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GPS OverviewGPS Overview

Chad HallChad HallTrimble Navigation LtdTrimble Navigation LtdMapping & GIS DivisionMapping & GIS Division

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Overview of the PresentationOverview of the PresentationHistory of GPSHistory of GPSThree Segments of GPSThree Segments of GPSHow to Get a PositionHow to Get a PositionErrors Sources in GPSErrors Sources in GPSDifferential GPSDifferential GPS

PostPost--ProcessingProcessingRealReal--time Correctiontime Correction

Accuracy RequirementsAccuracy RequirementsQuestion & Answer Your location is:

37o 23.323’ N122o 02.162’ W

Question & Answer

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History of the Global Positioning SystemHistory of the Global Positioning System

Started development in 1973Started development in 1973First four satellites launched in 1978First four satellites launched in 1978Full Operational Capacity (FOC) reached in July 1995Full Operational Capacity (FOC) reached in July 1995System cost of $12 billionSystem cost of $12 billionMaintained by the Department of Defense (Maintained by the Department of Defense (DoDDoD))GPS provides both civilian and military positioning GPS provides both civilian and military positioning globallygloballyGPS comprised of three “segments”GPS comprised of three “segments”

Space Segment (the satellites)Space Segment (the satellites)Ground Segment (the ground control network)Ground Segment (the ground control network)User Segment (GPS receivers and their users)User Segment (GPS receivers and their users)

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GPS Space SegmentGPS Space SegmentGPS ConstellationGPS Constellation

24 satellites (Space 24 satellites (Space Vehicles or SVs)Vehicles or SVs)10,900 nautical miles 10,900 nautical miles above Earth above Earth One revolution every One revolution every 12 hours12 hours6 orbital planes (556 orbital planes (55°°inclination)inclination)4 satellites in each 4 satellites in each planeplane

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GPS Control SegmentGPS Control Segment

Ground Control & Monitoring StationsGround Control & Monitoring Stations1 Master Control 1 Master Control -- Falcon AFB, COFalcon AFB, CO4 Monitoring Stations4 Monitoring StationsResponsible for monitoring the Responsible for monitoring the

health of satellites and the upload of health of satellites and the upload of navigation messages for broadcast navigation messages for broadcast

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GPS User SegmentGPS User Segment

Multiple ApplicationsMultiple Applications••GIS / CAD data captureGIS / CAD data capture••Vehicle trackingVehicle tracking••SurveyingSurveying••ConstructionConstruction••Pipeline mappingPipeline mapping••Utility asset collectionUtility asset collection••AgricultureAgriculture••PhotogrammetryPhotogrammetry••Recreation (hiking etc.)Recreation (hiking etc.)

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How to Get a PositionHow to Get a PositionNeed signal from at least four Need signal from at least four satellites for a 3D positionsatellites for a 3D positionOne satellite provides a time One satellite provides a time referencereferenceDistance to three remaining Distance to three remaining satellites is determined by satellites is determined by observing the GPS signal observing the GPS signal travel time from satellite to travel time from satellite to the receiverthe receiverWith three known points, and With three known points, and distances to each, we can distances to each, we can determine the GPS receiver’s determine the GPS receiver’s position (trilateration) position (trilateration)

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Errors Sources in GPSErrors Sources in GPSObstructionObstructionAtmospheric DelayAtmospheric DelayMultipath Multipath Selective Availability (S/A) Selective Availability (S/A)

Before May 2000 = 100mBefore May 2000 = 100mAfter May 2000 = 15mAfter May 2000 = 15m

Question:Question: Is 15 meters of error good enough accuracy Is 15 meters of error good enough accuracy for mapping and survey professionals?for mapping and survey professionals?Answer: Answer: No, reason why one must postNo, reason why one must post--process or process or use a realuse a real--time differential correction source in order time differential correction source in order to achieve better accuracies.to achieve better accuracies.

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PostPost--ProcessingProcessing

Base Station (GPS receiver) at Base Station (GPS receiver) at a known location a known location

(1 cm of accuracy)(1 cm of accuracy)

SatellitesSatellites

Rover (GPS Receiver) Rover (GPS Receiver) 15 m of accuracy15 m of accuracy

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RealReal--Time Differential CorrectionTime Differential CorrectionSatellitesSatellites

SatelliteSatelliteCorrectionCorrection

ServiceService

Cell PhoneCell PhoneCorrectionsCorrections

CoastguardCoastguardBeaconBeacon

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Example of Uncorrected File (15 meters)Example of Uncorrected File (15 meters)

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Example of Corrected File (Example of Corrected File (<1 meter)<1 meter)

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Recreational Grade GPSRecreational Grade GPS

15 m15 m

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Mapping Grade GISMapping Grade GIS

22--3 m3 m

CoastguardCoastguardBeaconBeacon

PostPost--ProcessProcessCellularCellular

SatelliteSatelliteCorrectionCorrection

ServiceService

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Mapping Grade GISMapping Grade GIS

<1m<1m

SatelliteSatelliteCorrectionCorrection

ServiceServiceCoastguardCoastguardBeaconBeacon

PostPost--ProcessProcessCellularCellular

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High Accuracy GPSHigh Accuracy GPS

11--2 cm2 cm

RTK Base StationRTK Base StationCellularCellular

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Where Do You Need to Be?Where Do You Need to Be?

15 m15 m22--3 m3 m

<1m<1m

11--2 cm2 cm

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Questions?Questions?

Your location is:37o 23.323’ N

122o 02.162’ W