NAT'L INST. OF STAND & TECH

A111D7

NBS

PUBLICATIONS

U.S. DEPARTMENT OF COMMERCE • NATIONAL BUREAU OF STANDARDS

1976
A History of the NBS Time and Frequency

Dissemination Services

Mar. 1923 First scheduled broadcasts of WWV, Washington, DC

Apr. 1933 WWV gets first 20 kW transmitter, Beltsville, MD

Jan. 1943 WWV relocated to Greenbelt, MD

Nov. 1948 WWVH commenced broadcasts, Maui, HI

Jan. 1950 WWV added voice announcements

Jul. 1956 WWVB (KK2XEI) began 60 kHz broadcasts. Boulder, CO

Apr. 1960 WWVL began 20 kHz experimental broadcasts. Sunset, CO

Jul. 1963 WWVB began high power broadcasts. Ft. Collins, CO

Aug. 1963 WWVL began high power broadcasts, Ft. Collins, CO

Jul. 1964 WWVH added voice announcements

Dec. 1966 WWV relocated to Ft. Collins, CO

Jul. 1971 WWVH relocated to Kauai, HI

Jun. 1972 First "leap second" in history added to UTC time scale

Jul. 1972 WWVL transmissions curtailed

Jan. 1974 Voice announcements changed from Greenwich Mean Time to Coordinated Universal Time

(WWV and WWVH)

Mar. 1975 Frequency calibration using network color TV became a nationvdde service

Aug. 1975 Line- 10 time comparisons using TV synchronization pulses became a nationwide service
NBS

TIME AND FREQUENCY

DISSEMINATION SERVICES

Sandra L. Howe, Editor

Time and Frequency Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

UBIUBT

JUNiJ 1976

(Supersedes NBS Special Publication 236, 1974 Edition)

U.S. DEPARTMENT OF COMMERCE, Rogers C. B. Morton, Secretary

James A. Baker, III, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued January 1 976
National Bureau of Standards Special Publication 432

(Supersedes NBS Special Publication 236, 1974 and previous editions)

Nat. Bur. Stand. (U.S.), Spec. Publ. 432, 20 pages (Jan. 1976) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON, D.C. 1976

For sale by the Superinlendent of Documents. U.S. Government Printing Office, Wasliington, D.C. 20402

(Order by SD Catalog No. C 13. HI -.432)
FOREWORD

This publication presents a description of the time and frequency dissemination services of the National Bureau of

Standards. Those interested in information on the NBS atomic clock system, transmitting antennas, or discussion of

technological applications of the dissemination services should refer to Section 8, Other Publications

.

This SPECIAL PUBLICATION 432 replaces former Special Publication 236. It will be revised and reissued

only as necessary to update information.

iii
Contents

Page

Introduction 1

1 . WWV and WWVH 1 la. Accuracy and Stability 1 lb. Radiated Power, Antennas and Modulation 3

Ic. Time Announcements 3

Id. Standard Time Intervals 3

le. Standard Audio Frequencies 3

If. Official Announcements 3 Propagation Forecasts 5 Geophysical Alerts 5 Marine Storm Warnings 5

Ig. "Silent" Periods 6

Ih. BCD Time Code 6

li. UTl Time Corrections 6

2. WWVB 6

2a. Accuracy and Stability 6

2b. Station Identification 6

2c. Radiated Power, Antenna, and Coverage 6

2d. BCD Time Code 6

3. WWVL 7

4. Summary of Broadcast Services 7

5 . How NBS Controls the Transmitted Frequencies 9

6. Frequency Calibration Service Using Network Television 9

6a. Color Bar Comparator Method 10

6b. Digital Offset Computer Method 10

7. Time Comparisons Using Television Synchronization Pulses 10

8. Other Publications 11

Appendix 12

lA. Dating of Events in the Vicinity of Leap Seconds 13 2A. WWV/WWVH Time Code 13 3A. WWVB Time Code 15

iv
NBS Time and Frequency Dissemination Services

Sandra L. Howe

Detailed descriptions are given of the time and frequency dissemination services of the National Bureau of

Standards (NBS.) These services include the broadcasts from radio stations WWV, WWVH, WWVB, and WWVL (on an intermittent basis), and new time and frequency calibration services using television. This

publication shows the services available on January 1, 1976. It will be updated only when the services are re- vised or when new services are added. A list of other publications available from the Time and Frequency

Division of NBS is also included

.

Key words: Broadcast of standard frequencies; frequency cahbration; high frequency; low frequency; standard

frequencies; television color subcarrier; time calibration; time signals.

Introduction

The time and frequency community is a small community, generally unknown to the world at large, yet

vitally important to many of the basic activities of every- day living. Electric power companies, radio and tele- vision stations, telephone companies, and navigators of

ships and planes all depend heavily on precise frequency

and time information. They must have a constantly

available source —a reliable, nationally and internationally recognized standard —with which to compare and

regulate their own timing equipment. For over 50 years, the National Bureau of Standards (NBS) has been pro- viding this standard for most users in the United States.* Since the inception of the broadcast services from radio

station WWV in 1923, NBS has continually improved

and expanded its time and frequency dissemination

services to meet the ever growing needs of an ever widening community of users. Today, still striving for

better ways to serve its public, NBS is making major

contributions to the nation's space and defense programs,

to worldwide transportation and communications, and to a multitude of industrial operations, as well as pro- viding convenient, highly accurate time service to many

thousands of users throughout the world. Services are presently available from stations WWV and WWVB in Fort Collins, Colorado, and from WWVH in Kauai,

Hawaii. In addition, new calibration services using net- work television are also available . This booklet is offered

as a guide to these services.

1. WWV and WWVH

NBS broadcasts continuous signals from its highfrequency radio stations WWV and WWVH. The radio frequencies used are 2.5, 5, 10, 15, and 20 MHz. WWV

also broadcasts on an additional frequency of 25 MHz.

The U.S. Naval Observatory provides time standards for tlie Department of Defense and

other interested users.

All frequencies carry the same program, but because

of changes in ionospheric conditions, which sometimes

adversely affect the signal transmissions, most receivers

are not able to pick up the signal on all frequencies at

all times in all locations. Except during times of severe magnetic disturbances, however—which make all radio

transmissions almost impossible —listeners should be

able to receive the signal on at least one of the broadcast frequencies. As a general rule, frequencies above

10 MHz provide the best daytime reception while the lower frequencies are best for nighttime reception.

Services provided by these stations include:

Time announcements

Standard time intervals Standard frequencies

Propagation forecasts Geophysical alerts Marine storm warnings

UTl time corrections

BCD time code

Figure 1 gives the hourly broadcast schedules of these

services along with station location, radiated power, and

details of the modulation.

la. Accuracy and Stability

The time and frequency broadcasts are controlled by

the primary NBS Frequency Standard in Boulder,

Colorado. The frequencies as transmitted are accurate to within one part in 100 billion at all times. Deviations

are normally less than one part in 1 ,000 billion from day

to day. However, changes in the propagation medium

(causing Doppler effect, diurnal shifts, etc.) result in fluctuations in the carrier frequencies as received by the user that may be very much greater than the uncertainty

described above.

1
WWV BROADCAST FORMAT

VIA TELEPHONE: (303) 499-7111

(NOT A TOLL-FREE NUMBER)

U S DEPARTMENT OF COMMERCE

National Bureau of Standards STATION ID 440 Hz 1H0UR MARK

NBS RESERVED

STORM INFORMATION

PROPAGATION

FORECASTS

STATION ID 30 MINUTES

GEO ALERTS

• BEGINNING OF EACH HOUR IS IDENTIFIED BY 0.8-SECOND LONG, ISDO-Hz TONE. • BEGINNING OF EACH MINUTE IS IDENTIFIED BY

0.8-SECOND LONG. 1000-Hz TONE. • THE 29th & 59th SECOND PULSE OF EACH MINUTE IS OMITTED WWVH BROADCAST FORMAT VIA TELEPHONE: (808) 335-4363 (NOT A TOLL-FREE NUMBER)

STATION ID MINUTES 440 Hz 1-HOUR MARK

NBS RESERVED

STORM INFORMATION

GEO ALERTS

NO AUDIO TONE

V75

STATION ID

• BEGINNING OF EACH HOUR IS IDENTIFIED BY 0.8-SECOND LONG, 1500-Hz TONE. • BEGINNING OF EACH MINUTE IS IDENTIFIED BY 0.8-SECOND LONG, 120a Hz TONE. • THE 29th 8> 59th SECOND PULSE OF EACH MINUTE IS OMITTED.

Figure 1 . The hourly broadcast schedules of WWV and WWVH.
lb. Radiated Power, Antennas and Modulation

Freauenc V MHz

Radiated Power, kW

wwv WWVH

2.5 2.5 5.0

J .u 10.0 10.0

10.0 10.0 10.0

15.0 10.0 10.0

20.0 2.5 2.5

25.0 2.5

The broadcasts on 5, 10, 15, and 20 MHz from

WWVH are from phased vertical half-wave dipole

arrays. They are designed and oriented to radiate a

cardioid pattern directing maximum gain in a westerly

direction. The 2.5 MHz antenna at WWVH and all antennas at WWV are half-wave dipoles that radiate omnidirectional patterns. At both WWV and WWVH, double sideband amplitude modulation is employed with 50 percent modulation on the steady tones, 25 percent for the BCD time code,

100 percent for seconds pulses, and 75 percent for voice.

Ic. Time Announcements

Voice announcements are made from WWV and

WWVH once every minute. To avoid confusion, a man's

voice is used on WWV and a woman' s voice on WWVH

The WWVH announcement occurs first —at 15 seconds

before the minute —while the WWV announcement

occurs at IV2 seconds before the minute. Though the announcements occur at different times, the tone markers referred to are transmitted simultaneously from

both stations. However, they may not be received at the same time due to propagation effects. The time referred to in the announcements is "Coordinated Universal Time" (UTC). It is coordinated

through international agreements by the International Time Bureau (BIH) so that time signals broadcast from

the many stations such as WWV throughout the world

will be in close agreement.

The specific hour and minute mentioned is actually the time at the time zone centered around Greenwich, England, and may be considered generally equivalent to the more well-known "Greenwich Mean Time" (GMT).

UTC time differs from your local time only by an integral

number of hours. By knowing your own local time zone and using the chart of world time zones in figure 3, the appropriate number of hours to add or subtract from UTC

to obtain local time can be determined. The UTC time

announcements are expressed in the 24-hour clock system —i.e., the hours are numbered beginning with 00 hours

at midnight through 12 hours at noon to 23 hours, 59

minutes just before the next midnight.

Id. Standard Time Intervals

The most frequent sounds heard on WWV and WWVH

are the pulses that mark the seconds of each minute,

except for the 29th and 59th seconds pulses which are omitted completely. The first pulse of every hour is an

800-millisecond pulse of 1 500 Hz . The first pulse of every minute is an 800-millisecond pulse of 1000 Hz at WWV

and 1200 Hz at WWVH. The remaining seconds pulses

are brief audio bursts (5-millisecond pulses of 1000 Hz

at WWV and 1200 Hz at WWVH) that resemble the ticking of a clock. All pulses commence at the beginning

of each second. They are given by means of doublesideband amplitude modulation.

Each seconds pulse is preceded by 10 milliseconds of

silence and followed by 25 milliseconds of silence to avoid interference which might make it difficult or impossible to pick out the seconds pulses. This total 40-

millisecond protected zone around each seconds pulse is illustrated in figure 2.

TONE- _0.010

SEC 0.025

SEC

-TONE

1000 Hz

WWV

1200 Hz

WWVH

Figure 2. Format of WWV and WWVH seconds pulses.

le. Standard Audio Frequencies

In alternate minutes during most of each hour, 500 or 600 Hz audio tones are broadcast. A 440 Hz tone, the musical note A above middle C, is broadcast once each

hour. In addition to being a musical standard, the 440 Hz

tone can be used to provide an hourly marker for chart recorders or other automated devices.

If. Official Announcements

Forty-five-second announcement segments (see fig. 1) are available on a subscription basis to other Federal

agencies to disseminate official and public service in- formation. The accuracy and content of these announce- ments are the responsibility of the originating agency, not necessarily NBS.

3
Figure 3. Standard time zones of the world and their relationship to UTC.

4
Most segments except those reserved for NBS use and

the semi-silent periods (see section Ig) are available. Arrangements for use of segments may be made through

the Time and Frequency Services Section, 277.06,

National Bureau of Standards, Boulder, CO 80302.

Propagation Forecasts

The propagation forecasts are given in voice at 14 minutes after each hour from WWV only . They are short- term forecasts of propagation conditions along North

Atlantic paths (such as Washington, D.C. to London or New York to Berlin) along with descriptions of current geomagnetic activity, K-index values (a measure of the

earth's magnetic field) and solar flux data (a measure of

the overall level of solar activity) . These forecasts are also applicable to high latitudes provided the appropriate time

correction is made.

The propagation forecast announcements are given as a phonetic and a numeral. The phonetic identifies the radio propagation quality at the time the forecast is issued (0100, 0700, 1300, or 1900 UTC). The numeral indicates

the quality expected during the six-hour period after the

forecast is issued. The meanings of the phonetics and

numerals are:

Phonetic

Whiskey

Uniform

November

Numeral

One

Two

Three

Four

Five

Six

Seven

Eight

Nine

Meaning

Disturbed

Unsettled

Normal

Meaning

Useless

Very poor

Poor

Poor-to-fair Fair

Fair-to- good

Good

Very good

Excellent

If, for example, propagation conditions are normal and

expected to be good during the next six hours, the coded

forecast announcement would be "November Seven."

The K-index is a measure of variation, or disturbance,

in the earth's magnetic field during the three-hour period

ending about one hour prior to issue of the forecast. The

K-figures range from 0 (very quiet) to 9 (extremely

disturbed) . The solar flux measurements are taken at 2800

MHz three times daily. The flux value is closely

associated with the well-known daily sunspot number and

is coming to be preferred to sunspot number as a measure

of solar activity

.

A typical announcement might be:

"The radio propagation quality forecast at 0100 is good. Current geomagnetic activity is normal. The coded forecast is November

Seven" (and then repeated). "The K-index at 0100 UTC is 2"

(repeated), "tending to increase. The 2800 Megahertz solar flux index is 70 units" (repeated), "tending to remain constant."

The forecasts are prepared by the Telecommunications

Services Center, Ojfice ofTelecommunications , Boulder

,

CO 80302. Information regarding these forecasts may be

obtained by writing to this address.

Geophysical Alerts

Current geophysical alerts (Geoalerts) are broadcast in voice from WWV at 1 8 minutes after each hour, and from

WWVH at 45 minutes after each hour. The messages are changed daily at 0400 UTC with provisions to provide

real-time data alerts of outstanding occurring events. These are followed by a summary of selected solar and

geophysical events during the previous 24 hours. In- quiries regarding these messages should be addressed to the Space Environment Laboratory, National Oceanic

and Atmospheric Administration , Boulder, CO 80302.

Marine Storm Warnings

Weather information about major storms in the Atlantic and eastern North Pacific are broadcast in voice from

WWV at 8, 9, and 10 minutes after each hour. Similar storm warnings covering the eastern and central North

Pacific are given from WWVH at 48, 49, and 50 minutes

after each hour. An additional segment (at 1 1 minutes

after the hour on WWV and at 51 minutes on WWVH)

may be used when there are unusually widespread storm

conditions . The brief messages are designed to tell mariners of storm threats in their areas. If there are no warnings

in the designated areas, the broadcasts will so indicate. The ocean areas involved are those for which the U .S . has

warning responsibility under international agreement.

The regular times of issue by the National Weather Ser- vice are 0500, 1 100, 1700, and 2300 UTC for WWV and

0000, 0600, 1200, and 1800 UTC for WWVH. These

broadcasts are updated effective with the next scheduled

announcement following the time of issue. Mariners might expect to receive a broadcast similar to the following:

"North Atlantic weather West of 35 West at 1700 UTC: Hurricane Donna, intensifying, 24 North, 60 West, moving northwest, 20 knots, winds 75 knots; storm, 65 North, 35 West, moving east, 10 knots; winds 50 knots, seas 15 feet."

Information regarding these announcements may be

obtained from the Director, National Weather Service,

Silver Spring , MD 20910.

5
Ig. "Silent" Periods

These are periods with no tone modulation. However,

the carrier frequency, seconds pulses, time announcements, and 100-Hz BCD time code continue. The main

silent periods extend from 45 to 51 minutes after the hour on WWV and from 15 to 20 minutes after the hour on WWVH. An additional 3-minute period from 8 to 1

minutes after the hour is silent on WWVH.

Ih. BCD Time Code

A binary coded decimal (BCD) time code is transmitted continuously by WWV and WWVH on a 100-Hz subcarrier. The 100-Hz subcarrier is synchronous with the code

pulses so that 10-millisecond resolution is attained. The

time code provides a standard timing base for scientific observations made simultaneously at different locations.

It has application, for example, where signals telemetered from a satellite are recorded along with the time code

pulses. Data analysis is then aided by having accurate, unambiguous time markers superimposed directly on the recording.

The WWV/WWVH time code format presents UTC

information in serial fashion at a rate of one pulse per

second. Groups of pulses can be decoded to ascertain the current minute, hour, and day of year. While the 100-Hz

subcarrier is not considered one of the standard audio

frequencies, the code does contain the 100-Hz frequency

and may be used as a standard with the same accuracy as the audio frequencies. A description of the time code is contained in the Appendix.

li. UTl Time Corrections

The UTC time scale broadcast by WWV and WWVH

runs at a rate that is almost perfectly constant because it is based on ultra-stable atomic clocks. This time scale meets

the needs of most users. Somewhat surprisingly, however, some users of time signals need time which is not

this stable. In applications such as very precise navigation

and satellite tracking, which must be referenced to the

rotating earth, a time scale that speeds up and slows down

with the earth's rotation rate must be used. The particular

time scale needed is known as UTl and is inferred from

astronomical observations. To be responsive to these users, information needed to obtain UTl time is included in the UTC broadcasts. This

occurs at two different levels of accuracy. First, for those

users needing to know UTl only to within about one

second (this includes nearly all boaters/navigators), occasional corrections of exactly one second —called "leap"

seconds —are inserted into the UTC time scale whenever

needed to keep the UTC time signals within ±0.9 second

of UTl at all times. These leap seconds may be either positive or negative and are coordinated under international agreement by the International Time Bureau (BEH)

in Paris. Ordinarily, a positive leap second must be added

about once per year (usually on June 30 or December 31),

depending on how the earth's rotation rate is behaving in each particular year. Information on how to assign dates

to events that occur near the time of a leap second inser- tion is given in the Appendix.

The second level of correction is included in the UTC

broadcasts for the very small number of users who need

UTl time to better than one second. These corrections, in units of 0.1 second, are encoded into the broadcasts by

using double ticks or pulses after the start of each minute

The amount of correction is determined by counting the number of successive double ticks heard each minute . The

1st through the 8th seconds ticks indicate a "plus" correction, and the 9th through the 16th, a "minus" correction. For example, if the 1st, 2nd, and 3rd ticks are doubled, the correction is "plus" 0.3 second: UTl = UTC + 0.3 second, or if UTC is 8:45:17, then UTl is 8:45:17.3. If the 9th, 10th, 11th, and 12th ticks are doubled, the correction is "minus" 0.4 second, or as in the above example, UTl = 8:45:16.6.

2. WWVB

WWVB transmits continuously on a standard radio

carrier frequency of 60 kHz. Standard time signals, time

intervals, and UT 1 corrections are provided by means of a BCD time code. The station is located on the same site as WWV. Effective coverage area is the continental U.S.

2a. Accuracy and Stability

The frequency of WWVB is normally within its pre- scribed value to better than 1 part in 100 billion. Deviations from day to day are less than 5 parts in 1 ,000 billion. Effects of the propagation medium on received signals are

relatively minor at low frequencies; therefore, frequency

comparisons to better than 1 part in 100 billion are possi- ble using appropriate receiving and averaging techniques.

2b. Station Identiflcation

WWVB identifies itself by advancing its carrier phase

45° at 10 minutes after every hour and returning to normal

phase at 15 minutes after the hour. WWVB can also be

identified by its unique time code.

2c. Radiated Power, Antenna, and Coverage

The effective radiated power from WWVB is 13 kW.

The antenna is a 122-meter, top-loaded vertical installed over a radial ground screen. Some measured field intensity contours are shown in figure 4.

2d. BCD Time Code

WWVB broadcasts time information in the form of a

6
Figure 4. Measured field intensity countours of WWVB at 13 kW ERP. BCD time code. The time code is synchronized with the 60-kHz carrier and is broadcast continuously at a rate of one pulse per second. Each pulse is generated by reducing

the carrier power 10 dB at the beginning of the second, so

the leading edge of every negative-going pulse is on time.

Details of the WWVB time code are presented in the Appendix.

3. WWVL

WWVL is an experimental station. Regular operations were curtailed on July 1, 1972, and it now broadcasts experimental programs only on an intermittent basis, de- pending upon need and availability of funds. Transmissions can be made available on a subscription basis to public organizations and other Federal agencies. Arrangements for use should be made through the Time and

Frequency Services Section, National Bureau of Standards, Boulder, CO 80302.

4. Summary of Broadcast Services

The services provided by the NBS radio stations are summarized in the following chart. Coordinates for the

stations are also listed.

&0H

STATION

DATE

SERVICE

BEGAN

RADIO

FREQUENCIES

AUDIO

FREQUENCIES

MUSICAL

PITCH

TIME

INTERVALS

TIME

SIGNALS

UTl

CORRECTIONS

OFFICIAL

ANNOUNCEME

wwv 1923 X X X X X X X

WWVH 1948 X X X X X X X

WWVB 1956 X X X X

WWVL 1960 X

COORDINATES:

WWV 40°40'49.0"N 105°02'27.0"W

WWVB 40°40'28.3"N 105°02'39.5"W

WWVL 40°40'51.3"N 105°03'00.0"W

WWVH 21°59'26.0"N 159°46'00.0"W

7
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Figure 5. The NBS frequency control system.

8
5. How NBS Controls the

Transmitted Frequencies

A simplified diagram of the NBS frequency control system is shown in figure 5. The entire system depends

upon the reference shown in this diagram as the NBS

Primary Time and Frequency Standard. This standard is comprised of a number of commercial cesium beam

clocks, up to two primary cesium beam frequency and

time standards, and computer-aided measurement and

computation methods which combine all of the clock data

to generate an accurate and uniform time scale, AT

(NBS). Another scale, UTC (NBS), is also generated by

adding leap seconds and small corrections to AT (NBS) as needed to keep UTC (NBS) synchronized with the inter- nationally coordinated time scale, UTC, which is maintained by the BIH.

Utilizing the line- 10 horizontal synchronizing pulses

from a local television station, the Fort Collins master

clock is compared on a regular basis with the UTC (NBS)

time scale. All other clocks and time-code generators at the Fort Collins site are then compared with the Fort Collins master clock. Frequency corrections of the WWVB and WWVL quartz crystal oscillators are based on their phase relative to the UTC (NBS) time scale.

The transmissions from WWV and WWVH are controlled by three commercial cesium standards located at each site. To insure accurate time transmission from each

station, the time-code generators are compared with the

stations' master clock several times each day.

Control of the signals transmitted from WWVH is based not only upon the cesium standards, but upon sig- nals from WWVB as received by phase-lock receivers. The cesium standards controlling the transmitted fre- quencies and time signals are continuously compared with

the received signals.

To insure that systematic errors do not enter into the system, the UTC (NBS) time scale is occasionally compared with the transmitting station clocks by the use of a very precise portable atomic clock.

6. Frequency Calibration Service Using

Network Television

For those users who require only frequency calibrations, an alternative to the radio broadcasts is available. This new service provides a means of calibrating oscillators traceable to NBS. It gives the user the option of

calibrating his oscillator quickly at very low cost with

modest accuracy or of expending more time and money

for higher accuracy.

A NBS FREQUENCY STANDARD

B NETWORK STANDARD

C USER'S OSCILLATOR

FREQUENCY STANDARDS

NETWORK

RUBIDIUM

FREQUENCY STANDARD

USER'S OSCILLATOR

NETWORK

COLOR TV BROADCASTS

COMPARE A-B

COMPARE B-C

SIGNALS RECEIVED

r

PHASE COMPARATOR

(TWO METHODS

COMPARE A-C = CALIBRATION OF C

COMPARED

published data

[easy, inexpensive!

L access J

ARE DESCRIBED BELOW)

CALIBRATIC

USER S SCILL/

I 1

ALIBR/

j|

\TED

Figure 6. How the frequency calibration service using color television works.

9
The service is very reliable because the networks use extremely stable rubidium oscillators to generate the 3.58

MHz color subcarrier frequency which is transmitted with

all color programs. The color signal is then used as a

transfer standard. Any oscillator that has a frequency of

10/N MHz, where N is any integer from 1 to 100, can be

calibrated.

If a user wants to make a calibration, he compares the color signal coming from the network centers in New

York City (or Los Angeles for those on the West Coast)

with his local oscillator. NBS monitors the same network

signals and publishes the difference between the network

oscillators and the NBS Frequency Standard in the monthly NB5 Time and Frequency Services Bulletin. A

user then knows two things: ( 1) the difference between his

oscillator and the network oscillators (by measurement)

and (2) the difference between the networks and NBS (by

publication). With this information, he can easily compute the difference between his oscillator and NBS (see

fig. 6). Thus, his calibration is traceable to the NBS

Frequency Standard. NBS has developed two methods for making these frequency calibrations. Equipment is commercially avail- able for both methods

6a. Color Bar Comparator Method

The color bar comparator is a simple circuit that connects to a standard color television set (fig. 7). It produces

a colored bar on the screen that changes color or moves

across the screen at a rate that depends on the frequency

difference between the user's oscillator and the TV net- work signal. By timing these changes with a stopwatch

and referring to the data published by NBS, an oscillator can be rapidly calibrated to an accuracy of 1 part in 1 billion.

Figure 7. Prototype of a color bar comparator.

6b. Digital Offset Computer Method

The second method, using a digital offset computer,

provides an automatic means of calibrating high-quality

crystal or atomic oscillators. It compares a signal from the

user's oscillator with the TV color signal and displays the frequency difference on the TV screen (fig. 8) as parts in 100 billion. If the measurements are averaged over about

15 minutes, a calibration accuracy of one part in 100

billion can usually be achieved.

More information on this service, including circuit details and lists of equipment manufacturers, is available upon request from the Time and Frequency Services Section, NBS, Boulder, CO 80302.

Figure 8. Prototype of a digital offset computer.

7. Time Comparisons Using Television

Synchronization Pulses

In the previous section, methods were described for using the frequency of a network television signal as a

transfer standard to link the user to the NBS Frequency

Standard. In a similar way, it is also possible to use a

particular synchronization pulse present in the normal

television picture signal as a time transfer standard to allow clock comparisons to be made with the UTC (NBS)

atomic time scale. To use this technique, a user first makes a simple time

difference measurement at a specified time during the day

between his local clock and a particular television signal

10
pulse (line- 10 (odd) horizontal synchronization pulse)

obtained from a normal television receiver. Commercial

equipment is available which can be used for this purpose

NBS also measures, at the same specified time, the time

difference between the TV synchronization pulse as re- ceived in Boulder, Colorado and the UTC (NBS) time

scale and publishes the data in the monthlyNB5 Time and

Frequency Services Bulletin. The difference between the

local measurement and the published NBS measurement

then represents the time difference between the user's clock and UTC (NBS) plus a propagation delay.

If the propagation delay can be determined —for example, by a portable clock measurement, then this part of the measurement can be subtracted out, leaving only the

actual time difference between the local and NBS clocks. Although the propagation delay of the TV signals through

the nationwide TV network distribution system has been

shown to be relatively constant to within a few microseconds for long periods extending over weeks or months,

occasional large changes of many milliseconds do occur due to network rerouting of TV signals. These large

changes are usually easy to recognize, especially if a

user regularly monitors more than one of the major TV

networks.

Even if the propagation delays are not measured or otherwise determined, the line-10 technique can still provide useful information about the stability performance of

a user's time scale or clock relative to NBS. As long as the delay remains constant, daily line-10 comparisons will show whether a user's clock is gaining or losing time

relative to NBS, even though the exact time difference cannot be determined without knowing the value for the propagation delay.

NBS publishes daily line-10 measurements for all three major television networks and for both East Coast and

West Coast-originated transmissions. The West Coast

data are suppled by the Hewlett-Packard Co. in Santa

Clara, California and are referenced to UTC (NBS) with

an accuracy of about 0.5 microsecond. West Coast data is for use only by those users in the Pacific Time Zone. For

current specific times during the day when each network

is measured, potential users of the line-10 time transfer technique should either consult a current issue of theNBS

Time andFrequency Services Bulletin or contact the Time

and Frequency Services Section, NBS, Boulder, CO

80302.

8. Other Publications

The Time and Frequency Division offers a variety of

publications about the NBS atomic time and frequency

standards, the associated dissemination services and how

to use them. These publications are available upon re- quest . For information about the atomic clock, primary time

and frequency standard, as well as special time and fre- quency calibration, test, and measurement services, write

to the Frequency and Time Standards Section, NBS,

Boulder, CO 80302 or call (303) 499-1000, x 3276. The

following are available:

General Information

Frequency Standards and Clocks: A Tutorial Introduction, Helmut Hellwig, Nat. Bur. Stand. (U.S.),

Tech. Note 616, 69 pages. Revised (March 1974).

Technical Publications

Accuracy Evaluation and Stability of the NBS Primary

Frequency Standards, D.J. Glaze, H. Hellwig, et

al, IEEE Trans, on Instr. &Meas., Vol. lM-23, No.

4 (Dec. 1974).

Atomic Frequency Standards: A Survey, H. Hellwig,

Proc. IEEE, Vol. 63, No. 2 (Feb. 1975).

Time and Frequency, H. Hellwig, D.W. Allan, F.L.

Walls, Proc. 5th Intl. Conf. on Atomic Masses and

Fundamental Constants (AMCO-5), Paris, France

(June 2-6, 1975).

An Accurate Algorithm for an Atomic Time Scale,

D. W. Allan, H. Hellwig, D. J. Glaze, Metrologia,

Vol. 11, (1975).

For more detailed technical information on how to use

the time and frequency dissemination services, the following publications are available from the Time &

Frequency Services Section, NBS, Boulder, CO 80302

(Phone (303) 499-1000, x3212):

Frequency Standard Hides in Every Color TV Set, D. Davis, Electronics (May 10, 1971).

Calibrating Crystal Oscillators with TV Color-refer- erence Signals, D. Davis, Electronics (March 20,

1975).

The Use of NBS High Frequency Broadcasts for Time

& Frequency Calibrations, N. Hironaka and C.

Trembath, Nat. Bur. Stand. (U.S.), Tech. Note

668 (May 1975).

Characterization and Concepts of Time-Frequency

Dissemination, J. L. Jespersen, B.E.Blair, and

L.E. Gatterer, Proc. IEEE, Vol. 60, No. 5, May

1972.

11
Appendix

lA. Dating of Events in the Vicinity of Leap Seconds

2A. WWV/WWVH Time Code

3A. WWVB Time Code

12
lA Dating of Events in the Vicinity of

Leap Seconds

When leap second adjustments are necessary to keep

the broadcast time signals (UTC) within ±0.9 second of

the earth-related UTl time scale, the addition or deletion of exactly 1 second occurs at the end of the UTC month.

By international agreement, first preference is given to December 31 or June 30, second preference to March 31

or September 30, and third preference to any other month.

When a positive leap second is required —that is, when

UTl is slow relative to UTC —an additional second is inserted beginning at 23h 59m 60s of the last day of the month and ending at jSh ^m ^s of the first day of the following month. In this case, the last minute of the month

in which there is a leap second contains 61 seconds. To

assign dates to events which occur around this extra sec- ond, refer to figure lA.

NORMAL MINUTB

(NO LEAP SECOND ADDED)

1

L

1

Doling of Event Shown:

56 58 0 2 30 June, 23^ 59"^ 59.5 ' UTC

57 59 EVENT -

30 June. 23^59"^ —>. / July, 0^ OTi

MINUTE WITH LEAP SECOND ADDED

57 59 0 1 56 58 60

Dating of Event Shown;

30 June, 23^1 59"! 60.5^ UTC

EVENT —I

30 June, 23^ 59 * ) July, O'' 0"!

Figure lA. Dating of events in the vicinity of a leap second.

Assuming that unexpected large changes do not occur

in the earth's rotation rate in the future, it is likely that positive leap seconds will continue to be needed about once per year. If, however, the earth should speed up

significantly at some future time, so that UTl runs at a

faster rate than UTC, then provision is also made for negative leap seconds in the UTC time scale. In this case,

exactly one second would be deleted at the end of some UTC month, and the last minute would contain only 59

seconds.

Positive leap seconds were inserted in all NBS broadcasts at the end of June 1972, December 1972, December

1973, December 1974, and December 1975.

2A. WWV/WWVH Time Code

The WWV/WWVH time code is a modified version of

the IRIG-H format. Data is broadcast on a 100-Hz subcarrier at a rate of one pulse per second. Certain pulses in succession comprise binary-coded groups representing

decimal numbers. The binary-to-decimal weighting

scheme is 1-2-4-8 with the least significant binary digit

always transmitted first . The binary groups and their basic decimal equivalents are shown in the following table:

Weight:

BINARY GROUP

12 4 8 0 0 0 0

10 0 0

0 1

1 1 0 0

1 0

0 1

1 1 0 0 0

1 0 0

0 0

0 0

DECIMAL

EQUIVALENT

0

12

34

56

7

In every case, the decimal equivalent of a BCD group is derived by multiplying each binary digit times the weight

factor of its respective column and then adding the four products together. For instance, the binary sequence 1010

in the 1-2-4-8 scheme means (1 x 1) -I- (0 x 2) + (1 x 4)

-I- (0 X 8) = 1 -h 0 4 -I- 0 = 5, as shown in the table. If fewer than nine decimal digits are needed, one or more of

the binary columns may be omitted.

In the standard IRIG-H code, a binary 0 pulse consists of exactly 20 cycles of 100-Hz amplitude modulation (200

milliseconds duration), whereas a binary 1 consists of 50

cycles of 100 Hz (500 milliseconds duration). In the WWV/WWVH broadcast format, however, all tones are suppressed briefly while the seconds pulses are trans- mitted (see sec. Ic). Because the tone suppression applies also to the 100-Hz

subcarrier frequency, it has the effect of deleting the first 30-millisecond portion of each binary pulse in the time

code. Thus, a binary 0 contains only 17 cycles of 100-Hz

amplitude modulation (170 milliseconds duration) and a binary 1 contains 47 cycles of 100 Hz (470 milliseconds

duration). The leading edge of every pulse coincides with

a positive-going zero crossing of the 100-Hz subcarrier, but it occurs 30 milliseconds after the beginning of the second.

Within a time frame of one minute, enough pulses are transmitted to convey in BCD language the current minute, hour, and day of year. Two BCD groups are needed

to express the hour (00 through 23); and three groups are needed to express the day of year (001 through 366).

When representing units, tens, or hundreds, the basic 1-2-4-8 weights are simply multiplied by 1, 10, or 100 as

13
appropriate . The coded information always refers to time

at the beginning of the one-minute frame . Seconds may be

determined by counting pulses within the frame. Each frame commences with a unique spacing of pulses

to mark the beginning of a new minute. No pulse is transmitted during the first second of the minute. Instead, a one-second space or hole occurs in the pulse train at that time. Because all pulses in the time code are 30 mil- liseconds late with respect to UTC, each minute actually

begins 1030 milliseconds (or 1.03 seconds) prior to the leading edge of the first pulse in the new frame.

For synchronization purposes, every ten seconds a so- called position identifier pulse is transmitted. Unlike the BCD data pulses, the position identifiers consist of 77

cycles of 100 Hz (770 milliseconds duration).

UTl corrections to the nearest 0. 1 second are broadcast

via BCD pulses during the final ten seconds of each

frame. The coded pulses which occur between the 50th

and 59th seconds of each frame are called control functions. Control function #1, which occurs at 50 seconds,

tells whether the UT 1 correction is negative or positive . If control function #1 is a binary 0, the correction is negative; if it is a binary 1, the correction is positive. Control

functions #7, #8, and #9, which occur respectively at 56, 57, and 58 seconds, specify the amount of UTl

correction. Because the UTl corrections are expressed in tenths of a second, the basic binary-to-decimal weights

are multiplied by 0.1 when applied to these control functions. Control function #6, which occurs at 55 seconds, is programmed as a binary 1 throughout those weeks when

Daylight Saving Time is in effect and as a binary 0 when

Standard Time is in effect. The setting of this function is changed at 0000 UTC on the date of change. Throughout

the U.S. mainland, this schedule allows several hours for the function to be received before the change becomes

effective locally —i.e., at 2:00 a.m. local time. Thus,

control function #6 allows clocks or digital recorders operating on local time to be programmed to make an

automatic one-hour adjustment in changing from Daylight Saving Time to Standard Time and vice versa. Figure 2A depicts one frame of the time code as it might

appear after being rectified, filtered, and recorded. In this example, the leading edge of each pulse is considered to be the positive-going excursion. The pulse train in the

figure is annotated to show the characteristic features of

FORMAT H, SIGNAL HOOl , IS COMPOSED OF THE FOLLOWING

1 ppm FRAME REFERENCE MARKER R = (P„ AND 1.03 SECOND "HOLE") BINARY CODED DECIMAL TIME-OF-YEAR CODE WORD (23 DIGITS) CONTROL FUNCTIONS (9 DIGITS) USED FOR UTi CORRECTIONS, ETC.

6 ppm POSITION IDENTIFIERS (P THROUGH P )

1 pps INDEX MARKERS ° '

^—1 .03 SECOND

-R-J HOLE IN CODE

FOR 0.8 SECOND

PULSE

-MINUTES- -HOURS- -DAYSp p ^ 1^ P P ii' 6 C 1^ p wuuumjuuuinjLnj^^

UTiCORRECTION

UTC AT POINT A

= 173 DAYS 21 HOURS

10 MINUTES

UTl AT POINT A

= 173 DAYS 21 HOURS

10 MINUTES

0.3 SECONDS

Pj-Pj POSITION IDENTIFIERS (0.770 SECOND DURATION)

W WEIGHTED CODE DIGIT (0.470 SECOND DURATION)

C WEIGHTED CONTROL ELEMENT (0.470 SECOND DURATION) CONTROL FUNCTION #6 ZERO^DURlL '^STANDARD '""^t"^ DURATION OF INDEX MARKERS, UNWEIGHTED CODE, AND UNWEIGHTED CONTROL ELEMENTS = 0.170 SECONDS

NOTE: BEGINNING OF PULSE IS REPRESENTED BY POSITIVE-GOING EDGE. 9/75

Figure 2A. WWV and WWVH time code format.

14
the time code format. The six position identifiers are denoted by symbols Pi, Pa, Pa, Pa, Ps, and Po. The

minutes, hours, days, and UTl sets are marked by brackets, and the applicable weighting factors are printed be- neath the coded pulses in each BCD group. With the exception of the position identifiers, all uncoded pulses

are set permanently to binary 0. The first ten seconds of every frame always include the

1 .03-second hole followed by eight uncoded pulses and

the position identifier Pi . The minutes set follows Pi and

consists of two BCD groups separated by an uncoded

pulse. Similarly, the hours set follows P2. The days set follows P3 and extends for two pulses beyond P4 to allow

enough elements to represent three decimal digits. The

UTl set follows Ps, and the last pulse in the frame is always Po.

In figure 2A, the least significant digit of the minutes

set is (0 X 1) -)- (0 X 2) + (0 X 4) (Ox 8) = 0; the most

significant digit of that set is (1 x 10) -I- (0 x 20) -I- (0 x 40)= 10. Hence, at the beginning of the 1 .03-second hole

in that frame, the time was exactly 10 minutes past the hour. By decoding the hours set and the days set, it is seen

that the time of day is in the 21st hour on the 173rd day of

the year. The UTl correction is -1-0.3 second. Therefore,

at point A, the correct time on the UTl scale is 173 days,

21 hours, 10 minutes, 0.3 second.

3A. WWVB Time Code

The WWVB time code is generated by shifting the power of the 60-kHz carrier. The carrier power is re- duced 10 db at the beginning of each second and restored

to full power 200 milliseconds later for a binary zero, 500 milliseconds later for a binary one, and 800 milli- seconds later for a reference marker or position identifier. Certain groups of pulses are encoded to represent

decimal numbers which identify the minute, hour, and

day of year. The binary-to-decimal weighting scheme is 8-4-2-1 with the most significant binary digit transmitted

first. Note that this weighting sequence is the reverse of the WWV/WWVH code. The BCD groups and their basic decimal equivalents are tabulated below:

BINARY GROUP

Weight: 8 4 2 1 0 0 0 0

0 0 0 1 0 0

0 0

0 0

0 0 0

0 0 1

DECIMAL

EQUIVALENT

0

12

34

56

7

89

The decimal equivalent of each group is derived by

multiplying the individual binary digits by the weight

factor of their respective columns and then adding the four products together. For example, the binary sequence 1001

in 8-4-2-1 code is equivalent to (1 x 8) + (0 x 4) -I- (0 x

2) -I- (1 X 1) = 8 -I- 0 -I- 0 -I- 1 = 9, as shown in the table.

If fewer than nine decimal digits are required, one or more

of the high-order binary digits may be dispensed with. Once every minute, in serial fashion, the code format

presents BCD numbers corresponding to the current minute, hour, and day on the UTC scale. Two BCD groups

identify the minute (00 through 59); two groups identify the hour (00 through 23); and three groups identify the day

of year (001 through 366). When representing units, tens, or hundreds, the basic 8-4-2-1 weights are multiplied by

1, 10, or 100 respectively. The coded information refers

to the time at the beginning of the one-minute frame.

Within each frame, the seconds may be determined by

counting pulses.

Every new minute commences with a frame reference pulse which lasts for 0.8 second. Also, every ten-second

interval within the minute is marked by a position identifier pulse of 0.8-second duration. UTl corrections to the nearest 0.1 second are transmitted at seconds 36 through 44 of each frame. Coded pulses

at 36, 37, and 38 seconds indicate the positive or negative

relationship of UTl with respect to UTC. Pulses at 36 and

38 seconds are transmitted as binary ones only if UTl is early with respect to UTC, in which case a correction must be added to the UTC signals to obtain UTl. The

pulse transmitted at 37 seconds is a binary one if UTl is late with respect to UTC, in which case the required UTl

correction must then be subtracted. The magnitude of the UTl correction is transmitted as a BCD group at 40, 41

,

42, and 43 seconds. Because UTl corrections are expres- sed in tenths of seconds, the basic 8-4-2-1 weight of that particular binary group is multiplied by 0.1 to obtain its proper decimal equivalent.

Figure 3A shows a sample frame of the time code in rectified or dc form. The negative-going edge of each

pulse coincides with the beginning of a second. Position

identifiers are labeled Pi , P2, P3, P4, Ps, and Po. Brackets

show the demarcation of the minutes, hours, days, and

UTl sets. The applicable weight factor is printed beneath

the coded pulses in each BCD group. Except for the position identifiers and the frame reference marker, all uncoded pulses are binary zeros. In figure 3A, the most significant digit of the minutes

set is (1 X 40) (0 X 20) -I- (0 x 10) = 40; the least significant digit of that set is (0 x 8) (0 x 4) -I- (1 x 2)

-H (0 X 1) = 2. Thus, at the beginning of the frame, UTC

was precisely 42 minutes past the hour. The sets for hours

and days reveal further that it is the 1 8th hour of the 258th

day ofthe year. The UTl correction is — 0.7 second, so at the beginning of the frame the correct time on the UTl

scale was 258 days, 18 hours, 41 minutes, 59.3 seconds.

15
TIME FRAME 1 MINUTE

(INDEX COUNT 1 SECOND)

0 1 0

I I I I I I I I I I I I I

20

I I I I I I I I I

30 40 50

I I I I I I I I I I I I I I I I I I I I I I I I I I I I 1 I I I I I

ON TIME POINT A -FRAME REFERENCE

MARKER

Pi 0.8 SECOND 0.2 SECOND 5 SECOND ).8 SECOND -| h- ^juuFiruTTUimmrwir^^

• MINUTES - HOURS DAYS

•s. ij-><. ^UTr UT, SET RELATIONSHIP

BINARY CODED DECIMAL TIME-OF-YEAR CODE WORD (23 DIGITS) CONTROL FUNCTIONS (15 DIGITS) USED FOR UTi CORRECTIONS

6 PPM POSITION IDENTIFIER MARKERS AND PULSES(Po THRU P5) (REDUCED CARRIER 0.8 SECOND DURATION PLUS 0.2 SECOND DURATION PULSE)

W - WEIGHTED CODE DIGIT (CARRIER RESTORED IN 0.5 SECOND - BINARY ONE)

U - UNWEIGHTED CODE DIGIT (CARRIER RESTORED IN 0.2 SECOND - BINARY ZERO)

UTC AT POINT A 258 DAYS

18 HOURS

42 MINUTES

NOTE: BEGINNING OF PULSE IS REPRESENTED BY NEGAT I VE - GOI NG EDGE.

UTI AT POINT A 258 DAYS

18 HOURS

41 MINUTES

59.3 SECONDS

9/75

Figure 3A. WWVB time code format.

tVU.S. government printing OFFICE; 1976-677-785

16
Telephone Time of Day Service

WWV and WWVH broadcasts may be heard via telephone. Since the RF carriers cannot be detected over telephone

circuits, only the audio portion of the broadcasts may be heard. Accuracy of the time signals as received anywhere in the contiguous 48 states is 30 milliseconds or better. By calling (303) 499-711 1 in Boulder, Colorado, the user will hear the live broadcasts as transmitted from WWV.

This service is automatically limited to three minutes per call . Similar time-of-day broadcasts from WWVH can be heard

by dialing (808) 335-4363 on the island of Kauai, Hawaii. NOTE: These are long distance toll calls for those users outside the local dialing area.

About the Announcers

The station identification and time-of-day announcements are pre-recorded —not "live." The regular announcer for WWV is Mr. Don Elliott of Atlanta, Georgia. Mrs. Jane Barbe, also of Atlanta, is the announcer for WWVH.

Tours

Guided tours are available at all of the NBS radio stations. Visiting hours at WWV, WWVB, and WWVL are every Wednesday, except holidays, from 2:00 to 4:00 p.m. Special tours may be scheduled at other times only by prior

arrangement with the engineer-in-charge. WWVH does not have regularly scheduled visiting hours —arrangements for

visiting the site should be made in advance. Tours of the NBS Boulder Laboratories, including visits to the atomic clock and the other dissemination services, are

available. Information can be obtained from the Program Information Office, NBS, Boulder, CO 80302.

Inquiries About the Stations

Correspondence pertaining directly to station operations may be addressed to:

Engineer-in-Charge

NBS Radio Stations WWV/WWVB/WWVL

2000 East County Road 58

Fort Collins, CO 80521

Telephone: (303) 484-2372

Engineer-in-Charge

NBS Radio Radio Station WWVH

P. O. Box 417

Kekaha, Kauai, HI 96752

Telephone: (808) 335-4361
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington.

D C. 20234

OfFICIAL BUSINESS

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