ThisserviceislimitedtousersspecificallyauthorizedbytheU.S.
P(Y)-codecapablemilitaryuserequipmentprovidesapredictablepositioning
accuracyofatleast22meters(95percent)horizontallyand27.7meters(95percent)
verticallyandtimetransferaccuracytoUTCwithin200nanoseconds(95 percent)
(DoDandDoT1995,A-36).Bythetimethe1992 FRPwaspublished,theprojected
1995 estimateof53,000civilusersofGPS exceededtheprojectednumberofmilitary
usersestimatedat19,000 (U.S.DoDandDoT 1993,3-41).Civilusersnowconstitute
themajorityofGPS users.The1994FRPestimatesthecurrenttotalnumberofGPS
usersatover500,000intheUnitedStatesalone(U. S. DoDandDoT 1995,3-7).
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.7
GPSSegments:
TheGlobalpositioningSystem(GPS)comprisesthreesegments:
The space segment (all functionsatellites)
The Controlsegment (allgroundstationinvolvedinthemonitoringofthesystem:
mastercontrolstation,monitoringstations&groundcontrol)
The usersegment (all civilandmilitaryGPSusers)
Figure1
SpaceSegment:
TheSpace Segmentisdesignedtoconsistof24 satellitesorbiting theearthat
approximately20200Kmevery12hours.At timeofwritingthereare26operational
satellitesorbiting theearth.Thespacesegmentissodesignedthattherewillbea
minimumof2to3satellitevisibleabovea15degcutoff angleatanypointofthe
earth'ssurface atanyonetime.EachGPS satellitehasseveralveryaccurateatomic
clockson board. Theclocksoperateatafundamentalfrequencyof10.23MHz. Thisis
usedtogeneratethesignalsthatarebroadcastfrom thesatellite.
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.8
Figure2
Positionofthe28GPSsatellitesat12.00hrsUTCon14th April 2001
Figure3
ControlSegment:
TheControlSegmentconsistsofonemastercontrolstation,5monitorstationsand4
groundantennasdistributedamong5locationsroughlyontheearthequator.The
ControlSegmenttrackstheGPSsatellites,updatestheirorbiting positionand
calibratesandsynchronizestheirclocks.
Afurtherimportantfunctionistodeterminetheorbit ofeachsatelliteandpredictits
pathforthefollowing24hours.Thisinformation isuploadedtoeachsatelliteand
subsequentlybroadcastfrom it.ThisenablestheGPS receivertoknowwhereeach
satellitecanbeexpectedtobefound.ThesatellitesignalsarereadatAscension,
Diego,Garcia&Kwajalein.Themeasurementsarethensenttothemastercontrol
stationinColoradoSpringswheretheyareprocessedtodetermineanyerrorsineach
satellite.Theinformationisthensentbacktothefourmonitoring stationsequipped
withgroundantennasanduploadedtothesatellites.
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.9
Figure4
UserSegment:
TheUserSegmentcomprisesofanyoneusingaGPS receivertoreceivetheGPS
signalanddeterminetheirpositionand/ortime.Typicalapplicationswithintheuser
segmentarelandnavigationforhikers,vehiclelocation,surveying,marine,
navigation,aerialnavigation, machinecontroletc.
Figure5
Figure 6a
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.10
Figure 6b
InformationTransmittedbythesatellites?
Thefollowingisthekeyinformationtransmittedbythesatellitesconstellationon
eitheracontinuousorperiodicalbasis.
1. SatelliteHealth
2. Ephemeredes
3. ConstellationAlmanac
4. Time
5. RangingSignals
6. AtmosphericCorrectionalData
TheEphemeredesdescribethedetailedorbitalcharacteristicsofthesatellitefrom
whichitistransmitted.Simplythisisthesatellite'smechanismfordescribing whereit
is.
ThesatellitesAlmanac describethecourseorbitaldataforallsatellitesinthe
constellation.Simplythisdatadescribeswhereall thesatellitesare,roughly, allowing
thereceivertoknowwheretolook,roughly,forasatellite.
ThisdataisbroadcasttotheUserSegmentsothatit canbestoredandemployedfor
initialsatelliteacquisitionandforvisibilityprediction.
Satellitesareidentifiedby:
1. SpaceVehicleNumber(SVN),and
2. PseudoRandomNoisenumber(PRN)
TheSpaceVehic1enumberindicatesthechronologicalorderinwhichthesatellites
werelaunched.MostGPS ReceiversemploythePRNtoidentifywhichsatellitethey
areobserving.
Howdoesit Work?
SomePrinciplestoAcknowledge
Thereare24operationalsatellites
Theyorbittheearthapproximatelyevery12hours
Theyarepositionedinsix(6)orbitalplanes
Thereforetherewill usuallybesomething intheorderof6to8satellitesvisibleabove
thehorizonatanypointintheworldandatanytimeoftheday.
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.11
1. Eachsatelliteemitsinformationrelatingtoitsposition,relativetotheearthand
timinginformation.Thistiming informationisderivedfrom extremelyaccurate
atomicclocks(cesium orrubidium)thataresynchronizedtoallothersatellite
clocksandtothegroundcontrolstations.
2. GPSReceiversareequippedwithquartzclocksthataresynchronizedtoGPS
timeviathedatatransmittedfromtheconstellation.
3. Timingisthebasisoflocationcomputation.
4. Thesatelliteradiatescodedsignalsthatarereceivedbytheuser’sGPS receiver.
5. Thecomputationinit simplestformistriangulation. SpaceBasedTriangulation.
ProducingLocations:
Thedeterminationofpositionisasimpleasthefollowing :
1. Asignalistransmittedfrom asatellitecontainingtheTimeofDepartureofthe
signal.
2. ThesignalisreceivedbytheGPSReceiverandtheTimeofArrivalis
registered.
3. WeknowthatRadiowaves(thesignal)travelattheSpeedofLight.
4. Weknowwherethesatelliteisfrom theinformationradiatedfromthesatellite.
Therefore,wecandeterminethedistance from ourreceivertoaparticularsatellite.
Thisallowstheconstructionofahemisphere,whosecentreisthesatelliteandwhose
radiusisthecalculateddistancefromaparticularsatellitetoourreceiver.
Whenthisprocess isrepeatedforanothersatellitethatisinview,thenthetwo
hemisphereswithcutthrougheachother.Repeatingthisprocess againwithathird
satelliteandtheintersectionofthethree hemisphereswillformapoint,whichis
whereyourreceiverislocated.
Thisallseemsabittopheavy,butrememberthatthesatellitesareconstantly
transmitting informationandthereceiverisusuallycapableof producingalocation
result up to10timeseverysecond.
Theacceptedruleformostreceiversisthatthereceivermustcontinuouslytracka
minimumoffour(4)satellitestoproducealocationthatcontainsalatitude,longitude
andaltitude.
Ofcoursemostreceiversavailabletodaywilltrackmanymoresatellitesthanfour(4).
Thisisimportant,asmentionedpreviously,theconstellationoperateson12 hour
orbits,thereforetheconstellationthatisvisible(beingusedbythereceiver) isalways
changing,andhencethereceiverneedstobelookingfornewsatellitesasthecurrent
inusesatellitesbegintodisappearfromview.
Conclusion:
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.12
Experiment2
Objectives:
1. UnderstandingtheprincipleofGPSSatellite.
2. UnderstandingthegenerationofL1carrier frequency.
3. UnderstandingtheoperationofGPSReceiver.
4. Establishingthelink betweentheGPSSatelliteand GPSTrainer.
Theory:
GPSSatelliteBlockdiagram:
OnboardtheSatellitehavefouratomicclocks.Thefollowingtimepulsesand
frequenciesrequiredforday-to-dayoperationarederivedfromtheresonantfrequency
ofoneoftheatomicclocksshowninfigure:
1. The50Hz datapulse.
2. TheC/A codepulse(Coarse/Acquisitioncode,PRN-Code,coarsereception
codeatafrequencyof1023MHz), whichmodulatesthedatausing anexclusive
oroperation(thisspreadsthedataovera1MHz bandwidth).
3. ThefrequencyofthecivilL1carrier(1575.42MHz)
ThedatamodulatedbytheC/A codemodulatestheL1carrierinturnbyusing
Bi-Phase-Shift-Keying(BPSK).Witheverychangeinthemodulateddatathereisa
180degchangeintheL1carrierphase.
Figure7
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.13
Figure8
ReceiverBlockdiagram:
Figure9
ThisisthesimplesttechniqueemployedbyGPSreceiverstoinstantaneouslygivea
positionandheightand/oraccuratetimetoauser.Theaccuracyobtainedisabetter
than100m(usuallyaroundthe30-50mmark)forcivilianusersand5-15m formilitary
users.
Procedure oftheExperiment:
Procedure :
Following stepshastobeperformwhiledoingtheexperiments.
Step1: Pleasegothroughthemanualbeforeperforming anypractical.
Step2: Installthesoftwarefrom theCDie.OpenWinZipfrom theCDandrunthe
setup file. Ifyoudon'thaveWinZipthenpleaseinstallWinZipfromtheCDitself.
Step3: Connectmainscordtothetrainer ST2276.Don’tswitchonthesystemnow.
Step4: Connectserialcabletotheportwhichisavailableonthetrainerconnect
anotherendofthecabletoPCserialport(COM 1,COM2,COM3etc.).
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.14
Step5: Connectthepatch. AntennatoSMA(subminiature)connectorofthe ST2276
trainer.
Step6: Placetheantennaintheopenspaceie.Placetheantennaoutsidethewindow.
Step7: Switchonthetrainer ST2276.
Step8: Precaution,don'ttouchtheantennaduringtheoncondition.
Step9: Opensoftwarefrom start/programfileIGPS Diag.Nowclickonoptionlike
COM1,ifitisnotpossibletodetectthencheckyourPCcomport.If yourPCcom
portisCOM2thenclickCOM2inthesoftware. Assoonasyouclickonanyofthese
com portaccordingtoyourPCthesoftwarewillstartdisplayingsomesignals.After
thisclickonstopbutton, belowIhaveshownyou thesoftwarewindow.
Figure10
Step10 : Hereintheabovesoftwarewindowwehavenotmentionanyreading,
actuallythisexperimentisonlytostudytheGPSSV,L1andGPS receiver.Butin
yourcaseyouwillgetsomereadingsbutdon'ttakeatpresentjustsee.Inthenext
experimentsyouhavetoanalysisall thisreading.
Conclusion:
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.15
Experiment3
Objective:
1. UnderstandingtheshapeofEarth.
2. Measurementoflatitude, longitude.
Theory:
EarthShape:
AsignificantproblemwhenusingtheGPSsystemisthatthereareverymanyco-
ordinatesystemsworldwide.Asaresult,thepositionmeasuredancalculatedbythe
GPSsystemdoesnotalwayscoincidewithone'ssupposedposition.
InordertounderstandhowtheGPS systemfunctions,itisnecessarytotakealookat
thebasicsofthescience thatdealswiththesurveyingandmapping oftheEarth
surface, geodesy. Withoutthisbasicknowledge, itisdifficult tounderstandwhywith
agoodportableGPSreceivertherightcombinationhastobeselectedfrom morethan
100differentmapreferencesystems.Ifanincorrectchoice ismade, apositioncanbe
outbyseveralhundredmeters.
DifferentEarthShapeslike:
1. Geoids
2. Spheroid
3. WorldwidereferenceellipsoidWGS-84
Formatoflatitudesandlongitudes:
Whereanumericlatitudeorlongitudeisgiven,thetwodigitsimmediatelytotheleft
ofthedecimalpointarewholeminutes,totherightaredecimalsofminutes,andthe
remaining digitstotheleftofthewholeminutesarewholedegrees.
Eg. 4533.35 is45degreesand33.35 minutes.".35"ofaminuteisexactly21seconds.
Eg.16708.033is167degreesand8.033minutes.“.033”ofaminuteisabout2
seconds.
Procedure :
Following stepshastobeperformwhiledoingtheexperiments.
Step1: Pleasegothroughthemanualbeforeperforming anypractical.
Step2: Installthesoftwarefrom theCDie.OpenWinZipfrom theCDandrunthe
setup file. Ifyoudon'thaveWinZipthenpleaseinstallWinZipfromtheCDitself.
Step3: Connectmainscordtothetrainer ST2276.Don'tswitchonthesystemnow.
Step4: Connectserialcabletotheportwhichisavailableonthetrainer.Connect
anotherendofthecabletoPCserialport(COM1, COM2, COM3etc.).
Step5: ConnectthepatchantennatoSMA(subminiature)connectorofthe ST2276
trainer.
Step6: Placetheantennaintheopenspaceie.Placetheantennaoutsidethewindow.
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.16
Step7: Switchonthetrainer ST2276.
Step8: Precaution,don'ttouchthe
Step9: Opensoftwarefrom start/programfile/GPSDiag.Nowclickonoptionlike
COM1,ifitisnotpossibletodetectthencheckyourPCcomport.If yourPCcom
portisCOM2thenclickCOM2inthesoftware. Assoonasyouclickonanyofthese
com portaccordingtoyourPCthesoftwarewillstartdisplayingsomesignals.After
thisclickonstopbutton,nowgototheobservationtablefornotingdownthevalues.
Step10: Takeatleastfourreading byplacingtheantennaatfourdifferentlocations.
Butswitchoff thepowerduring placingtheantennaondifferentlocation.
Sampleobservationtakenduringtesting:
Latitude Longitude City State Country
Observation:
Latitude Longitude City State Country
Result&Conclusion:
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.17
Experiment4
Objective:
1. UnderstandingtheprincipleofPRN codeinGPS.
2. UnderstandingtheprincipalofautocorrelationinGPS.
Theory:
PRN codes:
ThissectiondescribestheprincipleofthePRNcodethatisusedanditsuseforGPS.
PRNstandsforPseudo RandomNoise. Innormallanguageitmeansconsistsofalong
seriesofbits(0’sand1’s).At firstsighttheredoesn’tseemtobearegularpatternin
thebits.Butthereis!Thecodes-patternsusedforGPSrepeatthemselvesafterthe
1023rdbit.Thesecodescanbeeasilymadewithveryfewdigitalelements.Forthe
1023 bit pattern10shiftingregistersandsomedigitaladdersareneeded.Ingeneral
withnshifting registersaseriesof2n-1bitscanbegenerated.Forn=10 thiswill
become1024(=210)-1=1023bits.Thecodesaregeneratedwithaspeedof1.023
MHz(or1023000bitspersecond).Anexamplewithfourshiftingelementsisgiven
inthepicturebelow.
TheGPSsatellitesbroadcastthePRNcodesmixed(seepicturebelow)withtheother
GPSinformation,likeorbital(alsocalledephemeris)andclock-parameters,butalso
parametersconcerningtheothersatellites.BymixingthePRN-codewiththe50 Hz
datathetotalsignalisspreadoutoverabroadpartofthespectrum. Thistechniqueis
calledspreadspectrum.Thissectionwon'tgoverydeepinthiscomplexmatter,but
theresult isthatthesignalpowerisverylow,evenbeneaththenoisefloor.Inother
words:ithasbecomeveryhardtodistinguishthesignalfrom noise(thatisalways
presentonsignals).
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.18
Figure11
Figure12
WhentheGPSsignalsarereceivedbytheuserofGPS,thePRN-codeandGPSdata
havetobeseparated.Thisisdonebyagainmixing thereceivedsignalwithalocally
generatedPRN-code.ThismustbethesamePRN-codewhichhasbeengeneratedin
thesatellite.Itisimportantthatequalpartsofthecodearemixedwitheachother.
Thereforethecodegeneratedinthereceivermustbeshiftedintimeuntilthetwo
codesareexactlysynchronous.Inthisspecialcasewhenthereceiver‘locks’(also
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.19
referredasfull correlation)thetwocodescanblock.eachotheroutandtheGPS-data
remainsandcanbefurtherprocessed.Thismethod iscalleddispreading.
Figure13
EverysatellitehasitsownuniquePRN-codesothattheGPSreceivercandistinguish
thesignalsfrom varioussatellites.GPSreceiverisabletogenerate32PRN-codes.
Untilnowsomanysatelliteshavenotbeenlaunched.
WhentheGPS receiverhastostartupit doesn'tknowwhichGPSsignalisfrom
whichsatellite. Thereforeittriestolockwiththe32knownPRN-codesonebyone.If
onecodelocksthentheinformationofonesatellitecanbedecoded.Thisinformation
alsocontainsdataaboutothersatellitesandtherestcansoonbereceivedtoo.
Themainreasonforusing PRNcodesintheGPSsystemisthatthePRNcode
enlargestheunambiguousmeasurementrange.Onemustkeepinmindthatafter1023
bitsthecodeisrepeated.ItiscasethatGPS-receiverisawareit is‘looking’atthe
rightcodeandnotatitspredecessororsuccessor.Lookingatthewrongcodegivesa
navigationerrorof300 km(corresponding tothecodelengthof1millisecond).
Autocorrelation:
TheidealGPS receiverwouldhaveaninfinitelywidereceiverBWwhichwould
allowthereceivertocapture100%oftheGPSspreadspectrumsignal.The
normalizedautocorrelationfunctionforaninfinitelywideBW isgenerallyillustrated
asshowninfigurebelow.
HiK-Consulting

ST2276
ScientechTechnologies Pvt.Ltd.20
TheoreticalNormalizedAuto-correlationFunction
Figure14
Theauto-correlationpeakismaintainedbycontinuallyadjustingthelocallygenerated
codeforpeakcorrelatoroutput.TheunlimitedBW providesasharpcorrelation peak
andsteepearly/laterslopewhichfacilitatesaccurateerrorcorrectionforthecode-
lock-loop(alsocalledDelayLockLoop).
Inreality,aGPS receiverwouldneedabrickwall bandpassfilterwithaBWofat
leasttentimesthecodeC/A codechipping ratetobecapableofcapturing>99%of
theGPSspreadspectrumsignal.FormostGPSreceiverthisisgenerallynotpractical
toachieve.
Conclusion:
HiK-Consulting