WWV is the call sign of the United States National Institute of Standards and Technology's (NIST) HF ("shortwave") radio station located near Fort Collins, Colorado. WWV continuously transmits official U.S. Government frequency and time signals on 2.5, 5, 10, 15, and 20 MHz. The 25 MHz frequency was last used in 2014 on an experimental basis. It was later dropped due to an insufficient solar flux level needed for propagation of the higher HF or shortwave frequencies. This condition began to occur in the latter years of the current sunspot cycle 24.
These carrier frequencies and time signals are controlled by local atomic clocks traceable to NIST's primary standard in Boulder, Colorado by GPS common view observations and other time transfer methods. NIST also operates the very similar radio station WWVH in Kauai, Hawaii. WWV shares its site near Fort Collins with radio station WWVB which transmits carrier and time code (no voice) on 60 kHz in the LF ("longwave") band.
Both WWV and WWVH announce the Coordinated Universal Time each minute, and make other recorded announcements of general interest on an hourly schedule, including the GPS satellite constellation status and severe oceanic weather warnings. Since they share frequencies, WWV uses a male voice to distinguish itself from WWVH, which uses a female voice. WWV time signals can also be accessed by telephone.
Video WWV (radio station)
History
Launch
WWV is the oldest continuously-operating radio station in the United States, first going on the air from Washington, D.C. in May 1920, approximately six months before the launch of KDKA. The station first broadcast Friday evening concerts on 600 kHz, and its signal could be heard 40 kilometers (25 mi) from Washington. On December 15, 1920, WWV began broadcasting on 750 kHz, distributing Morse code news reports from the Department of Agriculture. This signal could be heard up to 300 kilometers (190 mi) from Washington. These news broadcasts ended on April 15, 1921.
Standard frequency signals
At the end of 1922, WWV's purpose shifted to broadcasting standard frequency signals. These signals were desperately needed by other broadcasters, because equipment limitations at the time meant that the broadcasters could not stay on their assigned frequencies. Testing began on January 29, 1923, and frequencies from 200 to 545 kHz were broadcast. Frequency broadcasts officially began on March 6, 1923. The frequencies were accurate to "better than three-tenths of one percent." At first, the transmitter had to be manually switched from one frequency to the next, using a wavemeter. The first quartz resonators (that stabilized the frequency generating oscillators) were invented in the mid-1920s, and they greatly improved the accuracy of WWV's frequency broadcasts.
In 1926, WWV was nearly shut down. Its signal could only cover the eastern half of the United States, and other stations located in Minneapolis and at Stanford University and the Massachusetts Institute of Technology were slowly making WWV redundant. The station's impending shutdown was announced in 1926, but it was saved by a flood of protests from citizens who relied on the service. Later, in 1931, WWV underwent an upgrade. Its transmitter, now directly controlled by a quartz oscillator, was moved to College Park, Maryland. Broadcasts began on 5 MHz. A year later, the station was moved again, to Department of Agriculture land in Beltsville, Maryland. Broadcasts were added on 10 and 15 MHz, power was increased, and time signals, an A440 tone, and ionosphere reports were all added to the broadcast in June 1937.
WWV was nearly destroyed by a fire on November 6, 1940. The frequency and transmitting equipment was recovered, and the station was back on the air (with reduced power) on November 11. Congress funded a new station in July 1941, and it was built 5 kilometres (3.1 mi) south of the former location, still referred to as Beltsville (although in 1961 the name used for the transmitter location would be changed to Greenbelt, Maryland). WWV resumed normal broadcasts on 2.5, 5, 10, and 15 MHz on August 1, 1943.
Time signals
WWV's primary purpose today (and for most of its existence) is to disseminate the "official U.S. time" (provided by government entities such as NIST and USNO) to ensure that uniform time is maintained throughout the United States and around the world. The time signals generated by WWV allow time-keeping devices such as radio-controlled clocks to automatically maintain accurate time without the need for manual adjustment. These time signals are used by commercial and institutional interests where accuracy is essential and time plays a vital role in daily operations - these include shipping, transport, technology, research, education, military, public safety and telecommunications. It is of particular importance in broadcasting, whether it be commercial, public, or private interests such as amateur radio operators. WWV provides a public service by making time information readily available at all hours and at no monetary charge (other than the cost of the receiving equipment itself).
WWV had been broadcasting second pulses since 1937, but these pulses were not tied to actual time. In June 1944, the United States Naval Observatory allowed WWV to use the USNO's clock as a source for its time signals. Over a year later, in October 1945, WWV broadcast Morse code time announcements every five minutes. Voice announcements started on January 1, 1950, and were broadcast every five minutes. Frequencies of 600 Hz and 440 Hz were broadcast during alternating minutes. By this time, WWV was broadcasting on 2.5, 5, 10, 15, 20, 25, 30, and 35 MHz. The 30 and 35 MHz broadcasts were ended in 1953.
A binary-coded decimal time code began testing in 1960, and became permanent in 1961. This "NASA time code" was modulated onto a 1000 Hz audio tone at 100 Hz, sounding somewhat like a monotonous repeated "baaga-bong". The code was also described as sounding like a "buzz-saw". On July 1, 1971, the time code's broadcast was changed to the present 100 Hz subcarrier, which is inaudible when using a normal radio (but can be heard using headphones or recorded using a chart recorder).
WWV moved to its present location near Fort Collins on December 1, 1966, enabling better reception of its signal throughout the continental United States. WWVB signed on in that location three years earlier. In April 1967, WWV stopped using the local time of the transmitter site (Eastern Time until 1966, and Mountain Time afterwards) and switched to broadcasting Greenwich Mean Time or GMT. The station switched again, to Coordinated Universal Time (UTC), in 1974.
The 20 and 25 MHz broadcasts were discontinued in 1977, but the 20 MHz broadcast was reinstated the next year. Starting on April 4, 2014, the 25 MHz signal was back on the air in an 'experimental' mode but has since been dropped again.
The voice used on WWV was that of Don Elliott Heald until August 13, 1991, when equipment changes required rerecording the announcer's voice. The one used at that time was that of John Doyle, but was soon switched to the voice of KSFO morning host Lee Rodgers.
The radio signals of WWV, WWVB and WWVH, along with the 4 atomic (cesium) clocks that their time signals derive from, are maintained by NIST's Time and Frequency Division, which is based in nearby Boulder, Colorado. The Time and Frequency Division is part of the NIST's Physics Laboratory, based in Gaithersburg, Maryland. NIST's predecessor, the National Bureau of Standards, previously maintained WWV as a part of the Department of Agriculture; NIST is currently part of the Department of Commerce.
WWVB's carrier frequency is maintained to an accuracy of 1 part in 1014 and can be used as a frequency reference. The broadcast time is accurate to within 100 ns of UTC and 20 ns of the national time standard.
WWV and Sputnik
WWV's 20 MHz signal was used for a unique purpose in 1958: to track the disintegration of Russian satellite Sputnik I after the craft's onboard electronics failed. Dr. John D. Kraus, a professor at Ohio State University, knew that a meteor entering the upper atmosphere leaves in its wake a small amount of ionized air. This air reflects a stray radio signal back to Earth, strengthening the signal at the surface for a few seconds. This effect is known as meteor scatter. Dr. Kraus figured that what was left of Sputnik would exhibit the same effect, but on a larger scale. His prediction was correct; WWV's signal was noticeably strengthened for durations lasting over a minute. In addition, the strengthening came from a direction and at a time of day that agreed with predictions of the paths of Sputnik's last orbits. Using this information, Dr. Kraus was able to draw up a complete timeline of Sputnik's disintegration. In particular, he observed that satellites do not fall as one unit; instead, the spacecraft broke up into its component parts as it moved closer to Earth.
Call Sign
WWV is one of a small number of radio stations west of the Mississippi River with a call sign beginning with W. The W call sign stems from the station's early locations in D.C. and Maryland--the call sign was maintained when the federal government moved the station to Colorado--and the fact that WWV, being a government station, does not fall within the FCC's jurisdiction with respect to call signs. How and why the call sign WWV was assigned to the time signal station are not known. However FCC regulations do dictate that time stations are to be issued call signs beginning with "WWV".
Maps WWV (radio station)
Broadcast format
On top of the standard carrier frequencies, WWV carries additional information using standard double-sideband amplitude modulation. WWV's transmissions follow a regular pattern repeating each minute. They are coordinated with its sister station WWVH to limit interference between them. Because they are so similar, both are described here.
Date and time
WWV transmits the date and exact time as follows:
- English-language voice announcements of time.
- Binary-coded decimal time code of date and time, transmitted as varying length pulses of 100 Hz tone, one bit per second.
In both cases the transmitted time is given in Coordinated Universal Time (UTC).
Per-second ticks and minute markers
WWV transmits audio "ticks" once per second, to allow for accurate manual clock synchronization. These ticks are always transmitted, even during voice announcements and silent periods. Each tick begins on the second, lasts 5 ms and consists of 5 cycles of a 1000 Hz sine wave. To make the tick stand out more, all other signals are suppressed for 40 ms, from 10 ms before the second until 30 ms after (25 ms after the tick). As an exception, no tick (and no silent interval) is transmitted at 29 or 59 seconds past the minute. In the event of a leap second, no tick is transmitted during second 60 of the minute, either.
On the minute, the tick is extended to a 0.8 second long beep, followed by 0.2 s of silence. On the hour, this minute pulse is transmitted at 1500 Hz rather than 1000. The beginning of the tone corresponds to the start of the minute.
Between seconds one and sixteen inclusive past the minute, the current difference between UTC and UT1 is transmitted by doubling some of the once-per-second ticks, transmitting a second tick 100 ms after the first. (The second tick preempts other transmissions, but does not get a silent zone.) The absolute value of this difference, in tenths of a second, is determined by the number of doubled ticks. The sign is determined by the position: If the doubled ticks begin at second one, UT1 is ahead of UTC; if they begin at second nine, UT1 is behind UTC.
WWVH transmits similar 5 ms ticks, but they are sent as 6 cycles of 1200 Hz. The minute beep is also 1200 Hz, except on the hour when it is 1500 Hz.
The ticks and minute tones are transmitted at 100% modulation (0 dBFS).
Voice time announcements
Voice announcements of time of day are made at the end of every minute, giving the time of the following minute beep. The format for the voice announcement is, "At the tone, X hour(s), Y minute(s), Coordinated Universal Time." The announcement is in a male voice and begins 7.5 seconds before the minute tone.
WWVH makes an identical time announcement, starting 15 seconds before the minute tone, in a female voice.
When voice announcements were first instituted, they were phrased as follows: "National Bureau of Standards, WWV; when the tone returns, Eastern Standard Time is [time in 12-hour format]." After the 1967 switch to GMT, the announcement changed to "National Bureau of Standards, WWV, Fort Collins, Colorado; next tone begins at X hours, Y minute(s), Greenwich Mean Time." However, this format would be short-lived. The announcement was changed again to the current format in 1971. "At the tone, X hour(s), Y minute(s), Greenwich Mean Time." The name "Greenwich Mean Time" was changed to "Coordinated Universal Time" in 1974.
Voice time announcements are sent at 75% modulation, i.e. the carrier varies between 25% and 175% of nominal power.
Standard frequencies
WWV and WWVH transmit 44 seconds of audio tone in most minutes. It begins after the 1-second minute mark and continues until the beginning of the WWVH time announcement 45 seconds after the minute.
Even-numbered minutes (except for minute 2) transmit 500 Hz, while 600 Hz is heard during odd-numbered minutes. The tone is interrupted for 40 ms each second by the second ticks. WWVH is similar, but exchanges the two tones.
WWV also transmits a 440 Hz tone, a pitch commonly used in music (A440, the note A above middle C) during minute 2 of each hour, except for the first hour of the UTC day. Since the 440 Hz tone is only transmitted once per hour, many chart recorders may use this tone to mark off each hour of the day, and likewise, the omission of the 440 Hz tone once per day can be used to mark off each twenty-four-hour period. WWVH transmits the same tone during minute 1 of each hour.
No tone is transmitted during voice announcements from either WWV or WWVH; the latter causes WWV to transmit no tone during minutes 43 through 51 (inclusive) and minutes 29 and 59 of each hour. Likewise, WWVH transmits no tone during minutes 0, 8, 9, 10, 14, 15, 16, 17, 18, 19, and 30.
Audio tones and other voice announcements are sent at 50% modulation.
Other voice announcements
WWV transmits the following 44-second voice announcements (in lieu of the standard frequency tones) on an hourly schedule:
- A station identification at :00 and :30 past each hour;
- marine storm warnings, provided by the National Weather Service, for the Atlantic Ocean at :08 and :09 minutes past, and for the Pacific Ocean at :10 past;
- at :14 and :15 past, GPS satellite health reports from the Coast Guard Navigation Center;
- at :18 past, a special "geophysical alert" report from NOAA is transmitted, containing information on solar activity and shortwave radio propagation conditions. These particular alerts were to be discontinued on September 6, 2011. However, as of June 17, 2011, WWV is announcing at :18 past that the decision has been retracted and that the geophysical alert reports "will continue for the foreseeable future".
Additional time slots are normally transmitted as a standard frequency tone, but can be preempted by voice messages if necessary:
- At :04 and :16 past the hour, NIST broadcasts any announcements regarding a manual change in the operation of WWV and WWVH, such as leap second announcements. These minutes are marked in the broadcast schedule as "NIST Reserved". When not used, a 500 Hz tone is broadcast. Prior to the shutdown of the OMEGA navigation system in 1997, an OMEGA status report was broadcast at :16 past the hour.
- Minute 11 is used for additional storm warnings if necessary. If not, a 600 Hz tone is transmitted.
WWVH transmits the same information on a different schedule. WWV and WWVH's voice announcements are timed to avoid crosstalk; WWV airs dead air when WWVH airs voice announcements, and vice versa. WWVH's storm warnings cover the area around the Hawaiian islands and the Far East rather than North America.
Digital time code
Time of day is also continuously transmitted using a digital time code, interpretable by radio-controlled clocks. The time code uses a 100 Hz subcarrier of the main signal. That is, it is an additional low-level 100 Hz tone added to the other AM audio signals.
This code is similar to, and has the same framework as, the IRIG H time code and the time code that WWVB transmits, except the individual fields of the code are rearranged and are transmitted with the least significant bit sent first. Like the IRIG timecode, the time transmitted is the time of the start of the minute. Also like the IRIG timecode, numeric data (minute, hour, day of year, and last two digits of year) are sent in binary-coded decimal (BCD) format rather than as simple binary integers: Each decimal digit is sent as two, three, or four bits (depending on its possible range of values).
Bit encoding
The 100 Hz subcarrier is transmitted at -15 dBFS (18% modulation) beginning at 30 ms from the start of the second (the first 30 ms are reserved for the seconds tick), and then reduced by 15 dB (to -30 dBFS, 3% modulation) at one of three times within the second. The duration of the high amplitude 100 Hz subcarrier encodes a data bit of 0, a data bit of 1, or a "marker", as follows:
- If the subcarrier is reduced 800 ms past the second, this indicates a "marker."
- If the subcarrier is reduced 500 ms past the second, this indicates a data bit with value one.
- If the subcarrier is reduced 200 ms past the second, this indicates a data bit with value zero.
A single bit or marker is sent in this way in every second of each minute except the first (second :00). The first second of each minute is reserved for the minute marker, previously described.
In the diagram above, the red and yellow bars indicate the presence of the 100 Hz subcarrier, with yellow representing the higher strength subcarrier (-15 dB referenced to 100% modulation) and red the lower strength subcarrier (-30 dB referenced to 100% modulation). The widest yellow bars represent the markers, the narrowest represent data bits with value 0, and those of intermediate width represent data bits with value 1.
Interpretation
It takes one minute to transmit a complete time code. Most of the bits encode UTC time, day of year, year of century, and UT1 correction up to ±0.7 s.
Like the WWVB time code, only the tens and units digits of the year are transmitted; unlike the WWVB time code, there is no direct indication for leap year. Thus, receivers assuming that year 00 is a leap year (correct for year 2000) will be incorrect in the year 2100. On the other hand, receivers that assume year 00 is not a leap year will be correct for 2001 through 2399.
The table below shows the interpretation of each bit, with the "Ex" column being the values from the example above.
The example shown encodes day 86 (March 27) of 2009, at 21:30:00 UTC. DUT1 is +0.3, so UT1 is 21:30:00.3. Daylight Saving Time was not in effect at the previous 00:00 UTC (DST1=0), and will not be in effect at the next 00:00 UTC (DST2=0). There is no leap second scheduled (LSW=0). The day of year normally runs from 1 (January 1) through 365 (December 31), but in leap years, December 31 would be day 366, and day 86 would be March 26 instead of March 27.
Daylight saving time and leap seconds
The time code contains three bits announcing daylight saving time (DST) changes and imminent leap seconds.
- Bit :03 is set near the beginning of the month which is scheduled to end in a leap second. It is cleared when the leap second occurs.
- Bit :55 (DST2) is set at UTC midnight just before DST comes into effect. It is cleared at UTC midnight just before standard time resumes.
- Bit :02 (DST1) is set at UTC midnight just after DST comes into effect, and cleared at UTC midnight just after standard time resumes.
If the DST1 and DST2 bits differ, DST is changing during the current UTC day, at the next 02:00 local time. Before the next 02:00 local time after that, the bits will be the same. Each change in the DST bits happens at 00:00 UTC and so 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 the seven hours 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.
During a leap second, a binary zero is transmitted in the time code; in this case, the minute will not be preceded by a marker.
Levels of modulation
The once-per-second "ticks" and minute and hour tones are modulated onto the carrier signal at 100 percent, or 0 dBc. The time code and audio tones are modulated at 50 percent, or approximately -3 dBc, and the maximum modulation level for the voice recordings is 75 percent, or approximately -1.25 dBc.
Transmission system
WWV broadcasts its signal on six transmitters, one per frequency. The transmitters for 2.5 MHz, 20 MHz, and the experimental 25 MHz put out an ERP of 2.5 kW, while those for the other three frequencies use 10 kW of ERP.
Each transmitter is connected to a dedicated antenna, which has a height corresponding to approximately one-half of its signal's wavelength, and the signal radiation patterns from each antenna are omnidirectional. The top half of each antenna tower contains a quarter-wavelength radiating element, and the bottom half uses nine guy wires, connected to the midpoint of the tower and sloped at one-to-one from the ground--with a length of ?2/4 times the wavelength--as additional radiating elements.
Half-hourly station identification announcement
WWV identifies itself twice each hour, at 0 and 30 minutes past the hour. The text of the identification is as follows:
WWV accepts reception reports sent to the address mentioned in the station ID, and responds with QSL cards.
Telephone service
WWV's time signal can also be accessed by telephone by calling +1 (303) 499-7111 (Boulder, Colorado). An equivalent time service operated by the U.S. Naval Observatory can be accessed by calling +1 (202) 762-1401 (Washington, D.C.). Telephone calls are limited to two minutes in length, and the signal is delayed by an average of 30 milliseconds.
See also
- CHU - Canadian shortwave time broadcast station
- Radio clock - time-signal receivers
- WWVB - longwave time broadcast station
- WWVH - Hawaiian shortwave time broadcast station
References
External links
- WWV Official Webpage
- Current WWV Geophysical alert text
- Audio history of NIST Radio Stations WWV & WWVH
- Patent by Heath Corporation for WWV-disciplined clock
- Patent by Precision Time Standard Co. for a later WWV-disciplined clock
- David L. Mills, University of Delaware: A Precision Radio Clock for WWV Transmissions (1997) Describes WWV time code decoding using DSP
- David L. Mills, University of Delaware: WWV/H Demodulator and Decoder (2004) Lecture slides for WWV time decoder DSP algorithms.
- WWV and WWVH time signals received, identified and decoded. Spectral images. By Nick Hacko, VK2DX
- Live streaming audio of WWV. Receiver located in Charlotte, NC USA
Source of article : Wikipedia
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