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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-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)

2008-12

-34

-328895.0  

-38.5

54818

54832

2008-12**  

-33

-328240.5

-38.5

54801

54818

2008-11

-33

-327085.5

-38.5

54771

54801

2008-10

-33

-326392.5

-38.5

54753

54771

2008-10**

-33

-325894.6

-38.3

54740

54753

2008-09

-33

-324745.6

-38.3

54710

54740

2008-08

-33

-323558.3

-38.3

54679

54710

2008-07

-33

-322792.3

-38.3

54659

54679

2008-07**

-33

-322369.9

-38.4

54648

54659

2008-06

-33

-321211.9

-38.6

54618

54648

2008-05

-33

-320594.3

-38.6

54602

54618

2008-05**

-33

-320018.3

-38.4

54587

54602

2008-04

-33

-319288.7

-38.4

54568

54587

2008-04**

-33

-318867.4

-38.3

54557

54568

2008-03

-33

-318178.0

-38.3

54539

54557

2008-03**

-33

-317684.0

-38.0

54526

54539

2008-02

-33

-316582.0

-38.0

54497

54526

2008-01

-33

-315974.0

-38.0

54481

54497

2008-01**

-33

-315405.5

-37.9

54466

54481

* 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).