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BIPM Time Department provides additional information on the use of PSFS in UTC

Starting from March 2020, the Time Department is providing additional graphical information concerning the use of Primary and Secondary Frequency Standards (PSFS) in UTC. A graphical representation of all evaluations of PSFS reported since Circular T 190 (October 2003) is available at:


The plot will report the measurements carried out using PSFS and which are used for the steering of TAI. This information, updated every month, will help to evaluate the impact of PSFS in UTC, in view of a possible redefinition of the second.


According to the roadmap towards a redefinition of the SI second (CCTF 2016); a regular contribution to TAI by the new frequency standards is considered one of the criteria for a new definition of the second.