GPSGlobal Positioning
SystemLecture 11
What is GPS?What is GPS?
The Global Positioning System.A system designed to accurately determining positions on the earth
The Global Positioning System.A system designed to accurately determining positions on the earth
What are the parts of GPS?
What are the parts of GPS?
Space SegmentControl SegmentUser Segment
Space SegmentControl SegmentUser Segment
GPS ComponentsGPS Components
Developed by the DoDDepartment of Defense
Funded in 1976 by Congress
Became fully functional in the early 1980’s
Developed by the DoDDepartment of Defense
Funded in 1976 by Congress
Became fully functional in the early 1980’s
Space SegmentSpace Segment
SatellitesAtomic ClockSolar PanelsTransmitter / Receiver (High frequency, low energy radio waves)
AntennaThere are 24 active satellites in orbit3 extra satellites in orbit that are not in use
SatellitesAtomic ClockSolar PanelsTransmitter / Receiver (High frequency, low energy radio waves)
AntennaThere are 24 active satellites in orbit3 extra satellites in orbit that are not in use
SatelliteSatellite
Atomic Clock9.2 billion oscillations per second
Each satellite transmits on two L Band frequencies
Every satellite can transmit its own unique code
The satellites can be repositioned by the DoD
Atomic Clock9.2 billion oscillations per second
Each satellite transmits on two L Band frequencies
Every satellite can transmit its own unique code
The satellites can be repositioned by the DoD
GPS SatelliteGPS Satellite
GPS Satellite OrbitsGPS Satellite Orbits
4 satellites in each of 6 orbits
Pass over a monitoring station once every 12 hours
12,600 mile high orbitThere are always more than 4 satellites visible anywhere on the planet
4 satellites in each of 6 orbits
Pass over a monitoring station once every 12 hours
12,600 mile high orbitThere are always more than 4 satellites visible anywhere on the planet
GPS Satellite OrbitsGPS Satellite Orbits
Control SegmentControl Segment
5 monitoring stationsGround station at Schreiver AFB in Colorado
Coordinates time with the Space SegmentMaster control station
HawaiiAscension IslandDiego GarciaKwajalein
Correction factors issued twice per dayTimingOrbital adjustments
5 monitoring stationsGround station at Schreiver AFB in Colorado
Coordinates time with the Space SegmentMaster control station
HawaiiAscension IslandDiego GarciaKwajalein
Correction factors issued twice per dayTimingOrbital adjustments
User SegmentUser Segment
1984 first commercial GPS receivers on the market
Receivers that interpret the signals received from the space segment and ground based transmitters.
Quartz clocks in the GPS receivers are corrected by the satellite signals
GPS unitsAccuracy ~10m - sub centimeter
1984 first commercial GPS receivers on the market
Receivers that interpret the signals received from the space segment and ground based transmitters.
Quartz clocks in the GPS receivers are corrected by the satellite signals
GPS unitsAccuracy ~10m - sub centimeter
GPS UnitsGPS Units
www.geodatasolutions.co.uk/ landsurvey.html
http://www.garmin.com http://www.bruceair.com/Multimedia_presentations.htm
ResultResult
Segments work together to establish a box where your location is
Segments work together to establish a box where your location is
TrilaterationTrilateration
TrilaterationTrilateration
Satellites send out a stream of coded radio signals that indicate their location in space and the exact time that the signal is being sent.
The time difference between the same part of the code indicates how far the satellite is from the receiverDistance = time * velocity
Velocity = 186,000 miles/second~.06 seconds travel time
Satellites send out a stream of coded radio signals that indicate their location in space and the exact time that the signal is being sent.
The time difference between the same part of the code indicates how far the satellite is from the receiverDistance = time * velocity
Velocity = 186,000 miles/second~.06 seconds travel time
TrilaterationTrilateration
Each satellite signal radiates outward in a sphere2 satellite signals overlap to produce a circle of probable location
3 satellite signals overlap to produce 2 points of probable locationOne can usually be discarded as not relevant
4 satellite signals overlap to produce 1 point of location with elevation
Each satellite signal radiates outward in a sphere2 satellite signals overlap to produce a circle of probable location
3 satellite signals overlap to produce 2 points of probable locationOne can usually be discarded as not relevant
4 satellite signals overlap to produce 1 point of location with elevation
TrilaterationTrilateration
http://www.digit-life.com/articles/gpssystem/index.html
http://www.gsm-modem.de
Possible Sources of Error
Possible Sources of Error
InterferenceAtmosphere, Solid Objects
EphemerisDisturbances of satellite orbit:
Sun and moon gravitationPressure of solar radiation
Clock ProblemAtomic clocks are nearly perfect so even very small errors can be significant
InterferenceAtmosphere, Solid Objects
EphemerisDisturbances of satellite orbit:
Sun and moon gravitationPressure of solar radiation
Clock ProblemAtomic clocks are nearly perfect so even very small errors can be significant
Sources of ErrorSources of Error
ReceiversWhat the receiver is designed forKind of GPS chipCan the unit reject errors
Multi PathReflection of signals off of objects so that the same signal reaches the receiver with different offsets
1/100 second discrepancy Misread of 1,860 miles
ReceiversWhat the receiver is designed forKind of GPS chipCan the unit reject errors
Multi PathReflection of signals off of objects so that the same signal reaches the receiver with different offsets
1/100 second discrepancy Misread of 1,860 miles
MultipathMultipath
http://www.kowoma.de
http://earthmeasurement.ca
Selective Availability (S/A)
Selective Availability (S/A)
A disruption to the signal to make commercial units less accurateA random timing introduced into the satellite signal
Accuracy off by up to 100 mMilitary units filter out S/A2000 - S/A was shut off so that commercial units received an unaltered signal10 - 15 m accuracy without correction
The government can turn on S/A or turn off the signal if needed.
A disruption to the signal to make commercial units less accurateA random timing introduced into the satellite signal
Accuracy off by up to 100 mMilitary units filter out S/A2000 - S/A was shut off so that commercial units received an unaltered signal10 - 15 m accuracy without correction
The government can turn on S/A or turn off the signal if needed.
Signal CorrectionSignal CorrectionDifferential GPS
Base stations are used to receive the same satellite signal (Base Station)
Signals to the base station and the mobile GPS receiver are compared to eliminate some sources of error
3 - 5 m accuracyWAAS - Wide Area Augmentation SystemCurrently only in North America
Asia - MSAS, Europe - EGNOSIssues local correction factors based on local conditions
Improves accuracy of GPS location< 3 m accuracy (to subcentimeter)
Differential GPSBase stations are used to receive the same satellite signal (Base Station)
Signals to the base station and the mobile GPS receiver are compared to eliminate some sources of error
3 - 5 m accuracyWAAS - Wide Area Augmentation SystemCurrently only in North America
Asia - MSAS, Europe - EGNOSIssues local correction factors based on local conditions
Improves accuracy of GPS location< 3 m accuracy (to subcentimeter)
Signal CorrectionSignal Correction
Tricks that are not correcting errors:Map-matching
Estimated location is snapped to a known path on the map
Interpolated positionGPS unit continues to track estimated location based on speed and direction of travel even when a signal is lost
Instant averagingGPS unit collects more than one position at each waypoint and averages them
Tricks that are not correcting errors:Map-matching
Estimated location is snapped to a known path on the map
Interpolated positionGPS unit continues to track estimated location based on speed and direction of travel even when a signal is lost
Instant averagingGPS unit collects more than one position at each waypoint and averages them