Our GPS System was actually invented by the U.S. Army for military purposes (eg missile guidance, tracking planes and soldiers, or navigation) Later, they made the system available to civilians.

The system consists of three parts

1. 24 - 32 Satelittes (only twelve are visible at once) in orbit around the Earth.
2. Ground stations for monitoring and controlling the satelittes.
3. A GPS receiver that calculates the position.

The twenty-four satelittes are in a certain orbit around our planet. They continously broadcast a signal down to Earth including the current time and the orbit information. The math and equations for determining a precise position is extremely complex, but the general concept is simple.

It takes four satelittes to determine your position. Each of those satelittes transmits a time signal. By comparing the time sent from the satelitte to the GPS Receiver's own clock, the elapsed time of the transmission can be determined. Because the position of the satelitte is known and because we know how fast the message travels through space, the distance between the receiver and the satelitte can be determined. The receiver then repeats this with the other three satelittes and triangulates a position.