Abstract :
The degree of precision attained by modern frequency standards has rendered Greenwich Mean Time unsuitable as a basis for the assessment of performance. In order to provide a more uniform time system for the estimation of absolute frequency, corrections for quasi-periodic errors must be applied. It is shown that the annual fluctuation observed from 1934 to 1949 is no longer applicable; the value determined for 1950¿51 shows a diminution of 40% in amplitude. This change caused difficulties in the Greenwich Time Service during the past two years, and gave rise to frequency errors amounting to 1¿ parts in 108. The existence of changes of this nature in the annual fluctuation is a hitherto unsuspected difficulty in the establishment of a uniform time system for the standardization of frequencies to an adequate accuracy. Attention is drawn to the difficulties inherent in the isolation and measurement of annual fluctuation, and a method has been developed for evaluating it over single twelve-month periods. This permits quarterly revision of the corrections adopted in current work. It is now possible to present a more reliable estimate of the stability attained by frequency standards of the latest type, and a numerical criterion of performance is proposed.
