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in o* NBS TECHNICAL NOTE 656

**»CAU O* *

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

Standard Time and Frequency:

Its Generation, Control, and

Dissemination by the

National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government. (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology,

\hc Institute for Computer Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system

with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Nuclear

Sciences - — Applied Radiation — Quantum Electronics ' — Electromagnetics 3 — Time

and Frequency * — Laboratory Astrophysics ' — Cryogenics '. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety 4 — Building

Environment * — Technical Evaluation and Application ' — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective

utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Institute consists of the following

divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington. DC. 20234.

' Part of the Center for Radiation Research.

' Located at Boulder, Colorado 80302.

' Part of the Center for Building Technology.
. 1974

10

r Its Generation, Control, and Dissemination

Standard Time and Frequency:

by the National Bureau of Standards

John B. Milton

Time and Frequency Division

Institute for Basic Standards

National Bureau of Standards

Boulder. Colorado 80302

<r •

; i

US DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

Issued June 1974
National Bureau of Standards Technical Note 656

Nat Bur. Stand. (U.S.), Tech. Note 656, 21 pages (June 1974 ) CODEN: NBTNAE

For »ale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. 03.46:656) *.35
CONTENTS

Page

1. INTRODUCTION 1

2. STANDARD FREQUENCY AND TIME GENERATION 3

2.1 Time and Frequency Division 3

2.1.1 The NBS frequency standard 3

2.1.2 Computed or "paper" time scales 4

2.1.3 Operational clock systems 4

2.2 Radio Station WWV, Fort Collins, Colorado 4

2.3 Radio Station WWVH, Kauai, Hawaii 7 2.4 Radio Station WWVB, Fort Collins, Colorado 7 2.5 Radio Station WWVL, Fort Collins, Colorado 7

3. TRANSMISSION OF TIME AND FREQUENCY 11

3.1 Transmission from WWV and WWVH 11

3.2 Transmission from WWVB 11

3.3 Transmission from WWVL (Experimental) 13

4. TIME AND FREQUENCY INTERCOMPARISONS 14

4.1 WWV Self-Comparisons 14

4.2 WWVH Self -Comparisons 14

4.3 WWV to WWVB and WWVL 14

4.4 Between the Fort Collins Standards and the

WWVH Standard 16

4.5 Between the Fort Collins Standards and the

NBS Standard 16

4.5.1 Portable clocks 16

4.5.2 TV synchronizing pulse method 16

5. SUMMARY 18

6. APPENDIX 18

6.1 Explanation of NBS Time Scales 18

6.1.1 AT(NBS) 18

6.1.2 UTC(NBS) 18

in
LIST OF FIGURES

Page

Figure 1. System for Generation of Computed Time

Scales AT(NBS) and UTC(NBS) 5

Figure 2. Standard Frequency and Time Interval

Generating System 6

Figure 3. WWV Time and Frequency Generation

One of Three Identical Systems Showing

Intercomparisons 8

Figure 4. WWVH Time and Frequency Generation

One of Three Identical Systems Showing

Intercomparisons 9

Figure 5 . WWVB Time and Frequency Generation and

Control—One of Three Semi-independent systems .... 10

Figure 6 . WWVL Frequency Generation and Control System 12

Figure 7. Clock Intercomparison WWV-WWVB-WWVL 15

Figure 8. Space-Time Diagram of TV Synchronization Method .... 17

iv
STANDARD TIME AND FREQUENCY: ITS GENERATION, CONTROL,

AND DISSEMINATION BY THE NATIONAL BUREAU OF STANDARDS

John B. Milton

The Time and Frequency Division of the National Bureau

of Standards maintains primary frequency standards, which

provide a realization of the internationally-defined second,

and two atomic time scales, AT(NBS) and UTC(NBS). AT(NBS) is

dependent upon the primary frequency standards, an ensemble

of commercial cesium clocks, and a computer algorithm to

process the data. The UTC(NBS) scale is derived from AT(NBS)

by the addition of small annual frequency adjustments and

leap second adjustments to keep its time nominally synchronous

with the international time scale UTC. The UTC(NBS) time

scale is used to calibrate the clocks and secondary standards

necessary for the operation of the NBS radio stations, WWV,

WWVH, WWVB, and WWVL. These stations transmit various

standard frequency and time signals throughout the world, and,

in addition, provide certain official announcements such as geoalert warnings, marine weather advisories, and radio

propagation forecasts.

Key Words: Clock synchronization; frequency and time dissemination;

primary frequency standard; standard frequency broadcasts; time

interval; time scales.

1 . INTRODUCTION

Since its inception in 1901, the National Bureau of Standards has

been involved in the design, construction, maintenance, and improvement

of standards of frequency and time interval. During the first two decades

of the 20th Century, the work on these standards was done by quite diverse

groups.

The people maintaining the standard of time interval were concerned

primarily with problems of navigation, while the standard of frequency

was maintained by a group concerned chiefly with interference between

adjacent channels in the radio spectrum.

Along with the maintenance and improvement of these standards came

the need to disseminate these standards. In 1923 the National Bureau

of Standards radio station WWV was established to disseminate the standard

of frequency via experimental broadcasts. Time pulses were added to

these broadcasts in 1935. Voice announcements of the time-of-day were

added in 1945.
In 1948, NBS radio station WWVH was placed in operation on the

island of Maui, Hawaii. In the 1950' s, very good commercial standards were controlling both of these high-frequency radio stations. At that

time, nothing was done at the stations beyond providing good crystal

oscillators. Utilizing the ultra-high precision oscillators located

at the NBS Boulder Laboratories, the time and frequency transmissions

of these stations were checked daily. Any deviations from the NBS standard were noted and corrective instructions were relayed to the radio stations.

In 1956, the Boulder Laboratories of the NBS began experimental

transmissions on 60 kHz from radio station WWVB. The 60 kHz driving

frequency for this station was derived directly from the NBS standard

of frequency located at the Boulder Laboratories. There was no control

of the transmissions beyond providing this standard driving frequency.

In 1960, utilizing an antenna borrowed from the Central Radio Propagation Laboratory of NBS, radio station WWVL was placed in experimental

operation on 20 kHz. This station was located some 10 miles west of

the laboratories in a high mountain valley. Direct generation of the

transmission frequency by the Boulder Laboratories was not feasible,

but a method of phase control using servo systems was employed. Since

January 1, 1960, when NBS began using a cesium beam device as its official

standard, all broadcast frequencies have been referenced to the cesium

resonance frequency.

WWVB and WWVL were moved to Fort Collins, Colorado, and began operation

there in July 1963. The original time and frequency control for these

stations was accomplished by continuously operating servo systems that

compared the phase of these stations as received in Boulder against

a phase reference derived from the NBS Frequency Standard. Phase corrective

information was then sent to Fort Collins via a radio link.

Primarily in order for NBS to increase the precision and accuracy

of the transmissions from station WWV, through more uniform U.S. coverage

and the replacement of obsolete transmitting and control equipment,

a new facility for that station was installed at Fort Collins, Colorado,

in 1966. Station WWVH was moved to a new facility in 1971. The description

of this new facility will be found in section 2.3.

Stations WWV, WWVB, and WWVL are, in essence, a composite facility

with the ability to disseminate standards of time and frequency from

19.9 kHz to 25 MHz . These stations provide standard atomic frequency

and a Universal Time scale, called UTC, which approximates the UT1 scale

based on the rotation of the earth.

Even though the time and frequency control of these stations is

provided by a highly precise ensemble of clocks and standards, the problem

remained of finding a simple, reliable, and inexpensive method of providing

a daily calibration of the Fort Collins standards with respect to the

NBS reference standards in Boulder.
This paper describes some of the current activities of the Time

and Frequency Division of the NBS . These include the generation

of the computed NBS time scales, the control of working clocks, and

the use of these clocks in coordination efforts with other standards

laboratories. The coordination of the Fort Collins site clocks and

frequency standards, the measurement and control of LF (and VLF) radiated

phase, and the method of coordination of the Fort Collins master clock

with that of NBS Boulder are also described. Additional information on

the broadcasts is published annually in NBS Special Publication 236,

available upon request from the Frequency-Time Broadcast Services Section

273.02, National Bureau of Standards, Boulder, CO 80302.

2. STANDARD FREQUENCY AND TIME GENERATION

2.1 Time and Frequency Division, Boulder, Colorado

With the advent of the first working atomic clock system in 1948, an ammonia device developed by Harold Lyons at NBS Washington, the era

of the highly accurate atomic time standards was born. The construction

of the original NBS cesium beam device, NBS-I, began in 1949. Much

of the initial work at NBS Boulder was under the project direction of

R. C. Mockler, and this early work included, among other things, refinement

of the NBS-I frequency standard.

In late 1959, the Atomic Frequency Standards Project, including

cesium beam and ammonia maser development work, was combined with the

project concerned with theoretical and practical aspects of atomic time

scales to form the Atomic Frequency and Time Standards Section. At

that time, this section was part of the Radio Physics Division.

In 1967, the Atomic Frequency and Time Standards Section was

combined with the Frequency-Time Broadcast Services Section and the

Frequency-Time Dissemination Research Section to form the Time and

Frequency Division.

2.1.1 The NBS Frequency Standard

From about 1959 to 1972 the NBS Frequency Standard (NBSFS) was

based on a series of thoroughly-evaluated laboratory cesium beam standards,

designated NBS-I, NBS-II, and NBS-III. During this period the specified

one-sigma fractional accuracy of the NBSFS —i.e., a statistical estimate

of how far the output frequency of the standard may deviate from the

"ideal" frequency of the isolated cesium atom, improved from several

parts in 1011 to 5 X 10_J- . The accuracy estimates are determined from

an extensive set of evaluative measurements to assess the magnitude

of possible frequency errors that might be introduced by various

components, subsystems, and environmental conditions of the overall

NBSFS system. Currently, the Quantum Electronic Frequency Standards

Program Team is evaluating two new, refined cesium standards, designated

NBS-4 and NBS-5 . These devices currently produce a documented accuracy

near 1 X lO--^. Further improvements may be expected in the future.

Present plans call for operation of this new NBSFS, consisting of the

two devices NBS-4 and NBS-5, on an intermittent basis in order to periodically calibrate the frequencies of the working ensemble of clocks in

the NBS time scale system. These data, as described next, are used

in producing the AT (NBS) and UTC(NBS) time scales.
2.1.2 Computed or "paper" time scales

The Atomic Time Standards Program Team utilizes nine oscillatorclock combinations that produce the computed time scales. One might

ask not only what is a "paper" time scale, but also how is it produced,

and then how is it used.

The paper time scales, AT(NBS) 1 and UTC(NBS) 1

, are in fact composite

time scales given by computer printouts which relate the indicated

time of each of the nine clocks in the ensemble to the computed scales

that are produced by appropriately weighting the individual contributing

oscillator-clocks. The computer utilizes the current measured or estimated

frequency of each oscillator with respect to the NBSFS, along with the

past history of each of the nine oscillators to apply a weighting factor

to each oscillator based on its performance. The computed composite

scales produced are more uniform than a scale produced by the best unit

of the ensemble. The computer output is a set of numbers that are the

time differences between each clock output and the computed scales.

With this information one has a mechanism for comparing the timekeeping

performance of individual clocks comprising the time scale system. This

comparison is with respect to AT(NBS). Figure 1 is a simplified diagram

of the system that produces the NBS time scales.

2.1.3 Operational Clock Systems

The Time and Frequency Division, through its Atomic Time Standards

Program Team, maintains a number of operational clock systems. The outputs

of these systems are electrical seconds pulses as well as visual displays

of time of day. Two of these clocks' outputs closely approximate the

UTC(NBS) time scale. Their deviations from the UTC(NBS) time scale seldom

exceed + 50 nanoseconds. A time comparator and alarm system are associated

with these two clocks. The alarm is actuated if the clocks diverge

by more than a small preset amount. An adjunct to the operational

clock systems is the clock comparison link to Fort Collins that utilizes

television synchronizing pulses. Figure 2 is a simplified diagram of

these operational clock systems.

2.2 Radio Station WWV, Fort Collins, Colorado

The heart of the time and frequency generation system at WWV is

a set of three commercial cesium beam frequency standards. These standards

are the basis for three identical generating units which provide to

the transmitters a composite RF signal containing the complete WWV format.

The cesium standards, through a series of dividers and distribution

amplifiers, drive the three WWV clocks, or more specifically the three

WWV time code generators. These time code generators provide the standard

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this rather complex format. Figure 3 shows a simplified drawing of

the WWV time and frequency generating system at Fort Collins.

2.3 Radio Station WWVH, Kauai", Hawaii

Station WWVH began operating at an old Navy site on the island

of Maui in 1948. After long and distinguished service, this station

was relocated near the town of Kekaha on the island of Kauai. The cesium

standards at WWVH are referenced indirectly against NBSFS vis the NBS

broadcasts, commercial and other government radio transmissions, and

portable clocks.

The generation system for WWVH, Kauai, Hawaii, is essentially the same as the system at WWV. Figure 4 shows this system.

2.4 Radio Station WWVB, Fort Collins, Colorado

The WWVB time and frequency generating system is somewhat similar

to that of WWV, although not as elaborate. WWVB uses a highly stable

quartz crystal oscillator as the standard frequency generator. This

crystal oscillator is referenced against NBSFS as noted later. Following

this quartz crystal oscillator is a device known as a frequency drift

corrector, which compensates for both frequency offset and frequency

drift of the quartz crystal oscillator. The format for the time code

and the 60 kHz driving frequency are produced by a special time code

generator, while two other generators are driven by a second quartz

oscillator. These generators, along with the oscillators and other

equipment, provide three semi-independent generating systems. Figure 5

shows the arrangement of this equipment.

2.5 Radio Station WWVL, Fort Collins, Colorado

Although regularly-scheduled WWVL transmissions were suspended

on July 1, 1972, 2 the essential equipment and antenna system remain

available for experimental use by NBS or by other government agencies on

a subscription basis. The following description applies to the WWVL

system in its most recent configuration.

' Effective July 1, 1972, regularly scheduled transmissions from WWVL

were discontinued. Contingent upon the need and the availability of

funds, the station broadcasts experimental programs thereafter on an intermittent basis only.

Transmissions may be made available on a subscription basis to organizations or agencies of the Federal government. Arrangements for use of

WWVL to broadcast experimental programs should be made through the FrequencyTime Broadcast Services Section 273.02, National Bureau of Standards,

Boulder, CO 80302.
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The frequency generation system for WWVL is like that of WWVB in

that quartz crystal oscillators and drift correctors are used as the

primary frequency generators. Since there is no complex time format

in this case, the one, two, or three operating frequencies, usually

selected from among 19.9, 20.0, and 20.9 kHz, are programmed to the

transmitter by NBS-built equipment. The synthesizers are units from

commercial VLF phase-tracking receivers. If multiple frequencies are being transmitted, the transmitter is shut off for about 0.1 second

out of each period to allow the frequency changeover. The carrier shutoff

is "on time" with respect to the UTC(NBS) time scale. The transmission

period of each frequency is usually ten seconds.

The local servo system for the generation and control equipment

is discussed in section 3. See figure 6 for a simplified diagram of

the WWVL control system.

3. TRANSMISSION OF TIME AND FREQUENCY

3.1 Transmission from WWV and WWVH

The multiple outputs from the RF driver units in the shielded control

room are supplied directly to the transmitters. The WWV and WWVH transmitters are simply high-power linear amplifiers and therefore do not contain

audio circuits or modulators. The delay from the time code generator

to the antenna is much less than the transmitted accuracy specified

for either station and is therefore negligible.

The specifications for WWV and WWVH are: frequency within + 1 X 10

of the NBS Frequency Standard; time within + 5 microseconds of the

UTC(NBS) time scale.

3.2 Transmission from WWVB

WWVB's transmitter and radiating system have an overall "Q" factor

of somewhat less than 100. This "Q" factor is sufficiently low to allow

operation without any phase control on the antenna. Future plans call

for a local servo system to continuously adjust the transmitted phase

at the antenna to be in agreement with the local station reference.

The local servo system compensates for the phase perturbations due to

wind shifting the antenna. These excursions seldom exceed 0.5 microsecond

peak to peak. To correct for discrete phase shifts that occur whenever

the antenna is tuned, a manual phase compensation is made based on a continuously operating phase monitor.

The relationship between a time pulse or carrier cycle and the

marker of a time scale becomes important when one operates a standard

time and frequency station in the LF range. The time at which a particular

positive-going carrier cycle crossover occurs for WWVB is published

on a daily basis in the monthly issues of the Time and Frequency Services

Bulletin. The first carrier crossover at the antenna occurs about six

microseconds after the marker pulse of UTC(NBS).

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The following points should be noted: (1) The radiated phase is

late relative to the time marker of UTC(NBS) because of the delays through

the WWVB transmission system. The master time code generator at WWVB

is maintained in close agreement with UTC(NBS), and any time difference

between UTC(NBS) and the WWVB time code generator is known to an adequate

accuracy. (2) When a phase or time error between the marker of UTC(NBS)

and the WWVB time code generator, and hence the WWVB radiated phase,

occurs, the error is corrected by changing the frequency of the WWVB

quartz crystal oscillator. The maximum rate of correction is limited

to 0.1 microsecond per four-hour period. In other words, during times

of phase correction, the frequency of WWVB can be in error with respect

to the NBS reference frequency by as much as 6 X 10" .

3.3 Transmission from WWVL (Experimental) 3

During periods when WWVL is operating, the transmitted phase is

controlled more precisely than that of any other NBS transmission.

There are two principal motivations for doing this: (1) The realizable

stability of the transmission through the medium in the 20 kHz region

is quite high; and (2) the susceptibility of the transmission system

to phase perturbations is also quite high. The "Q" of the antenna system

is of the order of 1000. With a tuning reactance of greater than 500

ohms, a change in this reactance of 0.2 percent causes a phase shift

of 45 or more than 6 microseconds. Every effort is made to hold the

transmitted phase to within +0.1 microsecond of its nominal value.

This has been accomplished through the use of a highly sensitive local

servo system.

This local servo begins with a pickup coil, located in the building

containing the antenna loading coil. During a transmission, the voltage

from this coil is supplied through a buried coaxial cable to the shielded

control room in the transmitter building. In the control equipment,

this signal is compared with the RF output of the WWVL synthesizer.

Any phase shifts detected at the antenna loading coil building can be

quickly compensated by the servo-driven phase shifter. Each transmitted

frequency has its own phase shifter, but the system contains a single

servo motor geared to all the phase shifters. Since each phase shifter

is continuously driven, any phase error is precorrected for the phase

shifter that will be controlling the transmitter during the upcoming

transmitting period. This precorrection reduces the phase noise of

the transmission considerably. In addition, the input phase to the

transmitter would be adjusted so that the zero voltage crossover of

the radiated field as measured at the antenna occurs in coincidence

with the time marker of the UTC(NBS) time scale.

"y " - - M ... — . _ See footnote .

13
4. TIME AND FREQUENCY INTERCOMPARISONS

4.1 WWV Self-Comparisons

As mentioned earlier, WWV has three independent time and frequency

generating systems.. These systems are intercompared at several points.

First, the phases of the 0.1 MHz outputs from the dividers following

the cesium standards are intercompared. The d-c outputs of the phase

detectors used are applied to a multi-channel chart recorder that has

a phase (time) full scale value of 1 microsecond. Second, the UTC(NBS)

time scale clocks which are part of the time code generator-programmers

and another set of UTC(NBS) clocks are intercompared several times daily.

The third comparison is accomplished at the time code generators.

The IRIG-H time code from each generator is monitored by a code comparator.

An alarm is sounded if the output from any unit diverges from any other

by 5 microseconds, or if a clock jumps phase such that its code is misaligned

with respect to that of any other unit. With these three comparison

systems, any standard, divider chain, or time code generator that fails can be immediately detected. See figure 3 for a simplified block diagram

showing the comparison systems.

4.2 WWVH Self-Comparisons

While the equipment complement at WWVH is essentially the same

as that of WWV, the Hawaiian staff has arranged the components in a slightly different manner. Again as at WWV, there are three points

of intercomparison of the triply-redundant systems. Initially the three

cesium standards are directly intercompared. Then the outputs of the

fail-safe dividers following the phase setting resolvers are compared.

Finally the IRIG-H code from each unit is continuously compared in a code comparator that is identical to the WWV unit. Figure 4 shows the

WWVH time and frequency generating system with the intercomparisons

indicated. Other aspects of the system, such as chart scale widths and

the use of phase error multipliers, are similar to those of WWV.

4.3 WWV-WWVB-WWVL Intercomparisons

The standard 100 kHz output from the WWV time and frequency generating

system designated "Rack C" is sent by coaxial cable to the WWVB/WWVL

control room. This standard frequency drives a digital clock at WWVB/WWVL.

All measurements of the WWVL and WWVB local clocks, which are driven

by the quartz crystal oscillators, are in terms of a high-stability

clock in Rack C at WWV. Thus the Rack C clock is the master clock for

the Fort Collins site.

This system of clock comparisons effectively prevents undetected

clock or time code generator failures. In addition to the clock comparisons,

the quartz crystal oscillators that form part of the LF and VLF generating

systems are continuously compared with the standard 100 kHz signal from

WWV. See figure 7.

14
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4.4 Fort Collins-WWVH Comparisons

Station WWVH uses all available information to maintain close ties

with the Fort Collins and NBS Boulder standards. This includes monitoring

of a Hawaiian Loran-C station, monitoring of station WWVB, portable

clock carries from NBS Boulder, and time comparisons via the NBS line10 TV system with a phase-stabilized Naval communications station on the island of Oahu. From portable clock trips and satellite comparisons,

the relationship between the master clocks at this Naval radio station

and the clocks at the U. S. Naval Observatory, and ultimately at Fort

Collins and Boulder, is known at all times to within about 1 microsecond.

4.5 Fort Collins-NBS Boulder Comparisons

4.5.1 Portable Clocks

A reliable high-precision method of comparing the Fort Collins

Master Clock with the NBS Boulder Master Clock is to physically carry

a cesium beam clock between the two locations. A highly accurate portable

clock of this sort normally loses or gains less than 0.1 microsecond

during the four-hour round trip. To perform this task on a daily basis

is expensive and Unnecessary. Nevertheless, a portable clock is occasionally carried to Fort Collins when circumstances dictate.

4.5.2 TV Synchronization Pulse Method

Because the portable clock method for clock synchronization is

expensive and time consuming, a new method had to be found. In 1967,

Tolman et al .

** described a method for comparing remote clocks using

television synchronizing pulses for time transfer. Since May 1968,

this method has been used to compare the Master Clock at Fort Collins

with the NBS Master Clock at Boulder.

The system uses what are called line-10 synchronized pulse generators. 5 These generators emit an electronic pulse each time line 10

(tenth line of the odd field) is received and processed by the TV sets.

The repetition rate of line 10 is such that the generators emit almost

30 pulses per second. The master clocks at Boulder and Fort Collins

are used to start a time interval measurement at each location at a particular second. The line-10 generator pulse is used to stop the

time interval measurement. If the master clocks at the two sites are synchronized, the difference between the time interval measurements

(using the same line-10 signal) would be the differential propagation

delay time of the TV signal only. This differential delay value is

known to be 257.9 microseconds for the particular TV channel employed.

H Tolman et al., "Microsecond clock comparison by means of TV synchronizing

pulses," IEEE Trans, on Instr. and Meas . , IM-16 , No. 3, September 1967.

5 D. D. Davis, B. E. Blair, and J. F. Barnaba, "Long-term continental

U.S. timing system via television networks," IEEE Spectrum, _8_, No. 8,

August 1971.

16
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This number was determined by carrying a portable cesium clock between

the two sites, synchronizing the master clock at Fort Collins, and then

measuring the delay time of the TV signal. If the time interval measurements indicate a delay time other than 257.9 microseconds, the difference

is attributed to Fort Collins master clock error. Figure 8 shows the

geography involved. As a result of these measurements, the difference

between the UTC(NBS) time scale and the Fort Collins master clock is

known with a precision of about 30 nanoseconds. The overall accuracy

of the measurement is about +0.2 microsecond.

5. SUMMARY

The Time and Frequency Division of NBS maintains the NBS Frequency

Standard, generates the NBS time scales, and disseminates standard time

and frequency information via NBS radio broadcasts.

This report has presented some aspects of the work of the Time

and Frequency Division. Methods of time scale generation were discussed.

Technical aspects of radio stations WWV, WWVH, WWVB, and WWVL were presented,

Methods of time scale comparisons between the various radio stations

and the Boulder Laboratories were described. Simplified block diagrams

of the radio stations' time and frequency generating systems as well

as those at the NBS Boulder Laboratories were included. Details and

more comprehensive diagrams are available to interested individuals

upon request from the Frequency-Time Broadcast Services Section, 273.02,

National Bureau of Standards, Boulder, CO 80302.

6 . APPENDIX

6.1 Explanation of NBS Time Scales

6.1.1 AT (NBS)

AT (NBS) is an atomic time scale, previously called NBS-A, whose

rate is determined by the primary frequency standards of the National

Bureau of Standards (NBSFS) . These standards provide the unit of time

interval, the second, as defined in the International System of Units

(SI). The origin marker of the scale was in agreement with UTC(NBS)

at 0000 UT 1 January 1958.

6.1.2 UTC(NBS)

UTC(NBS) is a coordinated time scale; i.e., the Bureau International

de l'Heure (BIH) , Paris, France, determines when steps of 1.0 second

should occur to keep this Universal Time Scale in approximate agreement

with earth time UT1. The rate of UTC(NBS) is occasionally adjusted

slightly to keep this scale in agreement with UTC(BIH). Because of

these adjustments, and because laboratory standards around the world

operate on slightly different frequencies, the rate of UTC(NBS) is usually

not exactly the same as the rate of AT(NBS).

18
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS TN-656

2. Gov't Accession

No.

4. TITLE AND SUBTITLE

STANDARD TIME AND FREQUENCY: ITS GENERATION, CONTROL, AND

DISSEMINATION BY THE NATIONAL BUREAU OF STANDARDS

7. AUTHOR(S)

John B. Milton

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

3. Recipient's Accession No.

5. Publication Date June 1974

6. Performing Organization Code

273,02

8. Performing Organ. Report No.

10. Project/Task/Work Unit No. 2730128

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Same as 9.

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The Time and Frequency Division of the National Bureau of Standards maintains

primary frequency standards, which provide a realization of the internationallydefined second, and two atomic time scales, AT (NBS) and UTC(NBS). AT (NBS) xs dependent upon the primary frequency standards, an ensemble of conrmercxal cesium

clocks, and a computer algorithm to process the data. The UTC(NBS) scale is

derived from AT (NBS) by the addition of small annual frequency adjustments and

leap second adjustments to keep its time nominally synchronous with the xnternational time scale UTC. The UTC(NBS) time scale is used to calxbrate the clocks

and secondary standards necessary for the operation of the NBS radio stations,

WWV WWVH, WWVB, and WWVL. These stations transmit various standard frequency

and time signals throughout the world, and, in addition, provide certaxn offxcxal

announcements such as geoalert warnings, marine weather advisories, and radxo

propagation forecasts.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Clock synchronization; frequency and time dissemination; primary frequency standard;

standard frequency broadcasts; time interval; time scales.

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