International Atomic Time

International Atomic Time

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International Atomic Time (TAI, from the French
French language
French is a Romance language spoken as a first language in France, the Romandy region in Switzerland, Wallonia and Brussels in Belgium, Monaco, the regions of Quebec and Acadia in Canada, and by various communities elsewhere. Second-language speakers of French are distributed throughout many parts...

 name Temps Atomique International) is a high-precision atomic coordinate
Coordinate time
In the theory of relativity, it is convenient to express results in terms of a spacetime coordinate system relative to an implied observer. In many coordinate systems, an event is specified by one time coordinate and three spatial coordinates...

 time standard
Time standard
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...

 based on the notional passage of proper time
Proper time
In relativity, proper time is the elapsed time between two events as measured by a clock that passes through both events. The proper time depends not only on the events but also on the motion of the clock between the events. An accelerated clock will measure a smaller elapsed time between two...

 on Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's geoid
The geoid is that equipotential surface which would coincide exactly with the mean ocean surface of the Earth, if the oceans were in equilibrium, at rest , and extended through the continents . According to C.F...

. It is the basis for Coordinated Universal Time
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

 (UTC), which is used for civil timekeeping all over the Earth's surface, and for Terrestrial Time
Terrestrial Time
Terrestrial Time is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of the Earth....

, which is used for astronomical calculations. Since 2008-12-31 when the last leap second was added, TAI has been exactly 34 seconds ahead of UTC. This is still true as of 2011-09-01. 34 seconds results from the initial difference of 10 seconds at the start of 1972, plus 24 leap second
Leap second
A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

s in UTC since 1972.

Time coordinates on the TAI scales are conventionally specified using traditional means of specifying days, carried over from non-uniform time standards based on the rotation of the Earth. Specifically, both Julian Dates and the Gregorian calendar
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

 are used. TAI in this form was synchronised with Universal Time
Universal Time
Universal Time is a time scale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time , i.e., the mean solar time on the Prime Meridian at Greenwich, and GMT is sometimes used loosely as a synonym for UTC...

 at the beginning of 1958, and the two have drifted apart ever since, due to the changing motion of the Earth.


TAI as a time scale is a weighted average of the time kept by over 200 atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

s in about 70 national laboratories worldwide. The clocks are compared using GPS signals and Two-Way Satellite Time and Frequency Transfer
Two-Way Satellite Time and Frequency Transfer
Two-way satellite time and frequency transfer is a high-precision long distance time and frequency transfer mechanism used between time bureaux to determine and distribute time and frequency standards....

. Due to the averaging it is far more stable than any clock would be alone (see signal averaging
Signal averaging
Signal averaging is a signal processing technique applied in the time domain, intended to increase the strength of a signal relative to noise that is obscuring it...

 for a discussion). The majority of the clocks are caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...

 clocks; the definition of the SI
Si, si, or SI may refer to :- Measurement, mathematics and science :* International System of Units , the modern international standard version of the metric system...

 second is written in terms of caesium.

The participating institutions each broadcast, in real time, a frequency signal with time code
Time code
A timecode is a sequence of numeric codes generated at regular intervals by a timing system.- Video and film timecode :...

s, 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 in this case identifies the National Physical Laboratory, UK
National Physical Laboratory, UK
The National Physical Laboratory is the national measurement standards laboratory for the United Kingdom, based at Bushy Park in Teddington, London, England. It is the largest applied physics organisation in the UK.-Description:...

. 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
International Bureau of Weights and Measures
The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...

 (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
Canonical is an adjective derived from canon. Canon comes from the greek word κανών kanon, "rule" or "measuring stick" , and is used in various meanings....

 TAI. This time scale is expressed in the form of tables of differences UTC-UTC(k) (equivalent 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. Doing so does not create another version of TAI; it is instead considered to be creating a better realisation of Terrestrial Time
Terrestrial Time
Terrestrial Time is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of the Earth....



Atomic timekeeping services started experimentally in 1955, using the first caesium atomic clock at the National Physical Laboratory, UK (NPL). Early atomic time scales consisted of quartz clocks with frequencies calibrated by a single atomic clock; the atomic clocks were not operated continuously. The "Greenwich Atomic" (GA) scale began in 1955 at the Royal Greenwich Observatory. The United States Naval Observatory
United States Naval Observatory
The United States Naval Observatory is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation, and Timing for the U.S. Navy and the U.S. Department of Defense...

 began the A.1 scale 13 September 1956, using an Atomichron commercial atomic clock, followed by the NBS-A scale at the National Bureau of Standards, Boulder
Boulder, Colorado
Boulder is the county seat and most populous city of Boulder County and the 11th most populous city in the U.S. state of Colorado. Boulder is located at the base of the foothills of the Rocky Mountains at an elevation of...

, Colorado. The International Time Bureau
International Time Bureau
The Bureau International de l'Heure or the International Time Bureau, seated at the Paris Observatory, was the international bureau responsible for combining different measurements of Universal Time. The bureau also played an important role in the research of time keeping...

 (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. Both the BIH scale and A.1 was defined by an epoch
Epoch (reference date)
In the fields of chronology and periodization, an epoch is an instance in time chosen as the origin of a particular era. The "epoch" then serves as a reference point from which time is measured...

 at the beginning of 1958: it was set to read Julian Date 2436204.5 (1 January 1958 00:00:00) at the corresponding UT2 instant. The procedures used by the BIH evolved, and the name for the time scale changed: "A3" in 1963 and "TA(BIH)" in 1969. This synchronisation was inevitably imperfect, depending as it did on the astronomical
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 realisation of UT2. At the time, UT2 as published by various observatories differed by several centiseconds.

The SI second was defined in terms of the caesium atom in 1967, and in 1971 it was renamed International Atomic Time (TAI).

Also in 1961, UTC began. UTC is a discontinuous time scale composed from segments that are linear transformations of atomic time, the discontinuities being arranged so that UTC approximated UT2 until the end of 1971, and UT1 thereafter. This was a compromise arrangement for a broadcast time scale: a linear transformation of the BIH's atomic time meant that the time scale was stable and internationally synchronised, while approximating UT1 means that tasks such as navigation
Navigation is the process of monitoring and controlling the movement of a craft or vehicle from one place to another. It is also the term of art used for the specialized knowledge used by navigators to perform navigation tasks...

 which require a source of Universal Time continue to be well served by public time broadcasts.

In the 1970s, it became clear that the clocks participating in TAI were ticking at different rates due to gravitational time dilation
Gravitational time dilation
Gravitational time dilation is the effect of time passing at different rates in regions of different gravitational potential; the lower the gravitational potential, the more slowly time passes...

, and the combined TAI scale therefore corresponded to an average of the altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...

s of the various clocks. Starting from Julian Date 2443144.5 (1 January 1977 00:00:00), corrections were applied to the output of all participating clocks, so that TAI would correspond to proper time at mean sea level (the geoid). Because the clocks had been on average well above sea level, this meant that TAI slowed down, by about 10−12. The former uncorrected time scale continues to be published, under the name EAL (Echelle Atomique Libre, meaning Free Atomic Scale).

The instant that the gravitational correction started to be applied serves as the epoch for Barycentric Coordinate Time
Barycentric Coordinate Time
Barycentric Coordinate Time is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to orbits of planets, asteroids, comets, and interplanetary spacecraft in the Solar system...

 (TCB), Geocentric Coordinate Time
Geocentric Coordinate Time
Geocentric Coordinate Time is a coordinate time standard intended to be used as the independent variable of time for all calculations pertaining to precession, nutation, the Moon, and artificial satellites of the Earth...

 (TCG), and Terrestrial Time
Terrestrial Time
Terrestrial Time is a modern astronomical time standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of the Earth....

 (TT). All three of these time scales were defined to read JD 2443144.5003725 (1 January 1977 00:00:32.184) exactly at that instant. (The offset is to provide continuity with the older Ephemeris Time
Ephemeris time
The term ephemeris time can in principle refer to time in connection with any astronomical ephemeris. In practice it has been used more specifically to refer to:...

.) TAI was henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184 s.

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."

See also

  • Time and frequency transfer
    Time and frequency transfer
    Time and frequency transfer describes mechanisms for comparing measurements of time and frequency from one location to another. The technique is commonly used for creating and distributing standard time scales such as International Atomic Time ....

  • Clock synchronization
    Clock synchronization
    Clock synchronization is a problem from computer science and engineering which deals with the idea that internal clocks of several computers may differ. Even when initially set accurately, real clocks will differ after some amount of time due to clock drift, caused by clocks counting time at...

  • Network Time Protocol
    Network Time Protocol
    The Network Time Protocol is a protocol and software implementation for synchronizing the clocks of computer systems over packet-switched, variable-latency data networks. Originally designed by David L...

  • Precision Time Protocol
    Precision Time Protocol
    The Precision Time Protocol is a protocol used to synchronize clocks throughout a computer network. On a local area network it achieves clock accuracy in the sub-microsecond range, making it suitable for measurement and control systems....

    , a related though separate technology
  • Magneto-optical trap
    Magneto-optical trap
    A magneto-optical trap is a device that uses both laser cooling with magneto-optical trapping in order to produce samples of cold, trapped, neutral atoms at temperatures as low as several microkelvins, two or three times the recoil limit.By combining the small momentum of a single photon with a...

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