Time & Freauency

Bulletin

No. 394 September 1990

(NISTIR 90-3940-9)

NIST TIME & FREQUENCY BULLETIN

(Supersedes No. 393 August 1990)

NO. 394 SEPTEMBER 1990

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

V

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

BIH

CCIR

CRL

Cs

CSIRO

GOES

GPS

lEN

INPL

LORAN

MC

MJD

NIST

NPL

NRG

NOAA

OP

PTB

SI

SV

TA

TAI

TAO

TUG

USNO

UTC

VLF

VSL

John Hopkins Universitv Applied Physics Laboratory

International Time Bureau, France

International Radio Consultative Committee

Communications Research Laboratories, Japan

Cesium standard

Commonwealth Scientific and Industrial Research Org

Geostationary Operational Environmental Satellite

Global Positioning System

National Institute of Electronics, Italy

National Physical Laboratory, Israel

Long Range Navigation

Master Clock

Modified Julian Date

National Institute of Standards & Technology

National Physical Laboratory, England

National Research Council, Canada

National Oceanic and Atmospheric Administration

Paris Observatory, France

Physical Technical Federal Laboratory, Germany

International System of Units

Space vehicle

Atomic Time

International Atomic Time

Tokyo Astronomical Observatory, Japan

Technical University of Graz, Austria

United States Naval Observatory

Coordinated Universal Time

very low frequency

Van Swinden Laboratory, Netherlands

anization, Australia

ns nanosecond

us - microsecond

ms millisecond

s second

min - minute

h hour

d day

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

1990 MJD

UTl - UTC(NIST)

(± 5 ms)

UTC - UTC(NIST)

(± 0.2 us)

UTC(USNO.MC) - UTC(NIST)

(± 0.04 us)

2 48105 -77 ms 0.3 us 0.60 MS

9 48112 -86 ms 0.3 us 0.52 MS

16 48119 -98 ms 0.2 us 0.44 MS

23 48126 -110 ms 0.2 us 0.35 MS

30 48133 -121 ms 0.2 us 0.29 MS

- 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 Centet (i) - UTC(NIST) by the simultaneous common-view approach (see references, page

6). The dav-co-dav 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

noted

BIPM for the computation on TAI and of UTC. All differential delays are 0 unless otherwise

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

UTC( i

)

PRN NUMBERS 48099 48109 48119 48129

APL 3.6,9, 12,13.14. 16,17, 20 -6 29 51 81

CRL 6,9 12 -207 -323 -503 -608

CSIRO ++ 26076 26343 26590 26825

lEN 12,13 20 974 811 579 359

NPL 2, 12,13 1920 1608 1396 1167

NRC+++ 2, 6,9, 12,13,14, 16,17, 20 -1228 -1385* -1316 -1241

OP 12,13,14, 16, 20 577 517 462 395

PTB 12, 16, 20 -3540 -3625 -3701 -3778

TAO 6,9, 12, 17, 20 2322 2041 1742 1513

TUG 2, 12,13,14, 16, 20 -548 -866 -1203 -1523

USNO.MC 3,6,9, 12,13 16,17, 20 673 563 444 327

VSL 2, 12,13,14, 16, 20 -2939 -3196 -3390 -3553

+ These values have been updated from those printed in last month's Bulletin. ++ UTC(CSIRO) - UTC(NIST) is computed from the average via CRL, TAO, & WWVH.

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

( 0 ). * NRC experienced a frequency shift of approximately +28.0 E-14 (NET) between MJD's 48110 - 48114.

Therefore, the value for MJD 48109 was computed from weighted and unfiltered data.

3. uri CORRBCnOilS 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 Jiine 1972, 31 December 1972-1979, 30 Jvme 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 UTl — UTC will always be held within i 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.

DUTl » UTl - UTC » 0.0 s beginning 0000 UTC on 10 May 1990

DUTl » UTl - UTC « -0.1 s beginning 0000 UTC on 26 July 1990 » -0.2 s beginning 0000 UTC on 20 September 1990

SPECIAL AIQIOUNCEMENr: The International Earth Rotation Service has announced that a leap second will be

inserted into the UTC time scale at the end of December 31, 1990.

- 2-
A. PHASE DEVIATIONS FOR WWVB AND LOKAN-C

WW\'B - The values shown K^r WWVh am the time difference between the time markers of the UTC(N1STI

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

antenna. The uncertuint\' of the individual measurements is f 0.5 us. The values listed are

for IJOO UTC.

LORAN-C - The values shown lor Loran-i represent the daiiv accumulated phase shift (in microseconds).

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

scale for a period of TA hours. 11 data were not recorded on a particular dav , the symbol

i, - ) is printed .

The stations monitored are Dana, Indiana (8970 M) and Fallon, Nevada (99A0 M). The

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

DATE

UTC(NIST) - WWVB(60 kHz)

ANTENNA PHASE

MJD (us)

DTC(NIST) - LORAN

LORAN-C (DANA)

(8970 M)

PHASE (in us

)

LORAN-C (FALLON)

(9940 M)

08/01/90 48104 5.64 +0.00 -0.19

08/02/90 48105 5.75 -0.29 -0.36

08/03/90 48106 5.74 (-) +0.23

08/04/90 48107 5.73 (-) +0.08

08/05/90 48108 5.71 ( - ) -0.09

08/06/90 48109 5.70 -0.09 -0.18

08/07/90 48110 5.70 -0.01 -0.08

08/08/90 48111 5.70 -0.04 -0.17

08/09/90 48112 5.73 +0.02 +0.04

08/10/90 48113 5.72 +0.12 +0.24

08/11/90 48114 5.73 +0.07 (-)

08/12/90 48115 5.74 -0.03 (-)

08/13/90 48116 5.74 -0.05 +0.02

08/14/90 48117 5.76 +0.02 +0.07

08/15/90 48118 5.75 -0.09 +0.07

08/16/90 48119 5.78 (-) (-)

08/17/90 48120 5.68 +0.17 +0.27

08/18/90 48121 5.66 +0.11 -0.01

08/19/90 48122 5.64 -0.31 -0.21

08/20/90 48123 5.62 -0.22 -0.15

08/21/90 48124 5.67 +0.24 +0.08

08/22/90 48125 5.64 -0.20 +0.06

08/23/90 48126 5.73 +0.02 +0.22

08/24/90 48127 5.73 (-) (-)

08/25/90 48128 5.74 +0.16 +0.05

08/26/90 48129 5.75 -0.21 -0.50

08/27/90 48130 5.77 -0.24 -0.14

08/28/90 48131 5.76 -0.03 -0.17

08/29/90 48132 5.75 -0.07 -0.08

08/30/90 48133 5.78 -0.07 -0.01

08/31/90 48134 5.66 (-) (-)
5. GOES TIME CODE INFORMATION

A. TIME CODE PERFORMANCE (1-31 August 1990)

GOES/East : Performance within normal limits during this period except for the following: During eclipse

periods from 0300-0500 UTC each day the GOES/East time code is transferred to GOES-7. The transmitted position

data during these periods may produce deviations exceeding the normal 100 microsecond limit.

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. FALL 1990 ECLIPSE OPERATIONS FOR GOES/EAST;

Due to the current use of an older satellite, GOES-2, as GOES/East, the GOES/East time code will have

to be shifted from GOES-2 to GOES-7 temporarily for two hours each day during the Eclipse season. Beginning on 7 August, the GOES/East time code will be shifted to GOES-7 at 98° West longitude from

0300-0500 UTC each day until 29 September. Satellite position data transmitted from GOES/East during

these periods will be approximately correct for GOES-7 and should not cause large errors in receivers

that correct for satellite position. Some users may experience loss of signal during these periods,

however, unless antennas are repointed appropriately.

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

satellites has been moved back to February 1992 at the earliest.

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 stsidard 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 "(^NBSCTO" 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 UWVB PHASE PEBTOSBATIGHS

OUTAGES PHASE PERTURBATIONS WVfVB 60 kHz

JULY BEGAN ENDED JULY 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 cf the time .scales, TA(NIST) and UTC(NIST), are calibrated with the NIST primary frequency

standards. The L'TClNISTl scale is coordinated within a microsecond of the internationallv 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:

>'LTC(NIST) 1987) yNBS-6 (J 1987) (-0.6 ±2(1 sigma)) x lO"'-^

for the date shown,

blackbody radiation,

the dates shown were

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

which is approximately (delta ygg) = -1.7 x

10'^^^. Using GPS^, the frequency of TAI for

measured to be:

2) yjAI 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 “ (+l-‘^ ± 2) x 10 (July 1987)

and

yiAI “ yCs(NIST) on geoid = ±0.7) x 10 (July 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-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.

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

- 5 -
Tahio 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 tne time and frequency differences between TA(NIST) and UTC(NIST) at the dates of leap seconds,

and/or frequency or frequencv drift changes.

table 7.1

FREQUENCY CHANGES

DATE (MJD) TA(NIST) UTC(NIST) TA(NIST1 - UTC(NIST) y[UTC(NIST)] - y[TA(NIST)]

1 Mar 89 47 586 0 -1.25 ns/d 24.045 121 622 s -4.58 E-13

1 Apr 89 47617 0 -1.50 ns/d 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/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/

24.045 128 060 s -5.35 E-13

1 Sep 89 47770 0 -1.00 ns/

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/d 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 25.045 135 724 s -6.16 E-13

1 Feb 90 47923 0 1.00 ns/d 25.045 137 382

*

s -6.21 E-13

1 Mar 90 47951 0 1.00 ns/

25.045 138 888

*

s -6.23 E-13

1 Apr 90 47982 0 1.00 ns/

25.045 140 560 s* -6.36 E-13

1 May 90 48012 0 1.00 ns/

25.045 142 241 s* -6.42 E-13

1 June 90 48043 0 1.00 ns/d 25.045 143 942 s* -6.34 E-13

1 July' 90 48073 0 0.50 ns/

25.045 145 580 s* -6.46 E-13

1 Aug 90 48104 0 0.00 ns/d 25.045 147 350 s -6.62 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.

"Due to the omission of the Dec. 31, 1989 Leap Second there has been an error of 1 second in the listed values

of TA(NIST) - UTC(NIST) in Bulletins No. 387 through No. 392, page 6.

REFERENCES

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

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.l3, 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 ANNOUNCatBn’S

AUTOMATED COMPUTER TIME SERVICE ( ACTS

Cn March 9, 1988, NIST iniLiaLed operation of a telephone time service designed to proviae computers with

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

compensation tor te lephone * i ine delay, advanced alert tor changes to and from daylight savings time and

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

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

nardware can aiSO be developed to set otner clock svstems.

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 -m77m. 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- 1 ine

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, B3 11 -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.

FREQUENCY CONTROL SYMPOSIUM ANNOUNCEMENT

The 45th Annual Frequency Control Symposium will be held May 29-31, 1991 in Los Angeles, CA. This

symposium is the leading technical conference addressing all aspects of frequency control and precision

timekeeping. Authors are invited to submit papers dealing with recent progress in research, development and

applications in areas represented by the following topics:

• Fundamental properties of piezoelectric crystals

• Theory and design of piezoelectric resonators

• Resonator processing techniques

• Surface acoustic wave devices (SAW)

• Quartz crystal oscillators

• Microwave and millimeter wave oscillators

• Signal processing and frequency control circuitry

• Atomic and molecular frequency standards

• Frequency and time coordination and distribution

• Sensors and transducers

• Applications of frequency control

• Measurement and specifications

Sponsors: lEEE-UFFC and the US Army Electronics Technology and Devices Laboratory (LABCOM).

Place: Los Angeles Airport Marriott, CA

Contact: Clark Wardrip, P. 0. Box 6147, Vandenberg AFB, CA 93437; (805) 865-3214.
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Boulder, Colorado 80303

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