Recommendation 1 of the 15th CCTF (2001)

Secondary representations of the second

The Consultative Committee for Time and Frequency,

considering that

  • the present definition of the second, based on the caesium-133 atom, remains unchanged,

  • there are a number of new atoms and ions being studied as potential bases for atomic frequency standards,

  • new optical-frequency measurement concepts may allow the use of optical transitions as practical frequency standards, since they provide for a direct microwave output from such standards,

  • new frequency standards based on other microwave transitions are being studied,

  • one of these new standards could eventually be considered as the basis for a new definition of the second;

intends to examine and approve accurate frequency measurements of atom and ion transition frequencies made relative to the caesium frequency standard as secondary representations of the second,

recommends that

  • a list of such secondary representations of the second be established,
  • the requirements for documentation of uncertainty that apply to secondary representations of the second be the same as those for primary caesium standards for use in International Atomic Time.

DOI : 10.59161/CCTF2001REC1E

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

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