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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 Date s 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. As of 2006 TAI is about 33 S ahead of Universal Time. OPERATION TAI as a Frequency standard is a weighted average of the time kept by about 300 Atomic Clock s in over 50 national laboratories worldwide. Many of these are Caesium atomic clocks, which are the standard by which the SI Second is defined. Due to the averaging it is far more stable than any clock would be alone. The participating institutions each broadcast in Real Time a frequency signal with Time Code s, which is their estimate of TAI. (Actually the time codes are usually published in the form of UTC .) The better laboratories' signals are mutually synchronised to within less than 10-7 s, but there are outliers up to 10-5 s out. These time scales are denoted in the form "TAI(NPL)" ("UTC(NPL)" for the UTC form), where "NPL" in this case identifies the National Physical Laboratory, UK . Some laboratories also publish their own atomic time scale, denoted in the form "TA(USNO)" ("USNO" identifies the United States Naval Observatory ). The clocks at different institutions are regularly compared against each other. The TAI. This time scale is expressed in the form of tables of differences UTC-UTC(x) and TAI-TA(x), for each participating institution x. 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 (TT). See the article on TT for more information. HISTORY Atomic timekeeping services started experimentally in realisation of UT2. At the time, UT2 as published by various observatories differed by several Centisecond s. A.1 was extrapolated backwards to 1956 . In 1961 the Bureau International De L'Heure (BIH) (later superseded by the BIPM ) constructed an atomic time scale named AM based on three atomic clocks. The clocks were compared by listening to radio Time Signal s based on them. The BIH's time scale was synchronised with A.1's epoch, and extrapolated back to 1955 using time signals from the first caesium clock at NPL. This time scale was soon renamed from AM to '''A3'''. Also in which require a source of Universal Time continue to be well served by public time broadcasts. In 1967 the SI Second was redefined in terms of the frequency supplied by a Caesium atomic clock. More clocks were added to the A3 time scale from 1967 , and it was renamed to TA. Finally in 1971 it was renamed '''TAI'''. In the 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 .) TAI is henceforth a realisation of TT, with the equation TT(TAI) = TAI + 32.184 s. In the 1990s annual periodic variations in the rate of some clocks were traced to Blackbody Radiation that varies with the ambient Temperature . It became clear that a correction for this was required. Accordingly, in 1997 the BIPM declared that the definition of the SI second referred to a caesium atom at rest and at Absolute Zero temperature. Temperature corrections were implemented in TAI from 1995 to 1998 , speeding TAI up by about 10-14.3. SEE ALSO
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