The global positioning system (GPS) - is a network of satellites that orbit the earth at fixed points above the planet and beam down signals to anyone on earth with a GPS receiver. These signals carry a time code and geographical data point that allows the user to pinpoint their exact position, speed and time anywhere on the planet.
GPS was originally designed for military and intelligence applications at the height of the Cold War in the 1960s, although inspiration came with the launch of the Soviet spacecraft Sputnik in 1957.
Transit was the first satellite system launched by the USA and tested by the US Navy in 1960. Just five satellites orbiting the earth allowed ships to fix their position on the seas once every hour. The 1967 successor to Transit was the Timation satellite, which demonstrated that highly accurate atomic clocks could be operated in space. GPS developed quickly for military purposes thereafter with a total of 11 "Block I" satellites being launched between 1978 and 1985.
However, it was the shooting down of a Korean passenger jet - flight 007 - in 1983 by the USSR led the Reagan Administration in the US to open up GPS for civilian applications so that aircraft, shipping, and transport the world over could fix their positions and avoid straying inadvertently into limited foreign territory.
The 1986 NASA space shuttle SS Challenger disaster slowed the upgrading of the GPS system and it was not until 1989 that the first Block II satellites were launched. By the summer of 1993, the US launched their 24th Navstar satellite into orbit, which completed the modern GPS constellation of satellites - a network of 24 - familiar now as the Global Positioning System, or GPS. 21 of the constellation of satellites were active at any one time, the other 3 satellites were spares. Today's GPS network has around 30 active satellites in the GPS constellation.
Today, GPS is used for dozens of navigation applications, route finding for drivers, map-making, earthquake research, climate studies, and an outdoor treasure-hunting game known as geocaching.