NIST Time & Frequency Service (callsign WWVB) is a longwave time signal radio station near Fort Collins, Colorado, and is operated by the United States Government via the National Institute of Standards and Technology (NIST). Most radio-controlled clocks in North America use WWVB's transmissions to set the correct time.
The normal signal transmitted from WWVB is 70 kW ERP and uses a 60 kHz carrier wave yielding a frequency uncertainty of less than 1 part in 10. The time code signal is derived from a set of atomic clocks located at the site, and transmitted using the IRIG "H" format and modulated onto the carrier wave using pulse-width modulation and amplitude-shift keying at one bit per second. A single complete frame of time code begins at the start of each minute, lasts one minute, and conveys the year, day of year, hour, minute, and other information .
WWVB is co-located with WWV, a time signal station that broadcasts in both voice and time code on multiple shortwave radio frequencies. WWVB is not an acronym or abbreviation but a call sign for the radio station.
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 can then apply time zone and daylight saving time offsets as needed to display local time. The time used in the broadcast is set by the NIST Time Scale, known as UTC(NIST). This time scale is the calculated average time of an ensemble of master clocks, themselves calibrated by the NIST-F1 and NIST-F2 cesium fountain atomic clocks.
In 2011, NIST estimated the number of radio clocks and wristwatches equipped with a WWVB receiver at over 50 million.
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History
LF and VLF (very low frequency) broadcasts have long been used to distribute time and frequency standards. As early as 1904, the United States Naval Observatory (USNO) was broadcasting time signals from the city of Boston as an aid to navigation. This experiment and others like it made it evident that LF and VLF signals could cover a large area using a relatively low power. By 1923, NIST radio station WWV had begun broadcasting standard carrier signals to the public on frequencies ranging from 75 to 2,000 kHz.
These signals were used to calibrate radio equipment, which became increasingly important as more and more stations became operational. Over the years, many radio navigation systems were designed using stable time and frequency signals broadcast on the LF and VLF bands. The best known of these navigation systems is the now-obsolete Loran-C, which allowed ships and planes to navigate via reception of 100 kHz signals broadcast from multiple transmitters.
What is now WWVB began as radio station KK2XEI in July 1956. The transmitter was located in Boulder, Colorado, and the effective radiated power (ERP) was just 1.4 watts. Even so, the signal was able to be monitored at Harvard University in Massachusetts. The purpose of this experimental transmission was to show that the radio path was stable and the frequency error was small at low frequencies.
In 1962, the National Bureau of Standards (NBS) — now known as National Institute of Standards and Technology (NIST) — began building a new facility at a site near Fort Collins, Colorado. This site became the home of WWVB and WWVL, a 20 kHz station that was moved from the mountains west of Boulder.
The site was attractive for several reasons, one being its exceptionally high ground conductivity, which was due to the high alkalinity of the soil. It was also reasonably close to Boulder (about ), which made it easy to staff and manage, but much farther away from the mountains, which made it a better choice for broadcasting an omnidirectional signal.
WWVB went on the air on July 4, 1963 (July 5 at 00:00 UTC), broadcasting a 5 kW ERP signal on 60 kHz. WWVL began transmitting a 0.5 kW ERP signal on 20 kHz the following month, using frequency-shift keying, shifting from 20 kHz to 26 kHz, to send data. The WWVL broadcast was discontinued in July 1972, while WWVB became a permanent part of the nation's infrastructure.
A time code was added to WWVB on July 1, 1965. This made it possible for clocks to be designed that could receive the signal, decode it, and then automatically synchronize themselves. The time code format has changed only slightly since 1965; it sends a decimal time code, using four binary bits to send each digit in binary-coded decimal (BCD).
The ERP of WWVB has been increased several times. It was raised to 7 kW and then 13 kW ERP early in its life and remained so for many years. As part of a WWVB modernization program in the late 1990s, the decommissioned WWVL antenna was refurbished and incorporated into the current phased array. Using both antennas simultaneously resulted in an increase until a major upgrade during 1998 boosted the ERP to 50 kW in 1999, and finally to 70 kW in 2005. The station also became able to operate on one antenna, with an ERP of 27 kW, while engineers could carry out maintenance on the other. The power increase made the coverage area much larger, and made it easier for tiny receivers with simple antennas to receive the signal. This resulted in the introduction of many new low-cost radio controlled clocks that "set themselves" to agree with NIST time.
WWVB, along with NIST's shortwave time code-and-announcement stations WWV and WWVH, were proposed for defunding and elimination in the 2019 NIST budget. However, the final 2019 NIST budget preserved funding for the three stations.
At midnight on April 7, 2024, WWVB's south antenna was disabled due to damage sustained during high winds. WWVB then transmitted exclusively from the north antenna, at a reduced power of 30kW. On May 20, 2024, NIST announced that the necessary replacement parts were being manufactured and shipped, with expected service restoration at the end of September 2024. As of October 10, 2024, WWVB was back to operating at full power.
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.For example, the dual-frequency time code receiver IC With the decommissioning of the Swiss longwave time station HBG on 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.
NIST explored two other ideas in 2012. One was to add a second transmission frequency at the current transmitter site. While it would not have helped signal strength, it would have reduced the incidence of interference and (frequency-dependent) multipath fading.
None of the ideas for a second transmitter were implemented.
Instead, NIST implemented the second idea, adding phase modulation to the WWVB carrier, in 2012. This requires no additional transmitters or antennas, and phase modulation had already been used successfully by the German DCF77 and French TDF time signals. A receiver that decodes the phase modulation can have greater process gain, allowing usable reception at a lower received signal-to-noise ratio than the PWM/ASK time code. The method is more fully described later in this article.
Antennas WWVB antenna helix house coordinates (WGS84) North South The WWVB signal is transmitted via a phased array of two identical antenna systems, spaced apart, one of which was previously used for WWVL. Each consists of four towers that are used to suspend a "top-loaded monopole" (umbrella antenna), consisting of a diamond-shaped "web" of several cables in a horizontal plane (a capacitive "top-hat") supported by the towers, and a downlead (vertical cable) in the middle that connects the top-hat to a "helix house" on the ground. In this configuration, the downlead is the radiating element of the antenna. Each helix house contains a dual fixed-variable inductor system, which is automatically matched to the transmitter via a feedback loop to keep the antenna system at its maximum radiating efficiency. The combination of the downlead and top-hat is designed to replace a single, quarter-wavelength antenna, which, at 60 kHz, would have to be an impractical tall.
Modulation format WWVB transmits data at one bit per second, taking 60 seconds to send the current time of day and date within a century. There are two independent time codes used for this purpose: An amplitude-modulated time code, which has been in use with minor changes since 1962, and a phase-modulated time code added in late 2012.
Amplitude modulation The WWVB 60 kHz carrier, which has a normal ERP of 70 kW, is reduced in power at the start of each UTC second by 17 dB (to 1.4 kW ERP). It is restored to full power some time during the second. The duration of the reduced power encodes one of three symbols: If power is reduced for one-fifth of a second (0.2 s), this is a data bit with value zero. If power is reduced for one-half of a second (0.5 s), this is a data bit with value one. If power is reduced for four-fifths of a second (0.8 s), this is a special non-data "mark", used for framing.
Each minute, seven marks are transmitted in a regular pattern which allows the receiver to identify the beginning of the minute and thus the correct framing of the data bits. The other 53 seconds provide data bits which encode the current time, date, and related information.
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 change to greater modulation depth was part of a series of experiments to increase coverage without increasing transmitter power.
Phase modulation An independent time code is transmitted by binary phase-shift keying of the WWVB carrier. A 1 bit is encoded by inverting the phase (a 180° phase shift) of the carrier for one second. A 0 bit is transmitted with normal carrier phase. The phase shift begins 0.1 s after the corresponding UTC second, so that the transition occurs while the carrier amplitude is low.
The use of phase-shift keying allows a more sophisticated (but still very simple by modern electronics standards) receiver to distinguish 0 and 1 bits far more clearly, allowing improved reception on the East Coast of the United States where the WWVB signal level is weak, radio frequency noise is high, and the MSF time signal from the U.K. interferes at times.
There are no markers as in the amplitude modulated time code. Minute framing is instead provided by a fixed pattern of data bits, transmitted in the last second of each minute and the first 13 seconds of the next one. Because the amplitude-modulated markers provide only 0.2 s of full-strength carrier, it is more difficult to decode their phase modulation. The phase-modulated time code therefore avoids using these bit positions within the minute for important information.
Allowance for carrier phase tracking receivers