The Global Positioning System (GPS) is a network of satellites, ground control stations and receivers that provide users with location information. The GPS network is made up of three primary components:
The current GPS constellation consists of about 30 satellites orbiting the Earth at about 11,000 nautical miles (20,370km). They are positioned so that from anywhere on Earth a GPS unit should be able to receive signals from at least six of the satellites.
Each satellite broadcasts information that indicates its current location and the exact time the signal was sent. The location is derived from the satellites known orbit path and the time is from an onboard atomic clock. Signals are broadcast on two different frequencies. One is available to the general public, the other (more accurate) signal is only available to authorized users.
The control component is a series of ground control stations that continually track and monitor the GPS satellites as they orbit the Earth. The control stations are located around the globe to maintain near constant contact with the satellite network. The GPS network is primarily managed from Schriever Air Force Base near Colorado Springs.
The user component consists of the many GPS receivers that are scattered all over the planet providing location information to their owners. The user component basically consists of everyone that uses the GPS network.
Just like an FM radio, GPS units passively receive signals - they do not transmit information back to the GPS satellites. Some GPS devices, such as smart phones, can be configured to send live location information via standard communication networks.