Nº 7 2013 > The future of time
To abolish or not to abolish the leap second?
Time-scales and the International Bureau of Weights and Measures
Elisa Felicitas Arias, Director, Time Department, International Bureau of Weights and Measures (BIPM)
The International Atomic Time (TAI) and Coordinated Universal Time (UTC) are maintained by the International Bureau of Weights and Measures (Bureau International des Poids et Mesures —BIPM). TAI constitutes the basis of UTC. Both time-scales are equally stable and accurate, but while TAI is continuous, UTC is adjusted from time to time by the insertion of a leap second to keep it synchronized with the Earth's rotation. The dates for inserting leap seconds in UTC are decided and announced by the International Earth Rotation and Reference Systems Service (IERS).
The development of the first cæsium frequency standard at the United Kingdom National Physical Laboratory in 1955 marked the beginning of the atomic era in frequency referencing and timekeeping. It was quickly recognized that the cæsium transition could serve as a reference for frequencies.
The unification of time on the basis of the atomic time-scale maintained in the 1960s by the Bureau International de l’Heure was recommended by the International Astronomical Union in 1967, by the International Union of Radio Science in 1969, and by ITU in 1970 — through its International Radio Consultative Committee (CCIR) — the forerunner of the Radiocommunication Sector (ITU–R). Finally, in 1971, official recognition was given by the General Conference for Weights and Measures, which introduced the designation International Atomic Time and the universal acronym TAI (as had been used by the Bureau International de l’Heure to designate atomic time since 1955).
In those days, however, one obstacle to the universal acceptance of TAI was the need to provide astronomical time (denominated UT1) to those users who — for the purposes of sea navigation and other domestic applications — required a time-scale based on the irregular rotation rate of the Earth. There was discussion about the wisdom of maintaining two different time-scales, and the need to avoid the enormous confusion that this might create. Ultimately, UTC was defined by ITU’s CCIR, and its use was endorsed by the General Conference for Weights and Measures in 1975. The definition of UTC was well adapted to the applications and technologies that existed in the early 1970s, and so this unique reference for time dissemination represented a good compromise for all users.
Despite objections, however, atomic time was increasingly used. TAI has never been disseminated directly; in fact it provides a frequency reference but has no practical use for measuring time intervals. It has no physical representation by clocks and in consequence is not disseminated by time signals. UTC is the time reference, calculated from TAI. Both UTC and TAI are calculated in post-processing, and available with a delay of 10 to 40 days. UTC is, however, needed in real time for some specific applications, including astronomical navigation, geodesy, telescope settings, space navigation and satellite tracking. The laboratories contributing to the formation of UTC at BIPM therefore maintain real-time realizations of UTC, indicated by UTC(k), where k is the designation of the laboratory concerned. These laboratories provide real-time access to UTC for practical applications and they disseminate UTC by various means.
Time shall unfold
Since 1972, UTC has differed from TAI by an integral number of seconds, changed whenever necessary by the insertion of a leap second to maintain the difference UT1 — UTC within 0.9 second. This system apparently works well. With four decades of experience, the procedures for inserting leap seconds have been refined and secured. However, with the emergence of ever-more sophisticated equipment and services, these procedures are becoming increasingly cumbersome and introduce an ambiguity in dating events when they occur. This has given rise to the creation — for particular applications — of continuous time-scales parallel to TAI but offset by a number of seconds. These alternative time-scales are broadcast, putting at risk the unification of time. Given the progress that has been made in communications, other means of providing UT1 in real time can be envisaged, and the future of UTC is now being discussed.
UTC has many applications in time synchronization at all levels of precision, from the minutes needed by the general public, to the nanoseconds required in the most demanding applications. The case of global navigation satellite systems is typical. The internal system time of the United States Global Positioning System (GPS), known as GPS Time, is closely synchronized with UTC as maintained by the United States Naval Observatory (USNO). GPS disseminates a good approximation to UTC, easily available at all levels of precision from a second to a few nanoseconds. Similar features will be adopted for Galileo, the future European satellite positioning system, and by the upcoming Chinese system BeiDou. The Russian system GLONASS follows UTC with leap seconds, synchronizing GLONASS Time to the realization of UTC in the Russian Federation.
Reliable time for science
TAI is the basis for the realization of time-scales used in dynamics, for modelling the motions of artificial and natural celestial bodies, with applications in the exploration of the solar system, tests of theories, geodesy, geophysics, and studies of the environment. In all these applications, relativistic effects are important. Different algorithms can be established, depending on requirements.
For an international reference such as UTC, the requirement is extreme reliability and long-term frequency stability. UTC therefore relies on the largest possible number of atomic clocks of different types, at present about 420, located in more than 70 institutes worldwide and connected in a network that allows precise time comparisons between remote sites. Each month the differences between the international time-scale UTC and the local approximations UTC(k) in contributing laboratories are reported in an official document called BIPM Circular T.
Defining, maintaining and realizing the reference time-scale is the result of continuous coordination between a group of organizations. The Metre Convention — a diplomatic treaty —was signed in 1875 and created BIPM, an intergovernmental organization under the authority of the General Conference for Weights and Measures. As at 6 February 2013, BIPM had 55 Member States, and 37 Associate States and Economies of the General Conference. BIPM acts in matters of world metrology, particularly concerning the demand for measurement standards of ever-increasing accuracy, range and diversity, and the need to demonstrate equivalence between national measurement standards. The General Conference for Weights and Measures adopts resolutions on the definition of units, and in particular on the definition of the second; it has adopted TAI and endorsed UTC.
BIPM is responsible for the provision of UTC and for its calculation, on the basis of international cooperation with national institutes; it gives metrological traceability to UTC to its local realizations.
ITU adopts recommendations relevant to the dissemination of time and frequency signals based on UTC. In particular, Recommendation ITU–R TF.460-6 describes the process for synchronizing UTC to UT1 at the level of 0.9 second.
The International Earth Rotation and Reference Systems Service (IERS) monitors the rotation of the Earth, and fixes and announces the dates of application of leap seconds in UTC. IERS produces and publishes predictions of the values of UT1-UTC, allowing access to UT1 with much higher precision than the coarse approximation of UTC. Finally, the more than 70 institutes that maintain the local realizations UTC(k) disseminate time for a variety of national and regional applications, ranging from civil timekeeping to enabling precise time synchronization for space and science activities.
In the event that ITU Member States approve a continuous reference time-scale, then IERS would have the essential role of guaranteeing the provision of the predicted values of UT1-UTC, and ITU would make specific recommendations for the wide dissemination of those values. BIPM would remain responsible for the maintenance of the reference time-scale, as part of a coordinated international effort.