An Introduction To GPS for Geocachers
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Transcript of An Introduction To GPS for Geocachers
An Introduction toGPS for Geocachers
Chris
aka BBWolf
Of Team BBWolf+3Pigs
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
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…
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
Transmitted Signals
• Each satellite transmits:– It’s approximate position– Exact orbit and location– Navigation signal
• Civilian• Military (more precise)
The Simple View• Knowing how far you are from at least
three fixed positions, you can determine your position (triangulation)
Your position
GPS Technology actually uses trilaterilization
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
Position Error
Position (Uncertainty) Error
• GPSr will display uncertainty values
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
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!
More information for the true “geeks”
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
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
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
Comparing Sequences
Satellite Sequence
GPSr Sequence
Time Offset x Speed Of Light = Distance From Satellite
• 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