The role of the hydrogen maser for frequency transfer from cesium fountains

Primary cesium frequency standards realized as cesium fountains offer the prospect of frequency accuracy in the 10-15 range, or better. Active hydrogen masers are good candidates for the source of a local time scale to disseminate frequency at this level of accuracy after calibration by a cesium fountain that operates intermittently. We develop a procedure for evaluating the frequency uncertainty associated with the resulting calibrated time scale. We apply the procedure to the expected performance of masers and cesium fountains, and find that current hydrogen masers can be used to deliver frequency uncertainties of less than 10-15, and might be used to approach 10-16. The procedure is appropriate to use, for example, when reporting uncertainties for cesium fountain average frequencies delivered to assist the International Bureau of Weights and Measures (BIPM) in steering International Atomic Time (TAI).