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Four rubidium fountains have been in continuous operation at USNO for more than 3 years. They contribute to the local timescale and are reported to the BIPM as continuously running clocks — as opposed to secondary standards — the first and only cold-atom clocks so reported. Measurements among individual fountains demonstrate fractional-frequency stability that routinely reaches the low 10∼16s. Intervals...
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 10−15 level and good agreement with the primary frequency standards contributing to TAI.
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