Recommendation 6 of the 20th CCTF (2015)

Development of national and international time and frequency links to improve methods for intercontinental clock comparisons and for dissemination to stakeholder

The Consultative Committee for Time and Frequency (CCTF),


  • that some optical frequency standards have already demonstrated fractional uncertainties in the low 10-18 and that the reduction in the uncertainty and instability of optical frequency standards developed in institutes around the world will continue,
  • that long-distance comparison with optical fibre links has been demonstrated with a stability and uncertainty that is compatible with the best present and future optical frequency standards,
  • the need for regular comparisons between these standards as an essential part of the preparation for a redefinition of the second and for other applications such as contributions to time scales, that the stabilities of time and frequency transfer techniques currently and routinely used for comparisons around the world, i.e. for the production of International Atomic Time (TAI), are insufficient for comparisons between the best optical frequency standards,
  • the growing interest of the Earth science and geodesy scientific communities for chronometric geodesy, i.e. new applications of optical frequency standards for determining gravitational potential differences and improving Earth gravity models and reference systems with these measurements,
  • the growing needs of industry for improving time and frequency capabilities using better transfer methods, in particular in the telecommunication and aerospace sectors,

recommends that

  • National metrology institutes (NMIs), optical fibre network providers, space agencies, national governments, regional metrology organizations (RMOs), International Telecommunication Union (ITU) and other relevant bodies:
    • vigorously support research and development of time and frequency transfer techniques matching the stability and uncertainty of the most advanced frequency standards. These techniques may include optical fibre links, advanced satellite microwave links, optical ground-to-space and space-to-space links and transportable frequency standards, and advanced space clocks,
    • help secure sustainable infrastructure of selected continental and intercontinental links forming a global time and frequency metrology backbone for these novel technologies,
    • make provisions for these novel technologies to be transferred with the relevant accuracy to other fields of science, industry and society,
  • the BIPM participates actively in these developments, notably by making preparations for exploiting, in time scale realization, clock comparison data issued from new time and frequency transfer methods,
  • those laboratories contributing to UTC and performing continuous time comparisons via fibre links regularly submit their results to the BIPM Time Department.

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

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