NIST-F1, a caesium fountain atomic clock used since 1999, has a fractional inaccuracy of less than. The planned performance of NIST-F2 is δf / f <. At this planned performance level the NIST-F2 clock will not lose a second in at least 300 million years.
Evaluated accuracy
The evaluated accuracy reports of various primary frequency and time standards are by the International Bureau of Weights and Measures. The first in-house accuracy evaluation of NIST-F2 reported a uB of. In March 2014 and March 2015 the NIST-F2 caesium fountain clock reported a uB of in the BIPM reports of evaluation of primary frequency standards. However, the uncertainty evaluation of NIST-F2's frequency shift due to distributed cavity phase, which currently limits many other accurate atomic fountain clocks, had a shortcoming that makes it difficult to assign an accuracy until this error is re-evaluated. In addition, the largest systematic error of NIST-F2 is a microwave amplitude dependent frequency shift, which includes frequency shifts due to the microwave lensing of atomic wave-packets by microwave dipole forces on the atoms and microwave leakage. An independent statistical analysis of the NIST measurements showed that this uncertainty was significantly too small and should be increased from to at least. As a result, the quoted uncertainty of NIST-F2 would increase to approximately, if the re-evaluation of the uncertainty due to distributed cavity phase shows that it is small. The March, 2015 report to the BIPM did not address the published criticism and there have been neither subsequent reports to the BIPM nor has an updated accuracy evaluation been published. The last submission of NIST-F1 to BIPM TAI was February 2016. At the request of the Italian standards organization, NIST fabricated many duplicate components for a second version of NIST-F2, known as IT-CsF2 to be operated by the :it:Istituto nazionale di ricerca metrologica|Istituto Nazionale di Ricerca Metrologica, NIST's counterpart in Turin, Italy. As of February 2016 the IT-CsF2 caesium fountain clock started reporting a uB of in the BIPM reports of evaluation of primary frequency standards.
