Measurements with Multiple Operational Fountain Clocks

The U.S. Naval Observatory USNO maintains an ensemble of atomic clocks for the generation of a Master Clock, the physical realization of UTCUSNO. As many as 20 hydrogen masers and 70 commercial cesium clocks can contribute to the ensemble. The timescale used to generate UTCUSNO relies on the short-term frequency stability of the masers and the long-term stability of the cesium clocks. To advance timekeeping activities, 7 rubidium fountain clocks are being added to the clock ensemble. The fountains will serve as a long-term frequency reference, similar to but better than the lower-stability cesium clocks, but will not provide any tie to the SI second. For a fountain to be integrated into the USNO clock ensemble like any other clock, it generates a continuous 5 MHz output that can be measured against the Master Clock. The 5 MHz is generated from a precision synthesizer referenced to a maser that also serves as the LO reference for a fountain the fountain adjusts the synthesizers output based on its measurement of the maser frequency. The maser can act as a flywheel to minimize the effect of interruptions to fountain operation, but the fountains have been designed to operate continuously for long periods of time. Performance of the first two operational rubidium fountains at the USNO is presented using relative measurements and comparisons against other timescales. Recent results with four fountains indicate frequency agreement at the 10 exp -15 level and good agreement with the primary frequency standards contributing to International Atomic Time TAI.