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 in most of 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 one-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. A single complete frame of time code begins on the second, lasts one minute, and conveys the year, day of year, hour, minute, and other information as of the beginning of the frame.

While most time signals encode the local time of the broadcasting nation, the United States spans multiple time zones, so WWVB broadcasts the time in Coordinated Universal Time (UTC). Radio-controlled clocks have to convert this to their own local time for display.

Antennas

WWVB antenna coordinates (WGS84)
North	40°40′50.6″N 105°03′01.7″W
South	40°40′28.9″N 105°02′42.3″W

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" T-aerial, consisting of a "cage" 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).

Modulation format

Station ID

WWVB 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 as a method of station identification. This phase step is equivalent to "cutting and pasting" 1/8 of a 60 kHz carrier cycle, or approximately 2.08 µs.

This station ID method is common for narrowband high power transmitters in the VLF and LF bands where other intervening factors forbid normal methods of transmitting call letters.

Modulation depth

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 effective radiated power (ERP) was 50 kW, the power reduction was 10 dB, resulting in a 5 kW signal. This change in modulation depth was part of a series of experiments to increase coverage without increasing transmitter power.

Data

The carrier, which is reduced in power at the beginning of each second, is returned to full power at one of three times within the second. The duration of the reduced carrier power encodes one "trinary digit" (having value of "zero," "one," or "marker") per second.

• If the period of reduced power is four-fifths of a second (0.8 s), this indicates a "marker."
• If the period of reduced power is one-fifth of a second (0.2 s), this indicates a data bit with value zero.
• If the period of reduced power is one-half of a second (0.5 s), this indicates a data bit with value one.

Each minute, seven markers are transmitted in a regular pattern. The other 53 seconds are filled with data bits which encode the current time.

Time code format

Each minute, WWVB broadcasts the current time in a binary-coded decimal format. While this is based loosely on the IRIG timecode, the order of the transmitted bits differs from any current or past IRIG time distribution standards.

• Markers are sent during seconds 0, 9, 19, 29, 39, 49 and 59 of each minute. Thus, the start of the second of two consecutive markers indicates the top of the minute, and serves as the on-time marker for the next frame of time code. Markers are important to allow receivers to properly frame the time code.
• A marker is also sent during leap seconds. In this exceptional event, three consecutive markers will be transmitted: one in second 59, one in second 60, and one in second 0. The start of the third marker indicates the start of the minute.
• There are 11 unused bits, transmitted as binary 0.
• The remaining 42 bits, zeros and ones, carry the binary time code and other information.

The on-time marker, the exact moment which the time code identifies, is the leading (negative-going) edge of the frame reference marker. Thus the time code is always transmitted in the minute immediately after the moment it represents, and matches the hours and minutes of the time of day a clock should be displaying at that moment, before time zone and daylight saving corrections are applied.

In the following diagram the cyan (0 dBr) blocks indicate the full strength carrier, and the dark blue (−17 dBr) the reduced strength carrier. The widest dark blue blocks—the longest intervals of reduced carrier strength—are the markers, occurring in seconds 0, 9, 19, 29, 39, 49, and 59. Of the remaining dark blue blocks, the narrowest represent reduced carrier strength of duration 0.2 seconds, hence data bits of value zero. Those of intermediate width (for example, in seconds :02 and :03) represent reduced carrier strength of duration 0.5 seconds, hence data bits of value one.

The example above encodes day 66 (March 6) of 2008, at 07:30:00 UTC. Since DUT1 is −0.3, UT1 is 07:29:59.7. DST is not in effect today, nor is it coming into effect. There is no leap second scheduled, but the current year is a leap year. The table below shows this in more detail, with the "Ex" column being the bits from the example above:

WWVB time code structure

Bit	Weight	Meaning	Ex		Bit	Weight	Meaning	Ex		Bit	Weight	Meaning	Ex
:00	FRM	Frame reference marker	M		:20	0	Unused, always 0.	0		:40	0.8	DUT1 value (0–0.9 s). DUT1 = UT1−UTC. Example:0.3	0
:01	40	Minutes (00–59) Example: 30	0		:21	0		0		:41	0.4		0
:02	20		1		:22	200	Day of year 1=January 1 365=December 31 (366 if a leap year) Example: 66 (March 6)	0		:42	0.2		1
:03	10		1		:23	100		0		:43	0.1		1
:04	0		0		:24	0		0		:44	0	Unused, always 0.	0
:05	8		0		:25	80		0		:45	80	Year (00–99) Example: 08	0
:06	4		0		:26	40		1		:46	40		0
:07	2		0		:27	20		1		:47	20		0
:08	1		0		:28	10		0		:48	10		0
:09	P1	Marker	M		:29	P3		M		:49	P5		M
:10	0	Unused, always 0.	0		:30	8		0		:50	8		1
:11	0		0		:31	4		1		:51	4		0
:12	20	Hours (00–23) Example: 07	0		:32	2		1		:52	2		0
:13	10		0		:33	1		0		:53	1		0
:14	0		0		:34	0	Unused, always 0.	0		:54	0	Unused, always 0.	0
:15	8		0		:35	0		0		:55	LYI	Leap year indicator	1
:16	4		1		:36	+	DUT1 sign. If +, bits 36 and 38 are set. If −, bit 37 is set.	0		:56	LSW	Leap second at end of month	0
:17	2		1		:37	−		1		:57	2	DST status value (binary): 00 = DST not in effect. 10 = DST begins today. 11 = DST in effect. 01 = DST ends today.	0
:18	1		1		:38	+		0		:58	1		0
:19	P2	Marker	M		:39	P4	Marker	M		:59	P0	Marker	M

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; the other DST bit at second 58 changes at the end of the UTC day. Therefore, if the DST bits differ, DST is changing at 02:00 local time during the current UTC day. Before the next 02:00 local time after that, the bits will be the same.

Each change in the DST bits will first be received in the mainland United States between 16:00 (PST) and 20:00 (EDT), depending on local time zone and on whether DST is about to begin or end. A receiver in the Eastern time zone (UTC-5) must therefore correctly receive the "DST is changing" indication within a seven hour period before DST begins, and six hours before DST ends, if it is to change the local time display at the correct time. Receivers in the Central, Mountain, and Pacific time zones have one, two, and three more hours of advance notice, respectively.

It is up to the receiving clock to apply the change at the next 02:00 local time if it notices the bits differ. If the receiving clock happens to not receive an update between 00:00 UTC and 02:00 local time the day of the change, it should apply the DST change on the next update after that.

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 mask 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, 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 can also operate on one antenna while the other is being maintained; WWVB radiates 27 kW of power when operating on only one antenna.

Service improvement plans

WWVB's Colorado location makes the signal weakest on the U.S. east coast, where urban density also produces considerable interference. In 2009, NIST raised the possibility of adding a second time code transmitter, on the east coast, to improve signal reception there and provide a certain amount of robustness to the overall system should weather or other causes render one transmitter site inoperative. Such a transmitter would use the same time code, but a different frequency.

Use of 40 kHz would permit use of dual-frequency time code receivers already produced for the Japanese JJY transmitters. With the decommissioning of the Swiss longwave time station HBG at 75 kHz, that frequency is potentially also available.

Plans were made to install the transmitter on the grounds of the Redstone Arsenal in Huntsville, Alabama, but the Marshall Space Flight Center objected to having such a high power transmitter so near to their operations. Funding, which was allocated as part of the 2009 ARRA "stimulus bill", expired before the impasse could be resolved, and it is now unlikely to be built.

As of 2011, two other possibilities are being explored. One is to add a second transmission frequency at the current transmitter site. While it would not help signal strength, it would reduce the incidence of interference and (frequency-dependent) multipath fading. A second possibility is to add phase modulation to the WWVB carrier, broadly similar to the current DCF77 signal. This would allow receivers with greater processing gain to decode the signal at a lower signal-to-noise ratio than the current AM time code.

See also

• Radio clock
• Watch (electronic movements)
• WWV (radio station)

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.
• How to get the time using a telephone, computer or radio signal From NIST's FAQ "What Time is It?"