The GPS network is made of three things namely the satellites, earth monitoring stations and the receivers. There are currently 24 satellites orbiting the earth. These work on the principle of triangulation thus revealing the exact location of the user.
Four satellites are needed since the process needs a very accurate local time, more accurate than any normal clock can provide, so the receiver internally solves for time as well as position. In other words, the receiver uses four measurements to solve for 4 variables - x, y, z, and t. These values are then turned into more user-friendly forms, such as latitude/longitude or location on a map, and then displayed to the user.
Each GPS satellite has an atomic clock, and continually transmits messages containing the current time at the start of the message, parameters to calculate the location of the satellite (the ephemeris), and the general system health (the almanac). The signals travel at a known speed - the speed of light through outer space, and slightly slower through the atmosphere. The receiver uses the arrival time to compute the distance to each satellite, from which it determines the position of the receiver using geometry and trigonometry.
Next is the GPS earth monitoring station. There are currently 4 unmanned stations and 1 master station operating in the US. Those working in the master station receive information from the four and update them regularly so people will always have the necessary data while on the move.
Lastly, is the receiver. This hand held device can be placed on the hood of the car and carried around after disembarking the vehicle. If the unit is able to get signals from three satellites in orbit, it can tell the user the longitude and latitude where you are. This means those that that receive data from four or more will get more information.
