31at Annual Precise Time .and Time Interval (PTTI) Meeting USEOFPRIMARYFREQUENCYSTANDARDS FORESTIMATINGTHEDURATIONOFTHE SCALEUNITOFTAI

Mesures The accuracy of International Atomic Time TAI is based on a small number of primary frequency standards (PFS) that aim at realizing the SI second, the unit of proper time. Following the ldh meeting of the Consultative Committee for Time and Frequency (April 1999), the BIPM time section is reconsidering how the PFS comparisons are used for evaluating the duration of the scale unit of TAI, uTAI, and how they are reported in Circular T and other BIPM publications. In the new procedure, it is proposed to break down the estimation of the uncertainty of the comparison in several elementary components and to report all of them in the BIPM publications in order to make the uncertainty evaluation more transparent and traceable. The BIPM also regularly computes an estimation of the duration of the scale unit of TAI using an accuracy algorithm combining the data of all PFS. The parameters used in this algorithm are re-evaluated and a new estimation of uTAI has been cam’ed out over the recent years. The standard uncertainty on this value is now estimated to be 3~10~‘~.