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1
Global Positioning SystemTheory and Practice
Matthew J. Gray
College of Agricultural Sciences and Natural Resources
University of Tennessee-Knoxville
Lecture Outline
• GPS Operation• Trilateration and GPS Errors• Differential Correction• Trimble Planning Software and Topozone.com
Reading Assignment:
1) Chapter 15: pp. 429-433 and 421-429:
• “Using Global Positioning Systems”
• “Using GIS in the Wildlife Profession”
• “Data Documentation”
What is GPS?
Worldwide Space-based Navigation System
•Determine position of all military forces continuously worldwide.
•Accurately deploy nuclear missiles from subs.
Funded, Controlled, and Maintained by
U.S. Department of Defense
Operated by U.S. Air Force
Original Objective
2
Components of GPS
1.
2.
3.
Components of GPS(1: Space Segment)
• 28 satellites in 6 nearly-circular orbits (4-5).• Orbital paths are 20,200 km MSL with a 55o angle of
inclination and 60o horizontal separation (360).• Global coverage 4-10 observable satellites
simultaneously w/ 15o elevational mask.
X 4 345 mihr
vel = ,Circle Earth
Twice Daily
12-hour Orbit Time
Components of GPS(2: Control Segment)
•Master Control Station•5 Monitor Stations
•3 Ground-based Uplink Antennas
Monitor Errors in Satellite Position
& Clocks
Transmit Corrections
3
Components of GPS(3: User)
GPS Receivers
Computes
3-Dimensional Location
3-4 Satellites
GPS Receivers
Five Steps of GPS (3-Dimensional Location)
• Geometric positioning (trilateration) of satellites• Measure satellite-specific distance
• Correct clock errors • Correct ephemeris (orbit) errors
• Correct atmospheric errors Remove
3 SVs
4 SVs
Trilateration: Satellites 1 and 2
(18,000 km)
Located within the Intersection of 2 Spheres
(19,000 km)
We’re somewhere on this sphere
18,000 km
1
4
Trilateration: Satellites 1-4
(18,000 km) (19,000 km)
(20,000 km) Usually Can Reject 1 of 2 Locations; however, 4th Satellite Needed for Time Correction
2 Possible Locations at Intersection of 3 Spheres
(21,000 km)
1
Geometric Configuration of Satellites (Geometric Dilution of Precision)
P Position Dilution of Precision (PDOP)< Horizontal Dilution of Precision (HDOP)< Vertical Dilution of Precision (VDOP)
PDOP < 6
1 per quadrant, >15o Elevation
60o
15o
SatView Demonstration: DOP(http://www.trimble.com/planningsoftware.html)
02/25/99: Lubbock, TX1400-1800 hrs: CDT1400-2300 hrs: CDT
Sample when satellites are positioned optimally!!
Their locations are known.
Free Trimble Software
5
2 Measuring Accurate Time(Atomic vs. Quartz Clocks)
Accuracy: 10-10 seconds(10 nanoseconds)
$100,000 each (4/satellite)
Satellite and Receiver Codes Must beGenerated Simulataneously
Accuracy: 10-3 seconds(300 km error)
$50 each (1/receiver)
4-Channel Receiver
4th Satellite
Receiver Calculates Time Correction andCalibrates Itself to Universal Time
and Perfect Synchrony
Determining Satellite Position(Ephemeris: Satellite Orbit)
3
High Altitude Orbits
Orbit ErrorsP Gravitational Pull (Moon & Sun)P Pressure from Solar Radiation
P Extremely StableP No Atmospheric DragP Maximize Coverage of EarthP Enhance Satellite Survivability
Monitored by Ground Control
& Errors Transmitted
4 Determining Satellite Position(Correcting Orbit Errors)
Monitor StationsGround
Antennas
Master Control P Colorado SpringsP HawaiiP AscensionP Diego GarciaP Kwajalein
P Pseudo-random CodeP Navigation Message
GPS Signal
6
Correcting Atmospheric Delays(Ionospheric and Tropospheric Errors)
5
IonosphereRegion of the atmosphere (50-300 km MSL) containing
ionized particles that reflects radio waves.
Ionospheric errors can be modeled and are incorporated as temporal-specific algorithms in GPS receivers.
Correcting Atmospheric Delays(Ionospheric and Tropospheric Errors)
5
TroposphereRegion of the atmosphere (0-20 km MSL) containing water
vapor, various gases, and temperature decrease with altitude.
Errors are minimal; nonetheless, tropospheric errors are modeled with ionospheric errors for “typical” meteorological conditions.
Additional Non-Intentional Errors
Receiver Noise
P Innate Receiver Error (i.e., technological limitations)
Multipath
P Signal Bounce or Reflection (i.e., same signal arrives >1 path)P Signal Rejection Techniquess
Expensive Receivers
7
Government-Induced Error(Selective Availability)
Error intentionally introduced randomly into satellite-specific clock data by the
U.S. Department of DefenseIntention of DOD is to limit accuracy of
non-U.S. military and government users for national security
SA can increase error in location by 20x (100 m)
No Longer Activated!
Differentially Corrected GPS (DGPS)
Real-time DGPSCorrections Transmitted via FM Radio Link
U.S Coast Guard
OmniStar, Inc.
•150 Broadcast Antennas
•FM Receiver & Antenna
•<100 km from Antenna
http://www.omnistar.com/
$300
Not good for mountainous
sites
Roving/Field UnitReceive Same
ErrorsReference/Base Station Unit< 300 km
apart
Differentially Corrected GPS (DGPS)
Post-processing DGPS
X
YZ
X
YZ
Error
Base Station
X
YZ
Rover
PathFinder Office Software
•http://www.rlageosystems.com/dgps.htm
Correction
8
Regional Community Base Stations
PSCC: Pellissippi State CollegeFRKN: Franklin, NC
White = 100 km
Tan = 200 km
Use Pathfinder Software
http://www.ngs.noaa.gov/CORS/cors-data.html
Accuracy and Cost of GPS ReceiversGeoExplorer XM PathFinder Pro XRGarmin Rino
10 m $250
2-5 m $2500
< 1 m $10000
SGPS
DGPS
Ephemeris Ionosphere Troposphere Noise Multipath Clock
2.5 0.60.55 0.3 1.5
3D
10.4
0 0.60.20.4 0.3 0 1.5
Good PDOP! http://users.erols.com/dlwilson/gps.htm
Helpful GPS Websites & ReferencesGPS Overview
http://www.trimble.com
GPS Products
http://www.magellandis.com
Community Base Stations
WAAS Enabled
http://www.garmin.com
Understanding GPS
E. D. Kaplan, editor
Artech House, Inc.
1996
GPS:Theory and PracticeHoffman-Wellenhof,
Lichtenegger, & Collins
Springer-Verlag1997
http://www.ngs.noaa.gov/CORS/Maps.html
http://www.trimble.com/gps/
http://gpsinformation.net/exe/waas.html
(7 m accuracy)