Time & Frequency

Bulletin

No. 392 July 1990

(NISTIR 90-3940-7)

NIST TIME & FREQUENCY BULLETIN

(Supersedes No. 391 June 1990)

NO. 392 JULY 1990

1. GENERAL BACKGROUND INFORMATION 1

2. TIME SCALE INFORMATION 1

International Timing Center comparisons via GPS common-view 2

3. UT1 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

IEN - 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 us - 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).

OOOO HOURS COORDINATED UNIVERSAL TIME

JUNE

1990 MJD

UT1 - UTC(NIST)

(± 5 ms) UTC - UTC(NIST)

(± 0.2 us) UTC ( USNO, MC) - UTC(NIST)

(± 0.04 us)

7 48049 -5 ms 0.5 us 1.13 us

14 48056 10 ms 0.5 us 1.08 us

21 48063 -24 ms 0.5 us 1.02 us

28 48070 -34 ms 0.5 us 0.95 us

-

1

-
INTERNATIONAL 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 interpolated to 0000 UTC for the particular MJD ending in 9. These data are prepared for the BIPM for the computation on TAI and of UTC. All differential delays are 0 unless otherwise

noted

UTC(i) - UTC(NIST) ( ns

MJD

UTC(i) SV NUMBERS 48039 48049 48059 48069

UTC (APL) - UTC(NIST) 3,6,9, 12,13 -49 -82(? -6 -23

UTC ( CRL ) - UTC(NIST) 6,9 12 481 404 304 190

UTC(CSIRO) - UTC(NIST) ++ 25468+ 25476 25509 25610

UTC(IEN) - UTC(NIST) 12,13 1396 1350 1290 1232

UTC(INPL) - UTC(NIST) VIA OP -175290+ -177351 -179456 -181813

UTC(NPL) - UTC(NIST) 3, 12,13 2919 2904 2845 2664

UTC(NRC) - UTC ( NIST )+++ 3,6,9, 12,13 -367+ -498 -605 -794

UTC (OP) - UTC(NIST) 12,13 1041 966 907 836

UTC(PTB) - UTC(NIST) 12, 16, 20 -3262 J -3314 -3353

UTC(TAO) - UTC(NIST) 6,9, 12 3856 3615 3390 3146

UTC (TUG) - UTC(NIST) 12,13 1308 986 689 357

UTC(USNO.MC) - UTC(NIST) 3,6,9, 12,13 1228 1131 1052 961

UTC(VSL) - UTC (NIST) 12,13 -2300 -2373 -2421 -2518

+ These values have been updated from those printed in Last month's Bulletin.

++ UTC(CSIRO) - UTC(NIST) is computed from the average of CRL, TA0, & WWVH.

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

( 0 ). @ APL experienced a time step of approximately +103 ns on MJD 48051. Therefore, the value for MJD 48049 was computed using raw, unfiltered data.

! Because of tracking schedule problems, no common-view data is available for PTB for MJD 48049.

3. UT1 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 UT1 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, 31 December 1987 and

1989. When future leap seconds are scheduled, advance notice will be provided in this bulletin.

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 GOES and are printed below. These corrections may be added to received UTC time signals in order to obtain UT1.

DUT1 = UT1 - UTC

= +0.1 s beginning 0000 UTC on 12 April 1990

= 0.0 s beginning 0000 UTC on 10 May 1990

= -0.1 s beginning 0000 UTC on 26 July 1990
4. PHASE DEVIATIONS FOR WWVB AND LORAN-C

WWVB - 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 1300 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

(ms ) LORAN-C (DANA)(8970 M)

LORAN-C (FALLON)

(9940 M)

06/01/90 48043 5.74 -0.25 -0.15

06/02/90 48044 5.73 -0.04 -0.16

06/03/90 48045 5.72 +0.33 +0.25

06/04/90 48046 5.72 +0.02 +0.20

06/05/90 48047 5.69 +0.10 +0.02

06/06/90 48048 5.68 -0.07 +0.06

06/07/90 48049 5.66 +0.07 +0.08

06/08/90 48050 5.70 +0.10 +0.06

06/09/90 48051 5.70 -0.15 -0.21

06/10/90 48052 5.69 +0.22 (-)

06/11/90 48053 5.69 (-) (-)

06/12/90 48054 5.64 (-) (-)

06/13/90 48055 5.69 (-) (-)

06/14/90 48056 5.65 +0.28 +0.01

06/15/90 48057 5.65 +0.13 +0.53

06/16/90 48058 5.64 -0.07 +0.10

06/17/90 48059 5.62 +0.14 -0.13

06/18/90 48060 5.61 -0.00 -0.13

06/19/90 48061 5.74 -0.33 +0.10

06/20/90 48062 5.64 (-) (-)

06/21/90 48063 5.69 -0.03 -0.03

06/22/90 48064 5.68 +0.13 +0.73

06/23/90 48065 5.67 +0.39 +0.11

06/24/90 48066 5.65 -0.02 -0.32

06/25/90 48067 5.64 -0.02 +0.19

06/26/90 48068 5.74 -0.01 -0.02

06/27/90 48069 5.74 -0.26 +0.11

06/28/90 48070 5.70 -0.11 -0.11

06/29/90 48071 5.70 -0.22 +0.39

06/30/90 48072 5.69 +0.42 -0.10

- 3 -
5. GOES TIME CODE INFORMATION

A. TIME CODE PERFORMANCE (1-30 June 1990)

GOES/East : Performance within normal limits during this period except for the following: (1) two brief

outages of the time code occurred during 1412-1423 UTC and 1630-1655 UTC on 29 June due to testing

operations at Wallops Island; and (2) variations exceeded normal 100 ms limits during portions of period

from 2000 UTC on 29 June to 2300 UTC on July due to poor-quality satellite orbital elements.

GOES/West : Performance within normal limits during this period.

B. SPECIAL REMINDER: Current satellite locations are about 60° West longitude for GOES/East and 135° West

longitude for GOES/West.

C. - SPECIAL NOTE

NOAA has announced that it will shift the GOES/East time code operations from the current

GOES-5 satellite to GOES-2 at 1400 UTC on 9 July 1990. This change is considered permanent

until new replacement satellites become available sometime after June 1991. The shift to GOES-2 is expected to have the following impact: (1) since GOES-2 is located at 60° West

longitude, satellite antennas may need to be repointed; and (2) since GOES-2 has a relatively large orbit inclination of about 8°, users of receivers that do not correct

automatically for satellite position changes will see larger diurnal variations of about 8 milliseconds peak-to-peak

D. FUTURE SATELLITE LAUNCHES: NOAA's present launch schedule for replacement of the current East and West

satellites is July 1991 and February 1992, respectively.

E. 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

MAY BEGAN ENDED MAY BEGAN ENDED

STATION 1990 MJD (UTC) (UTC) FREQUENCY 1990 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:

D yUTC(NIST) 1987) yNBS-6 ( Ju ly 1987) = (-0.6 ±2(1 sigma)) x 10' 33

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” 3

. Using GPS 3

, the frequency of TAI for

the dates shown were measured to be:

2) yTAj (July 1987) - yNBS-6 ( 3u3 y 1987 on geoid) = (+1.7 ±2(1 sigma)) x 10' 33

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)' 3ast calibration (July 1987) covered the period

from October 1986 through July 1987.

3 ) yCs(NIST) “ YNBS-6 - ( +3>4 ± 2) x 10 13 (July 1987)

and

4) yTAI - ycs(NisT) on geoid = < +0 - 3 * °- 7 > x 30

' 13

< Ju3-y 3987 >

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 33 /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-4 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-4 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.

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

- 5 -
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 Jan 89 47527 0 1.50 ns/

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/d 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/d 24.045 124 078 s -4.75 E- 13

1 Jun 89 47678 0 -1.00 ns/d 24.045 125 375 s -4.92 E- 13

1 Jul 89 47708 0 -1.00 ns/d 24.045 126 670 s -5.09 E- 13

1 Aug 89 47739 0 -1.00 ns/d 24.045 128 060 s -5.35 E- 13

1 Sep 89 47770 0 -1.00 ns/d 24.045 129 538 s -5.58 E- 13

1 Oct 89 47800 0 1.00 ns/

24.045 131 001 s -5.68 E- 13

1 Nov 89 47831 0 1.00 ns/

24.045 132 534 s -5.85 E- 13

1 Dec 89 47861 0 1.00 ns/

24.045 134 082 s -6.05 E- 13

1 Jan 90 47892 0 1.00 ns/d 24.045 135 724 s -6.16 E- 13

1 Feb 90 47923 0 1.00 ns/d 24.045 137 382 s -6.21 E- 13

1 Mar 90 47951 0 1.00 ns/d 24.045 138 888 s -6.23 E-13

1 Apr 90 47982 0 1.00 ns/d 24.045 140 560 s -6.36 E- 13

1 May 90 48012 0 1.00 ns/

24.045 142 241 s -6.42 E-13

1 June 90 48043 0 1.00 ns/d 24.045 143 942 s -6.34 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-14. Otherwise, y[UTC(NIST)] is maintained as stable as possible.

REFERENCES

Allan, D W. , Hellwig, H. and Glaze, D.J., "An accuracy algorithm for an atomic time scale," Metrologia, Vol.ll,

No. 3, pp. 133-138 (September 1975).

Glaze, D.J., Hellwig, H. , Allan, D.W., and Jarvis, S., "NBS-4 and NBS-6: The NIST primary frequency

standards," Metrologia, Vol.13, pp. 17-28 (1977).

Wineland, D.J., Allan, D.W., Glaze, D.J., Hellwig, H. , and Jarvis, S., "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, 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 & Frequency using GPS signals," Proc. 36th Annual

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

-6 -
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.

The test phase for this service is now complete and NIST is committed to long-term operation of the

service. Additional lines will be added as use expands. NIST requests that calling times be spread out so that the system is not heavily taxed in some narrow time frame (e.g., midnight). The service telephone number

is (303) 494-4774. 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 5|-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.

- 7 -
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