* One of the first applications of artificial Earth satellites was for navigation. Very early in the Space Age, researchers realized that constellations of satellites could be put in orbit to permit ships, aircraft, or other vehicles to precisely determine their locations. A number of navigation satellite constellations have been put into orbit, with the most prominent being the US "Global Positioning System (GPS)".
The GPS constellation was established by the US military for support of American forces in the field, but it is now in widespread use for public and commercial applications as well. It is now being complemented by other navigation satellite systems such as the Russian GLONASS system. This document describes the history, principles, and applications of navigation satellite systems.