NIST-F1: recent improvements and accuracy evaluations∗

In the last several years we have made many improvements to NIST-F1 (a laser-cooled Cs fountain primary frequency standard at the National Institute of Standards and Technology (NIST) in Boulder, Colorado) resulting in a reduction by over a factor of 2 in the uncertainty of the realization of the SI second. The two most recent accuracy evaluations of NIST-F1 had combined standard fractional uncertainties of 0.61 × 10−15 (June 2004) and 0.53 × 10−15 (January 2005), which were submitted to the Bureau International des Poids et Mesures with total fractional uncertainties (including time-transfer contributions) of, respectively, 0.88 × 10−15 and 0.97 × 10−15. Here we discuss the improvements and evaluation methods and present an updated uncertainty budget.