The document provides an introduction to GPS technology and its application in geocaching, highlighting the role of satellites and the trilateration method used to determine positions. It discusses the transition from selective availability to enhanced civilian accuracy post-May 2000, leading to the widespread popularity of geocaching. Additionally, it explains the technical aspects of GPS signals and how receivers calculate precise locations despite various interferences.

1. An Introduction to GPS for Geocachers Chris aka BBWolf Of Team BBWolf+3Pigs

2. What is Geocaching? “ Geocaching is a sport where you use a multi billion dollar satellite system to locate rubbermaid containers in the middle of the woods.” - Anonymous

3. Global Positioning System (GPS) GPS was developed by US DoD and is managed by the US Air Force $750 million annual cost to operate Now widely used in the civilian sector as well as military users Surveying Geology Hiking Maritime Airlines And on and on…

4. Three Segments Space Segment Minimum of 24 satellites (currently 30) in orbit around Earth at altitude 20,000 km Control Segment Satellites are tracked by ground stations Navigation updates sent to satellites Updates ephemeris data (position) and atomic clocks User Segment GPS Receiver Uses data from satellites to calculate user’s position, altitude and other data

5. Transmitted Signals Each satellite transmits: It’s approximate position Exact orbit and location Navigation signal Civilian Military (more precise)

6. The Simple View Knowing how far you are from at least three fixed positions, you can determine your position (triangulation) GPS Technology actually uses trilaterilization Your position

7. Exact Position Using three satellites, you should be able to determine exact position but other factors prevent “prefect” overlap Atmospheric interference Buildings Trees, etc Using additional satellites, the GPSr attempts to reconcile position by adjusting the offsets Same “interference” for all signals

8. Position Error GPSr will display uncertainty values Position (Uncertainty) Error

9. Other Data With multiple satellites can get altitude As you move, a GPSr will calculate Velocity Course Track (“bread crumbs”) Distance & Heading to marked position Elapsed time, average, speed, etc

10. Selective Availability & Geocaching Prior to May 2000, civilian GPS signal included a random error (Selective Availability) Civilian accuracy >30m Pres. Clinton turned off SA at 0000, 1 May 2000 Resulted in accuracy < 10m Benefit sportsmen, FAA, etc 3 May 2000 - Dave Ulmer placed first “cache” Now over 400,000 caches worldwide!

11. More information for the true “geeks”

12. Transmitted Signals Each satellite transmits ephemeris, almanac and navigation data Almanac data – coarse orbit & status info for all satellites Ephemeris – specific orbit and position data Navigation – Unique code every millisecond C/A (Coarse/Acquisition) – Civilian Encrypted P(Y) (Precision) - Military

13. Exact Position Using three satellites, you should be able to determine exact position but other factors prevent “prefect” overlap Atmospheric interference Buildings Trees, etc Using additional satellites, the GPSr attempts to reconcile position by adjusting the offsets Same “interference” for all signals

14. But how do you get distance? Satellites have atomic clock GPSr has accurate internal clock Each satellite’s C/A code is a unique string Allows for reception of signals from multiple satellites on same frequency GPSr reproduces each C/A code internally GPSr “lines up” the two sequences, and based on the offset, you can determine distance to satellite

15. Comparing Sequences 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 Satellite Sequence GPSr Sequence Time Offset x Speed Of Light = Distance From Satellite