An Overview of the Global Positioning Satellite System GPS

1
An Overview of the Global Positioning Satellite
System (GPS)

• Edward J. Delp
• N9YTE
• October 25, 2000
• n9yte_at_n9yte.net
• http//www.n9yte.net

2
Overview

• The Navigation Problem
• Earlier Approaches
• GPS Description
• How does GPS Work?

3
Acknowledgement

• Many of the graphical images used in this talk
  are courtesy of Peter H. Dana of the Department
  of Geography, University of Texas at Austin -
  
• http//www.utexas.edu/depts/grg/gcraft/
• notes/gps/gps.html
• The best web site for GPS

4
The Navigation Problem

• The ancient question Where am I?
• Earth coordinates latitude and longitude
• Lafayette N40/W86
• Latitude can be determined by Sun angle
• What about longitude?

5
Latitude and Longitude
6
Latitude and Longitude
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Longitude Problem

• No easy way to determine longitude
• On July 8, 1714 the Longitude Act was established
  in England to solve the longitude problem
  
• Two solutions were proposed
• -- use of stars and moons
• -- the time solution

8
Longitude

• Longitude The True Story of a Lone Genius
  Who Solved the Greatest Scientific Problem of His
  Time
  
• Dava Sobel

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Longitude Problem References

• http//www.pbs.org/wgbh/nova/longitude/

10
The Time Solution

• Where am I? What time is it in Greenwich,
  England?
  
• The ability to find ones position is based on
  how well one can tell what time is it?
  
• The development of the chronometer
• To find longitude to within 0.5 degree requires a
  clock that loses or gains no more than 3
  seconds/day

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Longitude

• How does this work?
• The earth turns 360 degrees in 24 hours
• 15 degrees 1 hour
• If you know the time in Greenwich when it is
  local noon at your location one can find your
  longitude relative to Greenwich
  
• Must know datum reference to use maps

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Datum Reference
Lone Pine Cemetery - N400 13.8 E260 17.24
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Satellite Navigation

• US Department of Defense has need for very
  precise navigation
  
• In 1973, the US Air Force proposed a new system
  for navigation using satellites
  
• The system is known as Navigation System with
  Timing and Ranging Global Positioning System or
  NAVSTAR GPS

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NAVSTAR GPS Goals

• What time is it?
• What is my position (including attitude)?
• What is my velocity?
• Other Goals
• - What is the local time?
• - When is sunrise and sunset?
• - What is the distance between two points?
• - What is my estimated time arrival?

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GSP System

• Simply stated The GPS satellites are nothing
• more than a set of clocks in the sky

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GPS Segments

• Space Segment the constellation of satellites
• Control Segment control the satellites
• User Segment users with receivers

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Space Segment
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Space Segment

• System consists of 24 satellites in the
  operational mode 21 in use and 3 spares
  
• 3 other satellites are used for testing
• Altitude 20,200 Km with periods of 12 hr.
• Current Satellites Block IIR- 25,000,000
• 2000 KG
• Hydrogen Maser Atomic Clocks

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Hydrogen Maser Clock

• These clocks lose one second every
• 2,739,000 million years

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GPS Orbits
21
GPS Orbits
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Control Segment

• Master Control Station is located at the
• Consolidated Space Operations Center
• (CSOC) at Flacon Air Force Station near
• Colorado Springs

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Control Segment
24
CSOC

• Track the satellites for orbit and clock
  determination
  
• Time synchronization
• Upload the Navigation Message
• Manage DOA

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Operational Capabilities

• Initial Operational Capability - December 8,
• 1993
• Full Operational Capability declared by the
• Secretary of Defense at 0001 hours on
• July 17, 1995

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GPS Transmitted Signal

• Two signals are transmitted on carriers
• L1 1575.42 MHz
• L2 1227.60 MHz
• These are derived from the system clock of
• 10.23 MHz (phase quadrature)
• Modulation used is Direct Sequence Spread
  Spectrum
  
• (code division multiple access - CDMA)

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GPS Signals
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GPS Clock Signals

• Two types of clock signals are transmitted
• C/A Code - Coarse/Acquisition Code available for
  civilian use on L1 provides 300 m resolution
  
• P Code - Precise Code on L1 and L2 used by the
  military provides 3m resolution

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Spread Spectrum

• Spread Spectrum is used because
• - resistance to jamming
• - masks the transmissions
• - resist multipath effects
• - multiple access
• All 24 GPS satellites transmit on the same two
  frequencies BUT use a different ID sequence
  

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GPS Signals

• The satellites transmit as part of their unique
  Spread Spectrum signal a clock or timing signal
  
• The range or distance to the satellite is
  obtained by measuring how long it takes for the
  transmitted signal to reach the receiver
  
• This is not the true range due to clock errors
  - what is obtained is know as the pseudo-range

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GPS Position

• By knowing how far one is from three satellites
  one can ideally find their 3D coordinates
  
• To correct for clock errors one needs to receive
  four satellites

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GPS How does it work?

• Typical receiver one channel C/A code on L1
• During the acquisition time you are receiving
  the navigation message also on L1
  
• The receiver then reads the timing information
  and computes the pseudo-ranges
  
• The pseudo-ranges are then corrected

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GPS How does it work?

• Corrected ranges are used to compute the
  position
  
• This is a very complicated iterative nonlinear
  equation

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Navigation Message

• To compute your position one must know the
  position of the satellite
  
• Navigation Message - transmitted on both L1 and
  L2 at 50 bits/s for 30 s
  
• Navigation message consists of two parts
• - satellite almanac
• - clock bias

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Why Do I Need To See 4 Satellites?

• The problem is that the clock signal from the
  satellite is corrupted by atmospheric refraction
  
• Another major problem is that the receivers
  clock is not very accurate
  
• For a 2D fix 3 satellites

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Why Do I Need To See 4 Satellites?
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Denial of Accuracy (DOA)

• The US military uses two approaches to prohibit
  use of the full resolution of the system
  
• Selective Availability (SA) - noise is added to
  the clock signal and the navigation message has
  lies in it
  
• Anti-Spoofing (AS) - P-code is encrypted
• The military sometimes turns off both DOA
  techniques

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Differential GPS

• Used to improve accuracy
• Put a satellite on the ground at a precise
  position
  
• Differential signal is not transmitted on
  standard satellite frequencies

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Uses of GPS

• Airplane and Boat Navigation
• Continental Drift
• Surveying
• Precise Timing
• Iceberg Tracking
• Archaeological Expeditions
• Mobile Multimedia

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GPS Clock Rollover

• GPS System Time rolled over at midnight 21-22
  August 1999, 132 days before the Year 2000
  
• On 22 August 1999, unless repaired, many GPS
  receivers claimed that it is 6 January 1980
  
• http//www.navcen.uscg.mil/gps/geninfo/
• y2k/gpsweek.htm

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Conclusion

• GPS will find more civilian uses
• DOD has promised to eliminate SA
• Russia has a system known as GLONASS
• The EU is discussing deploying its own system

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References

• B. Hofmann-Wellenhof, H. Lichtenegger, and J.
  Collins, GPS Theory and Practice, Third Edition,
  Springer-Verlag, 1994.
  
• T. Logsdon, The Navstar Global Positioning
  System, Van Nostrand, 1992.
  
• A. Leick, GPS Satellite Surveying, Second
  edition, Wiley, 1995.

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References

• T. A. Herring, "The Global Positioning System,"
  Scientific American, pp. 44-50, February 1996.
  
• N. J. Hotchkiss, A Comprehensive Guide to Land
  Navigation with GPS, Alexis, 1994.
  
• Special Edition on the Global Positioning System,
  Satellite Times, March/April 1996.
  
• D. Sobel, Longitude, Walker, 1995.

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Web Sites

• GPS Program Office
• http//www.laafb.af.mil/SMC/CZ/homepage/
• US Coast Guard Navaigation Center
• http//www.navcen.uscg.mil/default.htm
• GPS Precise Orbits
• http//www.ngs.noaa.gov/GPS/GPS.html
• GPS World Magazine
• http//www.gpsworld.com/