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NIST Time Scale Data Archive
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 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)
2007-12 -33 -314230.6 -37.9 54435 54466
2007-11 -33 -313813.7 -37.9 54424 54435
2007-11** -33 -313091.7 -38.0 54405 54424
2007-10 -33 -312635.7 -38.0 54393 54405
2007-10** -33 -311911.8 -38.1 54374 54393
2007-09 -33 -310768.8 -38.1 54344 54374
2007-08 -33 -309587.7 -38.1 54313 54344
2007-07 -33 -308940.0 -38.1 54296 54313
2007-07** -33 -308408.0 -38.0 54282 54296
2007-06 -33 -307762.0 -38.0 54265 54282
2007-06** -33 -307269.3 -37.9 54252 54265
2007-05 -33 -306738.7 -37.9 54238 54252
2007-05** -33 -306091.0 -38.1 54221 54238
2007-04 -33 -304948.0 -38.1 54191 54221
2007-03 -33 -304262.2 -38.1 54173 54191
2007-03** -33 -303764.3 -38.3 54160 54173
2007-02 -33 -302691.9 -38.3 54132 54160
2007-01 -33 -302079.1 -38.3 54116 54132
2007-01** -33 -301501.6 -38.5 54101 54116

* Provisional Value

** Rate Change in mid-month

Bibliography

Allan, D.W.; Hellwig, H.; and Glaze, D.J., " Requires Adove Acrobat Reader 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., Requires Adove Acrobat Reader “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., Requires Adove Acrobat Reader 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.; Requires Adove Acrobat Reader “Accuracy evaluation of the primary frequency standard NIST-7," Metrologia, Vol. 38, pp.427-458, (2001).

Weiss, M.A.; Allan, D.W.; Requires Adove Acrobat Reader “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).

Online: January 1998 - Last update: December 19, 2007 - ep

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