Global Positioning System

a network of satellites allowing users with portable GPS devices to determine precise locations on the surface of the Earth. The portable GPS device measures the exact time taken for signals to reach it from at least four different satellites; from this, the instrument can compute its location.

GPS was developed by the U.S. Department of Defense for military purposes, but it has plenty of civilian applications. Hikers and campers can use it to plot their position on maps. GPS-equipped automobiles can call up computerized maps showing their exact position. Police cars can transmit their exact position to the dispatcher automatically. Boats and ships can use GPS for practically effortless navigation.

To keep GPS from being accurate enough for enemy military use, the Department of Defense originally introduced constantly varying errors that limited GPS's accuracy to about 300 feet (100 m). This system of intentional errors was called Selective Availability. When Selective Availability was turned off in May 2000, the accuracy became approximately 100 feet (30 m).