Satellite navigation systems allow small electronic devices to determine their location (Longitude, Latitude, and Altitude) in within a few meters using time signals transmitted along a line of sight by radio from satellites. Receivers on the ground with a fixed position can also be used to calculate the precise time as a reference for scientific experiments. The Global Positioning System is the only fully functional satellite navigation system as of 2006. Built in navigation systems are usually available as part of a Manufacturer offer or package, and can run anywhere from $1500 to $2000. As the technology becomes more readily available, higher end, luxury car companies are offering navigation systems as a standard feature on their vehicles. More and more vehicles are offering Navigation systems as a package feature. Navigation systems are also available as an after market add-on, including the Magellan RoadMate 3000 and the TomTom GO Series. Granted some auto consumers will find that products like the Magellan RoadMate are out of their price range. Garmin has developed the Mobile 20 which will allow GPS navigation to be accessible on wireless mobile smartphones. As portability is an important feature, the Garmin Mobile 20 supports Nokia, Windows Mobile, and Treo Smartphones. Garmin's Mobile 20 is expected to debut July 2006.
The best known satellite navigation system is the United States' Global Positioning System (GPS), and as of 2006, the GPS is the only fully functional satellite navigation system. This consists of 24 to 27 satellites that orbit in six different planes. The exact number of satellites varies as satellites are replenished when older ones are retired. They orbit at an altitude of approximately 20,000 km with an inclination of 55 degrees. The satellites are tracked by a world-wide network of monitor stations. The tracking data is sent to a master control station that continuously updates position and clock estimates for each satellite. The updated data is then uplinked to the satellite via one of several ground antennas.