NIST Time Scale Data Archive

[an error occurred while processing this directive]

NIST Time Scale Data Archive - 2004
Notes on NIST time scale, primary standards & services

Return to Archive index 2002 2003 2005 2006 2007 2008 Current Year

Primary frequency standards developed and maintained by NIST are used to provide accuracy (rate) input to the BIPM. NIST-F1, a cesium fountain frequency standard, serves as the United States primary frequency standard. The uncertainty of NIST-F1 is less than 1 part 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)

2004-12	-32	-272712.15	-39.0	53361	53371
2004-12**	-32	-271891.05	-39.1	53340	53361
2004-11	-32	-270718.05	-39.1	53310	53340
2004-10	-32	-269505.95	-39.1	53279	53310
2004-09	-32	-268723.95	-39.1	53259	53279
2004-09**	-32	-268330.95	-39.3	53249	53259
2004-08	-32	-267898.65	-39.3	53238	53249
2004-08**	-32	-267110.65	-39.4	53218	53238
2004-07	-32	-266716.65	-39.4	53208	53218
2004-07**	-32	-265892.4	-39.25	53187	53208
2004-06	-32	-265342.9	-39.25	53173	53187
2004-06**	-32	-264722.9	-38.75	53157	53173
2004-05	-32	-264064.15	-38.75	53140	53157
2004-05**	-32	-263514.65	-39.25	53126	53140
2004-04	-32	-263004.4	-39.25	53113	53126
2004-04**	-32	-262334.6	-39.4	53096	53113
2004-03	-32	-261783.0	-39.4	53082	53096
2004-03**	-32	-261110.65	-39.55	53065	53082
2004-02	-32	-259963.7	-39.55	53036	53065
2004-01	-32	-259410.0	-39.55	53022	53036
2004-01**	-32	-258738.5	-39.5	53005	53022

* 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 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).