General description of what GPS is and how it works.
 

GPS (Global Positioning System) is the only system today able to show you your exact position on the Earth anytime, in any weather, anywhere. GPS satellites, 24 in all, orbit at 11,000 nautical miles above the Earth.

They are continuously monitored by ground stations located worldwide. The satellites transmit signals that can be detected by anyone with a GPS receiver. Using the receiver, you can determine your location with great precision.
 

The satellites are positioned so that we can receive signals from six of them nearly 100 percent of the time at any point on Earth. You need that many signals to get the best position information. Satellites are equipped with very precise clocks that keep accurate time to within three nanoseconds - that's 0.000000003, or three billionths, of a second. This precision timing is important because the receiver must determine exactly how long it takes for signals to travel from each GPS satellite. The receiver uses this information to calculate its position.
 

Although GPS was designed for military use, many thousands of civilians make use of it. The satellites actually broadcast two signals, one that is only for military use, and one that can be used by both military and civilians.

Since GPS is passive (you only need to receive the signal), there are no restrictions on who can use the signal available to civilians.
 

GPS technology can be used in a variety of fields besides providing surveyors with co-ordinate information they can also be used for navigation for vehicles on the sea, in the air and on the ground. GPS applications also include keeping track o f where a fleet of trucks, trains, ships or planes are and how fast they are moving; directing emergency vehicles to the scene of an accident; mapping where a city's assets are located