Orbit Altitude: 20,200 km
Orbit Type: MEO > Semi-Synchronous > Navigation

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The Global Positioning System (GPS) Satellite is a system of satellites
developed by the US Department of Defense to provide all-weather
round-the-clock navigation capabilities for military ground, ... sea, and air
forces. Since its implementation, GPS has also become an integral asset in
numerous civilian applications and industries around the globe, including
recreational uses (e.g. boating, aircraft, hiking), corporate vehicle fleet
tracking, and surveying.

GPS employs 24 spacecraft in 20,200 km circular orbits inclined at 55 degrees.
These spacecraft are placed in 6 orbit planes with four operational satellites
in each plane. All launches have been successful except for one launch failure
in 1981. The full 24-satellite constellation was completed on March 9, 1994.

GPS receivers use triangulation of the GPS satellites' navigational signals to
determine their location. The satellites provide two different signals that
provide different accuracies. Coarse-acquisition (C/A) code is intended for
civilian use, and is deliberately degraded. The accuracy using a typical
civilian GPS receiver with C/A code is typically about 100 meters. The
military's Precision (P) code is not corrupted, and provides positional
accuracy to within approximately 20 meters.

[Summary provided by NASA.]

http://msl.jpl.nasa.gov/Programs/gps.html