Artist's impression of a Global Positioning System (GPS) satellite. The GPS system is based on a network of orbiting Navstar satellites, first launched in 1978. Small ground-based receivers can lock on to three of these satellites. The time taken for radio signals to reach the receiver from the three satellites gives a highly-accurate indication of the user's position. Military receivers can demonstrate accuracies to 15m. Civil receivers, initially accurate to 30m, will soon only receive degraded signals offering accuracy to 100m for security reasons. The final operational network will have 21 Navstars with six in-orbit spares. Network completion is due in 1994.
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