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Global Positioning System Overview

Global Positioning System Overview. Author: Peter H. Dana Herman Li Oct 18, 2004. What is GPS. GPS = Global Positioning System Position, velocity, time Network of min. 24 (29) satellites, orbiting the earth every ~12 hours Funded and controlled by the US DOD Cost: 13 billion US.

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Global Positioning System Overview

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  1. Global Positioning System Overview Author: Peter H. Dana Herman Li Oct 18, 2004

  2. What is GPS • GPS = Global Positioning System • Position, velocity, time • Network of min. 24 (29) satellites, orbiting the earth every ~12 hours • Funded and controlled by the US DOD • Cost: 13 billion US NAVSTAR-2 GPS satellite

  3. Space Segment • Altitude of 20,000km • Positioned such that 5 – 8 satellites are in range at any time • Coded radio signals from 4 satellites can pinpoint location on earth • 6 orbital planes

  4. Trilateration • 1 satellite – a sphere • 2 satellites – a circle • 3 satellites – 2 points • ECEF XYZ coordinates • Distance = Travel Time x Speed of Light • Where are the satellites • Time is different in the sky

  5. Control Segment • 1ns drift / 3 hours • 1ns = ~30cm error • 5 monitor stations • 3 ground antennas • Radars located around the world • GPS broadcasts received ephemeris and clock correction data to receivers • Practically few ns error = ~ 1m

  6. Positioning Services • Precise Positioning Service (PPS) • 22m horizontal, 27.7m vertical accuracy • 200ns accuracy • Standard Positioning Service (SPS) • 100m horizontal, 156m vertical accuracy (SA included) • 340ns accuracy • SA turned off as of May 2, 2000

  7. GPS Satellite Signals • Two microwave carrier signals • L1 (1575.42MHz) • L2 (1227.60MHz) • L5 band coming • Speed of light varies • Ionosphere slows down lower frequencies more • Use L1 & L2 to remove ionosphere effects • Difference in arrival time removes error • Moisture in Troposphere also slows down signal

  8. GPS Satellite Signals • C/A (Coarse/Acquisition) Code • a.k.a Civilian Code • Unique pseudo-random-noise (PRN) code modulated on L1 and repeated every 1ms • P(Y)-Code (Precise Code) • PRN code modulated on both L1 and L2 and repeated every 7 days • Codes used for downloading ephemeris every 30 sec and almanac every 12.5 min

  9. GPS Data

  10. GPS Signals

  11. Code Phase Tracking (Navigation)

  12. Clock synchronization • Satellites have 4 atomic clocks • Receivers are cheap, 1us drift / sec • Assuming distance from satellites already known, 4th satellite solves the extra variable

  13. Differential Code Phase GPS • Base station (known position) computes corrections for each satellite signal • Corrections sent to rover receivers • Removes errors except multipath and receiver errors

  14. Carrier Phase Tracking (Survey) • Requires specially equipped receivers • Track L1 and/or L2 carrier signals • No time of transmission info • Requires differential calculations of receivers within 30km • Sub-cm accuracy possible

  15. Differential GPS • DGPS – Differential Code Phase GPS • Instantaneous results, less accurate • Real-time or post-processed • CPD – Carrier Phase Differential • Increased accuracy due to increased frequency • RTK – Real-time Kinematic • ie. Real-time Carrier Phase • Time to determine initial full cycles, accurate

  16. GPS Error Sources • PRN code noise (1m), receiver noise (1m) • Selective availability (no longer the case) • Uncorrected satellite clock error (1m) • Ephemeris data errors (1m) • Tropospheric delays (10m) • Unmodeled ionosphere delays • Multipath (0.5m) • Geometric Dilution of Precision (GDOP) • Bad when angles between receiver and satellites are similar

  17. GDOP Example

  18. User Equipment Segment • GPS receivers and user community • Cheap outdoor GPS ~ $180US • Outdoor GPS with map ~ $375US • Personal GPS with street map ~ $590US • Avionics GPS ~ $??

  19. Now and Beyond • WAAS (Wide Area Augmentation System) • FAA + DOT for precision flight approaches • Corrected differential messages broadcast by 2 geostationary satellites • GLONASS • Russian Federation’s satellite navigation system (2006) • Galileo • European Union and European Space Agency (2008)

  20. References • A GPS Tutorial • http://www.topconps.com/gpstutorial/TOC.html • FAA GPS FAQ • http://gps.faa.gov/FAQ/faq-gps.htm • GPS Overview • http://www.palowireless.com/gps/tutorial1.asp • How GPS Receivers Work • http://electronics.howstuffworks.com/gps.htm • The Fundamentals of GPS • http://www.directionsmag.com/article.php?article_id=228 • DGPS Explained • http://healthweb.ofs.gov.za/othersites/hwm/Medical%20Waste%20Management/Differential%20GPS_files/dgpsexp.htm • USNO GPS Timing Operations • http://tycho.usno.navy.mil/gps.html • Garmin • http://www.garmin.com/

  21. Time Dilation • “Net secular relativistic effect is 38.6s per day • Nominal clock rate is 10.23 MHz • Satellite clocks are offset by – 4.464733 parts in 1010 to compensate effect • Resulting (proper) frequency in orbit is 10229999.9954326 Hz • Observed average rate of satellite clock is same as clock on the geoid”  • “Relativity has become an important practical engineering consideration for modern precise timekeeping systems.These relativistic effects are well understood and have been applied successfully in the GPS.” http://www.navcen.uscg.gov/cgsic/meetings/ summaryrpts/41stmeeting/18%20Nelson%20.PPT

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