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Recommendation S 2 (CCDS, 1970), in CIPM, 1970
Definition of TAI

International Atomic Time (TAI) is the time reference coordinate established by the Bureau International de l'Heure on the basis of the readings of atomic clocks operating in various establishments in accordance with the definition of the second, the unit of time of the International System of Units.




In 1980 the definition of TAI was completed as follows (declaration of the CCDS, BIPM Com. Cons. Déf. Seconde, 1980, 9, S15 and Metrologia, 1981, 17, 70):

TAI is a coordinate time scale defined in a geocentric reference frame with the SI second as realized on the rotating geoid as the scale unit.




Reference:
PV, 38, 110 and Metrologia, 1971, 7, 43