NIST-F1: recent improvements and accuracy evaluations

In the last several years we have made many improvements to NIST-Fl (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-FI had combined standard fractional uncertainties of 0.61 x 10(-15) (June 2004) and 0.53 x 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 X 10(-15) and 0.97 x 10(-15). Here we discuss the improvements and evaluation methods and present an updated uncertainty budget.