Preliminary accuracy evaluation of a cesium fountain primary frequency standard at NIST

fountain primary frequency standard has been constructed at the National Institute of Stan-dards and Technology (NIST) in Boulder, CO. The new standard, NIST-F1, is described here, including details of systematic fre-quency biases for which frequency corrections are made. We have submitted two evaluations to the Bureau International des Poids et Mesures (BIPM), which have Type A (statistical) uncertainties less than 1 5 10 15 and Type B (systematic) uncertainties less than 0 9 10 15. I