International Time Scales and the B.I.P.M.
"...and that old common arbitrator, Time." -- W. S.
As a matter of policy, the U.S. Naval Observatory timescale, UTC(USNO), is kept within a close but unspecified tolerance of the international atomic timescale published by the Bureau International des Poids et Mesures (International Bureau of Weights and Measures [BIPM]) in Sevres, France. The world's timing centers, including USNO, submit their clock measurements to BIPM, which then uses them to compute a free-running (unsteered) mean timescale (Echelle Atomique Libre [EAL]). BIPM then applies frequency corrections ("steers") to EAL, based on measurements from primary frequency standards and intended to keep the International System's basic unit of time, the second, constant. The result of these corrections is another timescale, TAI (Temps Atomique International or International Atomic Time). The addition of leap seconds to TAI produces UTC. The world's timing centers have agreed to keep their real-time timescales closely synchronized ("coordinated") with UTC. Hence, all these atomic timescales are called Coordinated Universal Time (UTC), of which USNO's version is UTC(USNO).
The difference between UTC (computed by BIPM) and any other timing center's UTC only becomes known after computation and dissemination of UTC, which occurs about two weeks after the fact. This difference is presently limited mainly by the long-term frequency instability of UTC. UTC(USNO) has been kept within 26 nanoseconds of UTC during the past year through frequency steering of our Master Clocks to our extrapolation of UTC. Since synchronization is never perfect, we provide the latest data below on the differences between UTC and the UTC of other timing centers, including USNO, as well as on the mean timescale provided by our atomic clock ensemble (a computed "paper" timescale, as opposed to a real-time timescale), to which our Master Clocks are steered. The near real-time publication of this information has been recommended by the Comite Consultatif pour la Definition de la Seconde (Consultative Committee on the Definition of the Second [CCDS]), in 1997 renamed the Comite Consultatif du Temps et des Frequences (Consultative Committee on Time and Frequency [CCTF]), of the Comite International des Poids et Mesures (International Committee on Weights and Measures [CIPM]).
A Real-Time Reference
All of our reference clocks are real-time approximations of UTC, and as such are denoted UTC(USNO). Our official real-time reference clock is steered on the short term to the mean timescale of our clock ensemble, which is itself steered to an extrapolation of UTC. The principle behind this arrangement is that measurements against the real-time reference are immutable facts, whereas improvements are made in our unsteered internal mean timescale A.1 as systematic disturbances are identified after the fact.
The requirement for the details mentioned above originates from the use of the MC (USNO) as the real-time reference for many timed systems, particularly for the Global Positioning System (GPS) satellites. Since these systems have global coverage and are widely used, there is need to approximate the international reference as closely as the state of the art allows - in real time.