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| Temps Atomique International | |
|---|---|
| Name | Temps Atomique International |
| Abbreviation | TAI |
| Formation | 1955 |
| Founder | International Bureau of Weights and Measures |
| Type | International time standard |
| Purpose | Timekeeping and frequency standardization |
| Headquarters | Sèvres, France |
| Region served | Worldwide |
| Parent organization | International Bureau of Weights and Measures |
Temps Atomique International is the international atomic time scale that provides a continuous, stable, and precise measure of proper time used for scientific, navigational, and technological purposes. It is maintained by a network of national metrology institutes and international organizations to realize an SI-based second, underpinning systems such as Global Positioning System, European Space Agency, International Space Station, Very Long Baseline Interferometry, and International Telecommunication Union standards. The scale traces its authority to coordination among institutions like the International Bureau of Weights and Measures, the International Astronomical Union, and the Bureau International des Poids et Mesures laboratories.
Temps Atomique International is defined as a weighted average of the readings of approximately 400 primary and secondary atomic clocks operated by national metrology institutes including NIST, LNE-SYRTE, PTB, NMIJ, NPLI, CSIRO, KRISS, and BNM-LNE. It implements the SI second as realized by the cesium-133 hyperfine transition established by the General Conference on Weights and Measures and articulated by the Comité International des Poids et Mesures. The purpose of the scale is to furnish a uniform temporal coordinate for applications across European Space Agency missions, NASA operations, global navigation satellite systems such as GLONASS, BeiDou, and timing infrastructures of financial markets in London, New York City, and Tokyo.
The concept evolved from mid-20th-century efforts by laboratories including National Physical Laboratory (UK), Physikalisch-Technische Bundesanstalt, and Bureau International de l'Heure that sought to replace astronomical time with atomic references. Early milestones include the 1955 development of coordinated atomic time by the International Bureau of Weights and Measures, the 1967 redefinition of the SI second at the General Conference on Weights and Measures based on cesium, and subsequent international coordination through the International Astronomical Union and the International Telecommunication Union. Advances came with the deployment of hydrogen masers at institutions like Harvard-Smithsonian Center for Astrophysics and Observatoire de Paris, laser-cooled fountain clocks at PTB and NIST, and optical lattice clocks at JILA, NPL, NICT, and SYRTE that progressively improved stability and accuracy.
TAI is realized through metrological practices codified by the Comité International des Poids et Mesures and the Bureau International des Poids et Mesures. The realization uses cesium primary frequency standards, including cesium fountain clocks at NIST, PTB, LNE-SYRTE, and NMIJ, supplemented by hydrogen masers for short-term stability at facilities such as INRIM and NPL. Recent developments incorporate optical clocks based on strontium, ytterbium, and aluminum ions demonstrated at JILA, PTB, NIST, NMIJ, and NIST JILA as potential future primary standards. Time and frequency transfer methods—two-way satellite time and frequency transfer employed by INTELSAT-era systems, Common-View GPS pioneered by University of Bern, and carrier-phase GNSS techniques used by ESA—enable synchronization among laboratories.
TAI is computed monthly by the Bureau International des Poids et Mesures using clock data submitted by participating institutes including NIST, PTB, LNE-SYRTE, NMIJ, KRISS, and CSIRO. The computation applies algorithms developed in consultation with the International Earth Rotation and Reference Systems Service and uses weighting schemes to produce the best estimate of coordinate time. Dissemination of TAI and derived scales such as Coordinated Universal Time occurs via signals from Global Positioning System satellites operated by the United States Department of Defense, timing messages from the International Telecommunication Union, and broadcasts by national time services like NIST Time and Frequency Division and UK National Physical Laboratory.
TAI serves as the realization of terrestrial time scales and is the reference for Terrestrial Time, Barycentric Coordinate Time, and Coordinated Universal Time. UT1, maintained by the International Earth Rotation and Reference Systems Service, differs from TAI due to irregularities in Earth's rotation, necessitating leap seconds in Coordinated Universal Time coordinated by the International Telecommunication Union and the International Bureau of Weights and Measures. GNSS times such as GPS Time, GLONASS Time, and BeiDou Time are offsets of TAI or derived independently but are interrelated through bias estimates provided by agencies including USNO and Roscosmos.
TAI underpins precision navigation for Global Positioning System and other global navigation satellite systems, synchronization of telecommunications networks overseen by the International Telecommunication Union, timestamping in financial centers like London Stock Exchange and New York Stock Exchange, and experimental tests in fundamental physics conducted at CERN, LIGO, and Institute for Quantum Optics and Quantum Information. It supports geodesy efforts by the International Association of Geodesy and climate science observations coordinated with the World Meteorological Organization. The increasing accuracy of optical clocks at laboratories such as NIST, PTB, JILA, and SYRTE raises prospects for a future redefinition of the second as recommended by the Comité Consultatif du Temps et des Fréquences.
Governance of TAI involves the Comité International des Poids et Mesures, operational computation by the Bureau International des Poids et Mesures, scientific guidance from the International Astronomical Union and the International Earth Rotation and Reference Systems Service, and contributions from national metrology institutes such as NIST, PTB, LNE-SYRTE, NMIJ, KRISS, CSIRO, and NPL. Coordination with satellite operators and agencies like ESA, NASA, USNO, and Roscosmos ensures dissemination and interoperability across navigation and space systems. The General Conference on Weights and Measures provides treaty-level authority and updates to the SI that affect TAI.