An Introduction to GPS 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

“ 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…

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

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)

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) GPS Technology actually uses trilaterilization Your position

Knowing how far you are from at least three fixed positions, you can determine your position (triangulation)

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

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 GPSr will display uncertainty values 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

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!

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

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

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

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 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

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