How gps works


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How gps works

  1. 1. Introduction to NAVSTAR GPSCharlie Leonard, 1999(revised 2001, 2002)
  2. 2. The History of GPS Feasibility studies begun in 1960’s. Pentagon appropriates funding in 1973. First satellite launched in 1978. System declared fully operational in April, 1995.
  3. 3. How GPS Works
  4. 4. Three Segments of the GPS Space Segment User Segment Control Segment Ground AntennasMaster Station Monitor Stations
  5. 5. Control Segment US Space Command Cape Canaveral Hawaii Kwajalein Atoll Diego Garcia Ascension Is.Master Control Station Monitor Station Ground Antenna
  6. 6. Space Segment
  7. 7. User Segment 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.
  8. 8. Four Basic Functions of GPS Position and coordinates. The distance and direction between any two waypoints, or a position and a waypoint. Travel progress reports. Accurate time measurement.
  9. 9. Position is Based on Time Signal leaves satellite at time “T” T Signal is picked up by the T+3 receiver at time “T + 3” Distance between satellite and receiver = “3 times the speed of light”
  10. 10. Pseudo Random Noise Code Time Difference Satellite PRNReceiver PRN
  11. 11. What Time is It? Universal Coordinated TimeGreenwich Mean Time GPS Time + 13* Zulu Time Local Time: AM and PM (adjusted for local time zone) Military Time (local time on a 24 hour clock)* GPS Time is ahead of UTC by approximately 13 seconds
  12. 12. Signal From One Satellite The receiver is somewhere on this 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 time message, reducing position accuracy to some GPS users. S/A was designed to prevent America’s enemies from using GPS 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 Users Source 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! System and other flaws = < 9 meters User error = +- 1 km
  20. 20. Sources of Signal Interference Earth’s Atmosphere Solid Structures Metal Electro-magnetic Fields
  21. 21. Using GPS Receivers forPositioning and Navigation
  22. 22. GPS Navigation Terminology Active GOTO Waypoint t ypoin N (0000) g Le Distance to Wa e tiv Desired Track ) G) Ac 0 nd ver (X (SO (DTK) (xº) g Gr e d O in ar (CMG) (xº) N (00) Be e ou Sp XT E (CD I) Tracking (TRK) (xº)Active Course Made Good (CMG) PresentFrom LocationWaypoint
  23. 23. GPS Navigation: On the Ground Active GOTO L eg Waypoint N tive Bearing = 780 Ac COG = 3500Bearing = 650 XTE = 1/3 mi.COG = 50 XTE= 1/2 mi. Bearing = 400 COG = 1040 XTE = 1/4 mi. Course Over Ground (COG) =Location Where GOTO Bearing =Was Executed Cross Track Error (XTE) =
  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 the receiver’s true location. A position is not static. It changes constantly as the GPS receiver moves (or wanders due to random errors). A receiver must be in 2D or 3D mode (at least 3 or 4 satellites 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 GPS receiver’s memory. It can be either a saved position fix, or user entered coordinates. It can be created for any remote point on earth. It must have a receiver designated code or number, or a user supplied name. Once entered and saved, a waypoint remains unchanged in the receiver’s memory until edited or deleted.
  26. 26. Planning a Navigation Route = WaypointStart
  27. 27. How A Receiver Sees Your Route
  28. 28. GPS Waypoint Circle of Error X
  29. 29. GPS Dilution of Precision and Its Affects On GPS Accuracy
  30. 30. GPS Satellite Geometry Satellite geometry can affect the quality of GPS signals and accuracy of receiver trilateration. Dilution of Precision (DOP) reflects each satellite’s position relative to 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 most commonly in GPS to determine the quality of a receiver’s position. It’s usually up to the GPS receiver to pick satellites which provide the best position triangulation. Some GPS receivers allow DOP to be manipulated by the user.
  31. 31. Ideal Satellite Geometry NW E S
  32. 32. Good Satellite Geometry
  33. 33. Good Satellite Geometry
  34. 34. Poor Satellite Geometry NW E S
  35. 35. Poor Satellite Geometry
  36. 36. Poor Satellite Geometry
  37. 37. Differential GPS
  38. 38. Real Time Differential GPS x+5, y-3 x+30, y+60 x-5, y+3 Receiver DGPS Receiver DGPS SiteDGPS correction = x+(30-5) and True coordinates =y+(60+3) x+0, y+0True coordinates = x+25, y+63 Correction = x-5, y+3
  39. 39. NDGPS Ground Stations National 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 Stations National 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 System Geostationary GPS Constellation WAAS satellites WAAS Control Station (EastWAAS Control Local Area System (LAAS) Coast)Station (West Coast)
  42. 42. How good is WAAS?With Selective Availability setto zero, and under idealconditions, a GPS receiverwithout WAAS can achieve +-15 metersfifteen meter accuracy mostof the time.* +- 3 metersUnder 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.