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 cesium-beam
frequency standards, hydrogen masers, and rubidium fountains.
Frequency data from this ensemble are used to steer the frequency of another such
maser, forming our designated Master Clock (MC),
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) is computed daily.