La Crosse Technology WWVB Product Manual page 2

nature of the Earth's Ionosphere, reception is very limited during
daylight hours.
The Watch will search for a signal every night when reception is best.
The WWVB radio station derives its signal from the NIST Atomic
clock in Ft. Collins, Colorado. A team of atomic physicists is
continually measuring every second, of every day, to an accuracy of
ten billionths of a second per day. These physicists have created an
international standard, measuring a second as 9,192,631,770
vibrations of a Cesium-133 atom in a vacuum. This watch is
regulated by the WWVB transmitter.
For more information on WWVB and the NIST Atomic clock please
visit their website at
http://www.boulder.nist.gov/timefreq/stations/wwvb.htm
2