The Global Positioning System, or GPS, is a complex system that employs a group of satellites to allow a user to determine its exact position, velocity, and local time, under any weather condition and at any time of the day. The GPS was started in 1978 but completed only in 1994 with 24 satellites (21 active and 3 spare ones).

The GPS satellites are in orbit around Earth with a period of 12 sidereal hours and at an altitude of about 20,000 kilometers. These satellites are distributed in space such that any user has direct line of sight to at least 4 of the satellites from any location on Earth at any point in time. The satellites transmit unique codes that a user on earth receives for decoding and translation into the relevant information after a complex series of calculations. The satellite signals are transmitted at 1575.42 MHz and 1227.60 MHz. A user of the GPS system must have a GPS receiver which performs the operations needed to determine the precise location of the user.

The GPS system is controlled by a group of GPS network operators at several stations on Earth, with the master control station based in Colorado Springs in the U.S. This group of stations performs the following functions: 1) maintenance of the GPS time; 2) monitoring and control of the satellites' orbit positions; and 3) compensation of detected and/or predicted inaccuracies in the system.

Figure 1.   Photos of a GPS satellite (left) and a GPS receiver (right)

The theory of operation of the GPS system is complex in its details, but it may be simplified as follows: 

1) each of the satellites sends out a signal carrying information about the satellite's location and the time the signal was sent;

2) the GPS receiver of the user receives these signals directly from at least four satellites;

3) the receiver processes the information and calculates the time it took for each of these 4 signals to reach it from its corresponding satellite;

4) the receiver determines its distance from each of the 4 satellites it got the signals from;

5) the receiver calculates its exact location relative to these satellites and then computes its exact location relative to fixed points on the Earth's surface.

These processes can yield the longitude, latitude, and altitude data corresponding to the receiver/user. 

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