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Panoramic view of the clock hall at PTB with the four caesium clocks CS1, CS2, CSF1 and CSF2.

Unit of Time

Working Group 4.41

Comparisons of fountain clocks and TAI

The free atomic time scale EAL is formed by averaging the data of approximately 400 atomic clocks distributed worldwide. Comparisons of the scale unit of EAL and different types of atomic clocks (among them currently 12 fountain clocks worldwide, last updated summer 2022) are used to determine the difference d between the EAL scale unit and the SI second. The international atomic time scale TAI is obtained by correcting EAL accordingly so that d is minimized in the long term.  A sufficient amount of fountain clock data ensures the precision and long-term stability of TAI.

The figure shows the deviation d between the SI second realized by fountain clocks and other atomic clocks and the TAI scale unit as noted in the "Circular T" issues of the Time Section of BIPM. Depicted is d against the Modified Julian Date (MJD), with MJD 58849 corresponding to 2020-01-01. The data cover the period between January 2020 and summer 2022.

Diagram showing the deviation of the SI-second realized with fountain clocks and other atomic clocks and the scale unit of international atomic time TAI