Time & Freauency

Bulletin

No. 382 September 1989

(NISTIR 89-3910-9)

'^^4TES

NIST TIMK & FREQUENCY BULLETIN

(Supersedes No. 381 August 1989)

NO. 382 SEPTEMBER 1989

1. GENERAL BACKGROUND INFORMATION 1

2. TIME SCALE INFORMATION 1

International Timing Center comparisons via GPS common-view ...... 2

3. UTl CORRECTIONS AND LEAP SECOND ADJUSTMENTS 2

4. PHASE DEVIATIONS FOR WWVB AND LORAN-C 3

5. GOES TIME CODE INFORMATION 4

6. BROADCAST OUTAGES OVER FIVE MINUTES AND WWVB PHASE PERTURBATIONS .... 4

7. NOTES ON NIST TIME SCALES AND PRIMARY STANDARDS 5

8. SPECIAL ANNOUNCEMENTS 7

This bulletin is published monthly. Address correspondence to:

Gwen E. Bennett, Editor

Time and Frequency Division

National Institute of Standards & Technology

325 Broadway

Boulder, CO 80303

(303) 497-3295

NOTE TO SUBSCRIBERS: Please include your address label (or a copy)

with any correspondence regarding this bulletin.
1. GENERAL BACKGROUND INFORMATION

ABBREVIATIONS AND ACRONYMS USED IN THIS BULLETIN

APL John Hopkins University Applied Physics Laboratory

BIH - International Time Bureau, France

CCIR - International Radio Consultative Committee

CRL - Communications Research Laboratories, Japan

Cs - Cesium standard

CSIRO - Commonwealth Scientific and Industrial Research Organization, Australia

GOES - Geostationary Operational Environmental Satellite

GPS - Global Positioning System

lEN - National Institute of Electronics, Italy

INPL - National Physical Laboratory, Israel

LORAN - Long Range Navigation

MC - Master Clock

MJD - Modified Julian Date

NIST - National Institute of Standards & Technology

NPL - National Physical Laboratory, England

NRC - National Research Council, Canada

NOAA - National Oceanic and Atmospheric Administration

OP - Paris Observatory, France

PTB - Physical Technical Federal Laboratory, Germany

SI - International System of Units ns nanosecond

SV - Space vehicle MS microsecond

TA - Atomic Time ms millisecond

TAI - International Atomic Time s second

TAO - Tokyo Astronomical Observatory, Japan min - minute

TUG - Technical University of Graz, Austria h hour

USNO - United States Naval Observatory d day

UTC - Coordinated Universal Time

VLF - very low frequency

VSL Van Swinden Laboratory, Netherlands

2. TIME SCALE INFORMATION

The values listed below are based on data from the BIH, the USNO, and the NIST. The UTC — UTC(NIST) values are extrapolations since UTC is computed more than two months after the fact. The UTC(USNO,MC) - UTC(NIST) values

are averaged measurements from NAVSTAR satellites 3,4,6, and 8 (see references on page 6).

0000 HOURS COORDINATED UNIVERSAL TIME

1989 MJD

UTl - UTC(NIST)

(± 5 ms)

UTC - UTC(NIST)

(± 0.2 ms)

UTC ( USNO, MC) - UTC (NIST)

(± 0.04 ms)

3 47741 -416 ms -0.5 MS 0.53 MS

10 47748 -424 ms -0.5 MS 0.56 MS

17 47755 -430 ms -0.5 MS 0.59 MS

24 47762 -441 ms -0.5 MS 0.61 MS

31 47769 -447 ms -0.5 MS 0.63 MS

-

1 -
INTE31NATI0NAL TIMING CENTER COMPARISONS VIA GPS COMMON-VIEW

The table below is a weighted average of the indicated GPS satellites used as transfer standards to measure the

time difference of Timing Center (i) — UTC(NIST) by the simultaneous common-view approach (see references, page

6). The day-to-day variations of this technique are a few nanoseconds and the accuracy is about 10 ns. The

time of the measurement is

prepared for the BIPM for

noted.

interpolated to 0000

the computation on TAI

UTC for the

and of UTC.

particular MJD ending in

All differential delays

9. These data are are 0 unless otherwise

UTC(i) - UTC(NIST) (ns) MJD

UTC(i) SV NUMBERS 47739 47749 47759 47769

UTC(APL) - UTC(NIST) 3, 12,13 1352 1409 1662 1662(3

UTC(CRL) - UTC(NIST) 6, 12 809 891 855 720

UTC(CSIRO) - UTC(NIST) ** 21096* 21443 21784 22028

UTC(IEN) - UTC(NIST) 11,12,13,14 -103* -292 -480 -696//

UTC(INPL) - UTC(NIST) VIA OP -127492 -128805 -130148 -131513//

UTC(NPL) - UTC(NIST) 3, 11,12,13,14 481* 518 587 677//

UTC(NRC) - UTC(NIST)*** 3,6,9 ,11,12,13,14 13930* 14088 14250 14426//

UTC(OP) - UTC(NIST) 3, 11,12,13,14 707* 645 607 519//

UTC(PTB) - UTC(NIST) 11 -4552 -4449 -4399 -4364//

UTC (TAO) - UTC (NIST) 6, 14 3207* 3360 3544 3616

UTC (TUG) - UTC (NIST) 3. 11,12,13,14 -1442 -1664 -1872 -2096//

UTC(USNO,MC) - UTC (NIST) 3,6, 11,12,13,14 524 569 600 634//

UTC(VSL) - UTC(NIST) 3, 11,12,13,14 -1881* -1913 -1944 -2013//

These stations have updated values for MJD 47729 as follows: CSIRO = 20869, lEN = -57, NPL = 446,

NRC = 13797, OP = 796, TAO = 3106, VSL = -1782.

** UTC(CSIRO) - UTC(NIST) is computed from the average by CRL, TAO, and WWVH.

*** UTC(NRC) - UTC(NIST) has a differential delay of 41.2 ns; all other comparisons are computed using zero

( 0 ).

(3 The value for UTC(APL) - UTC(NIST) for MJD 47769 was extrapolated forward from MJD 47664.

// These values for MJD 47769 were extrapolated forward from 47768.

3. UTl CORRECTIONS AND LEAP SECOND ADJUSTMENTS

The master clock pulses used by the WWV, WWVH, WWVB, and GOES time code transmissions are referenced to the

UTC(NIST) time scale. Occasionally, 1 second 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 UTl astronomical time scale, which

changes slightly due to variations in the rotation of the earth.

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 timescale on 30 June 1972, 31 December 1972-1979, 30 June 1981-1983, 30 June 1985, and 31 December 1987.

When future leap seconds are scheduled, advance notice will be provided in this bulletin.

The use of leap seconds ensures that UTl — UTC will always be held within ± 0.9 s. The current value of

UTl — UTC is called the DUTl correction. DUTl corrections are broadcast by WWV, WWVH, WWVB, and GOES and are printed below. These corrections may be added to received UTC time signals in order to obtain UTl.

= -0.3 s beginning 0000 UTC 06 April 1989

DUTl = UTl - UTC = -0.4 s beginning 0000 UTC 08 June 1989 = -0.5 s beginning 0000 UTC 21 September 1989

- 2 -
4. PHASE DEVIATIONS FOR WWVB AND LORAN-C

VWVB — The values shown for WWVB are the time difference between the time markers of the UTC(NIST)

time scale and the first positive-going zero voltage crossover measured at the transmitting

antenna. The uncertainty of the individual measurements is ± 0.5 ms. The values listed are

for 1500 UTC.

LORAN-C - The values shown for Loran-C represent the daily accumulated phase shift (in microseconds).

The phase shift is measured by comparing the output of a Loran receiver to the UTC(NIST) time

scale for a period of 24 hours. If data were not recorded on a particular day, the symbol

(-) is printed.

The stations monitored are Dana, Indiana (8970 M) and Fallon, Nevada (9940 M). The

monitoring is done from the NIST laboratories in Boulder, Colorado.

UTC(NIST) - WWVB(60 kHz)

UTC (NIST) - LORAN PHASE (in ms)

DATE MJD

ANTENNA PHASE

(in ms)

LORAN-C (DANA)

(8970 M)

LORAN-C (FALLON)

(9940 M)

08/01/89 47739 5.66 (-) +0.01

08/02/89 47740 5.65 (-) (-)

08/03/89 47741 5.63 +0.00 -0.15

08/04/89 47742 5.64 -0.13 -0.19

08/05/89 47743 5.66 -0.15 -0.21

08/06/89 47744 5.68 +0.39 +0.35

08/07/89 47745 5.70 -0.11 -0.00

08/08/89 47746 5.64 -0.05 -0.10

08/09/89 47747 5.64 +0.01 +0.05

08/10/89 47748 5.61 +0.08 +0.02

08/11/89 47749 5.61 +0.19 -0.36

08/12/89 47750 5.63 -0.69 -0.10

08/13/89 47751 5.66 +0.34 +0.32

08/14/89 47752 5.68 +0.00 -0.08

08/15/89 47753 5.63 -0.17 -0.16

08/16/89 47754 5.62 +0.08 +0.01

08/17/89 47755 5.65 +0.11 +0.11

08/18/89 47756 5.65 -0.21 -0.24

08/19/89 47757 5.63 -0.01 -0.04

08/20/89 47758 5.61 (-) (-)

08/21/89 47759 5.60 (-) (-)

08/22/89 47760 5.63 (-) (-)

08/23/89 47761 5.61 (-) (-)

08/24/89 47762 5.68 -0.19 (-)

08/25/89 47763 5.63 -0.09 -0.09

08/26/89 47764 5.59 (-) (-)

08/27/89 47765 5.56 (-) (-)

08/28/89 47766 5.53 (-) (-)

08/29/89 47767 5.70 -0.83 -0.02

08/30/89 47768 5.68 +0.15 -0.83

08/31/89 47769 5.65 +0.04 -0.14

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5. GOES TIKE CODE INEORHATION

A. TIME CODE PERFORMANCE (1-31 August 1989)

GOES/East ; Performance within normal limits during this period.

GOES/West

Performance within normal limits during this period.

B. ECLIPSE OPERATIONS: Current Fall 1989 eclipse operations are not expected to adversely impact any GOES

time code performance aspects.

C. SPECIAL REMINDER: Current satellite locations are 65° W. for GOES/East and 135° W. for GOES/West. NOAA

is still considering the possibility of moving GOES/East from 65° back to the original 75° location, but no decision has been made as of this time.

D. GOES STATUS REPORTS

A brief message from the NIST giving current GOES time code status information is available from the U.S.

Naval Observatory's Automated Data Service computer system in Washington, DC. The message may be accessed

24 hours per day without charge by using a variety of terminals operating at 300, 1200, or 2400 Baud and even parity. Two different sets of telephone access numbers are available: (1) for 300 or 1200 Baud and

the Bell 103 standard use (202) 653-1079 (commercial), 653-1079 (FTS), or 294-1079 (Autovon); (2) for 1200 or 2400 Baud with either the CCITT V.22 standard or the Bell standard use (202) 653-1783 (commercial),

653-1783 (FTS), or 294-1783 (Autovon). To receive the GOES status message, use the following procedure:

1. Access the USNO computer database by dialing one of the appropriate telephone numbers above;

2. In response to the prompt for identification, type your name and the name of your organization,

followed by a carriage return;

3. Type "0NBSGO" followed by a carriage return to receive the status message at your terminal;

4. Disconnect by typing Control-D.

6. BROADCAST OUTAGES OVER 5 MINUTES AND WWVB PHASE PERTURBATIONS

OUTAGES PHASE PERTURBATIONS WWVB 60 kHz

AUGUST BEGAN ENDED AUGUST BEGAN ENDED

STATION 1989 MJD (UTC) (UTC) FREQUENCY 1989 MJD (UTC) (UTC)

WWVB NONE NONE

WWV NONE NONE

WWVH NONE NONE

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

The frequencies of the time scales, TA(NIST) and UTC(NIST), are calibrated with the NIST primary frequency

standards. The UTC(NIST) scale is coordinated within a microsecond of the internationally coordinated time

scale, UTC, generated at the BIH. It is used to control all of the NIST time and frequency services. The last

calibration of the relative frequency offset, y, of UTC(NIST) as generated in Boulder, Colorado, gave:

yUTC(NIST) 1987) - yNBs -5 (July 1987) = (-0.6 + 2 (1 sigma)) x 10"^^

for the date shown. This calibration includes a correction for the systematic offset due to room temperature

blackbody radiation, which is approximately (delta ygg) = -1.7 x 10"^^. Using GPS^, the frequency of TAI for

the dates shown were measured to be:

2) y-r;^! (July 1987) — y^BS-b 1987 on geoid) = (+1.7 ±2(1 sigma)) x 10“^^

where account has been taken of the gravitational "red shift."

Starting 1 January 1975, an accuracy algorithm was implemented to bring the second used in the generation of

TA(NIST) closer to the NIST "best estimate" of the SI second (see references, p. 6 ). The relative frequency

associated with this "best estimate" is denoted ycs(NIST)’ last calibration (July 1987) covered the period

from October 1986 through July 1987.

yCs(NIST) “ yNBS-6 = (+ 1 -^ ± 2 ) x lO'^^ ^987)

and

yTAI - yCs(NIST) on geoid = (+0*3 ± 0.7) x lO'^^ 1987 )

This algorithm should provide nearly optimum accuracy and stability for TA(NIST) since it uses all past

frequency calibrations with the NIST primary standards. These' calibrations are weighted proportionately to the

frequency memory of the clock ensemble that generates atomic time. This algorithm, therefore, capitalizes on a weighted combination of all the frequency calibrations with the primary standards in order to gain a "best

• estimate" of the SI Second while simultaneously obtaining the best uniformity available from the ensemble of

, working clocks in the atomic time scale system. The relative frequency of TA(NIST) is steered toward ycs(NIST)

, by slight frequency drift corrections of the order of 1 part in 10^^/yr.

TA(NIST) and UTC(NIST) are no longer simply related by an equation. TA(NIST) is now computed each month using

a Kalman algorithm which minimizes the mean square time dispersion. UTC(NIST) is now independently computed

using a different algorithm and is steered in frequency to keep its time within a microsecond of UTC(BIH).

Table 7.1 lists monthly values of the time difference between UTC(NIST) and TA(NIST). A linear interpolation

between monthly values will typically be within 10 ns of the actual time difference, TA(NIST) — UTC(NIST).

The primary standards of NIST (NBS-A and NBS-6) are used in either of two modes: as calibrators of frequency to

provide a reference for the SI second; or as member clocks of the NIST clock ensemble, to help keep the proper

time for TA(NIST) and the coordinated time for UTC(NIST). Operating in the clock mode, NBS-A and NBS-6 are only used and weighted according to their stability performance. Accuracy enters only when they are used as frequency calibrators, in which case clock operation is necessarily interrupted.

1 GPS is the Global Positioning System, a network of navigation satellites.

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Table 7.1 is a list of changes in the time scale frequencies of both TA(NIST) and UTC(NIST) as well as a list of the time and frequency differences between TA(NIST) and UTC(NIST) at the dates of leap seconds,

and/or frequency or frequency drift changes.

TABLE 7.1

FREQUENCY CHANGES

DATE (MJD) TA(NIST) UTC(NIST) TA(NIST) - UTC (NIST) y[UTC(NIST)] -- y[TA(NIST)]

1 Mar 88 47221 0 -1.25 ns/d 24.045 107 137 s -3.58 E-13

1 Apr 88 47252 0 -1.50 ns/d 24.045 108 130 s -3.85 E-13

1 May 88 47282 0 -1.50 ns/

24.045 109 170 s -4.29 E-13

1 Jun 88 47313 0 -1.50 ns/d 24.045 no 358 s -4.47 E-13

1 Jul 88 47343 0 -1.60 ns/d 24.045 111 523 s -4.64 E-13

1 Aug 88 47374 0 -0.40 ns/d 24.045 112 802 s -4.89 E-13

1 Sep 88 47405 0 -1.00 ns/

24.045 114 144 s -5.15 E-13

1 Oct 88 47435 0 1.00 ns/d 24.045 114 515 s -5.15 E-13

1 Nov 88 47466 0 1.25 ns/d 24.045 116 854 s -4.88 E-13

1 Dec 88 47496 0 1.50 ns/

24.045 118 088 s -4.69 E-13

1 Jan 89 47527 0 1.50 ns/d 24.045 119 325 s -4.57 E-13

1 Feb 89 47558 0 1.00 ns/

24.045 120 538 s -4.51 E-13

1 Mar 89 47586 0 -1.25 ns/

24.045 121 622 s -4.58 E-13

1 Apr 89 47617 0 -1.50 ns/

24.045 122 871 s -4.66 E-13

1 May 89 47647 0 -1.50 ns/

24.045 124 078 s -4.75 E-13

1 Jun 89 47678 0 -1.00 ns/

24.045 125 375 s -4.92 E-13

1 Jul 89 47708 0

1 O O ns/d 24.045 126 670 s -5.09 E-13

1 Aug 89 47739 0 o 0

1 ns/

24.045 128 060 s -5.35 E-13

UTC(NIST) is steered in time toward UTC by occasional frequency changes of the order of a few nanoseconds

per day; 1 ns/d is approximately 1.16E-1A. Otherwise, y[UTC(NIST)] is maintained as stable as possible.

REFERENCES

Allan, David W. , et al., "An accuracy algorithm for an atomic time scale," Metrologia, Vol.ll, No. 3,

pp. 133-138 (September 1975).

Glaze, D.J., et al., "NBS-4 and NBS-6: The NIST primary frequency standards," Metrologia, Vol.l3,

pp. 17-28 (1977).

Wineland, D.J., et al., "Results on limitations in primary cesium standard operation," IEEE Trans, on Instr. and Meas., Vol.IM-25, No. 4, pp. 453-458 (December 1976).

Allan, David W. and Weiss, Marc, "Accurate Time and Frequency Transfer During Common View of a GPS

Satellite," Proc. 34th Annual Symposium on Frequency Control, p.334 (1980).

Allan, David W. and Barnes, James A., "Optimal Time i Frequency using GPS signals," Proc. 36th Annual

Symposium on Frequency Control, p.378 (1982).

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8. SPECIAL ANNOUNCEMENTS

AUTOMATED COMPUTER TIME SERVICE (ACTS)

On March 9, 1988, NIST initiated operation of a telephone time service designed to provide computers with

telephone access to NIST time at accuracies approaching 1 ms. Features of the service include automated

compensation for telephone-line delay, advanced alert for changes to and from daylight savings time and

advanced notice of insertion of leap seconds. The ASCII-character time code should operate with standard

modems and most computer systems. While the system can be used to set computer time-of-day clocks, simple

hardware can also be developed to set other clock systems.

During the first six months, the service will be operated in a test phase to identify problems and obtain

feedback from users on both the format and operation of the service. After completion of the test phase, there

may be some revisions in the service. The service telephone number is (303) A94-4774. The number may be changed at a later date. A help message can be obtained by returning a ? during the first 6 s of transmission.

With appropriate user software, the NIST-ACTS service provides three modes for checking and/or setting

computer time-of-day clocks.

1. In the simplest form of the (1200 Baud) service, the user receives the time code and an on-time

marker/character which has been advanced a fixed period to nominally account for modem and telephone- line

delays. Accuracy in this mode should be no worse than 0.1 s unless the connection is routed through a satellite.

2. At 1200 Baud, if the user's system echoes all characters to NIST, the round-trip line delay will be

measured and. 'the on-time marker advanced to compensate for that delay. The accuracy in this mode should be

better than 10 ms. Our experience to date indicates that the asymmetry in conventional, 1200-Baud modems

limits the accuracy at this level. Repeatability is about 1 ms.

3. At 300 Baud the user can obtain the same type of service as described in item 2 above, but there is

generally less problem with modem asymmetry at this rate and our experience indicates that the accuracy is

about 1 ms.

The accuracy statements here are based upon the assumption that the telephone connection is reciprocal,

that is, that both directions of communication follow the same path with the same delay. Discussions with

telephone carriers indicate that this is the general mode of operation and our tests to date indicate that the

lines are both stable and reciprocal.

In order to assist users of the service, NIST has developed documentation of the features of the service,

some example software which can be used in conjunction with certain popular personal computers and simple

circuitry which can be used to extract an on-time pulse. This material is available on a 5i-in, 360-kbyte DOS

diskette with instructions for $35.00 from the NIST Office of Standard Reference Materials, B311 -Chemistry

Bldg, NIST, Gaithersburg, MD, 20899, (301) 975-6776. Specify the Automated Computer Time Service, RM8101.

Further technical questions and comments should be directed to NIST-ACTS, NIST Time and Frequency Division, 325

Broadway, Boulder, CO 80303.

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National Institute of Standards and Technology

Time & Frequency Division

Boulder, Colorado 80303

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