Global Positioning System Global Positioning System OverviewOverview

Author: Peter H. DanaAuthor: Peter H. Dana

Herman LiHerman LiOct 18, 2004Oct 18, 2004

What is GPSWhat is GPS

GPS = Global Positioning SystGPS = Global Positioning Systemem

Position, velocity, timePosition, velocity, time Network of min. 24 (29) satellNetwork of min. 24 (29) satell

ites, orbiting the earth every ites, orbiting the earth every ~12 hours~12 hours

Funded and controlled by the Funded and controlled by the US DODUS DOD

Cost: 13 billion USCost: 13 billion US NAVSTAR-2 GPS satellite

Space SegmentSpace Segment

Altitude of 20,000kmAltitude of 20,000km Positioned such that 5 – 8 satellites are in Positioned such that 5 – 8 satellites are in

range at any timerange at any time Coded radio signals from 4 satellites can Coded radio signals from 4 satellites can

pinpoint location on earthpinpoint location on earth 6 orbital planes6 orbital planes

TrilaterationTrilateration

1 satellite – a sphere1 satellite – a sphere 2 satellites – a circle2 satellites – a circle 3 satellites – 2 points3 satellites – 2 points ECEF XYZ coordinatesECEF XYZ coordinates

Distance = Travel Time x Speed of LightDistance = Travel Time x Speed of Light Where are the satellitesWhere are the satellites Time is different in the skyTime is different in the sky

Control SegmentControl Segment

1ns drift / 3 hours1ns drift / 3 hours 1ns = ~30cm error1ns = ~30cm error 5 monitor stations5 monitor stations 3 ground antennas3 ground antennas

Radars located around the worldRadars located around the world GPS broadcasts received ephemeris and clock GPS broadcasts received ephemeris and clock

correction data to receiverscorrection data to receivers Practically few ns error = ~ 1mPractically few ns error = ~ 1m

Positioning ServicesPositioning Services

Precise Positioning Service (PPS)Precise Positioning Service (PPS)• 22m horizontal, 27.7m vertical accuracy22m horizontal, 27.7m vertical accuracy• 200ns accuracy200ns accuracy

Standard Positioning Service (SPS)Standard Positioning Service (SPS)• 100m horizontal, 156m vertical accuracy (SA includ100m horizontal, 156m vertical accuracy (SA includ

ed)ed)• 340ns accuracy340ns accuracy• SA turned off as of May 2, 2000SA turned off as of May 2, 2000

GPS Satellite SignalsGPS Satellite Signals

Two microwave carrier signalsTwo microwave carrier signals• L1 (1575.42MHz)L1 (1575.42MHz)• L2 (1227.60MHz)L2 (1227.60MHz)• L5 band comingL5 band coming

Speed of light variesSpeed of light varies• Ionosphere slows down lower frequencies moreIonosphere slows down lower frequencies more• Use L1 & L2 to remove ionosphere effectsUse L1 & L2 to remove ionosphere effects• Difference in arrival time removes errorDifference in arrival time removes error• Moisture in Troposphere also slows down signalMoisture in Troposphere also slows down signal

GPS Satellite SignalsGPS Satellite Signals C/A (Coarse/Acquisition) CodeC/A (Coarse/Acquisition) Code

• a.k.a Civilian Codea.k.a Civilian Code• Unique pseudo-random-noise (PRN) code modulatUnique pseudo-random-noise (PRN) code modulat

ed on L1 and repeated every 1msed on L1 and repeated every 1ms

P(Y)-Code (Precise Code)P(Y)-Code (Precise Code)• PRN code modulated on both L1 and L2 and repeatPRN code modulated on both L1 and L2 and repeat

ed every 7 daysed every 7 days

Codes used for downloading ephemeris every Codes used for downloading ephemeris every 30 sec and almanac every 12.5 min30 sec and almanac every 12.5 min

GPS DataGPS Data

GPS SignalsGPS Signals

Code Phase Tracking Code Phase Tracking (Navigation)(Navigation)

Clock synchronizationClock synchronization

Satellites have 4 atomic clocksSatellites have 4 atomic clocks Receivers are cheap, 1us drift / secReceivers are cheap, 1us drift / sec Assuming distance from satellites already Assuming distance from satellites already

known, 4known, 4thth satellite solves the extra variable satellite solves the extra variable

Differential Code Phase GPSDifferential Code Phase GPS

Base station (known position) computes coBase station (known position) computes corrections for each satellite signalrrections for each satellite signal

Corrections sent to rover receiversCorrections sent to rover receivers Removes errors except multipRemoves errors except multip

ath and receiver ath and receiver errors errors

Carrier Phase Tracking Carrier Phase Tracking (Survey)(Survey)

Requires specially equipped receiversRequires specially equipped receivers Track L1 and/or L2 carrier signalsTrack L1 and/or L2 carrier signals No time of transmission infoNo time of transmission info Requires differential calculations of Requires differential calculations of

receivers within 30kmreceivers within 30km Sub-cm Sub-cm

accuracy accuracy possiblepossible

Differential GPSDifferential GPS

DGPS – Differential Code Phase GPSDGPS – Differential Code Phase GPS• Instantaneous results, less accurateInstantaneous results, less accurate• Real-time or post-processedReal-time or post-processed

CPD – Carrier Phase DifferentialCPD – Carrier Phase Differential• Increased accuracy due to increased frequencyIncreased accuracy due to increased frequency

RTK – Real-time KinematicRTK – Real-time Kinematic• ie. Real-time Carrier Phaseie. Real-time Carrier Phase• Time to determine initial full cycles, accurateTime to determine initial full cycles, accurate

GPS Error SourcesGPS Error Sources

PRN code noise (1m), receiver noise (1m)PRN code noise (1m), receiver noise (1m) Selective availability Selective availability (no longer the case)(no longer the case) Uncorrected satellite clock error (1m)Uncorrected satellite clock error (1m) Ephemeris data errors (1m)Ephemeris data errors (1m) Tropospheric delays (10m)Tropospheric delays (10m) Unmodeled ionosphere delaysUnmodeled ionosphere delays Multipath (0.5m)Multipath (0.5m) Geometric Dilution of Precision (GDOP)Geometric Dilution of Precision (GDOP)

• Bad when angles between receiver and satellites arBad when angles between receiver and satellites are similare similar

GDOP ExampleGDOP Example

User Equipment SegmentUser Equipment Segment

GPS receivers and user communityGPS receivers and user community Cheap outdoor GPS ~ $180USCheap outdoor GPS ~ $180US Outdoor GPS with map ~ $375USOutdoor GPS with map ~ $375US Personal GPS with street map ~ Personal GPS with street map ~

$590US$590US Avionics GPS ~ $??Avionics GPS ~ $??

Now and BeyondNow and Beyond

WAASWAAS (Wide Area (Wide Area Augmentation System)Augmentation System)• FAA + DOT for precision FAA + DOT for precision

flight approachesflight approaches• Corrected differential Corrected differential

messages broadcast by 2 messages broadcast by 2 geostationary satellitesgeostationary satellites

GLONASSGLONASS• Russian Federation’s satellite navigation system (2006)Russian Federation’s satellite navigation system (2006)

GalileoGalileo• European Union and European Space Agency (2008)European Union and European Space Agency (2008)

ReferencesReferences A GPS TutorialA GPS Tutorial

• http://http://www.topconps.com/gpstutorial/TOC.htmlwww.topconps.com/gpstutorial/TOC.html FAA GPS FAQFAA GPS FAQ

• http://gps.faa.gov/FAQ/faq-gps.htmhttp://gps.faa.gov/FAQ/faq-gps.htm GPS OverviewGPS Overview

• http://www.palowireless.com/gps/tutorial1.asphttp://www.palowireless.com/gps/tutorial1.asp How GPS Receivers WorkHow GPS Receivers Work

• http://electronics.howstuffworks.com/gps.htmhttp://electronics.howstuffworks.com/gps.htm The Fundamentals of GPSThe Fundamentals of GPS

• http://www.directionsmag.com/article.php?article_id=228http://www.directionsmag.com/article.php?article_id=228 DGPS ExplainedDGPS Explained

• http://healthweb.ofs.gov.za/othersites/hwm/Medical%20Wastehttp://healthweb.ofs.gov.za/othersites/hwm/Medical%20Waste%20Management/Differential%20GPS_files/dgpsexp.htm%20Management/Differential%20GPS_files/dgpsexp.htm

USNO GPS Timing OperationsUSNO GPS Timing Operations• http://tycho.usno.navy.mil/gps.htmlhttp://tycho.usno.navy.mil/gps.html

GarminGarmin• http://www.garmin.com/http://www.garmin.com/

Time DilationTime Dilation ““Net secular relativistic effect is 38.6s per dayNet secular relativistic effect is 38.6s per day

• Nominal clock rate is 10.23 MHzNominal clock rate is 10.23 MHz• Satellite clocks are offset by – 4.464733 parts in 1010 to compSatellite clocks are offset by – 4.464733 parts in 1010 to comp

ensate effectensate effect• Resulting (proper) frequency in orbit is 10229999.9954326 HzResulting (proper) frequency in orbit is 10229999.9954326 Hz• Observed average rate of satellite clock is same as clock on thObserved average rate of satellite clock is same as clock on th

e geoid” e geoid” ““Relativity has become an important practical enginRelativity has become an important practical engin

eering consideration for modern precise timekeeping eering consideration for modern precise timekeeping systems. These relativistic effects are well understood systems. These relativistic effects are well understood and have been applied successfully in the GPS.”and have been applied successfully in the GPS.”

http://www.navcen.uscg.gov/cgsic/meetings/http://www.navcen.uscg.gov/cgsic/meetings/summaryrpts/41stmeeting/18%20Nelson%20.PPTsummaryrpts/41stmeeting/18%20Nelson%20.PPT