NIST Time Scale Data Archive

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NIST Time Scale Data Archive - 2003
Notes on NIST time scale, primary standards & 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 NIST-7, which has served as the U.S. primary standard since 1994, has been replaced by NIST-F1, a cesium fountain frequency standard.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 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_o)

Month	x_ls(s)	x (ns)	y (ns/day)	T_o (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

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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 promary 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).