The Global Positioning System
More about GPS
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.
The global positioning system is operated by the Department of Defence in the USA and relies on between 24 and 32 satellites that sit in a "constellation" orbiting the planet at medium earth orbit. This is a distance of least 20,000 km above the earth's surface, but no higher than the orbit used by TV, communications and internet satellites, and weather satellites, the so-called geostationary orbit, which is at an altitude of approximately 35,000 km.
The more satellites in the global positioning system the better is the accuracy of the data received. If one satellite fails or sends out faulty signals, these are cancelled out by readings from another satellite in the constellation.
The global positioning system was designed with military and intelligence applications in mind at the height of the Cold War during the 1960s. In 1983, however, when a Korean passenger plane strayed into Soviet territory and was shot down by the USSR, the US' President Reagan ordered that a civilian version of GPS be made available to anyone. Currently, accessing the GPS costs nothing, there are no subscription or maintenance fees, although a GPS receiver is needed to make use of the system.
Today, GPS is used for dozens of applications including aircraft and shipping navigation, route finding for drivers, map-making, earthquake research, climate studies, and outdoor treasure-hunting games known as geocaching.
A typical GPS receiver has an antenna tuned to the frequencies transmitted by the satellites. This picks up the signal and feeds it to the receiver-processor, which then displays location and time very accurately.