Ronald Beard, Chairman, ITU-R Working Party 7A
Broadcast services throughout the world use an international timescale. This atomic timescale, known as Coordinated Universal Time (UTC), is defined by ITU and is maintained by the International Bureau of Weights and Measures (BIPM) in cooperation with the International Earth Rotation and Reference Systems Service (IERS).
Measurements contributed by timing centres around the world are used to determine UTC, which is adjusted to within 0.9 s of Earth rotation time (UT1) by IERS-determined values of the Earth’s rotation. Adjustments, made in one-second steps known as leap seconds, were implemented in 1972 to permit UT1 to be recovered from broadcast values of UTC for celestial navigation.
Current telecommunication and navigation systems use continuous timing for their data transmissions. Consequently, deliberations have been ongoing within ITU on modifying the definition of UTC to make it a continuous timescale, rather than the stepped atomic timescale that it is now.
The process would be to stop applying leap seconds at an agreed point, and permit the difference between UTC and UT1 to increase at a rate of approximately one second per year. Knowledge of the precise difference between UTC and UT1 would continue to be monitored, so that any user desiring UT1 would have the information needed to correct the readings of UTC to UT1.
The benefits of changing from step adjustments using leap seconds would be a continuous timescale available for all modern electronic navigation and computerized systems to operate with and eliminate the need for specialized ad hoc time systems.
Since 2000, a study question on “The future of the UTC timescale”, has been established in the ITU Radiocommunication Sector. With a change to the definition of the UTC timescale, there could be a major improvement in synchronization of communication networks, navigation systems and time distribution performance. Recent consideration of preliminary draft revised Recommendation ITU-R TF.460-6 on standard-frequency and time-signal emissions has not resulted in consensus among the ITU membership, so it has been decided to send the text to RA-12 for consideration.
The proposed revision eliminates the use of leap seconds in the UTC timescale on 1 January 2018, in other words five years after the bringing into force of the Final Acts of WRC-12, should the conference decide to incorporate the revised recommendation into the Radio Regulations.
