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Radio Stations |
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Services |
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Standards |
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Time Transfer |
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Metrology |
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Research |
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Data |
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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-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1994, has been replaced by NIST-F1, a cesium fountain frequency standard.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 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) |
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2000-12 |
-32 |
-213746.5 |
-40.0 |
51879 |
51910 |
2000-11 |
-32 |
-212546.5 |
-40.0 |
51849 |
51879 |
2000-10 |
-32 |
-211337.5 |
-39.0 |
51818 |
51849 |
2000-09 |
-32 |
-210167.5 |
-39.0 |
51788 |
51818 |
2000-08 |
-32 |
-208943 |
-39.5 |
51757 |
51788 |
2000-07 |
-32 |
-207663 |
-40.00 |
51725 |
51757 |
2000-06 |
-32 |
-206495.75 |
-40.25 |
51696 |
51726 |
2000-05 |
-32 |
-205248 |
-40.25 |
51665 |
51696 |
2000-04 |
-32 |
-204033 |
-40.5 |
51635 |
51665 |
2000-03 |
-32 |
-202777.5 |
-40.5 |
51604 |
51635 |
2000-02 |
-32 |
-201603 |
-40.5 |
51575 |
51604 |
2000-01 |
-32 |
-200347.5 |
-40.5 |
51544 |
51575 |
1999-12 |
-32 |
-199896.5 |
-41.0 |
51533 |
51544 |
1999-12** |
-32 |
-199096.5 |
-40.0 |
51513 |
51533 |
1999-11 |
-32 |
-197896.5 |
-40.0 |
51483 |
51513 |
1999-10 |
-32 |
-196641 |
-40.5 |
51452 |
51483 |
1999-09 |
-32 |
-195426 |
-40.5 |
51422 |
51452 |
1999-08 |
-32 |
-194155 |
-41.0 |
51391 |
51422 |
1999-07 |
-32 |
-192884 |
-41.0 |
51360 |
51391 |
1999-06 |
-32 |
-191654 |
-41.0 |
51330 |
51360 |
1999-05 |
-32 |
-190383 |
-41.0 |
51299 |
51330 |
1999-04 |
-32 |
-189183 |
-40.5* |
51269 |
51299 |
1999-03 |
-32 |
-187927.5 |
-40.5 |
51238 |
51269 |
1999-02 |
-32 |
-186779.5 |
-41.0 |
51210 |
51238 |
1999-01 |
-32 |
-185493 |
-41.5 |
51179 |
51210 |
1998-12 |
-31 |
-184206.5 |
-41.5 |
51148 |
51179 |
1998-11 |
-31 |
-182961.5 |
-41.5 |
51118 |
51148 |
1998-10 |
-31 |
-181675 |
-41.5 |
51087 |
51118 |
1998-09 |
-31 |
-180445 |
-41.0 |
51057 |
51087 |
1998-08 |
-31 |
-179174 |
-41.0 |
51025 |
51057 |
1998-07 |
-31 |
-177887.5 |
-41.5 |
50995 |
51025 |
1998-06 |
-31 |
-176642.5 |
-41.5 |
50965 |
50995 |
1998-05 |
-31 |
-175340.5 |
-42.0 |
50934 |
50965 |
1998-04 |
-31 |
-174080.5 |
-42.0 |
50904 |
50934 |
1998-03 |
-31 |
-172763 |
-42.5 |
50873 |
50904 |
1998-02 |
-31 |
-171573 |
-42.5 |
50845 |
50873 |
1998-01 |
-31 |
-170255 |
-42.5 |
50814 |
50845 |
1997-12 |
-31 |
-168938 |
-42.5 |
50783 |
50814 |
1997-11 |
-31 |
-167678 |
-42.0 |
50753 |
50783 |
1997-10 |
-31 |
-166376 |
-42.0 |
50722 |
50753 |
1997-09 |
-31 |
-165101 |
-42.5 |
50692 |
50722 |
1997-08 |
-31 |
-163768 |
-43.0 |
50661 |
50692 |
1997-07 |
-31 |
-162435 |
-43.0 |
50630 |
50661 |
1997-06 |
-30 |
-161145 |
-43.0 |
50600 |
50630 |
1997-05 |
-30 |
-159812 |
-43.0 |
50569 |
50600 |
1997-04 |
-30 |
-158537 |
-42.5 |
50539 |
50569 |
* Provisional Value
** Rate Change in mid-month
Return to UTC(NIST)-AT1
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 (September 1975).
Allan, D.W. and Weiss, M.A.,
Accurate time and frequency transfer during common
view of a GPS satellite, Proc. 34th Annual Symposium on Frequency Control, p. 334
(1980).
Allan, D.W. and Barnes, J.A.,
Optimal time and frequency using GPS signals,
Proc. 36th Annual Symposium on Frequency Control, p. 378 (1982).
Drullinger, R.E.; Glaze, D.J.; Lowe, J.P.; and Shirley, J.H.,
The NIST optically
pumped cesium frequency standard, IEEE Trans. Instrum. Meas., IM-40, 162-164 (1991).
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