International Atomic Time

International Atomic Time (abbreviated TAI, from its French name temps atomique international) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide. It is a continuous scale of time, without leap seconds, and it is the principal realisation of Terrestrial Time (with a fixed offset of epoch). It is the basis for Coordinated Universal Time (UTC), which is used for civil timekeeping all over the Earth's surface and which has leap seconds.

UTC deviates from TAI by a number of whole seconds. As of 1 January 2017^[update], immediately after the most recent leap second was put into effect, UTC has been exactly 37 seconds behind TAI. The 37 seconds result from the initial difference of 10 seconds at the start of 1972, plus 27 leap seconds in UTC since 1972. In 2022, the General Conference on Weights and Measures decided to abandon the leap second by or before 2035, at which point the difference between TAI and UTC will remain fixed.

TAI may be reported using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both Julian days and the Gregorian calendar are used. TAI in this form was synchronised with Universal Time at the beginning of 1958, and the two have drifted apart ever since, due primarily to the slowing rotation of the Earth.

TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide. The majority of the clocks involved are caesium clocks; the International System of Units (SI) definition of the second is based on caesium. The clocks are compared using GPS signals and two-way satellite time and frequency transfer. Due to the signal averaging TAI is an order of magnitude more stable than its best constituent clock.

The participating institutions each broadcast, in real time, a frequency signal with timecodes, which is their estimate of TAI. Time codes are usually published in the form of UTC, which differs from TAI by a well-known integer number of seconds. These time scales are denoted in the form UTC(NPL) in the UTC form, where NPL here identifies the National Physical Laboratory, UK. The TAI form may be denoted TAI(NPL). The latter is not to be confused with TA(NPL), which denotes an independent atomic time scale, not synchronised to TAI or to anything else.

The clocks at different institutions are regularly compared against each other. The International Bureau of Weights and Measures (BIPM, France) combines these measurements to retrospectively calculate the weighted average that forms the most stable time scale possible. This combined time scale is published monthly in "Circular T", and is the canonical TAI. This time scale is expressed in the form of tables of differences UTC − UTC(k) (equal to TAI − TAI(k)) for each participating institution k. The same circular also gives tables of TAI − TA(k), for the various unsynchronised atomic time scales.

Errors in publication may be corrected by issuing a revision of the faulty Circular T or by errata in a subsequent Circular T. Aside from this, once published in Circular T, the TAI scale is not revised. In hindsight, it is possible to discover errors in TAI and to make better estimates of the true proper time scale. Since the published circulars are definitive, better estimates do not create another version of TAI; it is instead considered to be creating a better realisation of Terrestrial Time (TT).

Early atomic time scales consisted of quartz clocks with frequencies calibrated by a single atomic clock; the atomic clocks were not operated continuously. Atomic timekeeping services started experimentally in 1955, using the first caesium atomic clock at the National Physical Laboratory, UK (NPL). It was used as a basis for calibrating the quartz clocks at the Royal Greenwich Observatory and to establish a time scale, called Greenwich Atomic (GA). The United States Naval Observatory began the A.1 scale on 13 September 1956, using an Atomichron commercial atomic clock, followed by the NBS-A scale at the National Bureau of Standards, Boulder, Colorado on 9 October 1957.