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WRC-15 confirms that UTC will retain 'leap seconds' - it will consider the issue again in 2023

Members of the International Telecommunication Union (ITU) met at the World Radiocommunication Conference (WRC-15) in Geneva in November 2015 and decided to maintain Coordinated Universal Time (UTC) with leap second insertions until 2023, when another WRC will consider the issue again after wider consultation. The BIPM, as a member of the ITU Radiocommunication Sector, participated in WRC-15 as an observer, and contributed to the work of the relevant committee dealing with the choice of a method for achieving a continuous reference time scale.

The discussions on a possible modification of UTC have no precedent in the history of the ITU. Started in year 2000, partly as an initiative of the BIPM, no consensus could be reached on stopping the procedure of coarsely synchronizing the uniform atomic time scale to the time originated in the non-uniform rotation of the Earth (namely 'UT1').

The achievement of the atomic caesium frequency standard in 1955 and the development of commercial versions were fundamental to the decision of the General Conference on Weights and Measures (CGPM) in 1967 to redefine the second in terms of an atomic transition, abrogating the current, astronomical definition. More controversial was the adoption of atomic time scales. Objections to the adoption of atomic time were based on its departure from astronomical time. In spite of objections, atomic time was increasingly used. The unification of time on the basis of the atomic time scale of the Bureau International de l'Heure (BIH) was recommended by the International Astronomical Union (IAU, 1967), the International Union of Radio Sciences (URSI, 1969) and the International Radio Consultative Committee of the International Telecommunication Union (CCIR, 1970). The ultimate endorsement came from the 14th CGPM in 1971, which introduced the designation 'International Atomic Time', and the universal acronym TAI.

Nevertheless, TAI was never disseminated directly, and UTC, approximating to UT1 has continued to dominate time dissemination since 1972. Its definition was based on the need to access UT1 in real time for some specific applications including astronomical navigation, geodesy and telescope settings. Although this system worked well, leap seconds are increasingly cumbersome in many systems and introduce an ambiguity in dating events when they occur. This leads to the present situation, when continuous time systems, parallel to TAI, but with a time offset of an integral number of seconds, created for internal synchronization of satellite systems, are used as a time reference, putting the unification of time at risk.

The BIPM took over the responsibility for the maintenance of UTC from the BIH in 1988, on the basis of the contribution of national timing centres that are distributed world-wide. The International Earth Rotation and Reference Systems Service (IERS), has been charged with deciding on the dates of insertion of leap seconds into UTC based on their survey of the Earth's rotational behaviour. The Radiocommunication Sector of the ITU continued with its responsibility for the dissemination of time signals via radiocommunication systems. In this context, Recommendation ITU-R TF.460-6 defines the method of synchronization of UTC to UT1 with a maximum offset of 0.9 seconds.

Aiming at clarifying roles and responsibilities, the resolution made by the WRC-15 calls for reinforcing the links between the ITU and the BIPM, and confirms that the BIPM is responsible for establishing and maintaining the SI second and its dissemination through the reference time scale. Coordinated actions of the two, including decisions of the CIPM and the CGPM, together with other relevant organizations such as the IAU, IERS and URSI are expected in the coming years, involving studies on the impact and application of a future reference time scale.

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