Recommended values of standard frequencies
The CIPM has charged the CCL-CCTF Frequency Standards Working Group (CCL-CCTF-WGFS) with maintaining this list of recommended values of standard frequencies for applications including the practical realization of the definition of the metre and secondary representations of the definition of the second.
All the radiations in the list (apart from the 87Rb 6.8 GHz transition) may be used for the practical realization of the definition of the metre.
The radiations marked [SRS] may be used as secondary representations of the second.