Global positioning system overview
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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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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

NAVSTAR-2 GPS satellite


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


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


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


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


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


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


GPS Data


GPS Signals


Code Phase Tracking (Navigation)


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


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


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


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


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


GDOP Example


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 ~ $??


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)


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/


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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