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Geocaching NSW: How GPS works

History of GPS

GPS consists of a series of satellites, called NAVSTAR (navigation satellite timing and ranging) and a ground based GPS receiver. The NAVSTAR satellites orbit the Earth at a distance of approximately 19 000 kilometres and take 12 hours to complete an orbit of the Earth. The GPS receiver determines its location on the Earth's surface by collecting signals from the orbiting GPS satellites.

The United States Defence Force developed the GPS, with the first satellite launched in 1978. It was completed in 1994, when the last of the 24 satellites was launched into orbit around the Earth (there are currently 27 satellites in orbit – three are reserves).

It was originally created to assist the US Defence Force identify and coordinate its military units. The system also allows soldiers to move across terrain that they are either unfamiliar with, or is difficult to see due to fog or sand storms. The first successful use of GPS in combat was during the 1991 Gulf War in Kuwait and Iraq.

To prevent other countries using this technology, an error was introduced into the GPS signal that only US Defence Force units could unscramble – known as 'selective availability'. Since its completion, the GPS has proved to be an invaluable tool, and despite selective availability, many have seen it as a useful commercial device for tracking objects and assisting people in emergencies.

Because of its ability to accurately pinpoint location, in May 2001, the US Government removed selective availability. For the first time the GPS is available to everyone, allowing a location-finding accuracy of, at best, five metres.

Text courtesy of CSIRO Education