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GPS Overview
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What is GPS?What is GPS? Radio-based navigation system developed by Radio-based navigation system developed by
DoDDoD• Initial operation in 1993Initial operation in 1993• Fully operational in 1995Fully operational in 1995
System is called NAVSTARSystem is called NAVSTAR• NAVNAVigation with igation with SSatellite atellite TTiming iming AAnd nd
RRanginganging• Referred to as GPSReferred to as GPS
Series of 24 satellites, 6 orbital planes, 4 Series of 24 satellites, 6 orbital planes, 4 satellite vehicles (SV) on each plane satellite vehicles (SV) on each plane
Works anywhere in the world, 24 hours a day, Works anywhere in the world, 24 hours a day, in all weather conditions and provides: in all weather conditions and provides: • Location or positional fixLocation or positional fix• VelocityVelocity• Direction of travelDirection of travel• Accurate timeAccurate time
Global Navigation Satellite Systems Global Navigation Satellite Systems (GNSS) (GNSS)
NAVSTARNAVSTAR• USAUSA
GLONASSGLONASS• RussiansRussians
GalileoGalileo• EuropeansEuropeans
TrilaterationTrilateration• Intersection of spheresIntersection of spheres
SV RangingSV Ranging• Determining distance from SVDetermining distance from SV
TimingTiming• Why consistent, accurate clocks are requiredWhy consistent, accurate clocks are required
PositioningPositioning• Knowing where SV is in spaceKnowing where SV is in space
Correction of errorsCorrection of errors• Correcting for ionospheric and tropospheric Correcting for ionospheric and tropospheric
delaysdelays
GPS involves 5 Basic StepsGPS involves 5 Basic Steps
How GPS works?How GPS works?
Range from each satellite calculatedRange from each satellite calculated range = time delay X speed of lightrange = time delay X speed of light
Technique called Technique called trilaterationtrilateration is used to is used to determine you position or “fix” determine you position or “fix” • Intersection of spheresIntersection of spheres
At least 3 satellites required for 2D fix At least 3 satellites required for 2D fix However, 4 satellites should always be usedHowever, 4 satellites should always be used
• The 4The 4thth satellite used to compensate for satellite used to compensate for inaccurate clock in GPS receiversinaccurate clock in GPS receivers
• Yields much better accuracy and provides 3D fixYields much better accuracy and provides 3D fix
Determining RangeDetermining Range
Receiver and satellite use same Receiver and satellite use same codecode
Synchronized code generationSynchronized code generation Compare incoming code with Compare incoming code with
receiver generated codereceiver generated code
From satellite
Measure time difference between the same part of code
From receiver
Series of ones and zeroes repeatingevery 1023 bits. So Complicated alternation of bits that pattern looks random thus called“pseudorandom code”.
Signal StructureSignal Structure
Each satellite transmits its own unique Each satellite transmits its own unique codecode
Two frequencies usedTwo frequencies used• L1 CarrierL1 Carrier 1575.42 MHz 1575.42 MHz • L2 CarrierL2 Carrier 1227.60 MHz 1227.60 MHz
CodesCodes• CA Code use L1 (civilian code) CA Code use L1 (civilian code) • P (Y) Code use L1 & L2 (military code) P (Y) Code use L1 & L2 (military code)
Three SV ranges knownThree SV ranges known
20,000 Km radius22,000 Km radius
21,000 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 KeyAccurate Timing is the Key
SVs have highly accurate atomic clocksSVs have highly accurate atomic clocks Receivers have less accurate clocksReceivers have less accurate clocks Measurements made using “nanoseconds”Measurements made using “nanoseconds”
• 1 nanosecond = 1 billionth of a second1 nanosecond = 1 billionth of a second 1/1001/100thth of a second error could introduce of a second error could introduce
error of 1,860 miles error of 1,860 miles Discrepancy between satellite and receiver Discrepancy between satellite and receiver
clocks must be resolvedclocks must be resolved Fourth satellite is required to solve the 4 Fourth satellite is required to solve the 4
unknowns (X, Y, Z and receiver clock error)unknowns (X, Y, Z and receiver clock error)
Satellite PositioningSatellite Positioning
Also required in the equation to solve Also required in the equation to solve the 4 unknowns is the actual location the 4 unknowns is the actual location of the satellite.of the satellite.
SV are in relatively stable orbits and SV are in relatively stable orbits and constantly monitored on the groundconstantly monitored on the ground
SV position is broadcast in the SV position is broadcast in the “ephemeris” data streamed down to “ephemeris” data streamed down to receiverreceiver
Sources of ErrorsSources of Errors
Largest source is due to the Largest source is due to the atmosphereatmosphere• Atmospheric refractionAtmospheric refraction
Charged particlesCharged particles Water vaporWater vapor
Ionosphere(Charged Particles)
Troposphere
Other Sources of ErrorsOther Sources of Errors
Geometry of satellite positions Geometry of satellite positions Satellite clock errorsSatellite clock errors SV position or “ephemeris” errorsSV position or “ephemeris” errors Quality of GPS receiverQuality of GPS receiver Multi-path errorsMulti-path errors
Dilution of Precision (DOP)Dilution of Precision (DOP) Geometric location of the satellites as Geometric location of the satellites as
seen by the receiverseen by the receiver The more spread out the satellites are in The more spread out the satellites are in
the sky, the better the satellite geometrythe sky, the better the satellite geometry PDOP (position dilution of precision) is a PDOP (position dilution of precision) is a
combination of VDOP and HDOPcombination of VDOP and HDOP The lower the PDOP value, the better the The lower the PDOP value, the better the
geometric strengthgeometric strength PDOP value less than 6 is recommendedPDOP value less than 6 is recommended
Selective AvailabilitySelective Availability
The intentional introduction of errors for The intentional introduction of errors for civilian users is called civilian users is called Selective Availability Selective Availability
SA was terminated on May 2, 2000SA was terminated on May 2, 2000 When SA was on, civilian users accuracy When SA was on, civilian users accuracy
was ~100 meters was ~100 meters Military has capability to degrade signal in Military has capability to degrade signal in
certain “theaters of operation” – this is certain “theaters of operation” – this is called called “spoofing” “spoofing”
Differential CorrectionDifferential Correction
Technique used to correct some of these errorsTechnique used to correct some of these errors Referred to as “differential GPS” or DGPSReferred to as “differential GPS” or DGPS In DGPS, two GPS receivers are usedIn DGPS, two GPS receivers are used One receiver is located at an accurately surveyed point One receiver is located at an accurately surveyed point
referred to as the “base station”referred to as the “base station” A correction is calculated by comparing the known A correction is calculated by comparing the known
location to the location determined by the GPS satelliteslocation to the location determined by the GPS satellites The correction is then applied to the other receiver’s The correction is then applied to the other receiver’s
(known as the “rover”) calculated position(known as the “rover”) calculated position
DGPS MethodsDGPS Methods
Post-processing Post-processing • Corrections performed Corrections performed after the data is after the data is
collectedcollected• Special software required Special software required
Real-time Real-time • Corrections are performed Corrections are performed while the data is while the data is
being collectedbeing collected• Need special equipment to receive the Need special equipment to receive the
DGPS signalDGPS signal
Wide Area Augmentation System - WAASWide Area Augmentation System - WAAS
New “real-time” DGPSNew “real-time” DGPS Satellite based Satellite based FAA initiative….now fully operationalFAA initiative….now fully operational Series of ~25 ground reference stations Series of ~25 ground reference stations
relay info to master control stationrelay info to master control station Master control station sends correction Master control station sends correction
info to WAAS satelliteinfo to WAAS satellite• http://http://gps.faa.gov/programs/waas/howitworks.htmgps.faa.gov/programs/waas/howitworks.htm
WAAS SatellitesWAAS Satellites
WAAS satellites are WAAS satellites are geo-stationarygeo-stationary On east coast, WAAS satellite sits off coast of Brazil over On east coast, WAAS satellite sits off coast of Brazil over
equator at 53.96equator at 53.96° West (#35 on Garmin)° West (#35 on Garmin)• http://www.lyngsat.com/tracker/inmar3f4.shtmhttp://www.lyngsat.com/tracker/inmar3f4.shtm
On west coast, WAAS satellite sits over Pacific ocean at On west coast, WAAS satellite sits over Pacific ocean at 178.0° East (#47 on Garmin)178.0° East (#47 on Garmin)• http://www.lyngsat.com/tracker/inmar3f3.shtmlhttp://www.lyngsat.com/tracker/inmar3f3.shtml
Ability to get signal deteriorates in northern latitudes Ability to get signal deteriorates in northern latitudes (satellite is lower on the horizon)(satellite is lower on the horizon)
If you can get WAAS satellite signal……..~3 meter accuracyIf you can get WAAS satellite signal……..~3 meter accuracy However, cannot always get signal due to obstructions However, cannot always get signal due to obstructions More WAAS satellites becoming available in futureMore WAAS satellites becoming available in future
• Europeans (EGNOS)Europeans (EGNOS)• Japanese (MSAS)Japanese (MSAS)
GPS Accuracy ComparisonGPS Accuracy Comparison
GPS DeviceGPS Device AutonomoAutonomousus
WAASWAAS
DGPSDGPSReal-Real-timetime DGPSDGPS
Post-Post-processprocess DGPSDGPS
Garmin GPSMap 76s Garmin GPSMap 76s ~ 10 - 15~ 10 - 15 ~3~3 33 1 - 31 - 3
Rockwell – PLGRRockwell – PLGR
Federal Users OnlyFederal Users Only ~ 8 - 15~ 8 - 15 NANA 33 NANA
Trimble - GeoXTTrimble - GeoXT ~ 10~ 10 ~3~3 1-31-3 Sub-meterSub-meter
Some common GPS devices used by FWS:
Accuracy given in meters
GPS Accuracy Issues GPS Accuracy Issues
Ways to improve the accuracy of your GPS collected Ways to improve the accuracy of your GPS collected datadata• Standardize data collection methodsStandardize data collection methods• Establish protocols for your applicationsEstablish protocols for your applications• Employ averaging techniquesEmploy averaging techniques• Perform mission planningPerform mission planning• Utilize DGPS Utilize DGPS • Understand how the selection of datums and coordinate Understand how the selection of datums and coordinate
systems affect accuracysystems affect accuracy GPS data collected in wrong datum can introduce ~200 meters of error GPS data collected in wrong datum can introduce ~200 meters of error
into your GIS!into your GIS!
Some issues to consider when Some issues to consider when purchasing GPS devicespurchasing GPS devices
What is the accuracy level required for your application? What is the accuracy level required for your application? (10 meters or sub-meter) (10 meters or sub-meter)
How is unit going to be used in field?How is unit going to be used in field?• External antenna required, in heavy canopy, ease of use, External antenna required, in heavy canopy, ease of use,
durability, data dictionary capability, waterproof…durability, data dictionary capability, waterproof…
Cost…… from $100 to $12KCost…… from $100 to $12K
Staff expertise..training..support networkStaff expertise..training..support network
How well does unit interface with GIS?How well does unit interface with GIS?
Mobile mapping software for WindowsCE devices
TerraSync (Trimble)
ArcPad (ESRI)
Multi-path rejection technology
Trimble GeoXT
Bluetooth
Allows for cable free operation
Latest Technology
Bring GIS data into the field!
Custom forms for data collection
ArcPad Software
Integrate GPS with GIS
ArcPad TrainingArcPad Training
NCTC 3 day course (TEC7133)NCTC 3 day course (TEC7133) Utilize ArcPad Tools for ArcGISUtilize ArcPad Tools for ArcGIS
• Geodatabase - “check out” & “check in”Geodatabase - “check out” & “check in” Design custom forms for data collectionDesign custom forms for data collection
• Applets & ArcPad StudioApplets & ArcPad Studio Utilize GPSCorrect extensionUtilize GPSCorrect extension Customize the ArcPad interface Customize the ArcPad interface
Visit paraswadher.com & wadher.info for more