The Global Positioning System (GPS)

The Department of Defense developed the NAVSTAR Global Positioning System to allow the U.S. Military to determine the precise position of any object anywhere on the Earth. It consists of 21 active satellites (and a few spares) in 10,000 mile orbits, each circling the earth twice a day.

Each satellite continuously transmits its own position and a time code. By measuring the relative arrival times of signals from several satellites, a GPS receiver can determine its own position. Because radio signals travel so quickly, accurate positioning requires incredibly precise time information.

To meet these precise requirements, the GPS system's master clock is always kept within 1 microsecond of the U.S. Naval Observatory's Master Clock. This clock is, by law, the official time-keeper for the United States.

Each GPS satellite has four atomic clocks on board. The U.S. Air Force Space Command at Shriever Air Force Base in Colorado monitors these clocks and the precise position of each satellite through a network of monitoring stations spaced around the world. They then compute and broadcast corrections to keep these clocks so accurate that they are almost always within 250 nanoseconds.

Sources

We offer the GPSClock 200, a GPS receiver that is designed for time reference use from the ground up. It weighs about .8 pound and has threads on it to allow it be easily mounted to a PVC pole.

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Further Reading
The Next Level
The Capture Effect