Impact of new clock technologies on the stability and accuracy of the International Atomic Time TAI

The BIPM Time Section is in charge of the generation of the reference time scales TAI (International Atomic Time) and UTC (Coordinated Universal Time). Both time scales are obtained in deferred-time by combining the data from a number of atomic clocks spread world-wide. Since the end of 1992, the quality of the timing data received at the BIPM has evolved rapidly thanks to the extensive replacement of older designs of commercial caesium clocks by the new HP 5071A units and to the use of active auto-tuned hydrogen masers. Consequently, the stability and the predictability of the reference time scales has improved significantly: these are characterized by an Allan standard deviation σ_y(τ) of 2,6×10^-15 for averaging times τ=40 d. The accuracy of TAI is estimated by the departure between the duration of the TAI scale interval and the SI second as produced on the rotating geoid by primary frequency standards. It is now possible to estimate TAI accuracy through the combination of results obtained from six different primary standards: LPTF-FO1, PTB CS1, PTB CS2, PTB CS3, NIST-7 and SU MCsR 102, all corrected for the black-body radiation shift. This led to a mean departure of the TAI scale interval of +2,0×10^-14 s over 1995, known with a relative uncertainty of 0,5×10^-14 (1 σ)