Atomic frequency standards have been checked by laboratory investigations and by the intercomparison of different standards.

Existing ammonia Masers enable a frequency to be defined in terms of a spectral line with an accuracy of ±1 part in 108 and give a reproducibility under stated conditions of ±2 parts in 1010. The cesium atomic beam standard enables a frequency to be defined in terms of a spectral line with a precision of ±1 part in 1010. Comparison between the U.K. and U.S. by the radio transmissions of standard frequencies and time signals gave an average agreement of a few parts in 1010 but revealed some systematic discrepancies. Direct comparisons between three U.S. standards and the U.K. standard gave agreement to 2.2, 3.2, and 1.5 parts in 1010.