UTC: Past, present and future

Andreas Bauch, Senior Scientist, PTB (national metrology institute) and Karsten Buckwitz, Senior Spectrum Advisor, BNetzA (Federal Networks Agency), Germany

Reliable access to accurate time has long been considered, and will remain indispensable for the proper functioning of modern infrastructure worldwide. Most users, industries, and organizations, as well as timing experts, agree on the need for a unique universal reference time scale and the General Conference on Weights and Measures (CGPM) decided in 1975, and reaffirmed in 2018, its recommendation to use Coordinated Universal Time (UTC) as the unique time scale for international reference and the basis for civil time.

UTC offers substantial advantages over other time scales:

• It is maintained by the International Bureau for Weights and Measures (BIPM), under the authority of the CGPM, with contributions from timing laboratories of many countries and hundreds of physicists and metrologists involved. This keeps it independent from the influence of any specific country, political power, or commercial entity.
• While UTC is widely recognized as the practical source of time and frequency in many countries, only a few specify UTC plus the appropriate offset as legal time. Germany is one such example, following UTC plus an offset of one or two hours, depending on the season, in accordance with the Law on Units and Time from 2008.
• UTC is disseminated through a real-time local approximation, or UTC(k), by the National Metrology Institute (NMI) or Designated Institute (DI) in each country. These institutes are entrusted domestically with the realization and dissemination of UTC(k) and in some cases of legal time. In Germany, the Federal Physical-Technical Institute (Physikalisch-Technische Bundesanstalt — PTB) disseminates legal time via the standard-frequency radio station DCF77 at 77.5 kilohertz (kHz), but also via the public telephone network and the Internet.

ITU’s role in time signal dissemination

The ITU Radiocommunication Sector (ITU–R) and its Working Party 7A play a vital role in providing the technical and regulatory basis for uninterrupted dissemination of timing signals worldwide. The definition of new time codes and the protection of standard frequency and time signal services remain important and ongoing tasks for Working Party 7A, with the support of concerned international organizations.

In decade-long discussions among all involved parties, consensus has been reached that alternative time scales should not be utilized as timing sources.

Instead, the practice of realizing UTC should be adapted to the needs of the 21st Century.

Previous World Radiocommunication Conferences conducted trials on updating UTC, and the 2015 conference (WRC‑15) considered the feasibility of achieving a continuous reference time scale, whether by modification of UTC or some other method. This WRC‑15 discussion took place in accordance with a decision in the previous cycle through Resolution 653 (WRC‑12).

Under the subsequent Resolution 655 (WRC‑15), the Director of the ITU Radiocommunication Bureau will report to the upcoming WRC‑23 on the outcomes of activities related to the various aspects of current and potential future reference time scales, as well as on the content and structure of time signals to be disseminated by radiocommunication systems.

Germany’s perspective

Several countries have supported Working Party 7A in preparing for the WRC‑23 discussion, like Germany, with a preparatory team working intensively on the topic between 2015 and 2022. Germany welcomed the final report, “Content and structure of time signals to be disseminated by radiocommunication systems and various aspects of current and potential future reference time scales, including their impacts and applications in radiocommunication” (ITU–R TF.2511), together with the associated note to the Director of the ITU Radiocommunication Bureau.

Germany fully supports the report’s conclusions and suggestions, which are likely to form the basis of the Bureau Director’s report to WRC‑23.

Updating ITU’s time-scale resolution

Many administrations, including Germany, have called for revising Resolution 655 (WRC‑15), “Definition of time scale and dissemination of time signals via radiocommunication systems”. This is essential to finalize the process and adapt UTC for the future.

Notable key findings and lessons have emerged through discussions on the topic since 2012:

BIPM is responsible for the definition and publication of the reference time scale UTC in accordance with Resolution 2 of the 26th CGPM, held in 2018;

• CGPM will determine the timeline and a future limit on the quantity UT1‑UTC, as laid down in Resolution 4 of the 27th CGPM in 2022; and
• the task of defining the properties of UTC is not part of spectrum regulation within ITU–R.

Continuing the dialogue, open to all

Interested users, industries and organizations — and also, of course, timing experts — are invited to join the vital and necessary dialogue on these issues in Working Party 7A. This ongoing discussion is taking place in accordance with the Memorandum of Understanding between ITU and BIPM, but also on the expected update of Resolution 655 at WRC‑23.

This article first appeared in ITU News Magazine: The future of Coordinated Universal Time – part of a series of editions on topics to be discussed at the World Radiocommunication Conference (WRC-23), from 20 November to 15 December in Dubai, UAE.

Download your copy of the ITU News Magazine: The future of Coordinated Universal Time.

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