Long-term comparison of caesium fountain primary frequency standards

Abstract

There are currently nine caesium fountain primary frequency standards regularly reporting calibrations of International Atomic Time to the Bureau International des Poids et Measures (BIPM). An investigation has been carried out using data from the BIPM publication Circular T to evaluate the frequency differences among these standards and to determine whether these offsets are consistent with the stated uncertainties. The fractional frequency uncertainties of some Cs fountains are now in the range of 4 × 10-16 to 5 × 10-16. The results of this investigation show that the standards agree well with each other. An overall estimate of the caesium frequency is made using the weighted mean of all the fountains. © 2010 BIPM and IOP Publishing Ltd.