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NIST Time Scale Data Archive

Notes on NIST time scale, primary standards and services

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 1016.

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 nist-equation to use is the one for which the desired T is greater than or equal to the entry in the T0 column and less than the entry in the last column. The values of xls, x, and y for that month are then used in the nist-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 xls 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 = xls + x + y*(T-T0)

Month xls (s) x (ns) y (ns/d) T0 (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

Bibliography

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 October 1, 2019