WWVB is a special NIST time signal radio station in Fort Collins, Colorado, co-located with WWV. WWVB is the station that radio-controlled clocks throughout North America use to synchronize themselves. The signal transmitted from WWVB is a continuous 60 kHz carrier wave, derived from a set of atomic clocks located at the transmitter site. A 1 bit-per-second time code, which is derived from the same set of atomic clocks, is then pulse-width modulated onto the carrier wave. The time in this code is given in UTC, which the radio-controlled clocks then have to convert to their own local time.
At the start of each UTC second, the WWVB 60 kHz carrier, which has a normal power of 50 kW, is reduced in power by 10 dB. The type of bit transmitted on each second is determined by when the carrier wave is returned to normal power within that second. If the carrier power is returned to normal one-fifth of a second after it is reduced, the bit is a zero. If the carrier is reduced in power for one half-second, the bit is a one. If the carrier remains at reduced power for four-fifths of a second, the bit is a reference bit. If two reference bits are sent consecutively, the start of the second reference bit indicates the top of the UTC minute.
Since WWVB's longwave signal tends to travel along the ground, it requires a shorter and less turbulent path to get to the radio receivers than WWV's shortwave signal, which bounces between the ionosphere and the ground. This results in the WWVB signal having greater accuracy than the WWV signal as received at the same site.