NIST TIME AND FREQUENCY BULLETIN 

NIST IR 6658-05 

No. 629 MAY 2010

 

1. GENERAL BACKGROUND INFORMATION ........................................................................................... 2 

 

2. TIME SCALE INFORMATION ............................................................................................................... 2 

 

3. BROADCAST OUTAGES OVER FIVE MINUTES AND WWVB 

 PHASE PERTURBATIONS ..................................................................................................................... 4 

4. NOTES ON NIST TIME SCALES AND PRIMARY STANDARDS ........................................................... 4 

5. UTC(NIST) – AT1 PARAMETERS ........................................................................................................... 5 

 

This bulletin is published monthly. Address correspondence to: 

 

Eyvon M. Petty, Editor 

Time and Frequency Division 

National Institute of Standards and Technology 

325 Broadway 

Boulder, CO 8O3O5-3328 

(3O3) 497-3295 

Email: pettye@boulder.nist.gov

U.S. DEPARTMENT OF COMMERCE, GARY LOCKE, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Patrick D. Gallagher, Director 

 

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2 

1. GENERAL BACKGROUND INFORMATION 

_______________________________________________________________________________________________________ 

ACRONYMS AND ABBREVIATIONS USED IN THIS BULLETIN

ACTS - Automated Computer Time Service 

BIPM - Bureau International des Poids et Mesures 

CS - Cesium Standard 

GPS - Global Positioning System 

IERS - International Earth Rotation Service 

LORAN - Long Range Navigation 

MC - Master Clock 

MJD - Modified Julian Date 

NIST - National Institute of Standards and Technology 

NOAA - National Oceanic and Atmospheric Administration 

NVLAP - National Voluntary Laboratory Accreditation Program ns - nanosecond 

SI - International System of Units μs - microsecond 

TA - Atomic Time ms - millisecond 

TAI - International Atomic Time s - second 

USNO - United States Naval Observatory min - minute 

UTC - Coordinated Universal Time 

_______________________________________________________________________________________________________ 

2. TIME SCALE INFORMATION 

 The values listed below are based on data from the IERS, the USNO, and NIST. The UTC(USNO,MC) - UTC(NIST) values are 

averaged measurements from all available common-view GPS satellites (see bibliography on page 5). UTC - UTC(NIST) data are 

on page 3. 

 0000 HOURS COORDINATED UNIVERSAL TIME 

APR 2010 MJD UT1-UTC(NIST) 

 (±5 ms) 

UTC(USNO,MC) - UTC(NIST) 

 (±20 ns) 

1 55287 016 ms 0 ns 

8 55294 009 ms -1 ns 

15 55301 -001 ms 1 ns 

22 55308 -009 ms 0 ns 

29 55315 -021 ms 0 ns 

 The master clock pulses used by the WWV, WWVH, and WWVB time-code transmissions are referenced to the UTC(NIST) time 

scale. Occasionally, 1 s is added to the UTC time scale. This second is called a leap second. Its purpose is to keep the UTC time 

scale within ±0.9 s of the UT1 astronomical time scale, which changes slightly due to variations in the Earth’s period of rotation. 

 NOTE: No leap second will be added at the end of June 2010. 

 Positive leap seconds, beginning at 23 h 59 min 60 s UTC and ending at 0 h 0 min 0 s UTC, were inserted in the UTC time 

scale on 30 June 1972, 1981-1983, 1985, 1992-1994, and 1997, and on 31 December 1972-1979, 1987, 1989, 1990,1995, 1998, 

2005, and 2008. 

 The use of leap seconds ensures that UT1 - UTC will always be held within ±0.9 s. The current value of UT1 - UTC is called the 

DUT1 correction. DUT1 corrections are broadcast by WWV, WWVH, WWVB, and ACTS and are printed below. These corrections 

may be added to received UTC time signals in order to obtain UT1. 

 +0.0 s beginning 0000 UTC 11 March 2010 

 +0.1 s beginning 0000 UTC 12 November 2009 

DUT1 = UT1 - UTC = +0.2 s beginning 0000 UTC 11 June 2009 

 +0.3 s beginning 0000 UTC 12 March 2009 

 -0.6 s beginning 0000 UTC 20 November 2008 
3 

 

The difference between UTC(NIST) and UTC has been within ±100 ns since July 6, 1994. The table below shows values 

of UTC - UTC(NIST) as supplied by the BIPM in their Circular T publication for the most recent 310-day period in which 

data are available. Data are given at 10-day intervals. Five-day interval data are available in Circular T. 

0000 Hours Coordinated Universal Time

DATE MJD UTC-UTC(NIST) ns 

Mar. 24, 1010 55279 -3.4 

Mar. 14, 2010 55269 -2.5 

Mar. 04, 2010 55259 -1.8

Feb. 22, 2010 55249 -2.2 

Feb. 12, 2010 55239 -0.4 

Feb. 02, 2010 55229 3.7 

Jan. 23, 2010 55219 5.7 

Jan. 13, 2010 55209 10.1 

Jan. 03, 2010 55199 15.0 

Dec. 24, 2009 55189 16.0 

Dec. 14, 2009 55179 17.9 

Dec. 04, 2009 55169 18.1 

Nov. 24, 2009 55159 15.6 

Nov. 14, 2009 55149 14.1 

Nov. 04, 2009 55139 7.8 

Oct. 25, 2009 55129 3.9 

Oct. 15, 2009 55119 0.6 

Oct. 05, 2009 55109 -5.2 

Sep. 25, 2009 55099 -10.2 

Sep. 15, 2009 55089 -13.8 

Sep. 05, 2009 55079 -15.1 

Aug. 26, 2009 55069 -15.8 

Aug. 16, 2009 55059 -16.4 

Aug. 06, 2009 55049 -14.7 

Jul. 27, 2009 55039 -12.3 

Jul. 17, 2009 55029 -9.6 

Jul. 07, 2009 55019 -4.9 

 
 4 

 

3. BROADCAST OUTAGES OVER FIVE MINUTES AND WWVB PHASE PERTURBATIONS 

OUTAGES OF 5 MINUTES OR MORE PHASE PERTURBATIONS 

2 ms 

Station Apr 

2010 

 MJD Began 

UTC 

Ended 

UTC 

Freq. Apr 

2010 MJD Began 

 UTC 

End 

UTC 

WWVB 60 kHz

WWV 

WWVH 

4. NOTES ON NIST TIME SCALES AND PRIMARY STANDARDS 

 Primary frequency standards developed and operated by NIST are used to provide accuracy (rate) input to the BIPM. NIST-F1, a 

cold-atom cesium fountain frequency standard, has served as the U.S. primary time and frequency standard since 1999. The 

uncertainty of NIST-F1 is currently about 3 parts in 1016. 

 The AT1 scale is run in real-time by use of data from an ensemble of cesium standards and hydrogen masers. It is a free-running 

scale whose frequency is maintained as nearly 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 by use of data published 

by the BIPM in its Circular T. Changes in the steering frequency will be made, if necessary, at 0000 UTC on the first day of the 

month, and occasionally at mid-month. A change in frequency is limited to no more than ±2 ns/day. The frequency of UTC(NIST) is 

kept as stable as possible at other times. 

 UTC is generated at the BIPM by use of 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 available data. 

 

References: 

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). 

Heavner, T.P.; Jefferts, S.R.; Donley; E.A.; Shirley, J.H. and Parker, T.E., “NIST F1; recent improvements and accuracy evaluations,” 

Metrologia, Vol. 42, pp. 411-422 (2005). 

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). 

Parker, T.E.; Jefferts, S.R.; Heavner, T.P.; and Donley, E.A., “Operation of the NIST-F1 caesium fountain primary frequency standard 

with a maser ensemble, including the impact of frequency transfer noise,” Metrologia, Vol. 42, pp. 423-430 (2005). 

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). 
 6 

The table below lists 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 offsets in time and 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 −T0) 

Month xls

(s) 

x 

(ns) 

y 

(ns/d) 

 T0

(MJD) 

Valid until 0000 on: 

(MJD) 

Jun 10 -34 -349222.9 -38.4* 55348 55378

May 10 -34 -348032.5 -38.4 55317 55348

 Apr 10 -34 -346880.5 -38.4 55287 55317*

 Mar 10 -34 -346150.9 -38.4 55268 55287

 Mar 10 -34 -345688.3 -38.0 55256 55268†

 Feb 10 -34 -344624.3 -38.0 55228 55256

 Jan 10 -34 -343446.3 -38.0 55197 55228

 Dec. 09 -34 -342952.3 -38.0 55184 55197

 Dec. 09 -34 -342261.1 -38.4 55166 55184†

 Nov 09 -34 -341838.7 -38.4 55155 55166

 Nov 09 -34 -341101.5 -38.8 55136 55155†

 Oct 09 -34 -340441.9 -38.8 55119 55136

 Oct 09 -34 -339895.9 -39.0 55105 55119†

 Sep 09 -34 -339271.9 -39.0* 55089 55105

 Sep 09 -34 -338730.1 -38.7 55075 55089†

 Aug 09 -34 -337917.4 -38.7 55054 55075

 Aug 09 -34 -337534.4 -38.3 55044 55054†

 Jul 09 -34 -336691.8 -38.3 55022 55044

 Jul 09 -34 -336349.8 -38.0 55013 55022†

 Jun 09 -34 -335209.8 -38.0 54983 55013

 May 09 -34 -334791.8 -38.0 54972 54983

 May 09 -34 -334027.8 -38.2 54952 54972†

 Apr 09 -34 -333225.6 -38.2 54931 54952

 Apr 09 -34 -332880.9 -38.3 54922 54931†

† Rate change in mid-month 

*Provisional value