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Geographic Information Systems

Geographic Information Systems. Global Positioning Systems (GPS). 1. GPS. A method used in surveying. It uses a constellation of satellites orbiting the earth at very high altitudes. GPS.

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Geographic Information Systems

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  1. Geographic Information Systems Global Positioning Systems (GPS)

  2. 1. GPS • A method used in surveying. It uses a constellation of satellites orbiting the earth at very high altitudes

  3. GPS .. • The GPS technology allows accurate geodetic surveys by using specially designed receivers that, when positioned at a point on the earth, measure the distance from that point to three or more orbiting satellites • Through the geometric calculations of triangulation, the coordinates of the point on the surface of the earth are determined

  4. X, Y, Z (position) and time from 4 satellites to calculate position Pamela E. Jansma, University of Arkansas

  5. 2. NAVSTAR GPS • NAVigation Satellite Timing And Ranging (NAVSTAR) Global Positioning System • Developed by U.S. Department of Defense • A constellation system of 28-30 satellites orbiting the earth at a high altitude twice a day • Transmitting precise time and position information • For world-wide and all weather navigations

  6. NAVSTAR .. • 24 satellites in 6 orbital planes • Orbit the earth at approx. 20,200 km • 550 inclination • Satellites complete an orbit in approximately 12 hours http://www.montana.edu/places/gps/lres357/assignments.html

  7. http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.htmlhttp://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

  8. 3. Operation Principles • The basis of GPS is triangulation • Satellite • GPS receiver • Determination of Location • Coordinate systems

  9. 3. Operation - Distance to One Satellite • Satellite - continuously broadcasts time and its location • GPS receiver - receives the signal and calculates the distance between the satellite and the receiver: Distance = Velocity x Time where time = the time a signal takes to travel from the satellite to the receiver, velocity = ?

  10. 3. Operation - Determine Location • Determination of location - 3 or more simultaneous distance measurements are needed to determine the location of the receiver • Coordinate systems - A GPS provides its position in geographical/UTM/ State Plane coordinates and altitude

  11. One measurement narrows down our position to the surface of a sphere http://www.montana.edu/places/gps/lres357/assignments.html

  12. A second measurement narrows down our position to the intersection of two spheres, which is a circle http://www.montana.edu/places/gps/lres357/assignments.html

  13. A third measurement narrows down our position to just two points, because the intersection of a circle and a sphere is two points http://www.montana.edu/places/gps/lres357/assignments.html

  14. A four measurement narrows our position down to one point http://www.bbk.ac.uk/geog/study/courses/pgis/GPSlecture10.pdf

  15. 3. Operation- Determine location .. Pamela E. Jansma, University of Arkansas

  16. 4. GPS Receiver • GPS receivers can be hand carried out or installed on aircraft, ships, tanks, submarines, cars and trucks

  17. GPS Receiver .. • Radio channels • Internal clock • Computer

  18. GPS Receiver .. • Radio channels It needs one or more channels to receive signals of     (1) the time a signal is sent, and         (2) a pseudorandom code to identify the signal • Internal clock - Synchronized with the satellite in order to calculate the signal travel time precisely

  19. GPS Receiver .. • Computer - Calculates distance and location - Stores location readings - Calculates the satellite availability

  20. 5. Autonomous vs. Differential GPS • Autonomous GPS: one receiver unit • Selective availability interference • Differential GPS • WAAS (Wide Area Augmentation System)

  21. Differential GPS .. • Selective availability interference -The Us government used to insert random errors in the signals in order to maintain optimum military effectiveness of the system. It was turned off in 2000 • Differential GPS

  22. Differential GPS • Selective availability interference • Differential GPS - Uses two receiver units, with one placed at a known location, such as a surveyed control point, to detect the inserted errors and calculate the needed corrections. The correction information is used by another receiver to overcome its selective availability interference

  23. Differential GPS (DGPS) .. • The DGPS corrections can be applied to the GPS data in real-time (radio modems). Or, can be done later on a PC • There are a series of radio beacons to transmit the DGPS corrections for accurate navigation

  24. 5. Differential Correction http://www.bbk.ac.uk/geog/study/courses/pgis/GPSlecture10.pdf

  25. 5. DGPS - WAAS Satellite DGPS • Wide Area Augmentation System (WAAS)   - a real-time differential correction service - is based on a network of approximately 25 ground reference stations that cover a very large service area http://www.montana.edu/places/gps/lres357/assignments.html

  26. WAAS Satellite DGPS .. - Signals from GPS satellites are received by wide area ground reference stations (WRSs) - Each of these precisely surveyed reference stations receive GPS signals and determine if any errors exist - These WRSs are linked to form the U.S. WAAS network. This service is primarily for aviation purposes

  27. WAAS Satellite DGPS .. • Improves the accuracy, integrity and availability of the basic GPS signals • Safety - critical navigation system for aviation • Owned and operated by Federal Aviation Administration (FAA) • Type: real-time • Cost: free

  28. WAAS Stations http://www.montana.edu/places/gps/lres357/assignments.html

  29. DGPS .. • Omnistar • Type: Real-time - Network of reference stations 70 reference stations 3 network control centers - Worldwide coverage • Cost: ~ $800/year http://www.montana.edu/places/gps/lres357/assignments.html

  30. 6. Static vs. Kinematic GPS • Static: two or more receivers receive data for a lengthy period of time at a fixed location • Kinematic: one receiver is placed at a known location, while one or more receivers move from point to point • Real Time Kinematic (RTK)

  31. 7. Advantages and Limitations • Advantages: fast, more accurate, lower cost than manual approaches • Disadvantages: obstructed locations

  32. 8. Readings • Chapter 2

  33. Omnistar – How it Works http://www.montana.edu/places/gps/lres357/assignments.html

  34. http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.htmlhttp://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

  35. http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.htmlhttp://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

  36. 5. DGPS - Omnistar CoverageNorth America http://www.montana.edu/places/gps/lres357/assignments.html

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