GNSS (Global Navigation Satellite System) is a satellite system that is used to pinpoint the geographic location of a user's receiver anywhere in the world. Two GNSS systems are currently in operation: the United States' Global Positioning System (GPS) and the Russian Federation's Global Orbiting Navigation Satellite System (GLONASS). A third, Europe's Galileo, is slated to reach full operational capacity in 2008. Each of the GNSS systems employs a constellation of orbiting satellites working in conjunction with a network of ground stations.
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Satellite-based navigation systems use a version of triangulation to locate the user, through calculations involving information from a number of satellites. Each satellite transmits coded signals at precise intervals. The receiver converts signal information into position, velocity, and time estimates. Using this information, any receiver on or near the earth's surface can calculate the exact position of the transmitting satellite and the distance (from the transmission time delay) between it and the receiver. Coordinating current signal data from four or more satellites enables the receiver to determine its position.
Depending on the particular technologies used, GNSS precision varies. For example, the United States Department of Defense originally used an intentional degradation (known as "Selective Availability," or "SA") of GPS signals to prevent potential military adversaries from using the positioning data. Because of SA, GPS accuracy was limited to a 100-meter range for civilian users, although military equipment enabled accuracy to within a single meter. In May 2000, a presidential order mandated that SA be discontinued. Without SA, all GPS receivers are potentially accurate to within 15 meters. When available, Galileo will provide position accuracy to within one meter.