Primary frequency standards developed and operated by NIST are used to provide accuracy (rate) input to the BIPM and to provide the best possible realization of the SI second. NIST-F1 and NIST-F2, cold-atom cesium fountain frequency standards, have served as the U.S. primary standards of time and frequency since 1999. The uncertainty of NIST-F2 is currently about 1 part in 10^{16}.

The AT1 scale is run in real-time by use of data from an ensemble of cesium standards and hydrogen masers. It is a free-running scale whose frequency is maintained as nearly constant as possible by choosing the optimum weight for each clock that contributes to the computation.

UTC is generated at the BIPM by use of a post-processed time-scale algorithm and is not available in real-time. The parameters that we use to generate UTC(NIST) in real-time are therefore based on an extrapolation of UTC from the most recent available data.

UTC(NIST) is generated as an offset from our real-time scale AT1. Time steps are never used. Instead, the frequency is steered so that the time output remains close to UTC. This is accomplished by using data published by the BIPM in its Circular T and by weekly estimates of UTC, which are published by the BIPM as rapid UTC or UTCr. Changes in the frequency may be made as often as once per week and are limited to ±2.3 x 10^{-14}. The frequency of UTC(NIST) is kept as stable as possible at other times.

The table lists the parameters that are used to define UTC(NIST) with respect to our real-time scale AT1. To find the value of UTC(NIST) - AT1 at any time T (expressed as a Modified Julian Day, including a fraction if needed), the appropriate equation to use is the one for which the desired T is greater than or equal to the entry in the T_{0} column and less than the entry in the last column. The values of x_{ls}, x, and y for that month are then used in the equation below to find the desired value. The parameters x and y represent the offset in time and in frequency, respectively, between UTC(NIST) and AT1; the parameter x_{ls} is the number of leap seconds applied to both UTC(NIST) and UTC as specified by the IERS. Leap seconds are not applied to AT1.

UTC(NIST) - AT1 = x_{ls} + x + y*(T-T_{0})

Month |
x _{ls}(s) |
x (ns) |
y (ns/d) |
T _{0}(MJD) |
Valid until 0000 on: (MJD) |
---|---|---|---|---|---|

2017-03* | -37 | -442062.85 | -36.85 | 57813 | 57814 |

2017-02* | -37 | -441325.85 | -36.85 | 57793 | 57813 |

2017-02 | -37 | -441029.45 | -37.05 | 57785 | 57793 |

2017-01 | -37 | -439880.9 | -37.05 | 57754 | 57785 |

2016-12 | -36 | -439547.45 | -37.05 | 57745 | 57754 |

2016-12** | -36 | -439287.05 | -37.2 | 57728 | 57745 |

2016-12** | -36 | -438770.45 | -36.9 | 57724 | 57738 |

2016-12** | -36 | -438733.45 | -37.0 | 57723 | 57754 |

2016-11 | -36 | -437694.75 | -37.0 | 57704 | 57723 |

2016-11** | -36 | -437694.75 | -37.3 | 57695 | 57704 |

2016-11** | -36 | -437620.95 | -36.9 | 57693 | 57695 |

2016-10 | -36 | -436956.75 | -36.9 | 57675 | 57693 |

2016-10** | -36 | -436510.35 | -37.2 | 57663 | 57675 |

2016-10** | -36 | -436472.9 | -37.5 | 57662 | 57663 |

2016-09 | -36 | -435910.4 | -37.5 | 57647 | 57662 |

2016-09** | -36 | -435350.9 | -37.3 | 57632 | 57647 |

2016-08 | -36 | -434936.7 | -37.65 | 57621 | 57632 |

2016-08** | -36 | -434191.7 | -37.25 | 57601 | 57621 |

2016-07 | -36 | -434079.95 | -37.25 | 57598 | 57601 |

2016-07** | -36 | -433041.2 | -37.1 | 57570 | 57598 |

2016-06 | -36 | -431923.7 | -37.25 | 57540 | 57570 |

2016-05 | -36 | -431476.7 | -37.25 | 57528 | 57540 |

2016-05** | -36 | -430771.8 | -37.1 | 57509 | 57528 |

2016-04 | -36 | -430697.6 | -37.1 | 57507 | 57509 |

2016-04** | -36 | -429931.05 | -36-5 | 57486 | 57507 |

2016-04** | -36 | -429672.75 | -36.9 | 57479 | 57486 |

2016-03 | -36 | -428636.75 | -37.0 | 57451 | 57479 |

2016-03** | -36 | -428521.05 | -37.3 | 57448 | 5745 |

2016-02 | -36 | -427816.15 | -37.3 | 57429 | 57448 |

2016-02** | -36 | -427556.45 | -37.1 | 57422 | 57429 |

2016-02** | -36 | -427446.05 | -36.8 | 57419 | 57422 |

2016-01 | -36 | -427014.25 | -36.8 | 57408 | 57419 |

2016-01** | -36 | -426313.25 | -36.4 | 57388 | 57408 |

*Provisional value

** Rate change in mid-month

Allan, D.W.; Hellwig, H.; and Glaze, D.J., "An accuracy algorithm for an atomic time scale," Metrologia, Vol.11, No.3, pp. 133-138 (1975).

Allan, D.W.; Davis, D.D.; Weiss, M.A.; Clements, A.; Guinot, B.; Granveaud, M.; Dorenwendt, K.; Fischer, B.; Hetzel, P.; Aoki, S.; Fujimoto, M.; Charron, L.; and Ashby, N., "Accuracy of International Time and Frequency Comparisons Via Global Positioning System Satellites in Common-view," IEEE Transactions on Instrumentation and Measurement, Vol. IM-34, pp. 118-125, (1985).

Jefferts, S.R.; Shirley, J.; Parker, T.E.; Heavner, T.P.; Meekhof, D.M.; Nelson, C., Levi, F.; Costanza, G.; De Marchi, A.; Drullinger, R.; Hollberg, L.; Lee, W.D.; and Walls, F.L., "Accuracy evaluation of NIST-F1," Metrologia, Vol. 39, pp. 321-336, (2002).

Lewandowski, W. and Thomas, C., "GPS time transfer," Proceedings of the IEEE, Vol. 79, pp. 991-1000, (1991).

Shirley, J.H.; Lee, W.D.; Drullinger, R.E., "Accuracy evaluation of the primary frequency standard NIST-7," Metrologia, Vol. 38, pp. 427-458, (2001).

Weiss, M.A.; Allan, D.W., "An NBS Calibration Procedure for Providing Time and Frequency at a Remote Site by Weighting and Smoothing of GPS Common View Data," IEEE Transactions on Instrumentation and Measurement, Vol. IM-36, pp. 572-578, (1987).

Created February 23, 2010, Updated June 2, 2021