7/1/2015 BeyondtheBasics(GPSandGIS)Part6

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BeyondtheBasics(GPSandGIS)Part6

PROJECT7H

{__}Withamapthathasgeographiccoordinatesonit,setuptheGPSreceiverwithanearbywaypoint.

{__}Takethereceiveroutsidesothatitiscalculatingpositions.Walk(or,ifyouhavetheremoteantennaandwishto,drive)withitandexperimentwiththevariousdisplayformatsandtheinformationpresented.

{__}Navigatetothewaypointyousetup.

OVERVIEW

RealTime,DifferentialGPSPositionFinding

Wediscussedpostprocessingcorrectioninsomedetail.Whileyoucangetmoreaccuratefixesfromsuchasystem,itrequiresanumberofadditional,timeconsumingsteps,afterdatacollection.Onceyouknowthesourceofthedifferentialcorrectionsfile,youhavetogetit,loaditontoyourcomputer,andexecutesoftwaretoproducethefinalcorrectedfile.WouldntitbeniceiftheGPSunitsimplygaveaccuratepositionsatthetimeyoutookthedata?

Further,therearesomeapplicationsinwhichtheresimplyisnttimetopostprocessGPSfixesbringingashipintoanarrowharbor,foroneexample.

Well,actuallythereisawaytoprovideinstantaneous,accuratefixes:RealTimeDifferentialGPS.

GettingCorrectionsforGPSMeasurementsRightNow!

RealTimeDifferentialGPS(RDGPS)maybeapproachedinseveralways:

Theusercansetupabasestationoveraknownpointandarrangefortransmissionofaradiosignalfromthebasestationtorovingreceivers.

SlipRingJinpatOfftheSelf&CustomizedSlipRingsJustTellusYourRequirements

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TheusersGPSsetupcanreceivecorrectionsignalsbroadcastfromanantennaintheareaoperatedbyacorporationoragovernmentagency.Suchinstallationsoperateabasestationcontinuouslyandbroadcastthecorrectiondata,sometimesaspartofanothersignalsuchasastandardFMbroadcast.TheU.S.CoastGuardmaintainssuchaserviceinsomecoastalareas,andcompaniessuchasAccupoint,Inc.andDifferentialCorrections,Inc.sellacorrectionservice.

TheusersGPSsetupcanreceivecorrectionsignalsfromacommunicationssatelliteparkedovertheEquator.ThesesignalscomefromdatatakenbybasestationslocatedindisparatepartsoftheUnitedStates.Thedataareanalyzed,packaged,andsenttoageostationarycommunicationsatelliteforrebroadcasttoEarth.OnesuchsystemusesthisapproachwithasatellitelocatedonameridianthatpassesthroughLakeMichigan.

Withallthreeofthesemethods,manyoftherequirementsforbasestationsremainthesame.Theymusttakedatafromallsatellitesthattherovermightuse.Theymusttakedatafrequently(everyfewseconds).Theyshouldhaveaseparatechannelforeachsatellitesoastotrackitcontinuously.

Withallthemethodsofdoingrealtimedifferentialcorrection,youmightwonderhowthesepiecesofequipmentmanagetotalktoeachother.Whilethefrequenciesonwhichbroadcaststakeplacemayvary,thecontentofrealtimedifferentialcorrectiondataisstandardized.ThecurrentstandardisRTCMSC104(version2),promulgatedbytheRadioTechnicalCommissionforMaritimeServices,inWashington,DC.Lookinmoredetailatthethreestepsforrealtimedifferentialcorrection.

AUserOperatedRealTimeBaseStation

Theusermayoperateherorhisownbasestation.Thosewhoneedreallyprecisecoordinatesrelativetosomenearbyknownpoint(e.g.,landsurveyors)usethismethodfrequently.Thecomplicationsgoupwithdifferentialcorrection(realtimeorotherwise),ofcourse.HeretheuserhastohaveatleasttwoGPSreceivers,plusatransmitterassociatedwithoneofthem(thebasestation)andreceiversforeachrover.InadditiontotherovingGPSreceiverandallofitssettingsandconditionsforgoodpositionfinding,theuserhastoalsobeconcernedaboutthesamefactorsforthebasestation,plusmakingtheradiolinkbetweenthestationsfunctionatsomedistance.Additionalcomplicationsrelatetomountingthebasestationantennaatapreciselyknownpoint.

ACentrallyLocatedRealTimeBaseStation

SinceGPSsignalerrorstendtobequitesimilaroverwidegeographicareas,thereareobviousadvantagestohavingasinglebasestationserveforanyrovingstationsinthatarea.Putanotherway,onecanthinkofvery

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few,ifany,reasonsforeachof28GPSusers,whoareinreasonablycloseproximitytoeachother,tocollectandrebroadcastidenticalcorrectiondata.Thisobviousfact,plustheentrepreneurialnatureofAmericansociety,hasproducedGPSdifferentialcorrectionservices,whereinausercontractsforequipmentandtherighttoreceivecorrectionstotherawGPSsignal.UserswhoaresufficientlyclosetoaU.S.CoastGuardGPSbeaconmaypickupasignalatnocostbesidesthatofthereceivingequipment.SuchstationsaresometimesreferredtoasContinuallyOperatingReferenceStations,CORS.

A"DifferentialCorrectionsAnywhere"System

Theproblemwith"centrallylocated"basestationsisthattheremaynotbeasignalfromonewhereyouaretryingtotakedata.Whiletherearemoreandmorebasestationseverymonth,vastgapsexistwheretheirsignalsdonotgo.Asolutiontothisproblemisasysteminwhichthebroadcastofthecorrectiondatacomesfromacommunicationssatellite.Thismeansthat,foralargeportionoftheEarthssurface,theusersreceiverisneveroutofrangeintheareacoveredbythesatellite,thoughlocaltopographymayblockthesignal.Thesatelliteofonesystemthatcurrentlyemploysthismethod(OMNISTAR)mightbeaslowas30abovethehorizoninnorthernpartsoftheUnitedStates.Thesatellitebroadcastsastraightlinesignalofabout4000megahertz.TheareacoveredisalloftheUnitedStates(exceptpartsofAlaska)andpartsofCanadaandMexico.AnothersystemthatprovidesasimilarserviceisRacalLandstar.Bothsystemsprovidecoverageoverlargepartsoftheearth.

OMNISTARhasaboutadozenbasestationslocatedontheperipheryoftheUnitedStates.ThesestationstransmitdataregardingtheerrorsintheGPSsignalsintheirareastoacentralnetworkcontrolcenter.Thesedataarethenanalyzedandrepackagedfortransmissiontothecommunicationssatellite.

Thebroadcastfromthesatelliteissuchthatthedatafromtheseveralstationscanbetailoredtotheusersposition.HowdoestheOMNISTARsystemknowwheretheuseris?TheusersGPSunittellsitscoordinatestotheattachedOMNISTARradio,whichthendecodesthesignalfromthecommunicationssatellitetoprovidethepropercorrectionsforthelocalarea.SincerealtimedifferentialcorrectioninformationisuselessunlessitarrivesattheGPSreceiverwithinmomentsofthetimeafixistakenbythereceiver,youcanseethatalothastohappeninanextremelyshortamountoftime.(AGPSsignalcodeleavesaGPSsatelliteitisreceivedbyyourreceiverandbytheOMNISTARbasestationsthebasestationstransmitthesignaltoacentrallocationwhereitisprocessedandsenttothecommunicationssatellite,whereitisresenttotheradionexttoyourGPSreceiverthatradiodeterminesthecorrectionyourGPSreceiverneedsandsuppliesit.Whew!)

Thissystemcanprovidemeasurementssuchthat95%ofthefixestakenliewithinhalfameterofthetruepoint,andthemeanvalueofanumberofpointsisaccuratetowithinacentimeter(horizontally),usingatopofthelinesurveygradereceiver.WithaGeoExplorer,youcanexpect95%ofthefixestoliewithinninemeters,withmeanswithintwometersofthetruepoint.TopracticeusingrealtimeDGPS,doPROJECT7I.

STEPBYSTEP

RealTime,DifferentialGPSPositionFindingPROJECT7I

RealTimeDifferentialGPSPositionFinding

Thisprojectassumesthatyouhaveaccesstoequipmentthatwillpermitrealtimedifferentialcorrection.Suchequipmentmightbe:

AsecondGPSreceiver,setupoveraknownpoint,transmittingRTCMformdatathatyourGeoExplorercanreceiveifaseparateradioisattachedtoit.

AradioreceiverconnectedtoyourGeoExplorerthatiswithinrangeofaU.S.CoastGuarddifferentialGPSbeacon.

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AradioreceiverconnectedtoyourGeoExplorerthatiswithinrangeofoneofseveralcommercialDGPSservicefacilities(e.g.,AccupointorDCI).

Acommunicationssatelliteantenna,receiver,anddemodulator,connectedtoyourGPSreceiver(i.e.,OMNISTAR,RacalLandstar).

Becauseofthewidevarietyofequipmentconfigurationsthatcanbeusedfordifferentialcorrection,thefollowingstepscanonlyserveasindicationsofwhatistobedone.

{__}Beforeleavingyourlaboroffice,makesureyouhaveagoodideaofwhatyouareabouttodo.RDGPSiswonderful,inthatitprovidesyouwithaccuratefixesinstantaneously.Thedownsideisthatyouhavealotmoreequipmenttocartaround,connect,andsupplyelectricalpowerto.Makesureyouunderstandandcheckoutalltheconnectionspriortotakingtheunitsintothefield.Datapacketsflowbetweentheradio13andtheGPSreceiverthroughthe"download"cableyouusedwhentransferringfilesfromtheGPSreceivertoaPC.

{_}Findasiteatwhichtotakedata.Ifyoucan,findasitewiththefollowingcharacteristics:

Aknownlocation,e.g.,asurveymonumentorbenchmark.

AclearviewoftheskyforreceivinggoodGPSsignals.

WithinrangeofagroundbasedRDGPSsignal,or,ifyouareusingOMNISTARorLandstar,withaclearviewoftheskytothesouthwherethecommunicationssatelliteisparked.(ThisisovertheEquator,duesouthofLakeMichigan,forOMNISTAR.)

Whereasourceofpowerisavailablefortheauxiliaryradioreceiver(possiblyanautomobileauxiliarypowerreceptacleoranadditionalbatterypack).

{_}StartacquiringGPSsignalswiththeGPSunit.

{__}Ifyoucan,assurethattheRDGPSradioisreceivingsignalsfromthetransmittingunit,butdonotyetconnectittotheGPSreceiver.

{__}Usingtheparametersfordatacollectionprovidedearlierinthistext,collectafileofabout20fixes,usingatimeintervaloffiveseconds.

{__}Connecttheoutputcablefromtheradiotothe"download"cablefromtheGeoExplorer.

{_}Under"Configuration~RTCM"youwillfindthismenu:

{_}Choose"Mode."Youwillseetheseoptions:

RTCM

"I.ModeGPS

2.PortB

3.Staletime010

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MsgCount0

MsgGaps0

Mode

Autonomous

Differential

AutoGPSGPD

CommPortA

1.ProtocolXMDM

2.Baud9600

3.ParityNone

4.DataBits8

5.StopBits1

6.Precision2

7.SendAt2s

"Autonomous"meansthereceivercalculatesonlyunconnectedGPSpositions.

"Differential"forcesthereceivertocalculate,present,and/orfileonlypositionscorrectedbyRTCMsignals.

"AutoGPSGPD"tellsthereceivertocalculatecorrectedpositionswhentheRTCMsignalisavailable,andtocalculateuncorrectedpositionsotherwise.(Theuncorrectedpositionscanbecorrectedlaterbypostprocessingifyouwish.)

{_}Choose"Differential."ThisisalsoshownasGPDmodeonsomeGeoExplorerscreens.

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{__}Under"Port"youprobablywant"A,"butthisdependsontherequirementsandcablesrelatedtotheradio.Onceyouselecttheport,youmustselectthecommunicationparametersthatrelatetothatportbychoosingoptionsonascreenlikethis:

{__}ChangetheProtocol:ThedefaultprotocolisXMDM,whichisusedwhentheGPSunitiscommunicatingwithaPC.Youneedtochangethissettingtotheoneappropriateforyourradio.Probablythiswillbeeither"RTCM"14or"RTCM/NMEA."

TheGeoExplorerhastheabilitytosenditspositionandvelocitydatathroughthedatacableinaformatcalled"NMEA0183."15Youusethiscapabilitywithadevicethatwantstoknowwhereyourreceiveris,suchastheOMNISTARorLandstarradio.

(Withmostmethodsofrealtimedifferentialcorrection,theapproximatelocationofyourreceiverisnotamystery,becauseithastobewithinrangeofasignalgeneratedlocallyandtransmittedfromanantennaonEarth.ButtheOMNISTARorLandstarsignalcomesfromasatellite22,000milesawayandcanbeusedalmostallovertheUnitedStates[andinpartsofCanadaandMexico].Sincedifferentialcorrectionsvaryfromplacetoplace,itisimportantthatyoureceivethesignalstailoredforyourarea.ThusyourreceivermusttelltheOMNISTARorLandstarradiowhereitis.)

SeetheRDGPSreceiveroperationmanualforspecificsontheprotocolsforsendingdatafromtheGPSunittotheradio.

{__}Setthebaud,parity,databits,andstopbitsasrequiredbyyourradio.

{__}IfyouareusingtheRTCM/NMEAprotocol(asyouwouldifyouwereusingOMNISTARorLandstar)youmustsettheNMEAprecision,whichindicatesthenumberofdecimalplacesinthelatitudeandlongitudeoutput.Avalueof"2"issuggested.(Thisprovidesforaprecisiontothenearest0.01degree.Thatincrementoflatitudeamountstoabout0.7milescertainlycloseenoughtochooseadifferentialcorrectionarea.Theruleofthumbisthateach60milesonthegroundgeneratesabouta1metererrorinthereading.)

{__}Settheoutputrate("SendAt")oftheNMEAsignal.(Twosecondsisadequate.)

{__}Backunder"Configuration~RTCM"youwillfind"StaleTime."ThisisthedurationthattheGPSreceiverwilltoleratebeforegivinguponreceivingaRTCMsignalwithwhichtocorrectfixes.InGPDmode,datapresentationandcollectionwillstopifnoRTCMmessageisreceivedduringthistime.(InGPS/GPDmode,afterthestaletimehaselapsed,thereceiverwillcontinuetocalculatefixes,buttheywillnotbecorrected.)

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{_}Observethe"MsgCount"and"MsgGaps"indicatorsonthissamescreen.Asyouwatch,youshouldseethemessagecountincreasebyatleastseveralperminute.ThenumberofmessagesreceivedastimepassesisagoodindicationthatyouarereceivingRTCMdatafromtheradio.Ifthisnumberdoesnotincrease,orresetstozerofrequently,youprobablyarenotgettingsatisfactoryinformationfromtheradio.Theproblemcouldbeinanynumberofplaces.Amongthem:

thebasestationnottransmitting,

yourRDGPSradionotreceiving,

badcableconnections,or

communicationparameterssetwrong.

Ifthingsdontseemtobeworking,recheckyoursetup.Ifthisdoesntwork,askyourinstructor.

{_}CheckthatyourGPSreceiverisstillcomputingfixes.Lookatthesatellitetrackingscreen(under"GPSStatus")andalsobecertainthatthe"Position"screendoesnotindicate"OLD."

{_}Assumingthateverythingisinorder,collect180fixesinafile.

{_}ChangetheRTCMmodebackto"Autonomous"andcollectanotherfileofabout20fixes.

{__}ChangetheRTCMmodebacktoitsdefault:"GPS/GPD."

{_}Changetheprotocolfortheportyouwereusingbackto"XMDM"andthecommunicationparametersbacktothosenecessarytocommunicatewithyourPC.

{_}Backatyourlaboroffice,transferthethreefilesyoutooktoaPC.Examinethem.Thefirstandthirdfilesshouldshowconsiderablevariation,whilethesecondfileshouldshowatightgroupofpoints.

TheGeo3canalsodorealtimecorrections.TheprocessbearsresemblancetothatfortheGeoExplorers.Differencesincludethewaytheinterfacewiththecablefromthecorrectionsreceiverissetup(thereisanattachmentthatgoesonthebackoftheGeo3)andwhereyoufindthevariousoptionstosetup(underCOMMSandOther).Bythistime,ifyouhavebeenworkingwiththeGeo3,youhaveaprettygoodfeelforthephilosophybehinditandthekindsofactionsyouhavetotaketogetittodowhatyouwantUsingitinrealtimemodewillnotbeasnap,butyoushouldbeabletodoPROJECT7Iabovewithouttoomuchdifficulty.

Nextpost:ThePresentandtheFuture(GPSandGIS)Part1

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