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1 History a Time Code Was Added to WWVB on July 1, 1965

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1 History a Time Code Was Added to WWVB on July 1, 1965 WWVB WWVB is a time signal radio station near Fort Collins, which became increasingly important as more and more Colorado and is operated by the National Institute stations became operational. Over the years, many ra- of Standards and Technology (NIST).[1] Most radio- dio navigation systems were designed using stable time controlled clocks in North America use WWVB’s trans- and frequency signals broadcast on the LF and VLF missions to set the correct time. The 70 kW ERP signal bands. The most well-known of these navigation systems transmitted from WWVB is a continuous 60 kHz car- is LORAN-C, which allows ships and planes to navigate rier wave, the frequency of which is derived from a set via reception of 100 kHz signals broadcast from multiple of atomic clocks located at the transmitter site, yielding transmitters. 12 a frequency uncertainty of less than 1 part in 10 .A What is now WWVB began as radio station KK2XEI in one-bit-per-second time code, which is based on the IRIG July 1956. The transmitter was located in Boulder, Col- “H” time code format and derived from the same set of orado, and the effective radiated power (ERP) was just atomic clocks, is then modulated onto the carrier wave 1.4 watts. Even so, the signal was able to be monitored using pulse width modulation and amplitude-shift keying. at Harvard University in Massachusetts. The purpose of A single complete frame of time code begins at the start this experimental transmission was to show that the radio of each minute, lasts one minute, and conveys the year, path was stable and the frequency error was small at low day of year, hour, minute, and other information as of frequencies. the beginning of the minute. WWVB is co-located with WWV, a much older time signal station that broadcasts In 1962, NIST (then called the National Bureau of Stan- on multiple short wave frequencies. dards or NBS) began building a new facility at a site near Fort Collins, Colorado. This site became the home While most time signals encode the local time of of WWVB and WWVL, a 20 kHz transmitter that was the broadcasting nation, the United States spans mul- moved from the mountains west of Boulder. tiple time zones, so WWVB broadcasts the time in Coordinated Universal Time (UTC). Radio-controlled The site was attractive for several reasons, one being its clocks can then apply time zone and daylight saving time exceptionally high ground conductivity, which was due offsets as needed to display local time.[2] The time used to the high alkalinity of the soil. It was also reasonably in the broadcast is set by the NIST Time Scale, known close to Boulder (about 80 km, 49.3 mi), which made it as UTC(NIST). This time scale is the calculated average easy to staff and manage, but much farther away from the time of an ensemble of master clocks, themselves cal- mountains, which made it a better choice for broadcasting ibrated by the NIST-F1 and NIST-F2 cesium fountain an omnidirectional signal. [3] atomic clocks. WWVB went on the air on July 5, 1963, broadcasting a 5 In 2011, NIST estimated the number of radio clocks and kW ERP signal on 60 kHz. WWVL began transmitting a wristwatches equipped with a WWVB receiver at over 50 500 W ERP signal on 20 kHz the following month, using million.[4] 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. 1 History A time code was added to WWVB on July 1, 1965. This made it possible for clocks to be designed that could re- LF and VLF (very low frequency) broadcasts have long ceive the signal, decode it, and then automatically syn- been used to distribute time and frequency standards. chronize themselves. The time code format has changed As early as 1904, the United States Naval Observatory only slightly since 1965; it sends a decimal time code, (USNO) was broadcasting time signals from the city of using four binary bits to send each digit in binary-coded Boston as an aid to navigation. This experiment and decimal (BCD). others like it made it evident that LF and VLF signals The ERP of WWVB has been increased several times. could cover a large area using a relatively small amount It was raised to 7 and then 13 kW ERP early in its life. of power. By 1923, NIST radio station WWV had be- There it remained for many years until a major upgrade gun broadcasting standard carrier signals to the public on during 1998 boosted the power to 50 kW in 1999, and frequencies ranging from 75 to 2,000 kHz. finally to 70 kW in 2005. The power increase made the These signals were used to calibrate radio equipment, coverage area much larger, and made it easier for tiny re- 1 2 3 MODULATION FORMAT ceivers with simple antennas to receive the signal. This Each consists of four 122-meter (400 ft) towers that are resulted in the introduction of many new low-cost ra- used to suspend a “top-loaded monopole” (T-aerial), con- dio controlled clocks that “set themselves” to agree with sisting of a diamond-shaped “web” of several cables in a NIST time. 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 1.1 Service improvement plans this configuration, the downlead is the radiating element of the antenna. Each helix house contains a dual fixed- WWVB’s Colorado location makes the signal weakest on variable inductor system, which is automatically matched the U.S. east coast, where urban density also produces to the transmitter via a feedback loop to keep the antenna considerable interference. In 2009, NIST raised the pos- system at its maximum radiating efficiency. The combi- sibility of adding a second time code transmitter, on the nation of the downlead and top-hat is designed to replace east coast, to improve signal reception there and provide a a single, quarter-wavelength antenna, which, at 60 kHz, certain amount of robustness to the overall system should would have to be an impractical 1,250 meters (4,100 ft) weather or other causes render one transmitter site inop- tall.[8] erative. Such a transmitter would use the same time code, As part of a WWVB modernization program in the late [5] but a different frequency. 1990s, the decommissioned WWVL antenna was refur- Use of 40 kHz would permit use of dual-frequency time bished and incorporated into the current phased array. code receivers already produced for the Japanese JJY Using both antennas simultaneously resulted in an in- transmitters.[6] With the decommissioning of the Swiss crease to 50 kW (later 70 kW) ERP. The station also be- longwave time station HBG at 75 kHz, that frequency is came able to operate on one antenna, with an ERP of 27 potentially also available. kW, while engineers could carry out maintenance on the other.[8] 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. Fund- 3 Modulation format ing, which was allocated as part of the 2009 ARRA “stim- ulus bill”, expired before the impasse could be resolved,[7] WWVB transmits data at one bit per second, taking 60 and it is now unlikely to be built. seconds to send the current time of day and date within a century. There are two independent time codes used for NIST explored two other ideas in 2012. One was to add this purpose: an amplitude-modulated time code, which a second transmission frequency at the current transmit- has been in use with minor changes since 1962, and a ter site. While it would not have helped signal strength, phase-modulated time code added in late 2012.[9] it would have reduced the incidence of interference and (frequency-dependent) multipath fading. None of the ideas for a second transmitter were imple- 3.1 Amplitude modulation mented. The WWVB 60 kHz carrier, which has a normal ERP Instead, NIST implemented the second idea, adding of 70 kW, is reduced in power at the start of each UTC phase modulation to the WWVB carrier, in 2012. This second by 17 dB (to 1.4 kW ERP). It is restored to full requires no additional transmitters or antennas, and phase power some time during the second. The duration of the modulation had already been used successfully by the reduced power encodes one of three symbols: German DCF77 and French TDF time signals.[7] A re- ceiver that decodes the phase modulation can have greater • processing gain, allowing usable reception at a lower re- If power is reduced for one-fifth of a second (0.2 s), ceived signal-to-noise ratio than the PWM/ASK time this is a data bit with value zero. code. The scheme is more fully described later in this • If power is reduced for one-half of a second (0.5 s), article. this is a data bit with value one. • If power is reduced for four-fifths of a second (0.8 2 Antennas s), this is a special non-data “marker,” used for fram- ing. Coordinates: 40°40′41″N 105°02′49″W / 40.67806°N Each minute, seven markers are transmitted in a regular 105.04694°W pattern which allows the receiver to identify the beginning The WWVB signal is transmitted via a phased array of of the minute and thus the correct framing of the data bits.
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