The establishment of the TAI is done in two steps: the construction of the EAT from clocks in laboratories, then its steering by primary frequency standards. Great progress in the precision and stability of EAL was made by the generalized use of GPS time transfer and by the introduction of hydrogen masers and new very performant clocks. Only two primary frequency standards are sufficiently accurate to steer the frequency of TAI, but new standards are now being assessed. The present stability of TAI is 5.10−15 and the accuracy 2.10−14. Improvements should occur in the years to come.

The establishment of the International Atomic Time (TAI) and the resulting Coordinated Universal Time (UTC) by the time section of BIPM is done in two steps (Guinot and Thomas, 1988). At first, a free atomic scale, EAL (Echelle atomique libre) is built in two month blocks combining data from about 200 atomic clocks kept by 60 laboratories and regularly reported to the BIPM by 45 centres which maintain a local coordinated universal time, UTC(k). Then, the duration of the scale interval of EAL is compared with data from primary caesium standards producing the SI second which, in turn, is converted on the rotating geoid as the unit scale of TAL A linear function of time with the necessary slope is added to EAL to ensure the accuracy of the TAI scale interval.