What Is International Atomic Time?

International Atomic Time (TAI, from the French Temps Atomique International) is a continuous, high-precision time scale computed by the International Bureau of Weights and Measures (BIPM) from the readings of more than 400 atomic clocks distributed across metrology institutes and observatories in over 30 countries. It serves as the foundation of modern timekeeping, providing a uniform and stable reference against which national time scales and the global standard Coordinated Universal Time (UTC) are anchored.

TAI was formally named by international agreement between 1971 and 1975, following a 1971 request from the Conférence Générale des Poids et Mesures (CGPM) for a definitive international atomic time reference. Earlier precursors include the Bureau International de l'Heure's atomic time scale, which began in July 1955. The formal definition of TAI was codified by the CGPM in 2018, consolidating decades of metrological practice.

Computing TAI from Atomic Clocks

The BIPM produces TAI using the ALGOS algorithm, which aggregates clock data from participating national laboratories. Rather than simply averaging the raw clock outputs, ALGOS weights each clock's contribution by its measured frequency stability. A statistical cap on individual weights prevents any single clock, even the most precise available, from dominating the ensemble and introducing correlated errors. The intermediate result of this averaging is called EAL (Échelle Atomique Libre, or free atomic time scale). Since 1977, TAI has been derived from EAL by applying small frequency steering corrections, keeping TAI aligned with the SI definition of the second to within a few parts in 10^13. The BIPM publishes monthly Circular T bulletins that document the difference between TAI and each contributing clock.

The SI Second and Caesium Standards

TAI is calibrated to realize the SI second, which since 1967 has been defined as exactly 9,192,631,770 periods of the radiation corresponding to the ground-state hyperfine transition of the caesium-133 atom at rest at 0 K. Primary frequency standards, including caesium fountain clocks operated at national laboratories such as NIST, PTB, and SYRTE, are used to steer TAI's frequency. These primary standards are not continuously running clocks but periodic measurement devices that evaluate the output frequency of the EAL ensemble and signal corrections when drift exceeds defined tolerances. Optical lattice clocks, which operate at frequencies roughly 100,000 times higher than caesium microwave standards, are under active evaluation by the BIPM Time and Frequency Department as candidate contributors to future redefinitions of the second.

TAI and Coordinated Universal Time

TAI is a continuous scale with no discontinuities; it has never been adjusted by leap seconds or any similar intervention. UTC, the time scale used for civil timekeeping worldwide, is derived from TAI but differs from it by an integer number of seconds. Leap seconds are inserted into UTC periodically to keep it within 0.9 seconds of UT1, the astronomical time scale tied to Earth's rotation. As of 2024, UTC lags TAI by 37 seconds, a gap that has grown since TAI was set to approximately match astronomical time on 1 January 1958. Because TAI is strictly uniform and never jumps, it is the preferred reference for applications that cannot tolerate discontinuities, such as deep-space navigation and high-frequency financial trading. The IERS Rapid Service and Prediction Centre tracks the divergence between TAI and Earth's rotation and issues advance notice of leap second insertions.

Applications

International Atomic Time has applications in a wide range of disciplines, including:

• Global navigation satellite systems (GPS, GLONASS, Galileo) that require synchronized atomic time references
• Telecommunications network synchronization and frequency distribution
• Fundamental physics experiments testing general and special relativity
• High-precision geodesy and Earth rotation monitoring
• Deep-space mission planning and spacecraft trajectory calculations