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.
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).