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  Page principale : UIT-R : Conférences et réunions : : [ITU/BIPM Workshop
Future of the International Time Scale
Geneva, 19-20 September 2013]
Secteur des radiocommunications (UIT-R)  
[ITU/BIPM Workshop
Future of the International Time Scale
Geneva, 19-20 September 2013]

[Background paper]
[The development of the first atomic frequency standard in the 1950s can be considered the origin of the atomic definition of the SI second by the 13th General Conference on Weights and Measures (1967) and the consequent adoption of the atomic time scale known as International Atomic Time (TAI).]

[The consideration of adopting an atomic time scale prompted a request from the maritime navigation community that broadcast time services provide access to a time scale following the irregular rotation of the Earth, represented by a form of universal time indicated by UT1. It is for this reason that Coordinated Universal Time (UTC) was defined in 1972 by Recommendation ITU-R TF.460 of the International Radiocommunication Union - Radiocommunication sector (ITU-R) to provide an atomic time maintained to within 1 second with the Earth's rotational time. The increasing difference between UTC and UT1 is offset through the insertion of one-second "steps" in UTC whenever necessary to resynchronize the atomic time scale to the Earth's rotational time. These one-second steps at irregular intervals are referred to as "leap seconds".]

[The International Bureau of Weights and Measures (BIPM) is responsible for the generation and maintenance of UTC based on data from some 400 atomic clocks spread world-wide. The International Earth Rotation and Reference Systems Service (IERS) monitors the irregularities of the Earth's rotation, and determines the dates of application of leap seconds in UTC. Finally, the time keeping institutes maintaining National and local atomic time scales disseminate UTC following the standards established by the ITU-R.]

[In the forty years since the definition of UTC the offset between TAI and UTC has grown to 35 s. The procedure for inserting a leap second into UTC in Recommendation ITU-R TF.460-6 also stipulates that the change be made at the same UTC instant all over the world. The dates for applying leap seconds are determined from observations of the earth's motion and announced some months in advance but cannot be accurately predicted, which adds to the complexity of the process.]

[Over the last ten years, a number of user communities have expressed their dissatisfaction with this stepped UTC system. These users require a continuous time reference to permit precise synchronization of their systems. This is the case of time synchronization by networks, telecommunications, electrical power control, internal synchronization of global navigation satellite systems (GNSS) and others. Furthermore, since the advent of GNSS, maritime navigation no longer relies on the Earth's rotational time as the determination of the earth's rotation angle from the prime meridian for navigation.]

[However, access to rotational time UT1 will remain essential for many users requiring accurate knowledge of the earth's rotation angle with time, including astronomers. Today's observing techniques for space and Earth associated with refined algorithms and today's networks of powerful computers allow the computation and real time dissemination of the difference between UTC and UT1 with a much higher precision than previously possible when the current UTC system was established.]

[Now forty years after the adoption of the definition of UTC the ITU-R is close to taking a decision on whether or not to decouple it from the rotation of the Earth as embodied in UT1. There has been considerable discussion on this over the last ten years, mainly at the ITU-R but also by other relevant organizations consulted.]

[While the details of the discussion are familiar to experts in time keeping and to the relevant users, they may not be familiar to other sectors, including some faced with making the decision on the future of UTC. This ITU/BIPM workshop is addressed to all sectors with the aim of raising awareness of the key issues, so that the most appropriate definition for serving all modern timekeeping applications will be adopted with the consensus of the different sectors.]


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Mis à jour le : 2013-02-15