NIST Time Scale Data Archive - 2004 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-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 = xls + x + y(T - To) Month xls(s) x (ns) y (ns/day) To (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).