Published by Australian Academy of Science
GPS and never having to say: 'Where am I?' Box 1 | A short history of GPS

The space age was born in October 1957, when the Soviet Union launched Sputnik – the world's first satellite. It took just one day for US researchers to work out the precise orbit of the satellite by observing how the frequency of its radio signal appeared to increase as it approached their receiver and decrease as it departed – an effect known as the Döppler shift. (The Döppler shift is also used by police radar to estimate the speed of approaching vehicles.) The researchers reasoned that they could also reverse this procedure. If they knew the exact position and orbit of a satellite, they could use its radio signals to precisely locate the position of a receiver on the ground.

The US military were particularly interested in developing satellite navigation systems to help guide their submarines and missiles. In 1965, the US Navy established the Transit system consisting of six satellites circling the Earth continuously in polar orbits. Submarines would receive radio signals from the satellites and analyse them to work out their Döppler shift. From this, submariners could determine the distance to the satellites and they could then determine their own location. The process took 10-15 minutes.

GPS was a natural next step. In 1973, the US Department of Defense proposed GPS as an instantaneous satellite positioning system that would be operational at all times from anywhere on the globe. The essential components of GPS are the Navstar satellites that orbit the Earth every 12 hours in a formation that ensures that every point on the planet will always be in radio contact with at least four satellites. The first operational GPS satellite was launched in 1978, and the system reached full 24-satellite capability in 1993. Additional satellites are now in orbit as operating spares.

At first, the US military had exclusive use of GPS through a secret signal and an encrypted code. When it was obvious that GPS had huge commercial potential, the Americans provided a separate, civilian signal, freely available worldwide for non-military use. The civilian signal can be used by anyone with a receiver to pinpoint their location to an accuracy of 20 metres. The error can be reduced down to centimetres by special techniques in complex receivers. The military signal remains encrypted and offers accuracy down to a few metres.

Related site

• The Global Positioning System: A national resource by Robert A. Nelson (Via Satellite, November 1999, Applied Technology Institute, USA)

Other boxes

Box 2. Improving GPS