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.
WWVB also has a method of station identification, which it transmits by a 45° shift in the phase of its carrier wave at ten minutes past the hour, and a -45° shift five minutes later. This phase step is equivalent to approximately 2.08 µs.
Since WWVB's longwave signal tends to propagate better along the ground, it requires a shorter and less turbulent path to get to the radio receivers than WWV's shortwave signal, which is strongest when it bounces between the ionosphereThe ionosphere is the part of the atmosphere that is ionized by solar radiation, and too tenuous to be cooled by contact with other air. It forms the inner edge of the magnetosphere and has practical importance because it reflects radio waves to distant p and the ground. This results in the WWVB signal having greater accuracy than the WWV signal as received at the same site. Also, since longwave signals tend to propagate much further at night, the WWVB signal can reach a larger coverage area during that time period, which is why many radio-controlled clocks are usually programmed to automatically synchronize themselves with the WWVB time code during local nighttime hours.
Another time signal station, WWVL, began transmitting a 500 Watt signal on 20 kHz in August 1963. It used Frequency Shift Keying ( FSKFrequency-shift keying (FSK is frequency modulation in which the modulating signal shifts the output frequency between predetermined values. Note 1: Usually, the instantaneous frequency is shifted between two discrete values termed the " mark frequency" a), shifting from 20 kHz to 26 kHz, to send data. The WWVL broadcast was discontinued in July 1972.
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