The GPS System

GPS, which stands for Global Positioning System, is a radio navigation system belonging to the American Ministry of Defense, that allows land, sea, and airborne users to determine their exact location, velocity, and time 24 hours a day, in all weather conditions, anywhere in the world.

The complete name of the system is NAVSTAR GPS, which means 'Navigation Satellite Timing And Ranging Global Positioning System'. It was born as a top secret project of the American Department of Defense during the final years of the Cold War so initially it was intended just for military purposes.

Today the GPS service is provided free of charge by the United States Air Force to the entire world. It is a constellation of satellites (21 active and 3 spare ones) orbiting at 11,000 nautical miles above the Earth and a series of ground stations that control and monitor those satellites. The satellites are spaced so that from any point on Earth, four satellites will be above the horizon.

On the ground, any GPS receiver contains a computer that 'triangulates' its own position by getting bearings from three of the four satellites. The result is provided in the form of a geographic position - longitude and latitude - for most receivers, with in a few metres. If the receiver is also equipped with a display screen that shows a map, the position can be shown on the map. When a fourth satellite can be received, the receiver/computer can calculate the altitude as well as the geographic position. If you are moving, your receiver may also be able to calculate your speed and direction of travel and give you estimated times of arrival to specified destinations.