fetlonaJ Bureau of Standards

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NBS

379

Standard Time and Frequency: Its Generation,

Control, and Dissemination from the

National Bureau of Standards

Time and Frequency Division

JOHN B. MILTON

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

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NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards ! was established by an act of Congress March 3, 1901 . Today,

in addition to serving as the Nation's central measurement laboratory, the Bureau is a principal

focal point in the Federal Government for assuring maximum application of the physical and

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divisions:Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic and Molecular Physics—Radio Physics -—Radio Engineering '-' —Time and Frequency -—Astrophysics - —Cryogenics.

-

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provides technical services designed to aid Government agencies in the selection, acquisition, and effective use of automatic data processing equipment; and serves as the principal focus

for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center consists of the following offices and divisions: Information Processing Standards—Computer Information — Computer Services — Sys- tems Development—Information Piocessing 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 Measure- ment System, and 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

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- Located at Boulder, Colorado 80302.

' Located at 5285 Port Royal Road, Springfield, Virginia 22151.
UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

NBS L_ kj I I ^ I v/#V L I

> ISSUED AUGUST J969

Nat. Bur. Stand. (U.S.), Tech. Note 379, 27 pages (August 1969)

CODEN: NBTNA

STANDARD

MD DSSSEMI

FREQUENCY: ITS GENERATION, CONTROL,

ION FROM THE NATIONAL BUREAU OF STANDARD!

TIME AND FREQUENCY DIVISION

JOHN B. MILTON

Frequency -Time Broadcast Services Section

Time and Frequency Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

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

Price: 35 cents

TABLE OF CONTENTS

Pag<

ABSTRACT . . . .

1. INTRODUCTION

2. STANDARD FREQUENCY AND TIME GENERATION . . . 4

2. 1 Atomic Frequency and Time Standards Section ... 4

2. 1. 1 The NBS frequency standard 5 2.1.2 Computed or "paper" time scales 5

2. 1. 3 Operational clock systems 6

2.2 Radio Station WWV, Fort Collins, Colorado .... 8

2. 3 Radio Station WWVB, Fort Collins, Colorado .... 8

2.4 Radio Station WWVL, Fort Collins, Colorado. ... 11

2.5 Radio Station WWVH, Maui, Hawaii 15

3. TRANSMISSION OF TIME AND FREQUENCY 15

3. 1 From WWV . 15

3.2 From WWVB 16

3. 3 From WWVL (Experimental) 17

3.4 From WWVH 18

4. TIME AND FREQUENCY INTERCOMPARISONS ..... 18

4. 1 Among the Fort Collins Standards . 18

4. 1. 1 WWV self comparisons 18

4.1.2 WWV to WWVB and WWVL ....... 19

4. 2 Between the Fort Collins Standards and the

WWVH Standard 21

4. 3 Between the Fort Collins Standards and the

NBS Standards 21

4. 3. 1 Portable clocks 21

4.3.2 TV synchronizing pulse method 21

a. Theory 21

b. Results . 22

Table 1. . . 24

APPENDIX- -EXPLANATION OF NBS TIME SCALES 26

in
LIST OF FIGURES

Page

Figure 1: System for Generation of Computed Time

Scales AT(NBS), SAT(NBS), and UTC(NBS) 7

Figure 2: Standard Frequency and Time Interval

Generating System, Time and Frequency

Division, Section 273. 04 ............. 9

Figure 3: WWV Time and Frequency Generation- -One

of Three Identical Systems Showing Intercomparisons, Ft. Collins, Colorado 10

Figure 4: WWVB Time and Frequency Generation and

Control- -One of Three Semi-independent

Systems, Ft. Collins, Colorado 12

Figure 5: WWVL Frequency Generation and Control

System, 1967-1969, Ft. Collins, Colorado 13

Figure 5a: New WWVL Frequency Generation and Control

System, Ft. Collins, Colorado 14

Figure 6: Clock Intercomparison WWV- WWVB- WWVL,

Ft. Collins, Colorado 20

Figure 7: Space-Time Diagram of Boulder to Ft. Collins

Clock Synchronizing. Diagram at One Instant

of Time 23

IV
STANDARD TIME AND FREQUENCY: ITS GENERATION, CONTROL,

AND DISSEMINATION FROM THE NATIONAL BUREAU OF STANDARDS

TIME AND FREQUENCY DIVISION

John B. Milton

The Time and Frequency Division of the National

Bureau of Standards produces the NBS time scales,

AT(NBS), SAT(NBS), and UTC(NBS). These time scales

are developed by utilizing the properties of the NBS

frequency standard, NBS-III. The main byproduct of

these time scales is the operational clock systems.

These operational clock systems are used, among other

things, to calibrate the clocks and secondary standards

necessary for the operation of the NBS radio stations,

WWV, WWVB, WWVL, and WWVH. These stations

transmit SAT (NBS), UTC(NBS), and various tones,

alerts, and corrections for time-of-day information.

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 Twentieth 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 radio

interference between adjacent channels in the radio spectrum.
The operational time-interval standard has progressed from a pendulum clock with electrical pulse output through crystal clocks to

the present-day "atomic clock." During the same period, the operational standard of frequency has evolved from a tuning fork to a quartz

crystal oscillator to an atomic beam device. While there may be only

a superficial connection between a pendulum clock and a tuning fork,

the present-day standards of time interval and frequency are, by their

very nature, inseparable.

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 to these broadcasts in 1945.

In 1948, NBS radio station WWVH was placed in operation. 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 of these stations were measured 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 I960, 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 ten

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, I960, 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.

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

facility disseminating standards of time and frequency from 19.9 kHz to

25 MHz. At the present time, these stations provide information at

both atomic frequency and an internationally agreed upon offset frequency.

The offset frequency is used to generate a Universal Time scale, called

UTC, which closely approximates the UT2 scale based on the rotation

of the Earth.
Even though the time and frequency control of these stations was

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 efforts of the Time and

Frequency Division of the NBS. These include the generation of the

computed NBS time scales, their translation into real 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.

2. STANDARD FREQUENCY AND TIME GENERATION

2. 1 Atomic Frequency and Time Standards Section

With the advent of the first working atomic clock system in 1948,

developed by Harold Lyons at NBS/Washington, the era of the highly

accurate atomic standards was born. The first work at NBS/Boulder

was under the project direction of Dr. R. C. Mockler. This early work

included, among other things, the refinement of the original NBS cesiumbeam device, NBS-I.

In late 195 9, the Atomic Frequency Standards Project, including

the 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.
2. 1. 1 The NBS Frequency Standard

The operations of the Atomic Frequency and Time Standards

Section are presently centered on a device known as NBS-III, which

serves as the NBS Frequency Standard. This machine, which is a longbeam cesium resonance standard, is probably the most thoroughly

evaluated standard of its kind. The most important requirement for a primary frequency standard is high accuracy, where the term "accuracy"

refers to an estimate of how far the output frequency of the standard

may deviate from the ideal frequency defined with respect to an isolated

cesium atom. In order to determine the limits of accuracy of such a standard, the possible causes, the cures in some cases, and the estimated magnitude of any and all small frequency shifts must be known:

hence the continuing study of NBS-III by the Section personnel. NBS-III

does not operate continuously; its main function is the frequency measurement of the frequencies of the working ensemble of clocks. These data, as described in the next section, are used in producing the AT(NBS) time

- 12

scale. The accuracy of NBS-III is ± 5 x 10 , based on 3 0" estimates

of all uncertainties.

2.1.2 Computed or "paper" time scales

The Section maintains six oscillator -clock combinations that are

used in producing three 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) * SAT(NBS)/'

; and UTC(NBS)/

are in fact computer printouts which relate the indicated time of each

of the six clocks in the ensemble to the computed scales that are

See Appendix.
produced by the computer. The computer utilizes the current measured

frequency of each oscillator with respect to NBS-III along with the past

history of each of the six oscillators to apply a weighting factor to each

oscillator based on its performance. The computed 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 clocks nationally or internationally

and also for comparing the timekeeping performance of individual clocks

comprising the time scale system. Figure 1 is a simplified diagram of

the system that produces the NBS time scales. More comprehensive

versions of all drawings in this document are available from the FrequencyTime Broadcast Services Section of the Time and Frequency Division.

2.1.3 Operational clock systems

The Section maintains three operational clock systems. The outputs of these systems are electrical seconds pulses as well as visual

displays of epoch. The time scale based on the primary NBS clock,

designated as Clock #7, closely approximates the AT(NBS) time scale.

Corrections for Clock #7 relative to the AT (NBS) time scale are computed

at regular intervals. Probably the most useful clock in a practical sense

is the unit designated Clock #8. This clock's output closely approximates

the UTC(NBS) time scale. The daily deviations of Clock #8 from the

UTC(NBS) time scale are published monthly in the NBS Time and

Frequency Services Bulletin. The deviation of this clock from the

UTC(NBS) time scale is constrained to always be less than 1 microsecond

and seldom exceeds ±0.2 microsecond. The third system, operating

from an independent quartz crystal oscillator, is a backup atomic rate

clock designated Clock #0. A time comparator and 24-hour alarm system
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are associated with Clock #0 and Clock #7. This alarm is actuated if Clock #7 diverges from Clock #0 by more than 5 microseconds. All of

the frequency standards in the operational system operate on the "atomic'

frequency. To produce an operational clock whose rate more nearly

approximates that of the UTC clock based on the earth's rotation, a

fixed frequency offset generator is placed between the atomic standard

and the operational clock, such as Clock #8. This offset rate for 19&9

is -300 x 10 . 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 Cs standards, through a series of dividers and distribution

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

three WWV time-code generators. These time-code generators provide

the standard 600 Hz, 440 Hz, 1 kHz, and all the gates, codes, etc. , necessary to produce 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 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
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stable quartz crystal oscillator as the standard frequency generator.

This crystal oscillator is referenced against NBS-III as noted later.

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

drift corrector, which compensates for both frequency offset and rate

change 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 4 shows the arrangement of this equipment.

2.4 Radio Station WWVL, Fort Collins, Colorado

The frequency generation 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 or two operating frequencies are programmed to the transmitter by NBS-built equipment. The synthesizers are units from

commercial VLF phase -tracking receivers. At present, two frequencies

are transmitted alternately every ten seconds. The transmitter is shut

off for about 0. 1 second out of each ten-second period to allow the frequency changeover. The carrier shutoff is "on time" with respect to

the UTC(NBS) time scale. See fig. 5 for a simplified diagram of this

system.

A second complete generating and programming system for WWVL

is nearing completion. The new system will allow the transmission

format to encompass from one to three operating frequencies. The

order of frequency transmission and the time length of each can be

controlled. The local servo system for the new generation and control

equipment is discussed in section 3. Figure 5a shows this new system.

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2.5 Radio Station WWVH, Maui, Hawaii

Station WWVH began operation in 1948 and is now the oldest existing

facility operated by the NBS Time and Frequency Division. Because the

generation and control systems and equipment have been little modified

since 1948, they remain rather primitive. A good quality quartz crystal

oscillator provides the driving frequency at 2.5 MHz. This oscillator

is referenced indirectly against NBS-III via the NBS VLF broadcasts.

The 440 Hz, 600 Hz, and 1200 Hz tones for the format are generated by

NBS- constructed equipment. WWVH has one standby clock system.

WWVH is soon going to be completely rebuilt on a new site at Kauai,

Hawaii, with operation at the new facility commencing about January 1,

1971. The generation and control equipment for the new WWVH will

mirror the system now in operation at WWV. Figure 3 shows the

WWV system.

3. TRANSMISSION OF TIME AND FREQUENCY

3. 1 Transmission from WWV

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

room are supplied directly to the transmitters. The WWV 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

WWV and is therefore neglected.

The specifications as published in the Time and Frequency Services

-12

Bulletin are: frequency within ±5x10 "of the NBS Frequency Standard;

time within ± 5 microseconds of the UTC(NBS) and UTC(USNO) time

scales (these two time scales have been coordinated since October 1968

and are identical to within state-of-the-art comparison techniques).

15
3. 2 Transmission from WWVB

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

factor of somewhat less than ]00. 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

epoch 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 carrier crossover occurs for WWVB is published on a daily

basis in the monthly issues of the Time and Frequency Services Bulletin.

This first carrier crossover at the antenna occurs about 6 microseconds

after the marker pulse of SAT(NBS).

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

late relative to the epoch of SAT(NBS) because of the delays through the

WWVB transmission system. The master time-code generator at

WWVB is maintained in close agreement with SAT(NBS) and any time

difference between SAT(NBS) and the WWVB time-code generator is

known at all times. (2) When a phase or time error between the epoch

of SAT(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. ] microsecond per four-hour period. In other words,

during times of phase correction, the frequency of WWVB can be in error

-1 2

with respect to the NBS reference frequency by as much as 6 x ]

16
3. 3 Transmission from WWVL (Experimental)

The transmitted phase of WWVL is controlled more precisely

than that of any other NBS transmission. There are two reasons for

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%

causes a phase shift of 45 or more than six microseconds. Every

effort is made to hold the transmitted phase to within ±0.1 microsecond

of its nominal value. This calls for a highly sensitive "local" servo

system.

This local servo begins with a pickup loop located in the building

containing the antenna loading coil. The voltage from this coil is

supplied through a buried coaxial cable to the shielded control room in

the transmitter building. This signal, through an AGC amplifier, is

compared with the output of the WWVL rf synthesizers. Any phase

shifts detected at the loading coil building are quickly compensated for

by the local servo-driven phase shifter. There is one of these phase

shifters for each transmitted frequency. In addition, the input phase

to the transmitter is adjusted so that the zero voltage crossover of the

radiated field as measured at the antenna occurs at the epoch of the

UTC(NBS) time scale. This is done for all transmitted frequencies.

Mentioned briefly in section 2.4 was the new WWVL generation

and control system, which has been tested but is not yet in use. This

new system is much improved over the existing control servo. For

instance, the "dead-band" of the original system was about 0. 2 microsecond. The new system "dead-band" is of the order of 20 nanoseconds.

The new system has a single servo motor-amplifier driving the three

17
phase shifters. Since each phase shifter is continuously driven, any

error is "precorrected" by the phase shifter that will be controlling the

transmitter in the next transmitting 1 0-second period. This tends to

reduce the phase -noise of the transmission. This noise can be as high

as 2 microseconds peak-to-peak in the current system.

Again, as in the case of WWVB, any errors between the radiated

phase and the epoch of UTC(NBS) are known at all times, and corrected

by the same procedure and at the same maximum rate of?- 6 x 10

3.4 Transmission from WWVH

Unlike the high-frequency transmitters at WWV, the units at Maui

are high-level modulated standard AM transmitters. The 2. 5 MHz from

the controlling crystal oscillator is fed to each transmitter. Amplification or multiplication of this frequency is carried out by each transmitter

as necessary. The various time-ticks, tones, voice announcements, and

Morse Code information groups are fed to the modulators of the transmitters. Even though the WWVH master clock is nominally maintained

in close agreement with UTC(NBS), the transmitter's modulation circuits degrade this performance somewhat. Station specifications are:

frequency within ± 5 x 10 of the NBS Frequency Standard, and time

nominally within ± 20 microseconds of UTC(NBS) and UTC(USNO).

4. TIME AND FREQUENCY INTERCOMPARISONS

4. 1 Among the Fort Collins Standards

4. 1. 1 WWV self comparisons

As was mentioned earlier, WWV has three independent time and

frequency generating systems. These units are intercompared at three

different locations in the systems: First, the phases of the "atomic"

1 MHz outputs from the dividers following the cesium standards are

18
intercompared. The outputs of the phase detectors used are applied

through meter relays to a multi-channel chart recorder with a 1 -microsecond full scale. Second, following the motor-driven offset generator,

the phases of the 1 MHz outputs from the UT C dividers are intercompared.

The phase comparison record is arranged to be on the same multi-channel

recorder. The meter relays are so adjusted that a 1 -microsecond

change in the phase difference between any two standards results in an

alarm sounding. The third intercomparison is accomplished at the

time-code generators. The 36-bit NASA time code from each generator

is monitored by a code comparator. The 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, offset

generator, divider chain, or time-code generator that fails can be

immediately detected. See fig- 3 for a simplified block diagram showing

the comparison systems.

4.1.2 WWV to WWVB and WWVL

The standard 100 kHz output from the WWV time and frequency

generating system designated "Rack A" is sent by coaxial cable to the

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

designated as the "Fort Collins Master Clock. " All measurements of

the WWVL and WWVB local clocks, which are driven by the quartz

crystal oscillators, are in terms of this Master Clock. This Master

Clock pulse is also returned by coaxial cable to the WWV control room.

This system of clock intercomparisons 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 fig« 6).

19
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4. 2 Between the Fort Collins Standards and the WWVH Standard

Station WWVH uses all available information to maintain close

ties with the Fort Collins standards. This includes continuous monitoring

of stations WWVB and WWVL and frequent comparisons, via portable

clock, with a phase stabilized Naval communications station on the island

of Oahu. Via portable clocks and other means the relationship between

the master clock at this Naval radio station and the clocks at Fort

Collins and Boulder is known at all times to within a few microseconds.

4o 3 Between the Fort Collins Standards and the NBS Standards

4. 3= 1 Portable clocks

The most reliable high-precision method of comparing the Fort

Collins Master Clock with the NBS/Boulder Clock #8 is to physically

carry a cesium-standard-driven clock between the two locations. A

highly accurate 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. 3. 2 TV synchronizing pulse method

a. Theory

Because the portable clock method for clock synchronization is

expensive and time consuming, and the Boulder-Fort Collins continuous

phase loop had been eliminated because it added short-term phase errors

due to sky-wave interference, 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,

Tolman, et al, "Microsecond clock comparison by means of TV synchronizing pulses," IEEE Trans, on Instr. and Meas., vol. IM-16,

No. 3, September, 1967.

21
this method has been used to compare the Master UTC Clock at Fort

Collins with the NBS Clock #8 at Boulder.

Figure 7 is a space-time diagram of how the method is developed.

Assume fig. 7 is a map of the geographical area in question at one instant

in time. Located on the map are five TV synchronizing pulses as they

might be at this instant.

From the diagram, if the Boulder clock and the Fort Collins clock

agree, the delay time over the distance D - D is

D = + B - A microseconds, (1) c where — = one synchronizing pulse period, or 63.55 microseconds. The

c values B and A are found by using the local clock tick as the start pulse

of a time-interval measurement and a synchronizing pulse from a local

TV set as the stop pulse. In the case of fig. 7, pulse #1 stops the Boulder

time interval measurement at the value "A" and pulse #5 stops the Fort

Collins time interval measurement at the value "B". If now the Fort

Collins clock is early relative to the Boulder clock, the value B will

increase because the interval will be longer. From the diagram, this

longer interval B' will be

B' = B + At (2) e "where At is the amount early,

e Computing the delay time D Using great circle distance calculations

and substituting eq. (2) into eq. (1),

4\

At = — - D + (B 1 - A) microseconds. (3) e c

b. Results

The TV synchronizing pulse method of clock intercomparisons was

put in service in May 1968. The measurements have been made on a

daily basis since then. During the actual measurement, some ten to

22
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Page 23
twenty readings are taken. The spread over this number of readings has

seldom exceeded ±0.5 microsecond. The average of this group is re- corded. When the Fort Collins Master Clock has diverged from NBS

Clock #8 by about one-half microsecond, the Fort Collins clocks are all

reset to the correct time. These "correct" times will vary from clock to

clock. For instance, the master clock at WWVL is maintained at a nominal and arbitrary 18 microseconds early with respect to Clock #8,

to allow for easier clock intercomparisons at Fort Collins. The WWV

clocks are maintained "on time."

The following table shows the results of the comparisons between

the TV synchronizing pulse method and eight portable clock trips from

Boulder to Fort Collins, starting in May 1968 and ending in December

1968.

Table 1

Portable Clock Reading TV Reading

Date Fort Collins Master Clock - #8 Fort Collins Master Clock - #8

May 14 17.63 Us 17. 92 Us

May 20 17.99 Us 18.64 us July 5 17.34 Us 17.74 Us

July 8 17. 10 us 17.20 Ms

Oct. 23 17.65 Us 17.57 Us

Nov. 1 17.89 Us 17.79 Us

Nov. 15 18.20 Us 18.24 Us

Dec. 17.33 Us 17.26 Us

It seems probable from the data that as the staff became more

familiar with the measurement scheme, the accuracy with respect to the

portable clock readings improved.

v

24
The method was proved to be satisfactory- -the precision and

accuracy are of a high order. The drawbacks to the current system

are mainly in the measurement technique. The amplified synchronizing

pulses themselves are not used; the actual measurement is made with

the set's internal oscillator that is phase locked to the incoming pulses.

It will be obvious to servo- systems people that this method is fraught

with problems (the free- running frequency of the internal oscillator must

be preset, for example).

A new system under development at the NBS utilizes the amplified

pulses from a more stable color TV system. The new system has

promise of improving the accuracy of the measurements by an order

of magnitude.

25
APPENDIX

EXPLANATION OF NBS TIME SCALES

ATfNBSl

AT (NBS) is an Atomic Time Scale, previously called NBS- A, whose

rate is determined by the primary frequency standard of the National

Bureau of Standards (NBSFS). This standard (NBS-III cesium beam)

realizes the second as defined in the International System of Units (SI).

The epoch of the scale was in agreement with UTC(NBS) at 0000 UT

1 January 1958.

SAT(NBS)

Stepped Atomic Time is a coordinated time scale; i. e. , the International Time Bureau (BIH) determines when steps in epoch of 0. 2

second should occur to keep this scale in approximate agreement with

UT2. The rate for this scale -was the same as for AT(NBS) prior to

1 October 1968. The NBS and the U. S. Naval Observatory (USNO)

agreed upon a coordinate rate starting 0000 UT 1 October 1968 for

SAT (NBS), and this rate may be written as:

f (f ^^J (1 +.4 x 10~ 13

) from 10/1/68 to 5/1/69 coordinate v NBSFS / / / /

f ,. . - (f ne_J (1 + 5 x 10" 1

) from 5/1/69 to present coordinate NBSFS

In other words, SAT (NBS) presently runs at a rate higher in fractional

13 frequency than AT(NBS) by 5 parts in 10 . SAT(NBS) is the time scale

broadcast by Radio Station WWVB. Note: The difference in rate between

these scales is relevant only to international and national standards

laboratories and observatories such as the BIH and NBS, and this in no

way affects all other users of this transmitted signal.

26
UTC(NBS)

UTC(NBS) is a coordinated time scale; i.e., the BIH determines

when steps in epoch of 0. 1 second and changes in rate should occur to

keep this Universal Time Scale in approximate agreement with UT2. Near

1 October 1968, the time difference, UTC(USNO) - UTC(NBS), was zero.

The NBS and the USNO agreed on a coordinate rate for these two time

scales starting 0000 UT 1 October 1968. The coordinate rate was chosen

as an average of the two rates, as determined by portable clocks, prior

to 1 October 1968. The USNO and NBS intend to maintain synchronization

of the UTC scales to within about 5 microseconds by using an appropriate

coordinate rate. The present coordinate rate of UTC(NBS) is:

f_.__.-_. = (f ) (1 - 300 x 10" 10

) UTC(NBS) coordinate

or

fUTC(NBS)= (f NBSFS

» (1 - 2 "- 995xl0

" 10)

-

This last equation is, again, useful only to other standards laboratories.

27 GPO 852 " 858

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