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NIST Time Scale Data Archive

Notes on NIST time scale, primary standards and services


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Primary frequency standards developed and maintained by NIST are used to provide accuracy (rate) input to the BIPM. NIST-7, which has served as the U.S. primary standard since 1994, has been replaced by NIST-F1, a cesium fountain frequency standard. The uncertainty of the new standard is currently 1.3 parts in 1015.

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

UTC(NIST) is generated as an offset from our real-time scale AT1. It is steered in frequency towards UTC using data published by the BIPM in its Circular T. Changes in the steering frequency are usually made at 0000 UTC on the first day of any month, and the change in frequency in any month is limited to ±2 ns/day. The frequency of UTC(NIST) is kept as stable as possible at other times.

UTC is generated at the BIPM using 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 data available.

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 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 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 - To)

Month xls(s) x (ns) y (ns/day) To (MJD) Valid
until 0000
UTC on:
MJD)

2003-12

-32 -258343.5 -39.5 52995 53005

2003-12**

-32 -257516.1 -39.4 52974 52995

2003-11

-32 -256925.1 -39.4 52959 52974

2003-11**

-32 -256334.85 -39.35 52944 52959

2003-10

-32 -255783.95 -39.35 52930 52944

2003-10**

-32 -255112.45 -39.5 52913 52930

2003-09

-32 -253927.45 -39.5 52883 52913

2003-08

-32 -252702.95 -39.5 52852 52883

2003-07

-32 -252228.95 -39.5 52840 52852

2003-07**

-32 -251473.7 -39.75 52821 52840

2003-06

-32 -251076.2 -39.75 52811 52821

2003-06** 

-32 -250276.2 -40.0 52791 52811

2003-05

-32 -249652.2 -39.0 52775 52791

2003-05**

-32 -249052.2 -40.0 52760 52775

2003-04

-32 -248495.7 -39.75 52746 52760

2003-04**

-32 -247855.7 -40.0 52730 52746

2003-03

-32 -247415.7 -40.0 52719 52730

2003-03**

-32 -246607.7 -40.4 52699 52719

2003-02

-32 -246284.9 -40.35 52691 52699

2003-02**

-32 -245474.9 -40.5 52671 52691

2003-01

-32 -244906.5 -40.6 52657 52671

2003-01**

-32 -244218 -40.5 52640 52657

* 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).