Time & Frequency

Bulletin

No. 395 October 1990

(NISTIR 90-3940-10)

i

iI

I

NIST TIME & FREQUENCY BULLETIN

(Supersedes No. 394 September 1990)

NO. 395 OCTOBER 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 Organizat ion, 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 MS - microsecond

TA - Atomic Time ms millisecond

TAI - International Atomic Time s second

TAO - Tokyo Astronomical Observatory, Japan rain - 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, A, 6, and 8 (see references on page 6).

OOOO HOURS COORDINATED UNIVERSAL TIME

SEPTEMBER

1990 MJD

UT1 - UTC(NIST)

(± 5 ms)

UTC - UTC(NIST)

( + 0.2 us)

UTC ( USNO, MC) - UTC (NIST)

(t 0.04 us)

6 481A0 -133 ms -0.1 MS 0.21 us

13 48147 -146 ms -0.2 ms 0.14 ms

20 48154 -160 ms -0.2 ms 0.07 ms

27 48161 -173 ms -0.2 ms 0.01 ms

-

1 -
INTERNATIONAL TIMING CENTER COMPARISONS VIA GPS COMMON-VIEW

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

time Jifterence of Timing Center (i) - L'TC(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 accuracv 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 TAX and of UTC. All differential delays are 0 unless otherwise

noted

UTC ( i ) - UTC ( NIST ) (ns

)

MJD

UTC( i

> PRN NUMBERS 48129 48139 48149 48159

APL 3,6,9, 12,13,14, 16,17, 20 81 102 99 81

CRL 3,6,9 12 -608 -700 -795 -913

CSIR0++ 26825 26985 27068 27145

IEN 0 12,13 20 359 141 -61 -259

INPL VIA OP -3043 -1957 -840 290

NPL 3, 12,13 1167 908 746 690

NRC H > 2 ,3,6,9, 12,13,14, 16,17, 20 -1241 -1161 -1097 -1062

OP 2,3, 12,13,14, 16, 20 395 327 262 183

PTB 12, 16, 20 -3778 -3850 -3914 -3947

TAO 3,6,9, 12, 14, 17, 20 1513 1250 1014 762

TUG 2,3, 12,13,14, 16, 20 -1523 -1853 -2139 -2434

USN0 2, 6,9, 12,13,14, 16,17, 20 327 220 120 26

VSL 2,3, 12,13,14, 16, 20 -3553 -3745 -3881 -3938

++ UTC(CSIRO) - UTC(NIST) is computed from the average via CRL, TAO, & WWVH. +++ UTC(NRC) - UTC(NIST) has a differential delay of 41,.2 ns; all other comparisons are computed using zero

( 0 ).

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 = 0.0 s beginning 0000 UTC on 10 May 1990

DUT1 = UT1 - UTC = -0.1 s beginning 0000 UTC on 26 July 1990 * -0.2 s beginning 0000 UTC on 20 September 1990

= -0.3 s beginning 0000 UTC on 1 November 1990

SPECIAL ANNOUNCDIENT : 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 -
4. PHASE DEVIATIONS FOR WWVB AND LORAN-C

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

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

antenna. The uncertaintv of the individual measurements is t 0.5 us. The values listed are

for 1300 I’TC

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 tor a period of 2d 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 us)

DATE MJD

ANTENNA PHASE

( us ) LORAN-C (DANA)

(8970 M)

LORAN-C (FALLON)

(9940 M)

09/01/90 48135 5.63 +0.11 (-)

09/02/90 48136 5.63 -0. 12 -0.42

09/03/90 48137 5.63 -0.16 -0.01

09/04/90 48138 5.63 +0.04 -0.11

09/05/90 48139 5.69 +0.07 +0.16

09/06/90 48140 5.74 +0.02 -0.04

09/07/90 48141 5.73 +0.29 +0.37

09/08/90 48142 5.76 -0.04 +0.06

09/09/90 48143 5.79 (-) (-)

09/10/90 48144 5.83 -0.03 -0.11

09/11/90 48145 5.92 -0.03 +0.12

09/12/90 48146 5.94 (-) -0.10

09/13/90 48147 5.91 +0.07 +0.19

09/14/90 48148 5.99 (-) (-)

09/15/90 48149 5.98 +0.19 +0. 13

09/16/90 48150 5.97 -0.12 -0.18

09/17/90 48151 5.96 +0.07 +0.18

09/18/90 48152 5.95 -0.10 -0.01

09/19/90 48153 5.99 -0.01 +0.06

09/20/90 48154 5.96 -0.09 -0.22

09/21/90 48155 5.93 +0.34 +0.02

09/22/90 48156 5.95 -0.01 +0.18

09/23/90 48157 5.97 -0.23 -0.27

09/24/90 48158 5.99 -0. 19 -0.26

09/25/90 48159 5.86 +0.13 +0.23

09/26/90 48160 5.88 -0.04 -0.03

09/27/90 48161 5.86 +0.02 -0.13

09/28/90 48162 5.84 +0.15 +0.11

09/29/90 48163 5.78 +0.05 -0.08

09/30/90 48164 5.70 -0.21 -0.23

- 3 -
5. GOES TIME CODE INFORMATION

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

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

1) During eclipse periods from 0300-0500 UTC each dav the GOES/East time code was transferred to GOES-7.

The transmitted position data during these periods was approximately correct but may have produced

deviations exceeding the normal 100 microsecond limit;

During 0000-1600 UTC on 1 October, the Daylight Saving Time bits in the time code were erroneously

set to "standard" time, causing receivers to display time incorrect by one hour; and

3* During approximately 1300-1415 UTC on 2 October, an incorrect time code sync word was transmitted,

causing some receivers to decode incorrect time and position data.

GOES /West

Performance within normal limits during this period except for the following:

1) During 0000-1600 UTC on 1 October, the Daylight Saving Time bits in the time code were erroneously

set to "standard" time, causing receivers to display time incorrect by one hour; and

2) During approximately 1300-1415 UTC on 2 October, an incorrect time code sync word was transmitted,

causing some receivers to decode incorrect time and position data.

B. SPECIAL ANNOUNCEMENT: Some users have reported that receivers appeared to be locking to a spurious GOES

satellite signal at times, producing a delay different by several milliseconds from the usual values.

Through the efforts of Mr. Ron Henderson of State University of New York, who operates an extensive

network of 100 GOES receivers in a lightning-detection network, the source of these signals was traced to

a spurious emission from the GOES-7 satellite at 98° West longitude. Sidelobe radiation from the Wallops

Island GOES/West uplink antenna was apparently getting into the GOES-7 system and was being re-radiated.

The GOES-7 DCPI transponder has now been deactivated in order to eliminate this spurious emission.

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

longitude for GOES/West.

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 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 "@NBSGO" 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

STATION

SEPTEMBER

1990 MJD

BEGAN

(UTC)

ENDED

(UTC) FREQUENCY

SEPTEMBER BEGAN

1990 MJD (UTC)

ENDED

(UTC)

WWVB 3 48137 1745 1820 60 KHz NONE

wwv 16 48150 2210 2310 20 MHz NONE

20 48154 0210 0250 20 MHz

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:

1) y[JTC(NIST) ( 3ul Y 198/) — V[\iBs -5 (July 1987) = (-0.6 i 2 (1 sigma)) x 10 13

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

blackbodv radiation, which is approximately (delta y BB ) = -1.7 x 10~ 14

. Using GPS 1

, the frequency of TAI for

the dates shown were measured to be:

-) YtaI ( Jul >' 1987) - vNBS _ 6 (July 1987 on geoid) = ( + 1.7 ± 2 (1 sigma)) x 10' 13

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)’ The last calibration (July 1987) covered the period

from October 1986 through July 1987.

3 ) yCs(NIST) ” lfNBS-6 = ( +1 * 4 ± 2) x 10 13 (July 1987)

and

4 ) yTAI - ycs(NIST) on geoid = (+0 - 3 ± °- 7 ) x 10 13 (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 13 /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 '.1 is a iisr of changes in the time scale frequencies of both TA(N’IST) and UTC(NIST) as well as a list of tne time and frequency differences between TA(N'IST) and UTC(NIST) at the dates of leap seconds,

ana/ or frequency or frequency drift changes.

TABLE 7.

FREQUENCY CHANCES

DAT! ( MJD

)

TA(NIST) UTC (nist; TA(NIST) - UTC(NIST) v ( UTC ( NIST ) ] -- yf TACNIST)

]

1 Apr 39 47617 0 -1.50 ns/d 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

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

1 Jui 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/d 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/d 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/

d

25.045 138 CO 00 00 s -6.23 E-13

1 Apr 90 47982 0 1.00 ns/

d

25.045 140 560 s -6.36 E-13

1 May 90 48012 0 1.00 ns/d 25.045 142 241 s -6.42 E-13

1 Junes 90 48043 0 1.00 ns/

d

25.045 143 942 s -6.34 E-13

1 July- 90 48073 0 0.50 ns/d 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

1 Sep 90 48135 0 -1.00 ns/

d

25.045 149 130 s -6.69 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

A I 'TOMATED COMPUTER TIME SERVICE (ACTS)

NIST initiated operation of a telephone time service designed to provide computers with

Cn

t accuracies approaching 1 ms. features of the service include automated March 9, 1988,

telephone access to alert for changes to and from daylight savings time and

compensation to ^ leap seconds. The ASCI I -character time code should operate with standard

While the svstem can be used to set computer time-of-day clocks, simple

nardware can also be developed to set other clock systems.

advanced notice of insertion ol morions and most computer 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) 494-4774. 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.

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: IEEE-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.
AiPOAT MO.

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