The NBS-A Time Scale-Its Generation and Dissemination

In conjunction with the United States Frequency Standard (USFS) located at the National Bureau of Standards, Boulder Laboratories, two time scales have been established. The NBS-A time scale is referenced to the USFS in accordance with the definition of the atomic second as adopted by the 12th General Conference of Weights and Measures in October, 1964. The epoch of this scale was set to be nearly coincident with the epoch of UT-2 at 00: 00 hour January 1, 1958. The NBS-UA time scale is also referenced to the USFS but the frequency offsets and shifts in epoch as announced by the Bureau International de l'Heure (BIH) in Paris are incorporated in this time scale. Thus, NBS-UA is an atomically interpolated approximation to UT-2. The accuracies of time interval measurements referenced to these time scales are believed to have essentially the accuracy of the USFS [1]-the accuracy of the USFS is about one part in 1011. The NBS-UA time scale is presently being disseminated by the NBS radio stations WWVL, WWVH, and WWV. The phases of WWVL transmissions have been maintained coherent with the NBS-UA time scale with a precision of about ±3 μs and the epoch of the WWV transmissions with a precision of about ±10 As since January, 1964. Since January 1, 1965, the WWVB phase has been maintained coherent with NBS-A.