An Introduction to GPS for Geocachers Chris aka BBWolf Of Team BBWolf+3Pigs
What is Geocaching? <ul><li>“ Geocaching is a sport where you use a multi billion dollar satellite system to locate rubber...
Global Positioning System (GPS) <ul><li>GPS was developed by US DoD and is managed by the US Air Force </li></ul><ul><li>$...
Three Segments <ul><li>Space Segment </li></ul><ul><ul><li>Minimum of 24 satellites (currently 30) in orbit around Earth a...
Transmitted Signals <ul><li>Each satellite transmits: </li></ul><ul><ul><li>It’s approximate position </li></ul></ul><ul><...
The Simple View <ul><li>Knowing how far you are from at least three fixed positions, you can determine your position (tria...
Exact Position <ul><li>Using three satellites, you should be able to determine exact position but other factors prevent “p...
Position Error <ul><li>GPSr will display uncertainty values </li></ul>Position (Uncertainty) Error
Other Data <ul><li>With multiple satellites can get altitude </li></ul><ul><li>As you move, a GPSr will calculate </li></u...
Selective Availability & Geocaching <ul><li>Prior to May 2000, civilian GPS signal included a random error (Selective Avai...
More information for the true “geeks”
Transmitted Signals <ul><li>Each satellite transmits ephemeris, almanac and navigation data </li></ul><ul><ul><li>Almanac ...
Exact Position <ul><li>Using three satellites, you should be able to determine exact position but other factors prevent “p...
But how do you get distance? <ul><li>Satellites have atomic clock </li></ul><ul><li>GPSr has accurate internal clock </li>...
Comparing Sequences <ul><li>Since you know the position of the satellite, and the distance from that satellite, you now ha...
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An Introduction To GPS for Geocachers

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A presentation on GPS technology and how it is used by geocachers. Presentation by Chris (aka Big Bad Wolf).

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An Introduction To GPS for Geocachers

  1. 1. An Introduction to GPS for Geocachers Chris aka BBWolf Of Team BBWolf+3Pigs
  2. 2. What is Geocaching? <ul><li>“ Geocaching is a sport where you use a multi billion dollar satellite system to locate rubbermaid containers in the middle of the woods.” </li></ul><ul><li>- Anonymous </li></ul>
  3. 3. Global Positioning System (GPS) <ul><li>GPS was developed by US DoD and is managed by the US Air Force </li></ul><ul><li>$750 million annual cost to operate </li></ul><ul><li>Now widely used in the civilian sector as well as military users </li></ul><ul><ul><li>Surveying </li></ul></ul><ul><ul><li>Geology </li></ul></ul><ul><ul><li>Hiking </li></ul></ul><ul><ul><li>Maritime </li></ul></ul><ul><ul><li>Airlines </li></ul></ul><ul><ul><li>And on and on… </li></ul></ul>
  4. 4. Three Segments <ul><li>Space Segment </li></ul><ul><ul><li>Minimum of 24 satellites (currently 30) in orbit around Earth at altitude 20,000 km </li></ul></ul><ul><li>Control Segment </li></ul><ul><ul><li>Satellites are tracked by ground stations </li></ul></ul><ul><ul><li>Navigation updates sent to satellites </li></ul></ul><ul><ul><li>Updates ephemeris data (position) and atomic clocks </li></ul></ul><ul><li>User Segment </li></ul><ul><ul><li>GPS Receiver </li></ul></ul><ul><ul><li>Uses data from satellites to calculate user’s position, altitude and other data </li></ul></ul>
  5. 5. Transmitted Signals <ul><li>Each satellite transmits: </li></ul><ul><ul><li>It’s approximate position </li></ul></ul><ul><ul><li>Exact orbit and location </li></ul></ul><ul><ul><li>Navigation signal </li></ul></ul><ul><ul><ul><li>Civilian </li></ul></ul></ul><ul><ul><ul><li>Military (more precise) </li></ul></ul></ul>
  6. 6. The Simple View <ul><li>Knowing how far you are from at least three fixed positions, you can determine your position (triangulation) </li></ul>GPS Technology actually uses trilaterilization Your position
  7. 7. Exact Position <ul><li>Using three satellites, you should be able to determine exact position but other factors prevent “prefect” overlap </li></ul><ul><ul><li>Atmospheric interference </li></ul></ul><ul><ul><li>Buildings </li></ul></ul><ul><ul><li>Trees, etc </li></ul></ul><ul><li>Using additional satellites, the GPSr attempts to reconcile position by adjusting the offsets </li></ul><ul><ul><li>Same “interference” for all signals </li></ul></ul>
  8. 8. Position Error <ul><li>GPSr will display uncertainty values </li></ul>Position (Uncertainty) Error
  9. 9. Other Data <ul><li>With multiple satellites can get altitude </li></ul><ul><li>As you move, a GPSr will calculate </li></ul><ul><ul><li>Velocity </li></ul></ul><ul><ul><li>Course </li></ul></ul><ul><ul><li>Track (“bread crumbs”) </li></ul></ul><ul><ul><li>Distance & Heading to marked position </li></ul></ul><ul><ul><li>Elapsed time, average, speed, etc </li></ul></ul>
  10. 10. Selective Availability & Geocaching <ul><li>Prior to May 2000, civilian GPS signal included a random error (Selective Availability) </li></ul><ul><ul><li>Civilian accuracy >30m </li></ul></ul><ul><li>Pres. Clinton turned off SA at 0000, 1 May 2000 </li></ul><ul><ul><li>Resulted in accuracy < 10m </li></ul></ul><ul><ul><li>Benefit sportsmen, FAA, etc </li></ul></ul><ul><ul><li>3 May 2000 - Dave Ulmer placed first “cache” </li></ul></ul><ul><ul><li>Now over 400,000 caches worldwide! </li></ul></ul>
  11. 11. More information for the true “geeks”
  12. 12. Transmitted Signals <ul><li>Each satellite transmits ephemeris, almanac and navigation data </li></ul><ul><ul><li>Almanac data – coarse orbit & status info for all satellites </li></ul></ul><ul><ul><li>Ephemeris – specific orbit and position data </li></ul></ul><ul><ul><li>Navigation – Unique code every millisecond </li></ul></ul><ul><ul><ul><li>C/A (Coarse/Acquisition) – Civilian </li></ul></ul></ul><ul><ul><ul><li>Encrypted P(Y) (Precision) - Military </li></ul></ul></ul>
  13. 13. Exact Position <ul><li>Using three satellites, you should be able to determine exact position but other factors prevent “prefect” overlap </li></ul><ul><ul><li>Atmospheric interference </li></ul></ul><ul><ul><li>Buildings </li></ul></ul><ul><ul><li>Trees, etc </li></ul></ul><ul><li>Using additional satellites, the GPSr attempts to reconcile position by adjusting the offsets </li></ul><ul><ul><li>Same “interference” for all signals </li></ul></ul>
  14. 14. But how do you get distance? <ul><li>Satellites have atomic clock </li></ul><ul><li>GPSr has accurate internal clock </li></ul><ul><li>Each satellite’s C/A code is a unique string </li></ul><ul><ul><li>Allows for reception of signals from multiple satellites on same frequency </li></ul></ul><ul><li>GPSr reproduces each C/A code internally </li></ul><ul><li>GPSr “lines up” the two sequences, and based on the offset, you can determine distance to satellite </li></ul>
  15. 15. Comparing Sequences <ul><li>Since you know the position of the satellite, and the distance from that satellite, you now have a sphere, centered on the satellite, you can be on </li></ul>Satellite Sequence GPSr Sequence Time Offset x Speed Of Light = Distance From Satellite

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