Abstract
Recent experimental data on the frequency stability of the National Bureau of Standards small passive hydrogen masers indicates that they are superior to any commercially available cesium standard for frequency comparisons or timekeeping out to periods of at least a month. Frequency drift between the small passive hydrogen maser and an ensemble of nine commercial cesium standards has been measured and is of order 1 ± 5 x 10-16/day averaged over 72 days. This is substantially less than the drift in active hydrogen masers. Timekeeping to a few nanoseconds per week has been demonstrated using the small passive hydrogen maser. These small masers are expected to be available in a 30 cm high rack mount in the near future. Using full-sized passive hydrogen masers, it appears possible to achieve frequency stabilities of order 10-15 over days and timekeeping to about one nanosecond per week. In order to take full advantage of the improved capabilities of the passive hydrogen masers, it will be necessary to use improved time comparison techniques.
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