The nation's LF time and frequency station, WWVB, has been testing a
different amplitude shift for transmission of its usual time code signal
during May. A 10 dB decrease in signal strength has long been used, but for
some parts of the month a 20 dB downward shift has been in effect.
FUll-amplitude carrier power remains the same. The latest such test ended on
May 26.

The 60 kHz station is operated by the National Institute of Standards and
Technology (NIST) near Boulder, Colorado. It underwent antenna and
transmitter improvements in the late 1990s to make its signal more available
to the wide variety of "atomic clock" timepieces now in use by businesses
and consumers all across North America. According to an analysis by K. C.
Allen of the National Telecommunication and Information Administration
(NTIA), changing the modulation depth could increase the signal-to-noise
ratio of clocks using matched filter receivers by the same amount as if
transmitter power were increased by a further 2.4 dB.

To keep track of the testing, and to find a short paper on the subject in
PDF format, visit the WWVB Outages page at
http://tf.nist.gov/stations/wwvboutages.htm.