Recommendation 5 of the 21st CCTF (2017)

On improving the uncertainty of Two-Way Satellite Time and Frequency Transfer (TWSTFT) for UTC Generation

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


  • that atomic frequency standards have achieved unprecedented accuracy, and that further rapid advances in this field are under way,
  • that time and frequency transfer data from the use of TWSTFT systems plays an important role in realization of UTC,
  • that the uncertainty of current TWSTFT systems for the realization of UTC is limited by technical constraints that can result in a daily variation (diurnal) in time transfer results;


  • that the experimental use of Software Defined Radio (SDR) receivers in the Asia-Pacific region (SDR receivers measure the timing signals generated by the currently used TWSTFT equipment) demonstrated that SDR TWSTFT significantly reduced the diurnal and measurement noise,
  • that a pilot study on using SDR TWSTFT for UTC was organized by the BIPM and the CCTF Working Group on Two-Way Satellite Time and Frequency Transfer (WGTWSTFT) in February 2016 to investigate the impact of SDR TWSTFT for the Asia-Asia, Asia-Europe, Europe-Europe, Europe-US and US-US links using different geostationary satellites,
  • that all UTC laboratories engaged in TWSTFT expressed their interest in participating in the pilot study and that twelve stations in Asia, Europe and the USA have installed SDR systems and carried out measurements contributing to the pilot study,
  • that the BIPM global validation of the SDR TWSTFT results has revealed a significant reduction of the diurnal and measurement noise in the inner-continental TWSTFT links with a gain factor of two to three in the time link stability as compared to the currently used TWSTFT results,
  • that a few digital TWSTFT modems have been under development in recent years, some of which are capable of supporting both code-phase and carrier-phase measurements, which could further improve the uncertainty of TWSTFT;

recommends that laboratories operating TWSTFT stations

  • continue to study the SDR TWSTFT technique by maintaining SDR operation or installing SDR systems in stations, and providing SDR TWSTFT data to the BIPM in parallel to the data from the currently used TWSTFT equipment, with the aim of improving the uncertainty of time links with the current TWSTFT by a reduction of the diurnal noise,
  • support and participate in the test of new digital TWSTFT modems as they become available with the perspective of further improving the uncertainty of TWSTFT, potentially making it practical for the comparison of optical clocks at the level of their accuracies;

recommends that the BIPM

  • work towards implementing the use of SDR TWSTFT data in UTC generation,
  • support studies to improve TWSTFT with redundant measurements in the TWSTFT network and with digital modems when data becomes available.

DOI : 10.59161/CCTF2017REC5E

The reader should note that the official version of this Resolution is the French text

CCTF logo

Menu Display