The National Bureau of Standards Atomic Time Scales: Generation, Dissemination, Stability, and Accuracy

The independent atomic time scale at the National Bureau of Standards AT(NBS), is based upon an ensemble of continuously operating cesium clocks calibrated occasionally by an NBS primary frequency standard. The data of frequency calibrations and interclock comparisons are statistically processed to provide nearly optimum time stability and frequency accuracy. The long-term random fluctuation of AT(NBS) due to nondeterministic perturbations is estimated to be a few parts in 1014, and the present accuracy is inferred to be 1 part in 1012. A small coordinate rate is added to the rate of AT(NBS) to generate UTC(NBS): this small addition is for the purpose of maintaining synchronization within a few microseconds of other international timing centers. UTC(NBS) is readily operationally available over a large part of the world via WWV, WWVH, WWVB, and telephone; also via some passive time transfer systems, eg., Loran-C and the TV line-10 system; and also experimentally via satellite and WWVL. The precision and accuracy of these dissemination systems will be discussed.