Comparing high accuracy frequency standards via TAI

Over the last few years the number of high accuracy (uncertainties in the low 10
^-15
or less) primary frequency standards contributing to TAI has increased significantly from only two over the period 2000 - 2002 to seven over the period 2002 - 2005. As a consequence the number of individual measurements of TAI frequency by such standards over the period 1999 to 2005 is >; 60 and increasing rapidly. Additionally, several other atomic transition frequencies (Rb, Yb
⁺
, Hg
⁺
, Sr
⁺
,...) have been repeatedly measured over the same period with comparable accuracies. The resulting overall set of data is now sufficiently large to be able to obtain meaningful statistics for the complete data set and, in some cases, for the contributions from individual standards. We show the ensemble of the data available to the BIPM at the time of the conference together with the results of some basic statistical analysis. In particular, we investigate the impact of these standards on the overall uncertainty of the realisation of the SI second via TAI, and the performance of some individual standards. Our results indicate that while there is good self coherence of most studied standards (especially for recent data) there seems to be significant disagreement between individual standards. We briefly discuss some potential sources of that disagreement (time transfer noise, uncertainties of the standards, instability of the reference timescale, etc...). We hope that, in the long term, this ongoing study will allow a better characterisation of the ensemble of standards and their impact on TAI.