GPS Overview

TEC7132April 2004

What is GPS?Radio-based navigation system developed by DoDInitial operation in 1993Fully operational in 1995System is called NAVSTARNAVigation with Satellite Timing And RangingReferred to as GPSSeries of 24 satellites, 6 orbital planes, 4 satellite vehicles (SV) on each plane Works anywhere in the world, 24 hours a day, in all weather conditions and provides: Location or positional fixVelocityDirection of travelAccurate time

Global Navigation Satellite Systems (GNSS)

NAVSTARUSA

GLONASSRussians

GalileoEuropeans

GPS involves 5 Basic StepsTrilaterationIntersection of spheresSV RangingDetermining distance from SVTimingWhy consistent, accurate clocks are requiredPositioningKnowing where SV is in spaceCorrection of errorsCorrecting for ionospheric and tropospheric delays

How GPS works?Range from each satellite calculated

range = time delay X speed of lightTechnique called trilateration is used to determine you position or fix Intersection of spheresAt least 3 satellites required for 2D fix However, 4 satellites should always be usedThe 4th satellite used to compensate for inaccurate clock in GPS receiversYields much better accuracy and provides 3D fix

Determining RangeReceiver and satellite use same codeSynchronized code generationCompare incoming code with receiver generated code

From satelliteMeasure time difference between the same part of codeFrom receiverSeries of ones and zeroes repeatingevery 1023 bits. So Complicated alternation of bits that pattern looks random thus calledpseudorandom code.

Signal StructureEach satellite transmits its own unique codeTwo frequencies usedL1 Carrier 1575.42 MHz L2 Carrier 1227.60 MHzCodesCA Code use L1 (civilian code) P (Y) Code use L1 & L2 (military code)

Three SV ranges known20,000 Km radius22,000 Km radius21,000 Km radiusLocated at one of these 2 points. However, one point can easily be eliminated because it is either not on earth or moving at impossiblerate of speed.

Accurate Timing is the KeySVs have highly accurate atomic clocksReceivers have less accurate clocksMeasurements made using nanoseconds1 nanosecond = 1 billionth of a second1/100th of a second error could introduce error of 1,860 miles Discrepancy between satellite and receiver clocks must be resolvedFourth satellite is required to solve the 4 unknowns (X, Y, Z and receiver clock error)

Satellite PositioningAlso required in the equation to solve the 4 unknowns is the actual location of the satellite.SV are in relatively stable orbits and constantly monitored on the groundSV position is broadcast in the ephemeris data streamed down to receiver

Sources of ErrorsLargest source is due to the atmosphereAtmospheric refraction

Charged particlesWater vapor

Other Sources of ErrorsGeometry of satellite positions Satellite clock errorsSV position or ephemeris errorsQuality of GPS receiverMulti-path errors

Dilution of Precision (DOP)Geometric location of the satellites as seen by the receiverThe more spread out the satellites are in the sky, the better the satellite geometryPDOP (position dilution of precision) is a combination of VDOP and HDOPThe lower the PDOP value, the better the geometric strengthPDOP value less than 6 is recommended

Selective AvailabilityThe intentional introduction of errors for civilian users is called Selective Availability SA was terminated on May 2, 2000When SA was on, civilian users accuracy was ~100 meters Military has capability to degrade signal in certain theaters of operation this is called spoofing

Differential CorrectionTechnique used to correct some of these errorsReferred to as differential GPS or DGPSIn DGPS, two GPS receivers are usedOne receiver is located at an accurately surveyed point referred to as the base stationA correction is calculated by comparing the known location to the location determined by the GPS satellitesThe correction is then applied to the other receivers (known as the rover) calculated position

DGPS MethodsPost-processing Corrections performed after the data is collectedSpecial software required

Real-time Corrections are performed while the data is being collectedNeed special equipment to receive the DGPS signal

Wide Area Augmentation System - WAASNew real-time DGPSSatellite based FAA initiative.now fully operationalSeries of ~25 ground reference stations relay info to master control stationMaster control station sends correction info to WAAS satellitehttp://gps.faa.gov/programs/waas/howitworks.htm

WAAS SatellitesWAAS satellites are geo-stationary On east coast, WAAS satellite sits off coast of Brazil over equator at 53.96 West (#35 on Garmin)http://www.lyngsat.com/tracker/inmar3f4.shtmOn west coast, WAAS satellite sits over Pacific ocean at 178.0 East (#47 on Garmin)http://www.lyngsat.com/tracker/inmar3f3.shtmlAbility to get signal deteriorates in northern latitudes (satellite is lower on the horizon)If you can get WAAS satellite signal..~3 meter accuracyHowever, cannot always get signal due to obstructions More WAAS satellites becoming available in futureEuropeans (EGNOS)Japanese (MSAS)

GPS Accuracy ComparisonSome common GPS devices used by FWS:Accuracy given in meters

GPS Device

Autonomous

WAASDGPS

Real-time DGPS

Post-process DGPS

Garmin GPSMap 76s

~ 10 - 15

~3

3

1 - 3 Rockwell PLGRFederal Users Only

~ 8 - 15

NA

3

NA

Trimble - GeoXT

~ 10

~3

1-3

Sub-meter

GPS Accuracy Issues Ways to improve the accuracy of your GPS collected dataStandardize data collection methodsEstablish protocols for your applicationsEmploy averaging techniquesPerform mission planningUtilize DGPS Understand how the selection of datums and coordinate systems affect accuracy

GPS data collected in wrong datum can introduce ~200 meters of error into your GIS!

Some issues to consider when purchasing GPS devicesWhat is the accuracy level required for your application?

(10 meters or sub-meter)

How is unit going to be used in field?External antenna required, in heavy canopy, ease of use, durability, data dictionary capability, waterproof

Cost from $100 to $12K

Staff expertise..training..support network

How well does unit interface with GIS?

Mobile mapping software for WindowsCE devicesTerraSync (Trimble)ArcPad (ESRI)

Multi-path rejection technology Trimble GeoXT

BluetoothAllows for cable free operation

Latest Technology

ArcPad Software

ArcPad TrainingNCTC 3 day course (TEC7133)Utilize ArcPad Tools for ArcGISGeodatabase - check out & check inDesign custom forms for data collectionApplets & ArcPad StudioUtilize GPSCorrect extensionCustomize the ArcPad interface

Next offering: September 1-3, 2004