Precise Time and the Master Clock

"Time and the hour runs through the roughest day."
-- W. S.

The U.S. Naval Observatory (USNO) is charged with the responsibility for precise time determination and management of time dissemination . Modern electronic systems, such as electronic navigation or communications systems, depend increasingly on precise time and time interval (PTTI). These systems are based on the travel time of the electromagnetic signals: an accuracy of 10 nanoseconds (10 one-billionths of a second) corresponds to a position accuracy of 10 feet. In fast communications, time synchronization is equally important. All of these official systems are referenced to the USNO Master Clock.

Thus, the USNO must maintain and continually improve its clock system so that it can stay one step ahead of the demands made on its accuracy, stability, and reliability.

The present USNO Master Clock is based on a system of dozens of independently operating cesium atomic clocks and a dozen hydrogen maser clocks.

"A man with a watch knows what time it is. A man with two watches is never sure."
-- Segal's Law

These clocks are distributed over 20 environmentally controlled clock vaults, to ensure their stability. By automatic intercomparison of all clocks every 100 seconds, the USNO time scale can be computed which is not only reliable but also extremely stable. Its rate does not change by more than about 100 picoseconds (0.000 000 000 1 seconds) per day from day to day.

On the basis of this computed time a clock reference system can be steered to produce clock signals which serve as the USNO Master Clock. The clock reference system is driven by a hydrogen maser atomic clock. Hydrogen masers are extremely stable clocks over short time periods (less than one week). They provide the stability, reliability and accessibility needed to maintain the accuracy of the Master Clock system.

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Dr. Demetrios Matsakis (

Time Service Dept., U.S. Naval Observatory, Washington, DC