WWVB
From Free net encyclopedia
WWVB is a special NIST time signal radio station near 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 modulated onto the carrier wave using a technique described equivocally as either pulse width modulation or amplitude-shift keying. The time in this code is given in UTC, which the radio-controlled clocks then have to convert to their own local time. A single complete frame of time code lasts one minute.
Contents |
Antennas
There are two identical antennas used to radiate the WWVB signal. Both antennas are 122 metres tall, and their centres are separated by 857 metres. The physical configuration of each antenna is a diamond-shaped "top loaded monopole", consisting of several cables spread on a flat plane from the top of their support towers, and a vertical cable that connects the top plane to a "helix house" on the ground. Each helix house contains a dual fixed-variable inductor system, which is used to keep the antenna system at its maximum radiating efficiency. The amount of cable used in each antenna is supposed to approach an optimum length of one-quarter wavelength, which for 60 kHz is almost 1.25 km.
Modulation Format
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 17 dB. Before July 12, 2005, the power reduction was 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 in one-fifth of a second, or 0.2 s, from when it was reduced, the bit is a zero. If the carrier power is returned to normal in one half-second, or 0.5 s, the bit is a one. If the carrier power is returned to normal in four-fifths of a second, or 0.8 s., the bit is a reference bit. If two reference bits are sent consecutively, the start of the second reference bit indicates both the top of the UTC minute and the "on-time marker" for the time code that follows it. In the exceptional case of three consecutive reference bits being transmitted, the second reference bit indicates the observance of a leap second.
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 "cutting and pasting" approximately 2.08 µs of the unshifted carrier wave.
Image:WWVB Time Code Format.png
Propagation
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 ionosphere 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.
The radiation pattern of WWVB antennas is designed to present a field strength of at least 100 μV/m² over most of the continental United States and Southern Canada during some portion of the day. Although this value is well above the thermal noise floor, man-made noise and local interference from a wide range of electronic equipment can easily swamp out the signal. Positioning receiving antennas away from electronic equipment helps to reduce the effects of local interference.
Antenna Reuse with former WWVL
Another time signal station, WWVL, began transmitting a 500 watt signal on 20 kHz in August 1963. It used Frequency Shift Keying (FSK), shifting from 20 kHz to 26 kHz, to send data. The WWVL broadcast was discontinued in July 1972.
As part of a recent WWVB modernization programme, the decommissioned WWVL antenna was used to radiate the WWVB signal. This allowed for a WWVB transmitter power increase to 50 kW, as well as providing a backup antenna that now facilitates routine maintenance.
| Time signal stations |
| BPM | CHU | DCF77 | GPS Technology | HBG | JJY | MSF | RWM | VNG | WWV | WWVB | WWVH | YVTO |