Time & Frequency

Bulletin

No. 393 August 1990

(NISTIR 90-3940-8)

NIST TIME h FREQUENCY BULLETIN

(Supersedes No. 392 July 1990)

NO. 393 AUGUST 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 WVVB 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 - dav

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(l'SNO,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

JULY

1990

UT1 - UTC(NIST)

MJD ( ± 5 ms

)

UTC - UTC(NIST)

(± 0.2 us)

UTC ( USNO, MC) - UTC (NIST)

(

t

0.04 us )

5 48077 -44 ms 0.3 us 0.89 us

12 48084 -51 ms 0.3 us 0.81 us

19 48091 -61 ms 0.3 us 0.75 us

26 48098 -69 ms 0.3 us 0.68 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(N'IST) 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 L'TC for the particular MJD ending in 9. These data are prepared for the 8IPM for the computation on TAI and of L'TC. All differential delavs are 0 unless otherwise

noted

.

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

LTC ( i

)

PRN NUMBERS -.8069 -.8079 u8089 48099

APL 3,6,9, 12,13 -23 - 57 -80 -6

CRL 6,9 12 190 90 -39 -207

CSIRO ++ 25610 25806 25912 26076

IEN 12,13 1232 1170 1093 974

INPL VIA OP -181813 -184271? f ?

NPL 12,13 2664 2447 2174 1920

NRC+++ 6,9, 12,13 -799+ -957 -1082 -1228

OP 12,13 16 836 731 637 577

PTB 12, 16, 20 -3353 -3407 -3466 -3540

TAO 6,9, 12 3146 2894 2624 2322

TUG 12,13 357 44 -219 -548

USNO.MC 3,6,9, 12,13 16,17, 20 961 868 771 673

VSL 12,13 -2518 -2574 -2721 -2939

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

( 0 ). 0 The value for UTC(INPL) for MJD 48079 was computed by extrapolating backwards 9.5 hours from weighted and

unfiltered data for MJDs 48079-48084.

! No data is available for UTC(INPL) for MJDs 48089 and 48099.

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

LTC(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 L'TC 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 ir. "re 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 am

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

.

= +0.1 s beginning 0000 UTC on 12 April 1990

DUT1 = UT1 - UTC = 0.0 s beginning 0000 UTC on 10 May 1990

= -0.1 s beginning 0000 UTC on 26 July 1990

SPECIAL ANNOUNCEMHIT : 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.
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 us. The values listed are

for 1300 U'TC .

*

LORAN-C - The values shown for Loran-C represent the daiiv 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 pr inted

.

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 (

N

1ST) - WWYB< 60 kHz

)

UTC (NIST) - LORAN PHASE ( Ln us J

DATE MJD

ANTENNA PHASE

( us ) LORAN-C (DANA

( 8970 M )

* LORAN-C (FALLON)

( 9940 M )

07/01/90 48 O 7 3 5.69 -0.48 -0.28

07/02/90 480 7 4 5 . 69 - 0.00 - 0.10

07/03/90 48075 5.71 *0.17 +0.24

07/04/90 480 76 5 . 66 - 0.22 -0.42

07/05/90 48077 5.61 - 0.00 +0.C9

07/06/90 48078 5.72 - 0 . 10 +0.02

07/07/90 48079 5.73 -0.03 -0.C3

07/08/90 48080 5.74 -0.26 ( -

)

07/09/90 u8081 5.75 - 0.02 (-)

07/10/90 48082 5.76 +0.15 -0.39

07/11/90 48083 5.77 +0.19 +0.11

07/12/90 -*8084 5.78 -0.09 -0.31

07/13/90 48085 5.80 - 0.01 +0.22

07/14/90 48086 5.74 +0.10 (-)

07/15/90 48087 5.70 - 0.02 (-)

07/16/90 48088 5.66 +0 . 48 +0.50

07/17/90 48089 5.67 - 0.22 +0.09

07/18/90 48090 5.67 +0.11 +0.16

07/19/90 48091 5.67 +0.05 +0.14

07/20/90 48092 5.65 +0.52 +0.30

07/21/90 48093 5.68 +0 . 64 +0.61

07/22/90 48094 5.70 -0.35 -0.32

07/23/90 48095 5.73 +0.25 -0.16

07/24/90 48096 5.69 +0.05 -0.07

07/25/90 48097 5.71 +0.30 +0.15

07/26/90 48098 5.72 (-) +0.04

07/27/90 48099 5.72 (-) +0.17

07/28/90 48100 5.68 (-) (-)

07/29/90 48101 5.66 (-) (-)

07/30/90 48102 5.64 -0.63 +0.08

07/31/90 48103 5.64 -0.31 -0.25
5. GOES TIME CODE INFORMATION

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

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

code was transferred from GOES-5 to the GOES-2 spare satellite, located at 60° W. longitude, at luOO L'TC on 9 July. During the transition period from about 1330-1420 L'TC, the GOES/East time code (when corrected for

satellite position) exceeded the normal 100 microsecond limits. See the special announcements below.

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 L'TC 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. IMPORTANT SPECIAL ANNOUNCEMENT - As previously announced, NOAA shifted the GOES/East time code operations from the 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 '!> 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.

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

satellites is July 1991 and February L992, respectively.

F. GOES STATUS REPORTS

A brief message from the NIST giving current GOES time code status information is available from the L'.S. Naval Observatory's Automated Data Service computer system in Washington, DC. The message mav 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 ,.2"'0 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 "?NBSG0" 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

JUNE

1990

BEGAN

MJD (UTC)

ENDED

(UTC) FREQUENCY

JUNE BEGAN ENDED

1990 MJD (UTC) (UTC)

WWVB NONE NONE

WWV NONE NONE

WWVH 29 48071 0258:00 0308:00 2.5,5,10,15 NONE
7. NOTES ON NIST TIKE SCALES AND PRIMARY STANDARDS

The frequencies of the tine scales, TA(NIST) and L'TC(NTST), are calibrated with the MIST primarv frequencv

standards. The L'TC(.NTST) scale is coordinated within a microsecond of- the internationally coordinated t me

scale, UTC, generated at the BIH. it is used to control all of the NIST tine and frequencv services. The Last calibration of the relative frequency offset, y, of (TCIS'IST) as generated in Boulder, Colorado, gave:

i ) -''UTC (NIST) lJul Y 1987 ) -'NBS-6 ( ^ u ^- v ''98') = (-0.6 t 2 (i sigma)) x LO*' 8

for the date shown. This calibration includes a correction for the systematic offset due to room t emperar ure black body' radiation, which is approx ina t e ly (delta '•'gg) - *1.' x 10 L

. ’.sing G PS ‘

, 'he frequencv of 7A1 for

the dates shown were measured to be:

2 ) >’ TA j (July 1987) — y'\gg - 5 (July 1987 on geoid) = (+1.7 t 2 (1 siena)) x 10 ^ 8

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

associated with this "best estimate" is denoted Ycs(NIST)' The *- ast calibration (July 1987) covered me ner:ou

from October 1986 through July 1987.

3) Ycs( NIST) —

YyjBS -6 — ( + 1 • 9 t 2 ) x 10 ^ (July 1987)

and

9) XrAI “ YCs(.NIST) on geoid = ' +0 • 3 t °- 7) x 10 13 (JuL -v i987 )

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

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

frequency memory of the clock ensemble that generates atomic time. This algorithm, therefore, capitalizes on. i 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 v"r s i yyyT 1 by slight frequency drift corrections of the order of 1 part in 10 ^/vr.

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 xeep its time within a microsecond of UTC(BIH).

Table 7.1 lists monthly values of the time difference between t'TC(NIST) and TA(.NIST). A linear interpo lat ion

between monthly values will typically be within 10 ns of the actual time difference, TA(NIST) - L'TC(NIST).

The primary standards of NIST (NBS-4 and NBS- 6 ) are used in either of two modes: as calibrators of frequencv r j 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 usee as frequency calibrators, in which case dock operation is necessarily interrupted.

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

-d-
Table 7.1 is a List of changes in the tune scale frequencies of both TA(NIST) and LTC(NIST) as well is 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 .

L

FREQUENCY CHANGES

DATE ( M JD ) T.A(NIST) UTC( NIST) TA( NIST) - l'TC(NIST) y [ l'TC( NIST) J -- v [ T.A( N 1ST )

L Feb 89 -.7553 0 l .00 ns/d 24 . 04 5 120 538 s “ 4 .,51 E- I 3

1 Mar 89 -.7586 0 -l . 25 n s / d

n / .045 121 622 s - 4 ., 58 E-13

1 Apr 89 •4/617 0 -1 . 50 ns / d 24 . 045 [ 1 2 871 s -4.,66 E-L3

1 May 89 4 7647 0 -1 . 50 ns/ d 24 .045 124 078 s - U ., 75 E-13

1 Jun 89 47673 0 -1 .00 ns/d 24 .045 125 375 s -4 ,. 92 E-13

1 Ju 1 89 •4 / / 0 8 0 -1 .00 ns/d 24 ,. 045 126 670 s - 5 .09 E-13

1 Aug 89 47739 0 -1 .00 ns/d ) ^ .045 128 060 s - 5

,

. 35 E-13

L Sep 89 47770 0 -1 .00 ns/d 24,.045 129 538 s - 5

.

. 58 E-13

1 Oct 89 4 7800 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 oo oo oo s -6..23 E-13

1 Apr 90 -.7982 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 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/d 25. 045 145 580 s -6. 46 E-13

UTC( N1 1ST

)

L S steered in time toward UTC by occasional frequency changes of the order of a few nanosecc"

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 listeu

of TA(N'IST) - 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," Metroiogia.

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 frequence

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

W'ineland, D.J., Allan, D.W., Glaze, D.J., Hellwig, H. , and Jarvis, S., "Results on limitations in pr.-a

cesium standard operation," IEEE Trans, on Instr. and Meas., Vol.IM-25, No. 4, pp. 453-458 (December

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 i ACTS )

On March 9, L988, NIST initiated operation of a telephone tine service designed to provide computers - i * telephone access to NIST time at accuracies approaching 1 ms. Features of the service include automated

compensation for te iephone - i ine delay, advanced alert for changes to and from daylight savings time and

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

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

hardware can also be developed to set other ciock 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 <.> that the system is not heavily taxed in some narrow rime frame te.g., midnight). The service telephone :mbe

is (303) 494-4774. The number may be changed at a later date. A help message can be obtained bv returning i during the first 6 s of transmission.

With appropriate user software, the NIST-ACTS service provides three modes for checking and/or set- .n.g computer tiine-of-day clocks.

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

marker /character which has been advanced a fixed period to nominally account for modem and telephone- I: ne delays. Accuracy in this mode should be no worse than 0.1 s unless tne connection is routed through a satellite

.

2. At 1200 Baud, if the user's system echoes ail characters to NIST, the round-trip line deiav wii. be measured and the on-time marker advanced to compensate for that deiav. 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 th

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 20

diskette with instructions for $35.00 from the NIST Office of Standard Reference Materials, B3 l

1 -Chemistry

Bldg, NIST, Gaithersburg, MD, 20899, (301) 975-6776. Specify the Automated Computer Time Service, RM8L01.

Further technical questions and comments should be directed to NIST-ACTS, NIST Time and Frequency Division, i Broadway, Boulder, CO 80303.
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