The World Radiocommunication Conference will decide in 2015
Coordinated Universal Time (UTC) is the international standard time-scale for all practical timekeeping in the modern world. UTC is calculated using atomic clocks. Its historical counterpart, Universal Time (UT1), is an astronomical time-scale based on the observation of the Earth’s rotation.
But the Earth’s rotation rate is irregular, and has been generally slowing down. So, since 1972, these two time-scales have been kept in step by adding a leap second to UTC whenever necessary to keep the difference between them to less than one second, as desired by the scientific community. An extra 25 seconds have been added to UTC and clocks around the world so far.
With the advent of global navigation satellite systems in particular, options for revising the definition of UTC and the future of the leap second have been keenly debated in various international organizations, notably ITU which is responsible for defining UTC. Some countries have called for an end to leap seconds, but others disagree arguing that the present definition of UTC with leap seconds is satisfactory.
The World Radiocommunication Conference in 2015 (WRC-15) will take a decision on the matter, based on studies which ITU is conducting now.
In the run-up to WRC-15, the September 2013 Special Edition of ITU News on “The future of time: To abolish or not to abolish the leap second?” sets out the pros and cons of keeping the leap second.
In his editorial, ITU Secretary-General, Dr Hamadoun I. Touré notes that timekeeping is critical to the functioning of modern society and highlights the crucial role of international cooperation and coordination.
The Director of the ITU Radiocommunication Bureau François Rancy, in his foreword “Modern times: Is the leap second history?” summarizes the arguments on both sides, and explains the different uses of UTC, including for global navigation satellite systems.
In The Past and Future of Coordinated Universal Time, Ronald Beard, Chairman of ITU-R Working Party 7A, reviews the history and possible future of the leap second.
Elisa Felicitas Arias, Director of the Time Department at the International Bureau of Weights and Measures (BIPM), the organization in charge of computing UTC, discusses the implementation of time-scales from an organizational perspective.
The International Earth Rotation and Reference Systems Service (IERS) determines and decides when leap seconds should be added to UTC and announces the dates for this insertion. Brian Luzum, Chair of the Directing Board of IERS, discusses this role.
In Blue Sky Thinking, the discussions on UTC that have taken place at the International Astronomical Union (IAU) are presented by Mizuhiko Hosokawa, President of IAU’s Commission 31 on Time.
Claude Boucher presents the views expressed within the International Union of Geodesy and Geophysics, in the context of using or adapting UTC in relation to geosciences.
David Finkleman, Senior Scientist of the Centre for Space Standards and Innovation, Colorado, and Kara Warburton, PhD Candidate from City University of Hong Kong, argue in What is in a name? that the ubiquitous term Coordinated Universal Time cannot be used to refer to a revised time-scale that is disconnected from Earth rotation.
W. Lewandowski from the International Bureau of Weights and Measures discusses the difficulties that leap seconds cause to global navigation satellite systems and their system times.
In Space Odyssey, Han Chunhao from the Beijing Satellite Navigation Center explores the internal time-scales used in global navigation satellite systems, such as GPS Time, GLONASS Time, Galileo Time and BeiDou Time.
Jörg Hahn, Engineering Manager of the European satellite navigation system Galileo — expected to become operational in 2014 — discusses Galileo and UTC leap seconds. Galileo is designed to accommodate leap seconds, but human actions are still needed to insert leap seconds, with the consequential risk of human errors.
Igor V. Zheltonogov, D. Aronov and S. Sorokin, from Geyser-Telecom, discuss the Russian Federation satellite navigation system GLONASS and UTC. Currently, the GLONASS system is the only navigation system that implements a time-scale with the leap second. The authors argue that if WRC-15 redefines UTC then it should ensure backward compatibility to enable existing equipment to continue operating.
Judah Levine from the Time and Frequency Division of the United States Institute of Standards and Technology discusses the impact of leap seconds on digital time services and Internet time servers. He concludes that keeping the difference between UT1 and UTC smaller than 1 second is not worth the associated difficulties.
Peter Whibberley, Senior Research Scientist from the Time and Frequency Group at the National Physical Laboratory in the United Kingdom, provides a British perspective on the future of UTC.The United Kingdom considers that the reported difficulties caused by leap seconds do not justify breaking the link between civil timekeeping and the Earth’s rotation.
Koichi Shibata from Japan’s Time Business Forum describes the impact of the leap second on Japan’s time-stamp system.Time stamps validate documents that have legal consequences, such as claims for intellectual property rights. Adjustments to accommodate leap seconds have, however, disrupted Japan’s time-stamp system.
Main Photo above shows Jim Gray, keeper of the American NBS-4 atomic clock. The NBS-4 is the long cylindrical object on the bench. It is a caesium clock, in which atoms of vaporized caesium-133 pass back and forth between magnets at each end. The caesium-133 atoms oscillate between two energy levels as they go. The standard second is based on counting these oscillations. This accurate timekeeping is passed to Paris, where signals from this and other atomic clocks are averaged to produce the worldwide Coordinated Universal Time (UTC).