The complete system consists of 24 satellites orbiting about 12,000 miles above the Earth, and five ground stations to monitor and manage the satellite constellation. These satellites provide 24-hour-a-day coverage for both two-and three- dimensional positioning anywhere on Earth.

Development of the $10 billion GPS satellite navigation system was begun in the 1970s by the US Department of Defense, which continues to manage the system, to provide continuous, worldwide positioning and navigation data to US military forces around the globe. However, GPS has an even broader civilian, commercial application. To meet these needs, GPS offers two levels of service, one for civilian access and the second encrypted for exclusive military use.

The GPS signals are available to an unlimited number of users simultaneously, and there is no charge for using the GPS Satellites either.

The basis of GPS technology is precise time and position information. Using atomic clocks and location data, each satellite continuously broadcast the time and its position. A GPS receiver receives these signals, listening to three or more satellites at once, to determine the users position on earth.

By measuring the time interval between the transmission and the reception of a satellite signal, the GPS receiver calculates the user and each satellite. Using the distance measurements of at least three satellites in an algorithm computation, the GPS receiver arrives at an accurate position fix. Information must be received from three satellites in order to obtain two-dimensional fixes(latitude and longitude), and four satellites are required for three-dimensional positioning (latitude, longitude and altitude).