WWVB

WWVB

Discussion
Ask a question about 'WWVB'
Start a new discussion about 'WWVB'
Answer questions from other users
Full Discussion Forum
 
Encyclopedia

WWVB is a NIST time signal
Time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...

 radio station near Fort Collins, Colorado
Fort Collins, Colorado
Fort Collins is a Home Rule Municipality situated on the Cache La Poudre River along the Colorado Front Range, and is the county seat and most populous city of Larimer County, Colorado, United States. Fort Collins is located north of the Colorado State Capitol in Denver. With a 2010 census...

, co-located with WWV. WWVB is the station that radio-controlled clocks
Radio clock
A radio clock or radio-controlled clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock...

 in most of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 use to synchronize themselves. The signal transmitted from WWVB is a continuous 60 kHz carrier wave, derived from a set of atomic clock
Atomic clock
An atomic clock is a clock that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element...

s 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
Amplitude-shift keying
Amplitude-shift keying is a form of modulation that represents digital data as variations in the amplitude of a carrier wave.Any digital modulation scheme uses a finite number of distinct signals to represent digital data. ASK uses a finite number of amplitudes, each assigned a unique pattern of...

. 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
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 spans multiple time zones, so WWVB broadcasts the time in UTC, which the radio-controlled clocks then have to convert to their own local time.

Antennas


There are two identical antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

s 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
T-aerial
A T-aerial is an antenna used for VLF, LF, MF and shortwave transmission or reception.It consists of a horizontal wire suspended between two radio masts or towers. A vertical wire is connected to the middle of the horizontal wire and hangs down close to the ground, where it is connected to the...

, 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
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

, which for 60 kHz is almost 1.25 km (4100 feet).

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
Microsecond
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...

.

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
Kw
kw or KW may refer to:* Kuwait, ISO 3166-1 country code** .kw, the country code top level domain for Kuwait* Kilowatt* Self-ionization of water Kw* Cornish language's ISO 639 code* Kitchener–Waterloo, Ontario, Canada...

, is reduced in power by 17 dB
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...

 to 1.4 kW. Before July 12, 2005, when WWVB's maximum effective radiated power
Effective radiated power
In radio telecommunications, effective radiated power or equivalent radiated power is a standardized theoretical measurement of radio frequency energy using the SI unit watts, and is determined by subtracting system losses and adding system gains...

 (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
Binary-coded decimal
In computing and electronic systems, binary-coded decimal is a digital encoding method for numbers using decimal notation, with each decimal digit represented by its own binary sequence. In BCD, a numeral is usually represented by four bits which, in general, represent the decimal range 0 through 9...

 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 second
    Leap second
    A leap second is a positive or negative one-second adjustment to the Coordinated Universal Time time scale that keeps it close to mean solar time. UTC, which is used as the basis for official time-of-day radio broadcasts for civil time, is maintained using extremely precise atomic clocks...

    s. 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
Ordinal date
An ordinal date is a calendar date typically consisting of a year and a day of year ranging between 1 and 366 , though year may sometimes be omitted...


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
DUT1
The time correction DUT1 is the difference between Universal Time , which is defined by Earth's rotation, and Coordinated Universal Time , which is defined by a network of precision clocks....

 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)
Gray code
The reflected binary code, also known as Gray code after Frank Gray, is a binary numeral system where two successive values differ in only one bit. It is a non-weighted code....

:

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
Leap year
A leap year is a year containing one extra day in order to keep the calendar year synchronized with the astronomical or seasonal year...

 and will include February 29. This lets a receiver implement the full Gregorian calendar
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

 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
Eastern Time Zone
The Eastern Time Zone of the United States and Canada is a time zone that falls mostly along the east coast of North America. Its UTC time offset is −5 hrs during standard time and −4 hrs during daylight saving time...

). 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. 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
Low frequency
Low frequency or low freq or LF refers to radio frequencies in the range of 30 kHz–300 kHz. In Europe, and parts of Northern Africa and of Asia, part of the LF spectrum is used for AM broadcasting as the longwave band. In the western hemisphere, its main use is for aircraft beacon,...

 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
Shortwave
Shortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used...

 signal, which is strongest when it bounces
Skywave
Skywave is the propagation of electromagnetic waves bent back to the Earth's surface by the ionosphere. As a result of skywave propagation, a broadcast signal from a distant AM broadcasting station at night, or from a shortwave radio station can sometimes be heard as clearly as local...

 between the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...

 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
Signal strength
In telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...

 of at least 100 μV/m over most of the continental United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 and Southern Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

 during some portion of the day. Although this value is well above the thermal noise floor
Noise floor
In signal theory, the noise floor is the measure of the signal created from the sum of all the noise sources and unwanted signals within a measurement system, where the noise is defined as any signal other than the one being monitored....

, 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
Time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.-Audible and visible time signals:...

 station, WWVL, began transmitting a 500 watt signal on 20 kHz in August 1963. It used frequency-shift keying
Frequency-shift keying
Frequency-shift keying is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK . BFSK uses a pair of discrete frequencies to transmit binary information. With this scheme, the "1" is called...

, 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
Commercial off-the-shelf
In the United States, Commercially available Off-The-Shelf is a Federal Acquisition Regulation term defining a nondevelopmental item of supply that is both commercial and sold in substantial quantities in the commercial marketplace, and that can be procured or utilized under government contract...

 for the Japanese JJY
JJY
JJY is the call sign of a low frequency time signal radio station.The station is located in Japan and broadcasts from two sites, one on Mount Otakadoya, near Fukushima, and the other on Mount Hagane, located on Kyushu Island...

 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
Redstone Arsenal
Redstone Arsenal is a United States Army base and a census-designated place adjacent to Huntsville in Madison County, Alabama, United States and is part of the Huntsville-Decatur Combined Statistical Area...

 in Huntsville, Alabama
Huntsville, Alabama
Huntsville is a city located primarily in Madison County in the central part of the far northern region of the U.S. state of Alabama. Huntsville is the county seat of Madison County. The city extends west into neighboring Limestone County. Huntsville's population was 180,105 as of the 2010 Census....

, but the Marshall Space Flight Center
Marshall Space Flight Center
The George C. Marshall Space Flight Center is the U.S. government's civilian rocketry and spacecraft propulsion research center. The largest center of NASA, MSFC's first mission was developing the Saturn launch vehicles for the Apollo moon program...

 objected to having such a high power transmitter so near to their operations. Funding, which was allocated as part of the 2009 ARRA
American Recovery and Reinvestment Act of 2009
The American Recovery and Reinvestment Act of 2009, abbreviated ARRA and commonly referred to as the Stimulus or The Recovery Act, is an economic stimulus package enacted by the 111th United States Congress in February 2009 and signed into law on February 17, 2009, by President Barack Obama.To...

 "stimulus bill", expired before the impasse could be resolved, and it is now unlikely to be built.

, 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
Phase modulation
Phase modulation is a form of modulation that represents information as variations in the instantaneous phase of a carrier wave.Unlike its more popular counterpart, frequency modulation , PM is not very widely used for radio transmissions...

 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
Signal-to-noise ratio
Signal-to-noise ratio is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. It is defined as the ratio of signal power to the noise power. A ratio higher than 1:1 indicates more signal than noise...

 than the current AM time code.

See also

  • Radio clock
    Radio clock
    A radio clock or radio-controlled clock is a clock that is synchronized by a time code bit stream transmitted by a radio transmitter connected to a time standard such as an atomic clock...

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

External links