Recommendation 4 of the 21st CCTF (2017)

On the utilization and monitoring of redundant time transfer equipment in the timing laboratories contributing to UTC

The Consultative Committee for Time and Frequency (CCTF), at its 21st session in 2017,

realizing that

• atomic frequency standards have achieved unprecedented accuracy, and that further rapid advances in this field are under way,
• the ability to compare these standards for the realization of UTC is limited by the accuracy and stability of time transfer equipment;

considering that

• time and frequency transfer data from the use of Global Navigation Satellite Systems (GNSS) signals and from Two-Way Satellite Time and Frequency Transfer (TWSTFT) systems play an important role in the realization of UTC,
• regular TWSTFT time link calibrations and GNSS equipment calibrations have been carried out through cooperation of the BIPM, the Regional Metrology Organizations (RMOs) and the UTC-contributing laboratories, resulting in calibration uncertainties at the level of order of 1 ns to 1.5 ns,
• hardware delay variations limit the validity of calibration results, and random variations in measurements also limit the ability to combine data from redundant and complementary techniques;

noting that

• Recommendation CCTF 4 (2012) recommended laboratories that contribute to UTC to upgrade their GNSS equipment towards multi-frequency multi-constellation receiving systems providing code- and carrier-phase measurements and to supply data from at least three receivers,
• the calibration of GNSS receivers of the Group One (G1) laboratories designated by the RMOs has special importance as they influence the calibrations or links of other multiple laboratories in a significant manner,
• variations of the common clock differences (CCD) between GNSS systems and variations of the double clock differences (DCD) between Global Positioning System (GPS) and TWSTFT links, which exceed the combined calibration uncertainty of the most recent calibrations, have been observed,
• several ongoing studies consider ways to mitigate the effects of time transfer systems’ calibration variations;

recommends that laboratories participating in UTC generation

• maintain at least two (three in the case of G1 laboratories) independent GNSS systems, some of which being state-of-the-art if resources are available,
• provide all data from redundant GNSS systems to the BIPM,
• monitor their CCDs and the internal characteristics of their laboratory reference signals, such as the shape of their pulse-per-second signal as seen at their time transfer systems, on an annual or other appropriate basis,
• document all setup and configuration changes relevant to time transfer and report this data to the BIPM,
• record and maintain both external and relevant internal temperature and humidity information and report this data to the BIPM,
• evaluate the correlation between the time transfer solutions and the local temperature and humidity measurements,
• calibrate at least one operational link every two years;

recommends that the BIPM and the CCTF WGs on TWSTFT and on GNSS Time Transfer

• coordinate TWSTFT and GNSS calibrations so that they can be compared and that the seasonal impacts can be studied and their effects minimized,
• study the impact of introducing calibration results on the long-term CCD and DCD series,
• define the format for the submissions of temperature and humidity information;

recommends that the BIPM

• continue to publish its computed time transfer differences (links and link comparisons) on its web pages in computer files with self-explanatory names,
• continue to publish the information about system configurations and calibration results on its web pages, along with the associated reports,
• continue to study the use of redundant time transfer systems in UTC generation,
• add the station temperature and humidity information to the Time Department database and make these data available to the WGs,
• add the station setup and configuration changes relevant to time transfer to the Time Department database.

DOI : 10.59161/CCTF2017REC4E