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6/29/2015 BasicConcepts(GPSandGIS)Part1

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BasicConcepts(GPSandGIS)Part1

INWHICHyouareintroducedtofactsandconceptsrelatingtotheNAVSTARGlobalPositioningSystemandhaveyourfirstexperienceusingaGPSreceiver.

OVERVIEW

AsportsclubinSeattledecidedtomountahuntingexpedition.Theyemployedaguidewhocamewellrecommended,andwhoseownviewsofhisabilitiesweregreaterstill.Unfortunately,aftertwodays,thegroupwascompletely,totallylost.YoutoldmeyouwerethebestguideintheStateofWashington,"fumedthepersonresponsibleforhiringtheguide."Iam,Iam"claimedthemandefensively."ButjustnowIthinkwereinCanada."

Storiesliketheoneaboveshouldbetoldnow(ifatall),beforetheyceasetobeplausible.Actually,evenatpresent,giventherightequipmentandamapofthegeneralarea,youcouldbeledblindfoldedtoanyspotinthegreatoutofdoorsanddetermineexactlywhereyouwere.Thishappycapabilityisduetosomeingeniouselectronicsandadozenbilliondollars1spentbytheU.S.government.IrefertoNAVSTAR(NavigationSystemwithTimeAndRanginginformallythe"NavigationStar")aconstellationoffrom24to32satellitesorbitingtheEarth,broadcastingdatathatallowsusersonorneartheEarthtodeterminetheirspatialpositions.ThemoregeneraltermintheUnitedStatesforsuchanentityis"GlobalPositioningSystem"or"GPS."TheRussianshavesuchanavigationsystemaswell,whichtheycallGLONASS(GlobalNavigationSatelliteSystem).(Onemightreflectthat,forsomepurposes,thecoldwarlastedjustlongenough.)Amoregeneral,recentacronymforsuchsystemsisGNSS,standingforGlobalNavigationSatelliteSystems.Inthewesternworld,GPSusuallyimpliesNAVSTAR,soIwillusethetwodesignationsinterchangeablyinthistext.



WhereAreYou?

Geography,andGeographicInformationSystems(GISs)particularly,dependontheconceptoflocation.Workingwith"location"seemstoimplythatwemustorganizeandindexspace.Howdowedothat?

Formally,weusuallydelineategeographicalspaceintwodimensionsontheEarthssurfacewiththelatitudelongitudegraticule,orwithsomeothersystembasedonthatgraticule.

Butinformally,andinthevastmajorityofinstances,weorganizespaceintermsofthefeaturesinthatspace.WefindagivenfeatureorareabasedonourknowledgeofotherfeatureswhetherwearedrivingtoVancouverorwalkingtotherefrigerator.Evenplanesandshipsusingradionavigationaldevicesdeterminetheirpositionsrelativetothelocationsoffixedantennae(thoughsomeoftheradiosignalsmaybeconvertedtograticulecoordinates).

Unlikekeepingtrackoftime,whichwasinitiallycomputedrelativetoasingle,spacebasedobject(thesun),humanskepttrackofspacefoundtheirwayonthegroundbyobservingwhatwasaroundthem.

Another,somewhatparallelwayoflookingatthisissueisintermsofabsoluteversusrelativecoordinates.IfItellyouthatLexington,Kentuckyisat38degrees(38)northlatitude,84.5westlongitude,Iamprovidingyouwithabsolutecoordinates.IfIsay,rather,thatLexingtonis75milessouthofCincinnati,Ohioand70mileseastofLouisville,Kentucky,Ihavegivenyourelativecoordinates.

Relativecoordinatesusuallyappealmoretoourintuitivecomprehensionof"location"thandoabsolutecoordinateshowever,relativecoordinatescanbequiteprecise.

Topassspatialinformationaround,humansdevelopedmapstodepictmountainsandroads,citiesandplains,radiostationsandsinkholes.Mapsaidboththeformalandinformalapproachesthathumansusetofindobjectsandpaths.Somemapshaveformalcoordinates,butmapswithoutgraticulemarkingsarecommon.Allmapsappealtoourintuitivesenseofspatialrelationships.Thecartographerusuallyreliesonourabilitytousethe"cognitivecoordinates"inourmemory,andourabilitiestoanalyze,toextrapolate,andto"patternmatch"thefeaturesonthemap.Itisgoodthatthismethodworks,since,unlikesomeamazingbirdandbutterflyspecies,humanshavenodemonstratedsenseofanabsolutecoordinatesystem.Butwithmaps,andanothertechnologicalinnovation,themagneticcompass,wehavemadeconsiderableprogressinlocatingourselves.

Idonotwanttoimplythatabsolutecoordinateshavenotplayedasignificantpartinourpositionfindingactivities.Theyhave,particularlyinnavigation.Atsea,orflyingoverunlitbodiesoflandatnight,captainsand



pilotsusedmethodsthatprovidedabsolutecoordinates.Onesposition,withinafewmiles,canbefoundby"shootingthestars"forashorttimewithdevicessuchassextantsoroctants.SotheGPSconceptfindinganearthlypositionfrombodiesinspaceisnotanentirelynewidea.Buttheabilitytodosoduringtheday,almostregardlessofweather,withhighaccuracyandalmostinstantaneously,makesamajorqualitativedifference.Asaparallel,considerthatahumancanmovebyfootorbyjetplane.Theyarebothmethodsoflocomotion,buttherethesimilarityends.

GPS,then,givespeopleaneasymethodforbothassigningandusingabsolutecoordinates.Now,humanscanknowtheirpositions(i.e.,thecoordinatesthatspecifywheretheyare)combinedwithmapand/orGISdatatheycanknowtheirlocations(i.e.,wheretheyarewithrespecttoobjectsaroundthem).Ihopethat,bythetimeyouvecompletedthistextandexperimentedwithaGPSreceiver,youwillagreethatNAVSTARconstitutesanastoundingleapforward.

WHATTIMEISIT?

WhilethisisatextonhowtouseGPSinGISandhenceisprimarilyconcernedwithpositionalissues,itwouldnotbecompletewithoutmentioningwhatmay,fortheaverageperson,bethemostimportantfacetofGPS:providingEarthwithauniversal,exceedinglyaccuratetimesource.Allowinganypersonorpieceofequipmenttoknowtheexacttimehastremendousimplicationsforthingswedependoneveryday(likegettinginformationacrosstheInternet,likesynchronizingtheelectricpowergridandthetelephonenetwork).Further,humanknowledgeisenhancedbyresearchprojectsthatdependonknowingtheexacttimeindifferentpartsoftheworld.Forexample,itisnowpossibletotrackseismicwavescreatedbyearthquakes,fromonesideoftheearth,throughitscenter,totheotherside,sincetheexacttime2maybeknownworldwide.3

GPSANDGIS

ThesubjectofthistopicistheuseofGPSasamethodofcollectinglocationaldataforGeographicInformationSystems(GIS).Theappropriatenessofthisseemsobvious,butletsexploresomeofthemainreasonsformakingGPSaprimarysourceofdataforGIS:

Availability:In1995,theU.S.DepartmentofDefense(DoD)declaredNAVSTARtohave"finaloperationalcapability."Deciphered,thismeansthattheDoDhascommitteditselftomaintainingNAVSTARscapabilityforciviliansatalevelspecifiedbylaw,fortheforeseeablefuture,atleastintimesofpeace.Therefore,thosewithGPSreceiversmaylocatetheirpositionsanywhereontheEarth.

Accuracy:GPSallowstheusertoknowpositioninformationwithremarkableaccuracy.Areceiveroperatingbyitself,canletyoulocateyourselfwithin10to20metersofthetrueposition.(Andlateryouwilllearnhowtogetaccuraciesof2to5meters.)Atleasttwofactorspromotesuchaccuracy:

First,withGPS,weworkwithprimarydatasources.ConsideronealternativetousingGPStogeneratespatialdata:thedigitizer.Adigitizerisessentiallyanelectronicdrawingtable,wherewithanoperatortraceslinesorenterspointsby"pointing"with"crosshairs"embeddedinaclearplastic"puck"atfeaturesonamap.

OnecouldconsiderthatthegroundbasedportionofaGPSsystemandadigitizerareanalogous:theEarthssurfaceisthedigitizingtable,andtheGPSreceiverantennaplaysthepartofthecrosshairs,tracingalong,forexample,aroad.ButdatagenerationwithGPStakesplacebyrecordingthepositiononthemostfundamentalentityavailable:theEarthitself,ratherthanamaporphotographofapartoftheEarththatwasderivedthroughaprocessinvolvingperhapsseveraltransformations.

Secondly,GPSitselfhashighinherentaccuracy.Theprecisionofadigitizermaybe0.1millimeters(mm).Onamapofscale1:24,000,thistranslatesinto2.4meters(m)ontheground.Adistanceof2.4miscomparabletotheaccuracyonemightexpectoftheproperlycorrecteddatafromamediumqualityGPSreceiver.Itwouldbehardtogetthisoutofthedigitizingprocess.Asecondaryroadonourmapmightberepresentedbyalinefive



timesaswideastheprecisionofthedigitizer(0.5mmwide),givingadistanceonthegroundof12m,orabout40feet.

Onlargerscalemaps,ofcourse,theprecisiononemightobtainfromadigitizercanexceedthatobtainedfromthesortofGPSreceivercommonlyusedtoputdataintoaGIS.Ona"200scalemap"(whereoneinchisequivalentto200feetontheground)0.1mmwouldimplyadistanceofapproximatelyaquarterofameter,orlessthanafoot.WhilethisdistanceiswellwithintherangeofGPScapability,theequipmenttoobtainsuchaccuracyisexpensiveandisusuallyusedforsurveying,ratherthanforgeneralGISspatialanalysisandmapmakingactivities.Insummary,ifyouarewillingtopayforit,attheextremesofaccuracy,GPSwinsoverallothermethods.SurveyorsknowthatGPScanprovidehorizontal,realworldaccuraciesoflessthanonecentimeter.

Easeofuse:AnyonewhocanreadcoordinatesandfindthecorrespondingpositiononamapcanuseaGPSreceiver.Asinglepositionsoderivedisusuallyaccuratetowithin10metersorso.ThosewhowanttocollectdataaccurateenoughforaGISmustinvolvethemselvesinmorecomplexprocedures,butthetaskisnomoredifficultthanmanyGISoperations.

GPSdataareinherentlythreedimensional:Inadditiontoprovidinglatitudelongitude(orother"horizontal"information),aGPSreceivermayalsoprovidealtitudeinformation.Infact,unlessitdoesprovidealtitudeinformationitself,itmustbetolditsaltitudeinordertoknowwhereitisinahorizontalplane.TheaccuracyofthethirddimensionofGPSdataisnotasgreat,usually,asthehorizontalaccuracies.Asaruleofthumb,variancesinthehorizontalaccuracyshouldbemultipliedby1.5(andperhapsasmuchas3.0)togetanestimateoftheverticalaccuracy.

ANATOMYOFTHETERM:"GLOBALPOSITIONINGSYSTEM"

Global:anywhereonEarth.Well,almostanywhere,butnot(ornotaswell):

insidebuildings

underground

inverysevereprecipitation

underheavytreecanopy

aroundstrongradiotransmissions

in"urbancanyons"amongsttallbuildings

nearpowerfulradiotransmitterantennasoranywhereelsenothavingadirectviewofasubstantialportionofthesky.TheradiowavesthatGPSsatellitestransmithaveveryshortlengthsabout20cm.Awaveofthislengthisgoodformeasuringbecauseitfollowsaverystraightpath,unlikeitslongercousinssuchasAMandFMbandradiowavesthatmaybendconsiderably.Unfortunately,shortwavesalsodonotpenetratematterverywell,sothetransmitterandthereceivermustnothavemuchsolidmatterbetweenthem,orthewavesareblocked,aslightwavesareeasilyblocked.

Positioning:answeringbrandnewandageoldhumanquestions.Whereareyou?Howfastareyoumovingandinwhatdirection?Inwhatdirectionshouldyougotogettosomeotherspecificlocation,andhowlongwouldittakeatyourspeedtogetthere?And,mostimportantlyforGIS,wherehaveyoubeen?

System:acollectionofcomponentswithconnections(links)amongthem.Componentsandlinkshavecharacteristics.GPSmightbedividedupinthefollowingway:4



TheEarth

ThefirstmajorcomponentofGPSisEarthitself:itsmassanditssurface,andthespaceimmediatelyabove.ThemassoftheEarthholdsthesatellitesinorbit.Fromthepointofviewofphysics,eachsatelliteistryingtoflybytheEarthatfourkilometerspersecond.TheEarthsgravitypullsonthesatelliteverticallysoitfalls.ThetrajectoryofitsfallisatrackthatisparalleltothecurveoftheEarthssurface.

ThesurfaceoftheEarthisstuddedwithlittle"monuments"carefullypositionedmetalorstonemarkerswhosecoordinatesareknownquiteaccurately.Theselieinthe"numericalgraticule"whichweallagreeformsthebasisforgeographicposition.Measurementsintheunitsofthegraticule,andbasedonthepositionsofthemonuments,allowustodeterminethepositionofanyobjectwechooseonthesurfaceoftheEarth.

EarthCirclingSatellites

TheUnitedStatesGPSdesigncallsforatotalofatleast24andupto32solarpoweredradiotransmitters,formingaconstellationsuchthatseveralare"visible"fromanypointonEarthatanygiventime.ThefirstonewaslaunchedonFebruary22,1978.Inmid1994all24werebroadcasting.Theminimum"constellation"of24includesthree"spares."Asmanyas28havebeenupandworkingatonetime.

ThenewestGPSsatellites(designatedasBlockIIR)areata"middlealtitude"ofabout11,000nauticalmiles(nm),orroughly20,400kilometers(km)or12,700statutemilesabovetheEarthssurface.Thisputsthemabovethestandardorbitalheightofthespaceshuttle,mostothersatellites,andtheenormousamountofspacejunkthathasaccumulated.TheyarealsowellaboveEarthsair,wheretheyaresafefromtheeffectsofatmosphericdrag.WhenGPSsatellites"die"theyaresenttoorbitsabout600milesfurtherout.

GPSsatellitesarebelowthegeostationarysatellites,usuallyusedforcommunicationsandsendingTV,telephone,andothersignalsbacktoEarthbasedfixedantennas.Thesesatellitesare35,763km(or19,299nmor22,223sm)abovetheEarth,wheretheyhangovertheequatorrelayingsignalsfromandtogroundbasedstations.

TheNAVSTARsatellitesareneitherpolarnorequatorial,butslicetheEarthslatitudesatabout55,executingasinglerevolutionevery12hours.Further,althougheachsatelliteisina12hourorbit,anobserveronEarthwillseeitriseandsetabout4minutesearliereachday.5Therearefourorfivesatellitesinslotsineachofsixdistinctorbitalplanes(labeledA,B,C,D,E,andF)set60degreesapart.Theorbitsarealmostexactlycircular.

ThecombinationoftheEarthsrotationalspeedandthesatellitesorbitsproducesawidevarietyoftracks



acrosstheEarthssurface.Figure11isaviewofthetrackswhichoccurredduringthefirsttwohoursafternoononSt.PatricksDay,1996.YouarelookingdownontheEarth,directlyattheequatorandata(northsouth)meridianthatpassesthroughLexington,Kentucky.Asyoucansee,byFigure11,thetracksneartheequatortendtobealmostnorthsouth.Thenumberofeachsatelliteisshownnearitstrackthenumbermarksthepointwherethesatelliteisattheendofthetwohourperiod.

GPSsatellitesmoveataspeedof3.87km/sec(8,653milesperhour).TheBlockIIRsatellitesweighabout1077kilograms(somewhatmorethanaton)andhavealengthofabout11.6meters(about38feet)withthesolarpanelsextended.Thosepanelsgenerateabout1100wattsofpower.Theradioonboardbroadcastswithabout40wattsofpower.(Comparethatwithyour"clearchannel"FMstationwith50,000watts.)TheradiofrequencyusedforthecivilianGPSsignaliscalled"GPSL1"andisat1575.42megaHertz(MHz).

Figure11.GPSsatellitetrackslookingfromspacetowardtheEquator.

Eachsatellitehasonboardfouratomicclocks(eithercesiumorrubidium)thatkeeptimetowithinabillionthofasecondorso,allowingusersonthegroundtodeterminethecurrenttimetowithinabout40billionthsofasecond.Eachsatelliteisworthabout$65millionandhasadesignlifeof10years.Figure12showsanimageofaNAVSTARsatellite.

GroundBasedStations

WhiletheGPSsatellitesarefreefromdragbytheatmosphere,theirtracksareinfluencedbythegravitationaleffectsofthemoonandsun,andbythesolarwind.Further,theyarecrammedwithelectronics.Thus,boththeirtracksandtheirinnardsrequiremonitoring.Thisisaccomplishedbyfourgroundbasedstationsneartheequator,locatedonAscensionIslandintheSouthAtlantic,atDiegoGarciaintheIndianOcean,andonKwajaleinAtoll,andinHawaii,bothinthePacific,plusthemastercontrolstation(MCS)atSchriever



(formerlyFalcon)AirForceBasenearColoradoSprings,Colorado.AsixthstationisplannedtobeginoperationatCapeCanaveral,Florida.Eachsatellitepassesoveratleastonemonitoringstationtwiceaday.Informationdevelopedbythemonitoringstationistransmittedbacktothesatellite,whichinturnrebroadcastsittoGPSreceivers.

Figure12.ANAVSTARGPSsatellite.

Subjectsofasatellitesbroadcastarethehealthofthesatelliteselectronics,howthetrackofthesatellitevariesfromwhatisexpected,thecurrentalmanac6forallthesatellites,andother,moreesotericsubjectswhichneednotconcernusatthispoint.Othergroundbasedstationsexist,primarilyforuploadinginformationtothesatellites.

Receivers

Thisisthepartofthesystemwithwhichyouwillbecomemostfamiliar.Initsmostbasicform,thesatellitereceiverconsistsof

anantenna(whosepositionthereceiverreports),

electronicstoreceivethesatellitesignals,

amicrocomputertoprocessthedatathatdeterminestheantennaposition,andtorecordpositionvalues,

controlstoprovideuserinputtothereceiver,and



ascreentodisplayinformation.

Moreelaborateunitshavecomputermemorytostorepositiondatapointsandthevelocityoftheantenna.Thisinformationmaybeuploadedintoapersonalcomputerorworkstation,andtheninstalledinGISsoftwaredatabase.AnotherelaborationonthebasicGPSunitistheabilitytoreceivedatafromandtransmitdatatootherGPSreceiversatechniquecalled"realtimedifferentialGPS"thatmaybeusedtoconsiderablyincreasetheaccuracyofpositionfinding.

ReceiverManufacturers

Inadditiontobeinganengineeringmarvelandofgreatbenefittomanyconcernedwithspatialissuesascomplexasnationaldefenseorasmundaneasrefindingagreatfishingspot,GPSisalsobigbusiness.DozensofGPSreceiverbuildersexistfromthosewhomanufacturejusttheGPS"engine,"tothosewhoprovideacompleteunitfortheenduser.Inthistextweexplaintheconceptsingeneral,butuseTrimbleNavigation,Ltd.equipmentsinceitworkswell,isquiteaccurate,hasaprogramofeducationaldiscounts,andislikelytobepartofeducationalGPSlabsthroughoutthecountry.

TheUnitedStatesDepartmentofDefense

TheU.S.DoDischargedbylawwithdevelopingandmaintainingNAVSTAR.Itwas,atfirst,secret.Fiveyearselapsedfromthefirstsatellitelaunchin1978untilnewsofGPScameoutin1983.Thestory,perhapsapocryphal,isthatPresidentReagan,atthetimeaKoreanairlinerstrayedintoSovietairspaceandwasshotdown,lamentedsomethinglike"thiswouldnthavehappenedifthedamnGPShadbeenup."AreporterwhooverheardwantedtoknowwhatGPSwas.InthealmosttwodecadessincedespitethefactthatpartsofthesystemremainhighlyclassifiedmerecitizenshavebeencashinginonwhatTrimbleNavigation,Ltd.calls"TheNextUtility."

ThereislittlequestionthatthedesignofGPSwouldhavebeendifferenthaditbeenaciviliansystem"fromthegroundup."Butthen,GPSmightnothavebeendevelopedatall.Manyissuesmustberesolvedinthecomingyears.APresidentialDirectiveissuedinMarchof1996designatedtheU.S.DepartmentofTransportationastheleadcivilianagencytoworkwithDoDsothatnonmilitaryusescanbloom.DoDislearningtoplaynicelywiththecivilianworld.Theyandweallhope,ofcourse,thatthecivilusesofGPSwillvastlyoutpacethemilitaryneed.

Oneimportantmatterhasbeenaddressed:ForyearsthemilitarydeliberatelycorruptedtheGPSsignalssothatasingleGPSunit,operatingbyitself(i.e.,autonomously),couldnotassureaccuracyofbetterthan100meters.Thispolicy(knownasSelectiveAvailability[SA])wasterminatedon2May2000.Nowusersofautonomousreceiversmayknowtheirlocationswithin10to20meters.

Users

Finally,ofcourse,themostimportantcomponentofthesystemisyou:the"youser,"asmyeightyearoldspelledit.Alargeandquicklygrowingpopulation,userscomewithawidevarietyofneeds,applications,andideas.FromtrackingicefloesnearAlaskatodigitizinghighwaysinOhio.Fromrescuingsailorstopinpointingtoxicdumpsites.Fromurbanplanningtoforestmanagement.Fromimprovingcropyieldstolayingpipelines.WelcometotheexcitingworldofGPS!



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