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The BIPM announces technical updates to Circular T

BIPM Circular T is the monthly publication that provides traceability to the SI second via UTC to its local realizations in national laboratories, known as UTC(k). Together they contribute data from about 85 laboratories maintaining approximately 450 atomic clocks world-wide. UTC(k) is disseminated in real time to the users through different means such as radio transmission or internet protocols.

To reflect recent technological advancements, the BIPM intends to update the content and format of Circular T. The modifications shall be implemented starting from issue 437 in June 2024. They concern the uncertainty information reported in sections 1 and 5 and relations of UTC with predictions of UTC broadcast by GNSS, reported in section 4, where information on Galileo and BeiDou systems will be added, henceforth covering all global navigation satellite systems.

All the planned updates have been discussed and approved by relevant Consultative Committee for Time and Frequency (CCTF) working groups.

BIPM Time Department Database

UTC - Coordinated Universal Time - is a time scale produced by the BIPM with the same rate as International Atomic Time (TAI). It differs from TAI only by an integral number of seconds.
Galileo - European Global Navigation Satellite System
BeiDou - Chinese Global Navigation Satellite System


Geographical distribution of laboratories that contribute to TAI and time transfer equipment (2024).

The updates imply structural changes in the Circular T, both ASCII and HTML, as well as in the data files available on the FTP server, through an Application Programming Interface (API) and in interactive plots. The uncertainties related to the calibration status of the GNSS time transfer equipment will be updated through application of a revised algorithm, to consider factors such as uncalibrated equipment, calibration ageing, uncontrolled events and provisional alignment.

The updates are expected to mainly affect laboratories without recognized calibration and/or very old calibrations. The impact on the comparison CCTF-K001.UTC has also been considered such that the uncalibrated laboratories will not lose the traceability in frequency.

Section 4 of Circular T will begin to include data for Galileo and BeiDou systems. This implementation has been made possible through a complete overhaul of the calculation process, following the method described by Defraigne et al.