As is typical for Gary Breed, editor of
High Frequency Electronics magazine, he wrote a great general
interest article about the time and frequency broadcasts put out by the National
Institute of Standards and Technology (NIST) – originally National Bureau of Standards (NBS). His piece
concerned the history of the
that have been used since their origin in 1920 by amateur and professional radio
operators to calibrate and synchronize time and frequency standards. I highly recommend
reading the one-page article.
There is yet another standard signal that is broadcast continually from the NIST
site in Boulder, Colorado, that just about everyone is familiar with whether they
know it or not. That would be the
used by all of the "radio-controlled" or "atomic" clocks that people have sitting
on their mantels, hanging on their walls, built into Wal-Mart electronics weather
stations, or are wearing on their wrists as a watch.
The WWVB radio signal is broadcast continually at 60 kHz, from an
antenna array located on the plains just east of the foothills of the Rocky Mountains,
near Boulder, CO (I have been by there when living in Colorado). The two sets of
antennas are spaced 2,812 feet (857 m) apart. Each antenna is a top loaded monopole
consisting of four 400 foot (122 m) high towers arranged in a diamond shape. A "top
hat" system of cable is suspended between the four towers. This top hat is electrically
isolated from the towers, and is electrically connected to a downlead suspended
from the center of the top hat, with the downlead serving as the radiating element.
A lot of other tricks that any Ham would love are employed to make the antenna as
efficient as possible (about 65% between the two antennas) for a system that operates
at a wavelength of about 2.7 miles (5 km).
The effective radiated power is 50 kW. A computer system is used to keep the
system tuned under all meteorological conditions (rain, ice, wind, etc). The frequency
uncertainty of the WWVB signal as transmitted is less than 1 part in a trillion
(1012). When the path delay to the remote receiver is removed, UTC time can be determined
with an uncertainty of less than 100 microseconds. A full description is provided
on the WWVB website.
The 60 kHz signal is broadcast continuously at a rate of 1 bit per second using
pulse width modulation to produce a binary coded decimal (BCD) format. The time
code contains the year, day of year, hour, minute, second, and flags that indicate
the status of Daylight Saving Time, leap years, and leap seconds. WWVB identifies
itself by advancing its carrier phase 45° at 10 minutes after the hour and returning
to normal phase at 15 minutes after the hour. Again, visit the website for details.
Coverage area maps are provided by NIST that plot two-hour increments
and show where the receive signal strength is expected to be 100 microvolts per
meter or greater. As you would expect for 60 kHz, the area varies throughout the
24-hour period, contracting during daylight hours and expanding at night. The western
half of the United States is pretty much fully covered all the time, but the east
coast falls well below the threshold for a large part of the day. This explains
why the directions that come with most radio-controlled clock products says that
it might take until the next morning after power-up to be able to synchronize with
WWVB is yet another one of those ubiquitous technologies that has been so absorbed
into our world that we barely notice or appreciate it. Sort of like GPS, which nowadays
inhabits not only aircraft and ship cockpits but extends to cell phones, laptop
computers, car dashboards and dog collars, WWVB-based clocks are all around us.
My house has a
weather station with a built-in "atomic clock." I see WWVB-synchronized clocks sitting
on peoples' desks at work. Some types of test equipment, including calibration lab
time standards, are now advertising integrated WWVB receivers. Look around and you
will certainly see something with the familiar radiating antenna symbol on it.
If you are really interested in learning what it takes to design a product that
uses the WWVB signal, here is a link to the complete designer's specification published
"WWVB Radio Controlled Clocks: Recommended Practices for Manufacturers
This pub, "NIST Time and Frequency Services," thoroughly explains the NIST
time and frequency standard system.
Additional Info: See
WWV Moves to Colorado (Part 1) and
WWV Moves to Colorado (part 2), in the ARRL's QST magazine.
Posted December 29, 2009