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