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GPS

GPS. GLOBAL POSITIONING SYSTEMS. “ “Trying to find out where you are and where you’re going is probably one of man’s oldest pastimes” – Trimble http://www.trimble.com/gps/. Pre-GPS. Navigation is critical, but historical navigational tools have limits:

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GPS

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  1. GPS GLOBAL POSITIONING SYSTEMS

  2. “ “Trying to find out where you are and where you’re going is probably one of man’s oldest pastimes” – Trimble http://www.trimble.com/gps/

  3. Pre-GPS Navigation is critical, but historical navigational tools have limits: • Sextant (Celestial) – contingent on weather and time of day • Compass – an acquired skill that usually works best with a map in hand. Can be altered by magnetic fields. • Maps and Landmarks – subject to change due to destruction, building, and environmental factors

  4. What is GPS? GPS is not a single UNIT! • GPS = Global PositioningSYSTEM • GPS was developed by the Department of Defense as a precise navigation system for the military. • Funding for the GPS was contingent on making the system available to the public.

  5. GPS is a SYSTEM There are three major components in this system: • Satellites • Ground Control Stations • GPS Receivers (or units)

  6. Satellites • There are 24 satellites (and 3 spares). • The DOD knows the EXACT location of each of the satellites at any given moment. • These satellites have VERY accurate clocks on board. • The satellites continuously send radio signals toward Earth. • The radio signals contain several pieces of information, including the satellite id#, a time stamp, and the satellite’s true position in space.

  7. Control Stations Thereare five control stations that monitor the satellites. • They are located in Hawaii, Colorado Springs, Ascension Island, Diego Garcia, and Kwajalein. • Control stations enable information on Earth to be transmitted to the satellites (updates and fine turning). • Control stations continuously track satellites, and update the positions of each satellite. • Without control stations, the accuracy of the system would degrade in a matter of days.

  8. GPS Receivers • GPS units are referred to as “receivers”. • They receive information (radio signals) from satellites. • These radio signals contain important information… • Signal containing: • Time stamp… • Satellite ID # • Exact position of satellite

  9. GPS Receivers • The Receiver knows exactly when the signal leaves the satellite (time stamp) and when the signal arrives at the receiver. • The Receiver is therefore able to calculate its distance from the satellite. -Distance = time x velocity • The receiver knows the exact position (location) of the satellite (via the signal).

  10. How GPS Works If the GPS receiver only obtains signals from 1 Satellite, then it “knows” that it is located somewhere on this sphere…

  11. How GPS Works

  12. How GPS Works If the GPS receiver only obtains signals from 2 satellites, then it “knows” that it is located somewhere where these 2 spheres intersect

  13. How GPS Works

  14. How GPS Works If the GPS obtains signals from 3 satellites, then it “knows” that it is located somewhere where these 3 spheres intersect (2 points)

  15. Selective Availability (SA) • For national security reasons, the military sometimes degrades the civilian (C)-code signal. This is called selective availability. • These errors are random.

  16. Selective Availability • It is possible to correct for Selective Availability. • This process is called Differential Correction • Here’s how it works…

  17. Differential Correction • There are already established base stations around Virginia • Surveyors have determined the precise location of these base stations already. • Each base station has a GPS receiver, which collects incoming (scrambled) signals. • The true (surveyed) location is then compared to the GPS coordinates. • The correction values are then sent to other GPS receivers in the field.

  18. WAAS • Wide Area Augmentation System • A network of ground stations and satellites that provide GPS signal corrections. • Better positional accuracy- Five times better!

  19. Tricks of the Trade: It is better for your receiver to get a fix on “distributed” satellites, then poorly distributed satellites. “Positional Dilution of Precision” Good Satellite Distribution Poor Satellite Distribution

  20. Other Tricks of the Trade: MultiPath Errors Try and stay away from buildings and other structures when using a GPS receiver.

  21. Final Tricks of the Trade: Tracking Satellites GPS has worldwide coverage, however… • You can lose satellite coverage (or received degraded signals) in areas with dense foliage, in “urban canyons”, etc. • You may also lose satellite coverage (or receive degraded signals) in deep valleys or gorges. This can introduce error.

  22. How accurate is a $120 GPS? • That’s the million dollar question…

  23. It depends… Acknowledgements: Dr. Phillip Rasmussen, Utah Geospatial Extension Specialist

  24. What can you do with a GPS? Determine location Where am I? • Locate the exact position of migrating animals. • Mark where snakehead fish have been caught in Virginia waterways. • Locate an injured hiker in the Appalachian Mountains. • Mark exactly where archaeological pieces were found at Jamestown. • Set up a coordinate system to plot the location of various tree species.

  25. What can you do with a GPS? Navigate: Where am I Going? • Collect and store points (positions). These are called waypoints. Waypoints could be trail heads, a creek crossings, mountain tops, camping grounds or even data collection points (for research). • Collect and store routes. A route is a path between two (or more) waypoints. The GPS can “guide you” from point “A” to point “B”.

  26. What can you do with a GPS? Tracking: Can I monitor where it will be next? • Commercial applications such as tracking delivery trucks and planning bus routes. • Following oil tanker routes in relation to known water hazards. • Identifying where humpback whales are headed for the winter.

  27. What can you do with a GPS? Mapping: Where is everything else? • Identify roads, mountains, rivers in terms of latitude and longitude as well as elevation. • Mark the location of important minerals and resources. • Identify the scope of natural disasters and calculate area.

  28. What can you do with a GPS? Timing: When will it happen? Remember: Each satellite has a highly accurate clock! • Synchronize events. For example, investment banking firms use GPS to guarantee that transactions are recorded simultaneously around the world. • Power companies used GPS to make sure that power is distributed appropriately at the right time through thousands of miles of lines.

  29. Current Applications of GPS • Public Safety • Environmental resource management • Aviation • Military • Local planning • Surveying • Recreation • Business

  30. GPS IN THE SOLS • Earth Science SOL 1 • The Student Will Plan And Conduct Investigations In Which Technologies, Including Computers, Probeware, And Global Positioning Systems (GPS), Are Used To Collect, Analyze And Interpret Data And To Demonstrate Concepts And Simulate Experimental Conditions.

  31. The Future is here.The applications are endless…

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