WWVB

WWVB antenna and support towers.

WWVB is a 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 based on the IRIG "H" format of time code and derived from the same set of atomic clocks, is then modulated onto the carrier wave using a technique described as pulse width modulation followed by 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.

Antennas

There are two identical antennas used to radiate the WWVB signal. Both antennas are 122 meters (400 feet) tall, and their centers are separated by 857 meters (2811 feet). 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, or downlead, 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 the top plane and downlead of each antenna is supposed to approach an optimum length of one-quarter wavelength, which for 60 kHz is almost 1.25 km (4100 feet).^[1]

Modulation Format

At the start of each UTC second, the WWVB 60 kHz carrier, which has a normal power of 70 kW, is reduced in power by 17 dB to 1.4 kW. Before July 12, 2005, when WWVB's maximum ERP was 50 kW, the power reduction was 10 dB, resulting in a 5 kW signal. 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 (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 (0.5 s), the bit is a one. If the carrier power is returned to normal in four-fifths of a second (0.8 s), the bit is a marker bit. Marker bits are sent during seconds 0, 9, 19, 29, 39, 49 and 59 of each minute; the other 53 seconds are normal bits which provide binary time code data. (11 unused bits are transmitted as binary 0.) Thus, the start of the second of two consecutive marker bits indicates the top of the minute, as well as serves as the on-time marker for the next frame of time code. A marker bit is also sent during leap seconds, so in this exceptional event, three consecutive marker bits will be transmitted.

WWVB also, as a method of station identification, advances the phase of its carrier wave by 45° at ten minutes past the hour, and returns to normal (a −45° shift) five minutes later. This phase step is equivalent to "cutting and pasting" 1/8 of a 60 kHz carrier cycle, or approximately 2.08 µs.

The on-time marker, the exact moment to which the time stamp applies, is the leading (negative-going) edge of the frame reference marker bit, thus the time stamp is always transmitted in the minute immediately after the moment it represents.

Announcement bits

Several bits of the WWVB time code give warning of upcoming events.

Bit 55 indicates that the current year is a leap year and will include February 29. This lets a receiver implement the full Gregorian calendar leap-year rules even though the time code does not include the century.

When a leap second is scheduled for the end of a month, bit 56 is set near the beginning of the month, and reset immediately after the leap second insertion.

The DST status bits indicate United States daylight saving time rules. The bits are updated daily at 00:00 UTC. The first DST bit, transmitted at 57 seconds past the minute, changes at the beginning of the UTC day that DST comes into effect or ends, providing a minimum of 6 hours' notification (of the EDT→EST change), and the other DST bit at second 58 changes at the end of the UTC day. It is up to the receiving clock to apply the change at 02:00 local time if it notices the bits differ.

An equivalent definition of the DST status bits is that bit 57 is set if DST will be in effect at 24:00Z, the end of the current UTC day. Bit 58 is set if DST was in effect at 00:00Z, the beginning of the current UTC day.

Propagation

Since WWVB's low frequency 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 farther 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/m 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 re-use 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 WWVB modernization program in the late 1990s, the decommissioned WWVL antenna was refurbished and used to radiate the WWVB signal. Using both antennas simultaneously allowed for a WWVB transmitter power increase to 50 kW (later 70 kW), as well as providing a backup antenna that now facilitates routine maintenance. WWVB radiates 27 kW of power when operating on only one antenna.

WWVB east plans

WWVB's Colorado location makes the signal weakest on the U.S. east coast, where urban density also produces considerable interference. NIST is considering adding a second time code transmitter, on the east coast, to improve signal reception there. Such a transmitter would use the same time code, but a different frequency.^[3] 40 kHz would permit use of dual-frequency time code receivers already produced for the Japanese JJY transmitters.

See also

• Radio clock
• Watch

References

External links

• NIST Radio Station WWVB
• WWVB Radio Controlled Clocks: Recommended Practices for Manufacturers and Consumers
• NIST Special Publication 250-67 with a detailed history and description of NIST time and frequency radio stations WWV, WWVH and WWVB.
• Entry at Skyscraperpage.com
• Simple Radio Clocks for PCs Jon Buzzard's excellent HOWTO page for making a WWVB-controlled (or MSF or DCF77) receiver for use with Network Time Protocol.
• WWVB-controlled USB radio clock Radio Clock for PCs with optional external antenna.
• WWVB-Based Precision Frequency Comparator Frequency standards (crystal or rubidium) characterized with WWVB receiver and stepper motor.