USNO MASTER CLOCK

CLOCK, n. A machine of great moral value to man, allaying his concern for the future by reminding him what a lot of time remains to him.
--Ambrose Bierce (1842-1914), "The Devil's Dictionary," 1911

The atomic clock timescale of the Observatory is based on an ensemble of approximately 50 cesium-beam frequency standards and a dozen hydrogen masers. Frequency data from this ensemble are used to steer the frequency of another such maser, designated Master Clock 2 (MC #2), until its time equals the average of the ensemble, thereby providing the physical realization of this "paper timescale."

Specifically, the frequency of a device called an Auxiliary Output Generator is periodically adjusted so as to keep the time of this maser synchronized as closely as possible with that of the computed mean timescale USNO timescale UTC (USNO), which in turn adjusted to be close to the predicted UTC. The unsteered internal reference timescale is designated as A.1, while the reference of the actual Master Clock is called UTC (USNO).

UTC (USNO) is usually kept within 10 nanoseconds of UTC. An estimate of the slowly changing difference UTC - UTC (USNO,MC #2) is computed daily.