The Time Synchronization Capabilities of the Navstar Global Positioning System

The ability to synchronize distant atomic clocks is required by a number of diverse technological disciplines including geodesy, astronomy, navigation, communications, computer networking, deep-space tracking, and data encryption. Portable atomic clocks often furnished acceptable synchronization, but in recent years, the precisely timed binary pulse trains transmitted by the Navstar Global Positioning System (GPS) satellites have provided an attractive alternative. Each Block II satellite carries two cesium and two rubidium atomic clocks with long-term stabilities exceeding 2 × 10^-13 for a 1-day averaging time. If the GPS clocks could be made to operate that long, they would require at least 160,000 years to lose or gain 1 second. Yet, they must be updated from the ground at least once per day to maintain a maximum lo timing error of 13 nanoseconds with respect to the other satellites in the GPS constellation.