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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

Global Positioning System Overview

Author: Peter H. Dana

Herman Li

Oct 18, 2004

what is gps
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
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
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
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
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
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 signals1
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
clock synchronization
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
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
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
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
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
user equipment segment
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
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
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
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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