International Atomic Time

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International Atomic Time (abbreviated TAI, from its French name temps atomique international [1] ) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. [2] TAI is a weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide. [3] 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.

Contents

UTC deviates from TAI by a number of whole seconds. As of 1 January 2017, when another leap second was put into effect, [4] UTC is currently 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.

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 weighted average of the time kept by over 450 atomic clocks in over 80 national laboratories worldwide. [3] The majority of the clocks involved are caesium clocks; the International System of Units (SI) definition of the second is based on caesium. [5] The clocks are compared using GPS signals and two-way satellite time and frequency transfer. [6] 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. [3] This combined time scale is published monthly in "Circular T", [7] 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 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. [8]

The International Time Bureau (BIH) began a time scale, Tm or AM, in July 1955, using both local caesium clocks and comparisons to distant clocks using the phase of VLF radio signals. The BIH scale, A.1, and NBS-A were defined by an epoch at the beginning of 1958 [lower-alpha 1] The procedures used by the BIH evolved, and the name for the time scale changed: A3 in 1964 [10] and TA(BIH) in 1969. [11]

The SI second was defined in terms of the caesium atom in 1967. From 1971 to 1975 the General Conference on Weights and Measures and the International Committee for Weights and Measures made a series of decisions that designated the BIPM time scale International Atomic Time (TAI). [12]

In the 1970s, it became clear that the clocks participating in TAI were ticking at different rates due to gravitational time dilation, and the combined TAI scale, therefore, corresponded to an average of the altitudes of the various clocks. Starting from the Julian Date 2443144.5 (1 January 1977 00:00:00 TAI), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at the geoid (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). [13]

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. [14] All three of these time scales were defined to read JD 2443144.5003725 (1 January 1977 00:00:32.184) exactly at that instant. [lower-alpha 2] TAI was henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184 s. [15]

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." [16]

Relation to UTC

Contrary to TAI, UTC is a discontinuous time scale. It is occasionally adjusted by leap seconds. Between these adjustments, it is composed of segments that are mapped to atomic time by a constant offset. From its beginning in 1961 through December 1971, the adjustments were made regularly in fractional leap seconds so that UTC approximated UT2. Afterward, these adjustments were made only in whole seconds to approximate 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. [17]

See also

Notes

  1. ↑ They were set to read Julian Date 2436204.5 (1 January 1958 00:00:00) at the corresponding UT2 instant. However, each observatory used its own value of UT2. [9]
  2. ↑ The 32.184 second offset is to provide continuity with the older ephemeris time.

Related Research Articles

Δ<i>T</i> (timekeeping) Measure of variation of solar time from atomic time

In precise timekeeping, ΔT is a measure of the cumulative effect of the departure of the Earth's rotation period from the fixed-length day of International Atomic Time. Formally, ΔT is the time difference ΔT = TT − UT between Universal Time and Terrestrial Time. The value of ΔT for the start of 1902 was approximately zero; for 2002 it was about 64 seconds. So Earth's rotations over that century took about 64 seconds longer than would be required for days of atomic time. As well as this long-term drift in the length of the day there are short-term fluctuations in the length of day which are dealt with separately.

The term ephemeris time can in principle refer to time in association with any ephemeris. In practice it has been used more specifically to refer to:

  1. a former standard astronomical time scale adopted in 1952 by the IAU, and superseded during the 1970s. This time scale was proposed in 1948, to overcome the disadvantages of irregularly fluctuating mean solar time. The intent was to define a uniform time based on Newtonian theory. Ephemeris time was a first application of the concept of a dynamical time scale, in which the time and time scale are defined implicitly, inferred from the observed position of an astronomical object via the dynamical theory of its motion.
  2. a modern relativistic coordinate time scale, implemented by the JPL ephemeris time argument Teph, in a series of numerically integrated Development Ephemerides. Among them is the DE405 ephemeris in widespread current use. The time scale represented by Teph is closely related to, but distinct from, the TCB time scale currently adopted as a standard by the IAU.
<span class="mw-page-title-main">Greenwich Mean Time</span> Time zone of Western Europe, same as WET

Greenwich Mean Time (GMT) is the local mean time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a particular time unless a context is given. The term GMT is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom.

<span class="mw-page-title-main">Leap second</span> Extra second inserted to keep civil time in sync with the Earths rotation

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time and imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation. The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently would run ahead of observed solar time unless it is reset to UT1 as needed. The leap second facility exists to provide this adjustment. The leap second was introduced in 1972. Since then, 27 leap seconds have been added to UTC, with the most recent occurring on December 31, 2016.

<span class="mw-page-title-main">Second</span> SI unit of time

The second is the unit of time in the International System of Units (SI), historically defined as 1⁄86400 of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. "Minute" comes from the Latin pars minuta prima, meaning "first small part", and "second" comes from the pars minuta secunda, "second small part".

Terrestrial Time (TT) is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of Earth. For example, the Astronomical Almanac uses TT for its tables of positions (ephemerides) of the Sun, Moon and planets as seen from Earth. In this role, TT continues Terrestrial Dynamical Time, which succeeded ephemeris time (ET). TT shares the original purpose for which ET was designed, to be free of the irregularities in the rotation of Earth.

A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.

Universal Time is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with respect to the International Celestial Reference Frame (ICRF), called the Earth Rotation Angle. UT1 is the same everywhere on Earth. UT1 is required to follow the relationship

<span class="mw-page-title-main">International Earth Rotation and Reference Systems Service</span> Body responsible for maintaining global time and reference frame standards

The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Parameter (EOP) and International Celestial Reference System (ICRS) groups.

<span class="mw-page-title-main">Solar time</span> Calculation of elapsed time by the apparent position of the sun

Solar time is a calculation of the passage of time based on the position of the Sun in the sky. The fundamental unit of solar time is the day, based on the synodic rotation period. Traditionally, there are three types of time reckoning based on astronomical observations: apparent solar time and mean solar time, and sidereal time, which is based on the apparent motions of stars other than the Sun.

The Time from NPL is a radio signal broadcast from the Anthorn Radio Station near Anthorn, Cumbria, which serves as the United Kingdom's national time reference. The time signal is derived from three atomic clocks installed at the transmitter site, and is based on time standards maintained by the UK's National Physical Laboratory (NPL) in Teddington. The service is provided by Babcock International, under contract to the NPL. It was funded by the former Department for Business, Innovation and Skills; as of 2017 NPL Management Limited (NPLML) was owned by the Department for Business, Energy and Industrial Strategy (BEIS), and NPL operated as a public corporation.

In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. Older systems use telescope observations.

<span class="mw-page-title-main">DUT1</span> Time scale with correction

DUT1 is a time correction equal to the difference between Universal Time (UT1), which is defined by Earth's rotation, and Coordinated Universal Time (UTC), which is defined by a network of precision atomic clocks.

<span class="mw-page-title-main">National Physical Laboratory of India</span>

The CSIR- National Physical Laboratory of India, situated in New Delhi, is the measurement standards laboratory of India. It maintains standards of SI units in India and calibrates the national standards of weights and measures.

<span class="mw-page-title-main">Unit of time</span> Measurement unit for time

A unit of time is any particular time interval, used as a standard way of measuring or expressing duration. The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom. The exact modern SI definition is "[The second] is defined by taking the fixed numerical value of the cesium frequency, ΔνCs, the unperturbed ground-state hyperfine transition frequency of the cesium 133 atom, to be 9 192 631 770 when expressed in the unit Hz, which is equal to s−1."

<span class="mw-page-title-main">NIST-F1</span> Atomic clock

NIST-F1 is a cesium fountain clock, a type of atomic clock, in the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and serves as the United States' primary time and frequency standard. The clock took less than four years to test and build, and was developed by Steve Jefferts and Dawn Meekhof of the Time and Frequency Division of NIST's Physical Measurement Laboratory.

<span class="mw-page-title-main">NIST-F2</span> Atomic clock used for US time standard

NIST-F2 is a caesium fountain atomic clock that, along with NIST-F1, serves as the United States' primary time and frequency standard. NIST-F2 was brought online on 3 April 2014.

<span class="mw-page-title-main">Coordinated Universal Time</span> Primary time standard

Coordinated Universal Time or UTC is the primary time standard globally used to regulate clocks and time. It establishes a reference for the current time, forming the basis for civil time and time zones. UTC facilitates international communication, navigation, scientific research, and commerce.

<span class="mw-page-title-main">Atomic clock</span> Extremely accurate clock

An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions between such states they interact with a very specific frequency of electromagnetic radiation. This phenomenon serves as the basis for the International System of Units' (SI) definition of a second:

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, , the unperturbed ground-state hyperfine transition frequency of the caesium-133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.

Bernard René Guinot (1925–2017) was a French astronomer. He is known for his contributions to the establishment of temps atomique international (TAI) and the geodetic reference system used in satellite navigation.

References

Footnotes

  1. ↑ Temps atomique 1975[ further explanation needed ]
  2. ↑ Guinot, B. (1986). "Is the International Atomic Time TAI a coordinate time or a proper time?". Celestial Mechanics. 38 (2): 155–161. Bibcode:1986CeMec..38..155G. doi:10.1007/BF01230427. S2CID   120564915.
  3. 1 2 3 BIPM Annual Report on Time Activities (PDF). Vol. 15. International Bureau of Weights and Measures. 2020. p. 9. ISBN   978-92-822-2280-5. ISSN   1994-9405. Archived (PDF) from the original on 14 August 2021. Retrieved 16 June 2022.
  4. ↑ Bizouard, Christian (6 July 2016). "Bulletin C 52". Paris: IERS. Archived from the original on 13 August 2017. Retrieved 31 December 2016.
  5. ↑ McCarthy & Seidelmann 2009, p. 207, 214.
  6. ↑ 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
  7. ↑ Circular T, International Bureau of Weights and Measures , retrieved 16 June 2022
  8. ↑ McCarthy & Seidelmann 2009, pp. 199–200.
  9. ↑ Guinot 2000, p. 181.
  10. ↑ 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
  11. ↑ McCarthy & Seidelmann 2009, pp. 200–201.
  12. ↑ McCarthy & Seidelmann 2009, pp. 203–204.
  13. ↑ McCarthy & Seidelmann 2009, p. 215.
  14. ↑ Brumberg, V.A.; Kopeikin, S.M. (March 1990). "Relativistic time scales in the solar system". Celestial Mechanics and Dynamical Astronomy . 48 (1): 23–44. Bibcode:1990CeMDA..48...23B. doi:10.1007/BF00050674. ISSN   0923-2958. S2CID   120112678.
  15. ↑ McCarthy & Seidelmann 2009, p. 218–219.
  16. ↑ "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.
  17. ↑ McCarthy & Seidelmann 2009, p. 227–229.

Bibliography

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