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

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


Gps data

GPS Data


Gps signals

GPS Signals


Code phase tracking navigation

Code Phase Tracking (Navigation)


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


Gdop example

GDOP Example


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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