February 1967 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
This is the second of a two-part series
on the move of the WWV transmitter stations operated by the National Bureau of Standards
(now called National Institute of Standards and Technology) from Greenbelt, Maryland,
to Boulder, Colorado. WWV Part I appeared in the January 1967 edition of the ARRL's
QST magazine. WWV began transmitting time / frequency standards in 1920
in order to provide a means for remote stations and laboratories to calibrate local
standards that would prevent transmitting stations from interfering with each other.
Although most people don't realize it, the 60 kHz signal that their 'atomic'
clocks and watches use to self-adjust time emanates from the WWVB antenna in Boulder.
The first installment of the article discusses the history and rationale for relocating
the WWV facility to a new location. This second part gets into the technical aspects
of the WWV facility's equipment and operation. As usual, I am amazed at the pioneers
who conceived of, designed, and implemented these kinds of operations.
Here are Part I
and Part II, and this
is a short story on
WWV Moves to Colorado
In Two Parts: Part II
By Yardley Beers, W0EXS
The earliest function of WWV was to transmit
standard carrier frequencies to provide frequency calibrations so that radio stations
could stay on their assigned frequencies and avoid mutual interference, and to allow
persons having receiving sets and wavemeters or frequency meters to calibrate them.
Indeed, it is this feature of the broadcasts which is of chief interest to amateurs
today. However, in the course of history the usefulness has been greatly increased
by superimposing time markers and other information by a suitable program of modulation.
In addition, mainly because of the needs of stations used for tracking of rockets
and space vehicles, the present accuracy is far in excess of that required for the
original purpose. However, for the moment, we shall confine our remarks to this
Quite clearly, if the objective of avoiding interference between stations on
adjacent channels is to be accomplished, all frequency measurements must be coordinated:
that is, they must be referred to a single standard. Similar reasoning holds for
countless other physical quantities: length, force, speed, voltage, current, resistance,
power, to name a few. However, most of these are not independent. They may be related
by definition. For example, if one measures the time necessary for an object to
travel some previously-measured distance, he can determine the speed. Or they may
be related by physical laws. For example, if a known current flows through a wire
of known resistance, the voltage drop between its ends, as measured by a voltmeter,
must agree with that computed by Ohm's Law.
Sadami Katahara, KH6DK, engineer-in-charge, makes adjustment
to frequency control equipment at WWVH.
Richard F. Carle K0LYM, engineer-in-charge of WWVB-WWVL, with
some of the frequency control equipment of that station.
Fig. 1 - Accuracy of WWV broadcasts as a function of time.
Solid curve gives accuracy of the transmitted signal. Dashed curve gives accuracy
of received signal at about 1,000 miles from transmitter. Degradation of accuracy
is due to propagation effects. See text for information on present accuracy.
Thus, to digress for a moment, we are led to the concept that all the physical
measurements within the U.S.A. form the National Measurement System. This System
is involved in essentially all activities of commercial and private life. It makes
it possible to assure a housewife that when she pays for a pound of meat in a Denver
supermarket she will receive the same amount of meat as when she shopped at her
former market in Chicago. It makes it possible for the piston of an automobile motor
made in Detroit to give proper performance when used in a cylinder block made in
New Jersey. This System is something far greater than the National Bureau of Standards,
but the Congress has given the Bureau the responsibility of providing "the basis
for accurate and consistent measurements": that is, for providing standards and
for leading the coordination of measurements within the System.
However, the coordination is not solely confined to measurements within the U.S.A.
Frequency measurements must be consistent with those outside if international QRM
is to be avoided, and manufactured goods on the international market must have specifications
given in units which are internationally significant. Therefore, NBS is further
given the responsibility of making international comparisons. As part of its coordinating
role, NBS must participate in the standardizing of the units and definitions of
quantities. As the result of the recommendations of international committees to
which NBS is a party, NBS has adopted for its own use the hertz as the unit of frequency,
and it is encouraging others to do the same. For this reason, this unit is used
in this article.
In the hierarchy of measurements, "frequency" and the very closely-associated
quantity "time" collectively play a very important and unique role. It has been
stated above that the measurements of many quantities are interrelated. In fact,
the interrelations are so numerous that it can be shown that the system of measurements
of physical quantities is based upon the units of just four quantities; frequency-time
is one of these, the others being length, mass, and temperature. Also, it is the
only unit for which the user can obtain instantaneously a virtually direct calibration
against the NBS Standard in his laboratory, factory, or home. For this latter reason,
there is an effort to convert measurements of other quantities into measurements
of frequency by the use of suitable transducers.
Fundamental Considerations of Time and Frequency
If the reader is to appreciate fully the significance of the WWV broadcasts,
it is necessary to review some fundamental concepts. The measurements of both time
interval and frequency are based upon some physical phenomenon which ideally is
perfectly repetitive - or which, if not exactly repetitive, is accurately predictable,
such as the oscillations of a piano string, the motion of a pendulum, or the electrical
oscillations of a resonant circuit composed of an inductor and capacitor. Until
recently, the most accurately-predictable repetitive phenomenon available was the
revolution of the moon around the earth (although the results are generally expressed
in terms of the time-equivalent motions of the earth about the sun). Until recently
all measurements of time and frequency have been referred to this. However, it is
now generally believed that the electromagnetic radiation emitted by atoms and molecules
is much more uniform, and that there is no physical reason to suppose that there
should be any variation provided the atoms are either isolated or maintained under
constant environmental conditions. Therefore, the best present standards of time
and frequency are based upon these atomic radiations. Measurements of the motions
of the earth and moon referred to them reveal nonuniformities in these motions which
are quite trivial from the human point of view but which, scientifically speaking,
are very significant.
A standard frequency is determined by the number of cycles of the reference phenomenon
per second of time. Conversely, if one counts the number of cycles of standard frequency
which occur between two events, he can ascertain the time interval between those
events. Thus, a clock consists of (a) a physical configuration which gives rise
to a repetitive phenomenon and (b) a device which counts oscillations. When one
is counting the intervals between large numbers of events, it is convenient to refer
measurements to one particular event, which is said to define the "epoch" of a time
scale. For example, when we speak of the year 1966 A.D., we are speaking of a time
scale for which the epoch is defined by the birth of Christ, and we are referring
to some event occurring 1966 years later, approximately.
The unit of time used by scientists is the second, and at present this has two
different definitions. One of these, called the "Ephemeris" second, is based upon
astronomical measurements, and we shall not give the details here. The other, the
"atomic" second, is the time interval required for exactly, 9,192,631,770 cycles
of the radiation absorbed or emitted by a certain transition of isolated cesium
atoms. This last statement is equivalent to saying that, by definition, the frequency
of this radiation is exactly 9,192,631,770 hertz. These two definitions of the second
are consistent; that is, if one were to refer any given time-interval measurement
either to appropriate astronomical observations or to the cesium atom, he would
obtain essentially the same result for the first several digits, but he would find
the measurements relative to the cesium atom to be more precise: that is, more of
the digits in his answer would be meaningful. Furthermore, if one has the proper
apparatus, measurements relative to the cesium atom can be carried out much more
conveniently than the astronomical ones. Mainly for the first of these two reasons,
atomic measurements are preferred for accurate scientific work.
Frequency and time are the primary business of WWV, and the physical facilities
described in Part I of this article exist solely for the dissemination of highly
accurate information about these two quantities. The article concludes with a description
of the basic standards used for this purpose, how they are used to control the transmitters,
a brief history of WWV and a description of the NBS organizational setup that is
At the present, two types of time scales have been established for people who have
need for accurate time measurements. One of these is Atomic Time, which does progress
uniformly, in principle. The other is Universal Time, which progresses in synchronism
with the slightly-irregular rotation of the earth; this is required for earth navigation.
To generate Universal Time from Atomic Time, it is necessary to compensate for the
irregularities of rotation. The most widely-used type of compensation involves a
combination of (a) offsetting the frequency of the oscillator in the clock by an
amount determined by the International Time Bureau in Paris and (b) by making occasional
step adjustments of 0.1 second, as determined by the International Time Bureau in
consultation with astronomical observatories all over the world. At present, the
frequency offset is three parts in a hundred million lower than specified by the
definition of the second, an amount much too small to be noticeable by radio amateurs
and many others, but large enough to be important to those who track satellites.
This offset is held constant within any calendar year, but frequently it has been
necessary to change it from year to year. Therefore, many people feel it would be
better to eliminate offsets completely and use only step adjustments, even though
there would be a few more of them. This subject received considerable attention
at the 1966 meeting of the CCIR (International Consultative Committee on Radio)
in Oslo, and as a result of agreements made there, the use of offsets, which has
been mandatory in standard-frequency transmissions on certain frequencies, is now
optional. It is likely that in the near future many standard frequency transmissions
will eliminate them.
At the present time, however, the frequencies of WWV, WWVH, and WWVL are offset,
and their time signals are on the so-called NBS-UA Time Scale, which is generated
by an atomic standard but which is compensated by the offset and by step adjustments
to agree with Universal Time within about 0.1 sec. On the other hand, 60 kHz is
not one of the frequencies covered by this international agreement, and at the present
time the frequency of WWVB is not offset; the time is broadcast in accordance with
the so-called Stepped Atomic Time Scale (SAT), which uses 0.2 sec. step adjustments
only to keep within approximately 0.1 sec. of Universal Time. The offset and step
adjustments of the NBS-UA Scale are determined by the International Time Bureau
on the basis of data supplied from many sources. Step adjustments of the SAT Scale
are determined by NBS on the basis of data supplied by the U.S. Naval Observatory.
When a step adjustment is made on either scale, it is made at the beginning of a
month and announced in advance in the Federal Register and in publications of the
While WWV, WWVH, and WWVL broadcast time on one scale and WWVB on another, Universal
Time on either scale can be obtained from any broadcast by means of corrections
partly coded in the broadcasts. The details of how this can be done, and much other
information concerning the broadcasts, can be obtained from a booklet.1
History and Accuracy of NBS Broadcasts
The first broadcasting conducted by NBS, in 1920, was on an experimental basis
in order to gain technical competence. For a brief time entertainment and market
reports were transmitted for the Department of Agriculture. Although speech and
music were transmitted, code was used for the reports. After a short time, this
work was discontinued. It should be noted, however, that these broadcasts preceded
the much more publicized pioneering broadcasts of KDKA.2
Regular standard frequency broadcasts from WWV started on March 6, 1923, with
the station originally located upon the grounds of NBS in Washington. (Some previous
experimental tests had shown that wavemeters owned by some of the listeners were
off by as much as seven percent!) Originally transmissions took place only a few
hours a day, and spot frequencies were transmitted in accordance with a previously
announced schedule. These frequencies ranged from below the a.m. broadcast band
to considerably above.
After a brief interlude in College Park, Maryland, the station was moved in 1932
to a site near the now-discontinued station in Greenbelt, Maryland. In 1931 essentially
continuous operation on 5 MHz was initiated and, while oscillators using quartz
crystals had been used for reference for some time previously, at that time the
frequency of the station became directly crystal controlled. Operation on various
spot frequencies continued as before for a time, but it tapered off while continuous
operation on 2.5 MHz and on integral multiples of 5 MHz were gradually added to
the service. On November 6, 1940, the station was demolished by fire. By ingenuity
and hard work, a new station was improvised and put on the air within a few days.
On August 1, 1943, the station went on the air with improved "permanent" facilities
in Greenbelt. In 1948, similar broadcasts were initiated from WWVH in Maui, Hawaii
on some of the same frequencies (at present 2.5, 5, 10, and 15 MHz.)
The accuracy of the carrier frequencies of these broadcasts, as to be expected,
increased with time by many orders of magnitude, as indicated by the solid curve
of Fig. 1, which gives the accuracy of the ground-wave signal. This improvement
was due to the innovation of improved reference standards and control methods, as
noted on the figure. At present, the accuracy is of the order of a few parts in
a million million (1012), and the control system is so good that this
closely approaches the accuracy of the NBS Atomic Standard. It is, of course, far
in excess of that needed by radio amateurs and other services for keeping their
transmitters on assigned frequencies. If an amateur can measure a 50-MHz signal
with an accuracy to 50Hz, he would probably consider that his accuracy is far greater
than he needs. Yet this would be only one part in a million (106). Needs
for other transmitting services are hardly any greater; only the tracking of satellites
and pure science require this extreme accuracy.
It is to be noted that a spectacular increase in accuracy resulted from the introduction
of the use of a cesium atomic standard. The cesium atoms give a resonance which
corresponds to a Q of about 100 to 1000 million, which exceeds that obtainable from
quartz crystals and is far in excess of that obtainable from inductors and capacitors.
Also, the cesium standard is far less vulnerable to environmental effects such as
changes in temperature. At present, NBS and many other laboratories are continually
improving atomic standards and investigating other types, such as the hydrogen maser,
which many people feel is superior to the cesium standard. It is amusing to note
that the quartz-controlled oscillator has passed from something which was avante
garde for a primary standard to something which is a consumer item, as contained
in walkie-talkies that can be purchased at many corner drugstores.
The NBS cesium beam atomic frequency standard NBS-with C. J.
Snider (left) and D.J. Glaze, W0YVZ
Howard E. Michel, Jr., K0BPY, checks the standing wave ratio
of the 15 MHz transmitter before turning on the power of the 10 kw. transmitter
connected to the dummy load equipment shown here.
Unfortunately, the accuracy of the signal from WWV as received at some distant
point, typically 1,000 miles away, is considerably poorer than that of the transmitted
signal. This is indicated by the dashed curve in Fig. 1. This deterioration
of accuracy is due to Doppler shifts resulting from motion of the ionosphere. The
horizontal slope of the latter portion of this curve indicates that the accuracy
of the received signals is ultimately limited by this effect. For distant users
the present frequency control of the station is far better than it need be.
On the other hand, there are special users who require these high accuracies,
and many of them are located at large distances. To satisfy their needs, other methods
had to be sought. One method is to use radio stations with much lower frequencies,
where propagation errors are far less serious. This approach led to the establishment
of WWVB and WWVL. Another approach is physically to carry high-quality quartz clocks
and small atomic clocks from NBS and other reference laboratories to field sites.
NBS and other organizations are presently engaged in such an activity on a limited
The establishment of WWVB and WWVL was indirectly assisted by the move of the
principal radio work and some of the low temperature work of NBS from Washington
to new laboratories at Boulder, Colorado, the home of the University of Colorado,
during 1951-1954. At about that time the standards work and propagation work were
separated organizationally into the Radio Standards Laboratory (RSL) and the Central
Radio Propagation Laboratory (CRPL).3
A principal purpose of the move was to provide access to better sites for propagation
studies, and in due time experimental standard-frequency transmissions with low
power were commenced on 60 kHz from the Boulder site and on 20 kHz with an antenna
strung across Sunset Canyon, a few miles to the west. It was shown that propagation
errors at distances up to 2,000 miles were negligible at 60 kHz. The 20-kHz transmission
gives poorer accuracy at these distances, but at distances above 2,000 miles its
errors are less than those of 60 kHz. However, should the accuracy of the frequency
standards improve by another factor of 10 greater than they are today, it is almost
certain that propagation errors at these two frequencies will tend to limit the
accuracy of the received signals, and the carrying of clocks will play an even more
As the result of the success of this experimental work, the present stations
WWVB and WWVL were constructed on a site approximately seven miles north of Fort
Collins, Colorado, which is known as the home of Colorado State University. NASA
provided the funds and sponsored the WWVL facility, and is the main potential user
of its signals for synchronizing distant satellite stations. This site provided
a large area of flat land of very high conductivity, which aided in improving the
antenna efficiencies at these lower frequencies. Also, it was far enough away from
Boulder (56 miles) to avoid serious interference to propagation studies from the
The question arises: Why was it necessary to rebuild WWV if these lower frequency
transmissions give greater accuracy? The answer is that it is a matter of convenience.
This article is concerned with WWV, and it would be inappropriate to give a long
discussion of the details of the construction of WWVB and WWVL and the methods of
using their signals. It suffices to say that most commonly-available receivers cannot
receive them at all, and if their signals are to be fully utilized, special and
rather expensive receivers are required. The users who need the full accuracy, such
as NASA, are very important but are few in number, and are of a type that usually
can afford these special receivers. On the other hand, WWV and WWVH can be received
on sets which are generally available either without modification or by the addition
of a very simple converter. The number of users whose needs can be met with the
accuracy of WWV, using simple receivers, is very large. Amateurs, of course, are
in this group. Therefore, it is considered justified to maintain and improve the
service offered to this larger group.
Modulation of WWV
Originally the WWV transmissions provided only standard radio frequencies, with
the principal purpose of making it possible for radio stations to stay on their
legally assigned frequencies. In 1935, the service was augmented by modulation of
the carrier to provide time pulses at one second intervals and standard musical
pitch. Later, other information - such as time, time-scale corrections, and propagation
warnings - was added to the modulation program, as described in detail in Reference
1. Incidentally, signals from WWV and WWVH may be partially distinguished, even
though they are on the same carrier frequencies, because the station announcements
are staggered and they have different silent periods (15 to 19 minutes after the
hour for WWVH and 45 to 49 minutes after the hour for WWV). Propagation forecasts
(mainly relevant to North Atlantic paths) are given only by WWV, and are given after
each station announcement at five-minute intervals. They consist of a letter ("N"
for normal, "U" for unsettled, and "W" for disturbed) and a number from one to nine
to indicate the quality. Audio tones of 440 and 600 Hz are transmitted on the schedule
contained in Reference 1, where one can also find explanations of other features
which are of less interest to most amateurs.
NBS Standards of Time and Frequency
The administration of WWV is related to the manner in which the frequency of
the station is controlled. Administratively, the station comes under the jurisdiction
of the Radio Standards Physics Division, which, with a full-time staff of slightly
more than 100 people, is one of the two technical divisions of the Radio Standards
Laboratory. About one-half of the Division's staff, distributed among three sections,
is devoted to time and frequency matters (the other three sections deal with radio
materials, quantum electronics, and plasma).
The Atomic Time and Frequency Section, headed by Dr. James A. Barnes, has the
responsibility of operating the NBS Atomic Frequency Standards. The standard in
use is a particular cesium atomic beam built by NBS and referred to as "NBS III."
However, the cesium beam standard is a passive device, and the present one is not
designed to give automatic calibrations twenty-four hours a day. Therefore, the
Section also operates continuously five very stable oscillators, some controlled
by quartz crystals and the others stabilized by atomic resonances. The frequencies
of these oscillators are continuously compared with each other, and the data are
recorded automatically. Also, once each working day they are compared to NBS III.
The output of one of the five oscillators is fed to a correction device consisting
of a driven phase shifter. The rate of drive of the phase shifter is adjusted to
(a) make output frequency correspond to the average of the five oscillators (or,
automatically, to the average of four of them if the fifth one is in serious disagreement)
and (b) to correct for the average drift through aging of the oscillators. The output
of this correcting device is referred to as the Drift Controlled Oscillator (DCO),
and this serves as the basis for the NBS time scales and for the control of the
radio stations. Connected to the DCO is also a device for producing the frequency
offset required by the NBS-UA Time Scale. Connected to these oscillators and other
circuits are a number of devices which count cycles and thus become clocks for telling
time on both the NBS-UA and NBS-A Time Scales. Originally the epoch of the NBS-A
Scale, which, unlike the Stepped Atomic Time Scale (SAT) broadcast by WWVB, has
never had any step adjustments, coincided with that of UT on approximately January
1, 1958. At the present time, NBS-UA, which has been adjusted to keep within close
synchronization with the rotation of the earth, lags NBS-A by about five seconds
due to the irregular motion of the earth, which during the immediate past has been
Besides routine operation and maintenance of these devices, the Atomic Time and
Frequency Section also does fundamental research and development of atomic frequency
standards, and it possesses several others of various types, partly for backup for
NBS III and partly for research purposes. An important part of this research is
the study of very-low-frequency noise. Additional important work on atomic frequency
standards is also carried out in the Quantum Electronics Section, headed by Dr.
Donald A. Jennings.
Control of the Radio Stations
The NBS radio stations administratively are part of the Frequency and Time Broadcast
Services Section, of which Mr. David H. Andrews is Chief. The headquarters of the
Section is in Boulder.
The stations are controlled by a system which makes direct reference to the NBS
standards in Boulder, and at the same time provides each station with high-quality
oscillators so that it can preserve a high approximation of synchronization with
the NBS III in case the control link fails for a time. Furthermore, each station
(counting WWVB-WWVL as one station for this purpose) is equipped with three oscillators.
On the assumption that it is unlikely that more than one oscillator will fail or
be in serious error at one time, it is assumed that should one disagree with the
other two, those two can be considered as being correct. The new Fort Collins WWV
has three new commercially-manufactured cesium-controlled oscillators.
The new WWV is located in a new building about one-quarter mile away from the
building housing the WWVB-WWVL transmitters, but the two stations are integrally
interconnected so that one person can monitor all of the transmitters on the site
and observe and correct any equipment that malfunctions. Furthermore, although the
WWV and WWVB-WWVL frequency-control systems are nominally independent, in the event
of failure of one of them, switches can be thrown to allow the remaining operating
system to control all transmitters. Also contained in the Fort Collins complex are
spare devices for generating frequency offsets.
The synchronization with the NBS standards in Boulder is accomplished by the
use of some monitoring receivers located in Boulder and operated continuously. The
received signals are compared to a signal supplied from the DCO in the Atomic Time
and Frequency Standards Section, as mentioned above. Electronic circuits determine
the sign and magnitude of any frequency error, and these control the modulation
of a 49.85-MHz transmitter which transmits the corrections to receivers at Fort
Collins. Demodulating circuits connected to these receivers generate correction
voltages which are applied to the oscillators that control the transmitters. Thus,
the system forms a closed-loop electronic servo, which not only corrects for drifts
in the oscillator, but also for changes in phase resulting from swaying of the antennas
in the wind. Incidentally, local v.h.f. amateurs regularly use the signal from the
49.85-MHz transmitter to check their 50-MHz receivers. However, in the near future
the 49.85-MHz link may be replaced by a microwave system.
Control of WWVH is accomplished primarily by monitoring the WWVB and WWVL signals
and correcting the oscillators. A check upon the time synchronization is made occasionally
by carrying portable clocks, partly by NBS personnel and partly by others. It is
the aim of Mr. Andrews to have at least four clock-carrying trips made to WWVH each
The Frequency and Time Dissemination Section, headed by Mr. A.H. Morgan, does
research on new ways of transmitting frequency and, especially, time calibrations
- for example, by the use of satellites and the use of VHF meteor-scatter propagation.
It studies propagation errors for correction of the WWVB and WWVL signals. It operates
a number of receivers for monitoring both NBS and some non-NBS standard frequency
broadcasts. Comparisons are continually made between the NBS standards and those
elsewhere, both in the U.S.A. and in foreign countries.
The theoretical group in the Division Office, headed by Dr. George E. Hudson,
also has made important contributions to the time and frequency program through
representing the Division on important committees such as the CCIR, in analyzing
data, and in aiding in the determination of frequency offsets and step adjustments.
As stated previously, Mr. David H. Andrews is Chief of the section which administers
the new station, and Mr. Peter P. Viezbicke is the engineer in charge of the design
and construction of the new station. Mr. R.S. Gray served as an assistant to Mr.
Viezbicke during early construction. Mr. John B. Milton is in charge of the design
of the frequency-control system of the new station.
The writer could not have prepared this article without the help of many individuals,
especially many of those mentioned in the article, in supplying information and
checking the manuscript for correctness. He is also indebted to Mr. Hugh J. Stewart
for help in obtaining the photographs; to Mr. Eldred C. Wolzien for correcting the
historical portions; and to Mmes. Donna Stolt and Eddyce Helfrich for typing and
During the last few months of operation of the Greenbelt station, it was necessary
to have extra staff in order to man both the old and new stations. During this period,
much of the manpower at Greenbelt was supplied under contract with the Philco Corporation
by a staff headed by Mr. W.M. Swartz.
1 NBS Miscellaneous Publication 236, "NBS Standard Frequency and Time
Services" (1966 Edition). For sale by Superintendent of Documents, U.S. Government
Printing Office. Washington, D.C., 20402 - Price 15 cents.
2 "Measures for Progress - A History of the National Bureau of Standards,
U.S. Department of Commerce, 1966.
3 In October, 1965, the organizational separation was made much more
complete when CRPL, although not moved geographically, was removed from the jurisdiction
of the National Bureau of Standards completely and joined to a new agency of the
Department of Commerce called the Environmental Science Services Administration
(ESSA). At that time CRPL was renamed the Institute for Telecommunication Sciences
and Aeronomy (ITSA). It was joined by other organizations, either removed to Boulder
from elsewhere or newly created, to form the Institutes of Environmental Research
(IER) which is a major unit of ESSA.