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The development of the first atomic frequency standard by Louis Essen in the 1950s is at the origin of the adoption of the atomic definition of the SI second by the 13th General Conference on Weights and Measures in 1967 and the consequent adoption of the atomic timescale.

After the short reign of ephemeris time as the world's reference timescale from 1954 until 1967, Coordinated Universal Time (UTC), synchronized to universal time UT1, appeared as the best compromise for satisfying the requests of all users. At the moment of the discussion on the adoption of an atomic timescale to replace ephemeris time, the possibility of having both an astronomical time and an atomic time to serve different purposes was discussed. In the words of Essen [1], this 'would cause endless confusion as well as involving duplication of equipment'.

Forty years after the adoption of the definition of Coordinated Universal Time at the International Telecommunication Union (ITU), we are close to the moment of making a decision on whether or not to decouple UTC from its tight link to the rotation of the Earth embodied in UT1. It has been a ten-year process of discussion, mainly at the ITU with the input of the International Astronomical Union, the BIPM, the Consultative Committee for Time and Frequency and other organizations. The majority opinion supported the change based on developers and users of systems that need time synchronization to a stable and continuous reference timescale; others insist on the necessity of keeping the leap-second strategy for serving some applications or just for tradition. It is our hope that, as happened in the seventies, the most appropriate definition to serve all modern applications will be adopted with the consensus of the different sectors.

The redirection of international timekeeping from astronomy to metrology can be considered the benchmark that started the era of modern timescales, all based on atomic properties. The aim of this special issue of Metrologia is to review timescales in use today, either the internationally recognized references or those adapted to some specific applications, to discuss new and future developments and to present the sometimes complex procedures for making international recommendations.

We are grateful to our colleagues who, without exception, accepted our invitation to contribute to this special issue.

Reference

Henderson D 2005 Metrologia42 S4–29

The pdf file contains an appendix: "Glossary of acronyms related to timescales used in this issue".

Since 1954 when the definition of the second first came under the authority of the intergovernmental organization of the Metre Convention, the range and complexity of time metrology have increased far beyond anything envisaged in those days. Today, the essential international coordination of this domain of metrology is through the organs of the Convention with the exception of the definition of Coordinated Universal Time, UTC. In this short article I suggest that this also should now come under the authority of the Metre Convention.

The International Telecommunications Union (ITU) is the leading United Nations agency for Radio and Telecommunications coordination worldwide. The process of managing overall frequency spectrum utilization is through Worldwide Radio Conferences, associated radiocommunication conferences and the activities of the Radiocommunication Study Groups. These Study Groups and their Working Parties, devoted to specialized technical areas, provide the mechanism for Member Nations to participate, study and recommend standards and practices to ensure equitable utilization and interference-free operation within the radio spectrum. An important underlying aspect of spectrum utilization is the facilitation of the determination and coordination of the international time scale. The international time scale is an atomic time scale used by broadcast services throughout the world known as Coordinated Universal Time (UTC). UTC is defined by the International Telecommunication Union (ITU-R) and is maintained by the International Bureau of Weights and Measures (BIPM) in cooperation with the International Earth reference and Rotation Service (IERS). Contributed measurements from timing centres around the world are used in the determination of UTC, which is adjusted to within 0.9 s of Earth rotation time (UT1) by IERS-determined values of the Earth rotation. The 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 telecommunications and navigation systems utilize continuous timing for their data transmissions; consequently, deliberations have been ongoing within the ITU-R on the issue of modifying the definition of UTC to a continuous time scale.

Astronomy has provided a means to mark the passage of time throughout history. One of the repeating phenomena that makes this possible is the Earth's rotation. The basic variability in its rotational speed, however, makes astronomical techniques unsuitable for timekeeping with the precision required for modern applications. Physical metrology from the first mechanical clocks to the most sophisticated atomic standards of today has assumed a growing role in timekeeping. Along with this progress in technology, more sophisticated concepts of timescales have appeared to take advantage of those improvements.

This paper reviews the present status of the timescales established at the International Bureau of Weights and Measures (BIPM). We focus our attention on the calculation and the characteristics of Coordinated Universal Time (UTC) and present its applications.

Local representations of the international time scale, Coordinated Universal Time (UTC), are maintained by approximately 69 national measurement institutes and other time laboratories. These laboratories contribute their clock and time transfer measurements for use in the computation of UTC. Although local representations of UTC, commonly called UTC(k) time scales, vary considerably, for example in the numbers of atomic clocks available, they also share many characteristics. In this paper, we examine the rationale and requirements for maintaining a local representation of UTC. Its applications might range from underpinning the reference time scale of a Global Navigation Satellite System to providing traceability for frequency and time dissemination services. We address the practical aspects of setting up and operating a UTC(k) time scale, including the equipment and algorithms that generate the time scale, optimize its performance and measure its offset from the similar time scales maintained by other laboratories. We conclude by considering briefly some future developments that may have an impact on the laboratories operating local representations of UTC.

Monitoring the Earth's rotation angle is essential in various domains linked to reference systems such as space navigation, precise orbit determinations of artificial Earth satellites including the Global Navigation Satellite Systems (GNSS), positional astronomy and for geophysical studies on time scales ranging from a few hours to decades.

Universal Time UT1 is based on the rotation of the Earth on its axis. Historically it was related to mean solar time on the meridian of Greenwich, sometimes known as Greenwich Mean Time. Monitoring Earth orientation, and in particular UT1, is the primary task of the International Earth Rotation and Reference Systems Service (IERS). The Earth Orientation Center is responsible for monitoring Earth orientation parameters (EOPs) including long-term consistency and leap second announcements. The Rapid Service/Prediction Center is in charge of the rapid, near real-time solution and predictions. These two complementary services of the IERS provide Earth orientation information from results derived predominantly from Very Long Baseline Interferometry with valuable input from GNSS observations and global atmospheric angular momentum for both the combination and prediction of EOPs.

The algorithms for relativistic time transfer in the vicinity of the Earth and in the solar system are derived. The concepts of proper time and coordinate time are distinguished. The coordinate time elapsed during the transport of a clock and the propagation of an electromagnetic signal is analysed in three coordinate systems: an Earth-Centred Inertial (ECI) coordinate system, an Earth-Centred Earth-Fixed (ECEF) coordinate system and a barycentric coordinate system. The timescales of Geocentric Coordinate Time (TCG), Terrestrial Time (TT) and Barycentric Coordinate Time (TCB) are defined and their relationships are discussed. Some numerical examples are provided to illustrate the magnitudes of the effects.

The International Conference held in 1884 at Washington defined a universal time as the mean solar time at the Greenwich meridian (GMT). Now, the Universal Time, version UT1, is strictly defined as proportional to the angle of rotation of the Earth in space. In this evolution, the departure of UT1 from GMT does not exceed one or two seconds. This is quite negligible when compared with the departure between the solar time and the legal time of citizens, which may exceed two hours without raising protests.

Numerous time scales exist to address specific user requirements. Accurate dynamical time scales (barycentric, geocentric and terrestrial) have been developed based on the theory of relativity. A family of time scales has been developed based on the rotation of the Earth that includes Universal Time (specifically UT1), which serves as the traditional astronomical basis of civil time. International Atomic Time (TAI) is also maintained as a fundamental time scale based on the output of atomic frequency standards. Coordinated Universal Time (UTC) is an atomic scale for worldwide civil timekeeping, referenced to TAI, but with epoch adjustments via so-called leap seconds to remain within one second of UT1. A review of the development of the time scales, the status of the leap-second issue, and user considerations and perspectives are discussed. A description of some more recent applications for time usage is included.

An astronomical or navigational almanac can best be thought of as a device for connecting an observer with celestial objects. For an observer with a known position and time the almanac allows the observer to identify the celestial objects. Conversely, if the observer knows what objects he is observing and when, the almanac allows him to determine his position. In either case, knowledge of the time is crucial in providing this link between the celestial objects and the observer through the almanac. This paper summarizes the various time scales used in the astronomical and nautical almanacs produced jointly by the US Naval Observatory and HM Nautical Almanac Office.

This paper describes some of the design considerations behind the timescale transformation software provided by the International Astronomical Union's SOFA collaboration. The solution adopted by SOFA includes two-part Julian dates, to safeguard precision, and individual treatment of specific transformations, rather than a single general-purpose routine. Correct handling of UTC leap seconds was a particular challenge.

I describe the statistical considerations used to design systems whose clocks are compared by the use of dial-up telephone lines or the Internet to exchange timing information. The comparison is usually used to synchronize the time of a client system to the time of a server, which is, in turn, synchronized to the time scale of a national timing laboratory. The design includes a dynamic estimate of the system performance and a comparison between the performance and a parameter that specifies the required stability based on external considerations. The algorithm adjusts the polling interval and other parameters of the algorithm to realize the specified performance at minimal cost, where the cost is taken to be proportional to the inverse of the interval between message exchanges using either the Internet or dial-up telephone calls.

The development and current status of BeiDou Navigation Satellite System are briefly introduced. The definition and realization of the system time scales are described in detail. The BeiDou system time (BDT) is an internal and continuous time scale without leap seconds. It is maintained by the time and frequency system of the master station. The frequency accuracy of BDT is superior to 2 × 10⁻¹⁴ and its stability is better than 6 × 10⁻¹⁵/30 days. The satellite synchronization is realized by a two-way time transfer between the uplink stations and the satellite. The measurement uncertainty of satellite clock offsets is less than 2 ns. The BeiDou System has three modes of time services: radio determination satellite service (RDSS) one-way, RDSS two-way and radio navigation satellite service (RNSS) one-way. The uncertainty of the one-way time service is designed to be less than 50 ns, and that of the two-way time service is less than 10 ns. Finally, some coordinate tactics of UTC from the viewpoint of global navigation satellite systems (GNSS) are discussed. It would be helpful to stop the leap second, from our viewpoint, but to keep the UTC name, the continuity and the coordinate function unchanged.

Global Navigation Satellite Systems (GNSSs) use internal reference time scales: GPS Time, GLONASS Time, Galileo System Time and BeiDou System Time. Constructed from a clock ensemble, they are designed for internal system synchronization, necessary to produce a navigation solution. They are usually steered to an external stable reference time scale, for example UTC(USNO), modulo 1 s, for GPS time. To achieve safe operation of a GNSS, a system time should preferably be a uniform time scale not affected by the leap seconds of Coordinated Universal Time (UTC). But this is not compatible with international recommendations that radio broadcast time signals should conform as closely as possible to UTC. This paper describes the various approaches chosen by GNSS providers and the relation between GNSS system times and UTC in terms of numbering of seconds. Different solutions for numbering seconds do not help the GNSS interoperability. This paper also explains why, on some occasions, GNSS system times play a role of alternative time scales with the consequent risk of confusion.