Global Positioning System (GPS)


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Global Positioning System (GPS)

  1. 1. IntroductiontoNAVSTAR GPSCharlie Leonard, 1999(revised 2001, 2002)
  2. 2.  Feasibility studies begun in 1960’s. Pentagon appropriates funding in 1973. First satellite launched in 1978. System declared fully operational in April, 1995.The History of GPS
  3. 3. How GPS Works
  4. 4. Control SegmentSpace SegmentUser SegmentThree Segments of the GPSMonitor StationsGroundAntennasMaster Station
  5. 5. Kwajalein AtollUS Space CommandControl SegmentHawaiiAscensionIs.Diego GarciaCape CanaveralGround AntennaMaster Control Station Monitor Station
  6. 6. Space Segment
  7. 7.  Military. Search and rescue. Disaster relief. Surveying. Marine, aeronautical and terrestrial navigation. Remote controlled vehicle and robot guidance. Satellite positioning and tracking. Shipping. Geographic Information Systems (GIS). Recreation.User Segment
  8. 8.  Position and coordinates. The distance and direction between any two waypoints, or aposition and a waypoint. Travel progress reports. Accurate time measurement.Four Basic Functions of GPS
  9. 9. Position is Based on TimeT + 3Distance between satelliteand receiver = “3 times thespeed of light”TSignal leaves satelliteat time “T”Signal is picked up by thereceiver at time “T + 3”
  10. 10. Pseudo Random Noise CodeReceiver PRNSatellite PRNTimeDifference
  11. 11. What Time is It?Zulu TimeMilitary Time(local time on a 24 hour clock)Universal Coordinated TimeGreenwich Mean TimeLocal Time: AM and PM (adjusted for localtime zone)GPS Time + 13** GPS Time is ahead of UTC by approximately 13 seconds
  12. 12. Signal From One SatelliteThe receiver issomewhere onthis sphere.
  13. 13. Signals From Two Satellites
  14. 14. Three Satellites (2D Positioning)
  15. 15. Triangulating Correct Position
  16. 16. Three Dimensional (3D) Positioning
  17. 17. Selective Availability (S/A) The Defense Department dithered the satellite timemessage, reducing position accuracy to some GPS users. S/A was designed to prevent America’s enemies from usingGPS against us and our allies. In May 2000 the Pentagon reduced S/A to zero meters error. S/A could be reactivated at any time by the Pentagon.
  18. 18. Sources of GPS ErrorStandard Positioning Service (SPS ): Civilian UsersSource Amount of Error Satellite clocks: 1.5 to 3.6 meters Orbital errors: < 1 meter Ionosphere: 5.0 to 7.0 meters Troposphere: 0.5 to 0.7 meters Receiver noise: 0.3 to 1.5 meters Multipath: 0.6 to 1.2 meters Selective Availability (see notes) User error: Up to a kilometer or moreErrors are cumulative and increased by PDOP.
  19. 19. Receiver Errors are Cumulative!User error = +- 1 kmSystem and other flaws = < 9 meters
  20. 20. Sources of Signal InterferenceEarth’s AtmosphereSolid StructuresMetal Electro-magnetic Fields
  21. 21. Using GPS Receivers forPositioning and Navigation
  22. 22. XTE(CDI)N (0000)(00)NDesired Track(DTK) (xº) Active LegDistancetoWaypointBearing(X0 )PresentLocationSpeedOverGround (SOG)Tracking (TRK) (xº)Active GOTOWaypointGPS Navigation TerminologyCourse Made Good (CMG)(CMG) (xº)ActiveFromWaypoint
  23. 23. Active GOTOWaypointBearing =Course Over Ground (COG) =Cross Track Error (XTE) =Location Where GOTOWas ExecutedBearing = 650COG = 50XTE= 1/2 mi.Bearing = 780COG = 3500XTE = 1/3 mi.Bearing = 400COG = 1040XTE = 1/4 mi.Active LegNGPS Navigation: On the Ground
  24. 24. Position Fix A position is based on real-time satellite tracking. It’s defined by a set of coordinates. It has no name. A position represents only an approximation of thereceiver’s true location. A position is not static. It changes constantly as the GPSreceiver moves (or wanders due to random errors). A receiver must be in 2D or 3D mode (at least 3 or 4satellites acquired) in order to provide a position fix. 3D mode dramatically improves position accuracy.
  25. 25. Waypoint A waypoint is based on coordinates entered into a GPSreceiver’s memory. It can be either a saved position fix, or user enteredcoordinates. It can be created for any remote point on earth. It must have a receiver designated code or number, or a usersupplied name. Once entered and saved, a waypoint remains unchanged inthe receiver’s memory until edited or deleted.
  26. 26. Planning a Navigation RouteStart= Waypoint
  27. 27. How A Receiver Sees Your Route
  28. 28. GPS Waypoint Circle of ErrorX
  29. 29. GPS Dilution of Precision and ItsAffects On GPS Accuracy
  30. 30. GPS Satellite Geometry Satellite geometry can affect the quality of GPS signals and accuracyof receiver trilateration. Dilution of Precision (DOP) reflects each satellite’s position relativeto the other satellites being accessed by a receiver. There are five distinct kinds of DOP. Position Dilution of Precision (PDOP) is the DOP value used mostcommonly in GPS to determine the quality of a receiver’s position. It’s usually up to the GPS receiver to pick satellites which provide thebest position triangulation. Some GPS receivers allow DOP to be manipulated by the user.
  31. 31. Ideal Satellite GeometryNSW E
  32. 32. Good Satellite Geometry
  33. 33. Good Satellite Geometry
  34. 34. Poor Satellite GeometryNSW E
  35. 35. Poor Satellite Geometry
  36. 36. Poor Satellite Geometry
  37. 37. Differential GPS
  38. 38. DGPS Sitex+30, y+60x+5, y-3True coordinates =x+0, y+0Correction = x-5, y+3DGPS correction = x+(30-5) andy+(60+3)True coordinates = x+25, y+63x-5, y+3Real Time Differential GPSDGPS ReceiverReceiver
  39. 39. NDGPS Ground StationsNational Differential Global Positioning SystemYellow areas show overlap between NDGPS stations. Green areas are little to no coverage.Topography may also limit some areas of coverage depicted here.
  40. 40. NDGPS Ground StationsNational Differential Global Positioning SystemYellow areas show overlap between NDGPS stations. Green areas are little to no coverage.Topography may also limit some areas of coverage depicted here.
  41. 41. Wide Area Augmentation SystemGeostationaryWAAS satellitesGPS ConstellationWAAS ControlStation (West Coast)Local Area System (LAAS)WAAS ControlStation (EastCoast)
  42. 42. How good is WAAS?+ -3 meters+-15 metersWith Selective Availability setto zero, and under idealconditions, a GPS receiverwithout WAAS can achievefifteen meter accuracy mostof the time.*Under ideal conditions aWAAS equipped GPSreceiver can achieve threemeter accuracy 95% of thetime.** Precision depends on good satellite geometry, open sky view, and no userinduced errors.