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The realization and dissemination of the international time scales is the responsibility of the Time Department of the BIPM. International Atomic Time (TAI) is the uniform time scale; it is kept as close as possible to the second of the SI. Coordinated Universal Time (UTC) is an atomic time scale derived from TAI, to provide a reference scale in step with the irregular rotation of the earth. Local realizations of UTC exist at the national time laboratories. These laboratories participate in the calculation of the international time scales by sending their clock data to the BIPM. Most of them are equipped with commercial caesium beams that provide a practical realization of the second sufficiently accurate for most applications. More accurate caesium standards exist in a small number of laboratories; for them, the uncertainties are estimated to be a few parts in 1015. New developments in clocks using trapped or cooled atoms or ions are leading to improvements well beyond this.
The atomic time scales TAI and UTC are disseminated monthly through the BIPM Circular T. The BIPM Annual Report on Time Activities provides all relevant information, data and results for the year previous to its publication. Reports on time-transfer techniques are also issued regularly.
Other activities related to the time scales are developed in the Department; these contribute to improving the calculation algorithms and increasing knowledge about time transfer techniques.
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Summary
