GPS is a navigation and positioning system of the U.S. Department of Defense (DoD) and is based on a system of 24 satellites which circle the earth twice a day at a height of 20,000 km. During the last 20 years, the U.S. developed and deployed this system primarily for military purposes.
Equipped with a GPS receiver, this navigation system can be used free of charge.
On 1 May 2000, the U.S. suspended the «selective availability», an artificial distortion of the signals. The user can now determine his position with an accuracy of a few meters (95% of the observations fall within a circle with a radius of 7 m). The accuracy depends mainly on the satellite configuration and on the type of GPS receiver. Higher accuracies may be reached when using special GPS survey methods.
