International Atomic Time

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International Atomic Time (abbreviated TAI, from its French name temps atomique international

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,[4] 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.[5]

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.

Operation

TAI is a

GPS signals and two-way satellite time and frequency transfer.[7] Due to the signal averaging TAI is an order of magnitude
more stable than its best constituent clock.

The participating institutions each broadcast, in

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.[3] This combined time scale is published monthly in "Circular T",[8] 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).

History

Early atomic time scales consisted of

National Bureau of Standards, Boulder, Colorado on 9 October 1957.[9]

The

VLF radio signals. The BIH scale, A.1, and NBS-A were defined by an epoch at the beginning of 1958[a] The procedures used by the BIH evolved, and the name for the time scale changed: A3 in 1964[11] and TA(BIH) in 1969.[12]

The SI second was defined in terms of the caesium atom in 1967. From 1971 to 1975 the

International Committee for Weights and Measures made a series of decisions that designated the BIPM time scale International Atomic Time (TAI).[13]

In the 1970s, it became clear that the clocks participating in TAI were ticking at different rates due to

mean sea level). Because the clocks were, on average, well above sea level, this meant that TAI slowed by about one part in a trillion. The former uncorrected time scale continues to be published under the name EAL (Échelle Atomique Libre, meaning Free Atomic Scale).[14]

The instant that the gravitational correction started to be applied serves as the epoch for Barycentric Coordinate Time (TCB), Geocentric Coordinate Time (TCG), and Terrestrial Time (TT), which represent three fundamental time scales in the Solar System.[15] All three of these time scales were defined to read JD 2443144.5003725 (1 January 1977 00:00:32.184) exactly at that instant.[b] TAI was henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184 s.[16]

The continued existence of TAI was questioned in a 2007 letter from the BIPM to the ITU-R which stated, "In the case of a redefinition of UTC without leap seconds, the CCTF would consider discussing the possibility of suppressing TAI, as it would remain parallel to the continuous UTC."[17]

Relation to UTC

Contrary to TAI, UTC is a

UT1. This was a compromise arrangement in order to enable a publicly broadcast time scale. The less frequent whole-second adjustments meant that the time scale would be more stable and easier to synchronize internationally. The fact that it continues to approximate UT1 means that tasks such as navigation which require a source of Universal Time continue to be well served by the public broadcast of UTC.[18]

See also

Notes

  1. UT2 instant. However, each observatory used its own value of UT2.[10]
  2. ^ The 32.184 second offset is to provide continuity with the older ephemeris time.

References

  • "History of TAI−UTC". Time Service Dept., United States Naval Observatory. 2009. Archived from the original on 19 October 2019. Retrieved 4 January 2010.
  • "International Atomic Time". International Bureau of Weights and Measures. Archived from the original on 10 January 2011. Retrieved 24 January 2020.

Footnotes

  1. ^ Temps atomique 1975[further explanation needed]
  2. S2CID 120564915
    .
  3. ^
    ISSN 1994-9405. Archived
    (PDF) from the original on 14 August 2021. Retrieved 16 June 2022.
  4. IERS. Archived
    from the original on 13 August 2017. Retrieved 31 December 2016.
  5. ISSN 0261-3077
    . Retrieved 23 October 2024.
  6. ^ McCarthy & Seidelmann 2009, p. 207, 214.
  7. ^ Explanatory Supplement of BIPM Circular T (PDF), International Bureau of Weights and Measures, 12 July 2021, archived (PDF) from the original on 9 October 2022, retrieved 16 June 2022
  8. ^ Circular T, International Bureau of Weights and Measures, retrieved 16 June 2022
  9. ^ McCarthy & Seidelmann 2009, pp. 199–200.
  10. ^ Guinot 2000, p. 181.
  11. ^ Allen, Steve. "The epoch of TAI is 1961-01-01T20:00:00 UT2". UCO/Lick Observatory. Archived from the original on 10 October 2021. Retrieved 21 January 2019. By 1964 BIH realized that some atomic chronometers were much better than others, and they constructed A3 based on the best 3
  12. ^ McCarthy & Seidelmann 2009, pp. 200–201.
  13. ^ McCarthy & Seidelmann 2009, pp. 203–204.
  14. ^ McCarthy & Seidelmann 2009, p. 215.
  15. S2CID 120112678
    .
  16. ^ McCarthy & Seidelmann 2009, p. 218–219.
  17. ^ "CCTF 09-27" (PDF). International Bureau of Weights and Measures. 3 September 2007. Archived from the original (PDF) on 16 March 2012. Retrieved 24 September 2018.
  18. ^ McCarthy & Seidelmann 2009, p. 227–229.

Bibliography
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