The international comparison of atomic standards of time and frequency

Atomic frequency standards are now in use in a number of countries. The measurement, in terms of these standards, of the frequencies of a number of radio transmissions which are closely controlled, and the interchange of the results obtained, enables the standards themselves to be compared. Some details are given of the various atomic standards and of the methods of measurement employed at the National Physical Laboratory. The results obtained between January, 1958, and December, 1960, for seven standards are tabulated. It is shown that the use of the v.l.f. transmissions GBR, Rugby (16kc/s), and NBA, Summit (18kc/s), enables day-to-day comparisons to be made with a precision of ±2 parts in 1010. There is some evidence that there are at present differences between the different standards of ±2 parts in 1010.