NIST Atomic Clock Broadcasts

Jim Whitehead (ejw@ICS.uci.edu)
Thu, 17 Sep 1998 14:48:58 -0700

So, last night Julia and I received a somewhat surprising wedding gift. The
gift was a digital clock, which initially made us very confused -- after
all, a digital clock is a bit pedestrian as a wedding gift. But, upon
closer examination, we discovered that the clock had an attached antenna and
claimed to be capable of setting its own time.

The documentation for the clock, from Oregon Scientific:

http://www.oregonscientific.com/timefram.html (the picture of the clock
appears to be a broken link right now).

States that the clock resets itself based on a time signal broadcast from
Boulder, Colorado, and gives some tips on how to adjust the antenna to get
the signal better. But, tantalizingly, it says nothing about the actual
signal that is sent.

AltaVista to the rescue.

After getting sidetracked into reading about the Time Service Department at
the US Naval Observatory:

http://tycho.usno.navy.mil/

Including a link to this page of time synchronization software:

http://www.eecis.udel.edu/~ntp/software.html

I found this page which gives a description of station WWVB, which
broadcasts the signal our clock picks up.

http://www.bldrdoc.gov/timefreq/wwvstatus.htm

For more detailed information, including the exact format of the broadcast
signal, there is this incredibly detailed report on radio stations WWV,
WWVH, and WWVB (WWV and WWVH are additional shortwave stations which also
broadcast a time signal):

NIST TIME AND FREQUENCY SERVICES
NIST Special Publication 432 (Revised 1990)

Roger E. Beehler and Michael A. Lombardi

Time and Frequency Division
Physics Laboratory
National Institute of Standards and Technology
Boulder, CO 80303-3328

http://www.bldrdoc.gov/timefreq/pubs/sp432/sp432.htm

Some stats on the signal:

* The transmitted accuracy of WWVB is normally better than 1 part in 100
billion (1 x E-11).
* Day-to-day deviations are less than 5 parts in 1000 billion (5 x E-12).
* The BCD time code can be received and used with an accuracy of
approximately 0.1 ms.
* When proper receiving and averaging techniques are used, the received
accuracy of WWVB should be nearly as good as the transmitted accuracy.

As of 19 December 1997 a new transmitter and antenna system at WWVB has been
radiating 23 kilowatts of power, up from the previous value of 10 kilowatts.

During 1998, a second antenna system and additional high-powered
transmitters will be placed in service to increase WWVB's total radiated
power once again (to between 35 and 40 kilowatts) and provide increased
reliability and additional signal strength.

There is a list of time-related publications available for download at:

http://www.bldrdoc.gov/timefreq/tfpub.htm

The main page for the Time and Frequency Division of NIST is at:

http://www.bldrdoc.gov/timefreq/index.html

- Jim