GPS requires precise clocks to provide astounding positional accuracy.
Atomic clocks in GPS satellites keep time to within three nanoseconds—three-billionths of a second. Position accuracy depends on the receiver. Most handheld GPS receivers are accurate to about 10 to 20 meters (33 to 66 feet).
Both military and civilian users can obtain higher accuracy by using a second GPS unit at a fixed nearby location—a method called Differential GPS. In this way, positions can be determined with an accuracy better than 1 centimeter (less than half an inch). For military users, additional encrypted signals can provide high accuracy.
All GPS satellites must transmit their data signals at the exact same time, so precise synchronization is essential. Their signals are monitored constantly and adjusted as needed. The GPS Operations Center at Schriever Air Force Base in Colorado Springs, Colorado, controls the constellation of satellites that provides navigation data to military and civilian users worldwide.