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