A radio clock or radio-controlled clock (RCC), and often colloquially (and incorrectly) referred to as an "

atomic clock An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions betwee ...

", is a type of

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

clock A clock or chronometer is a device that measures and displays time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month, a ...

watch A watch is a timepiece carried or worn by a person. It is designed to maintain a consistent movement despite the motions caused by the person's activities. A wristwatch is worn around the wrist, attached by a watch strap or another type of ...

that is automatically synchronized to a

time code A timecode (alternatively, time code) is a sequence of numeric codes generated at regular intervals by a timing synchronization system. Timecode is used in video production, show control and other applications which require temporal coordinatio ...

transmitted by a

radio Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...

transmitter connected to a

time standard A time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of cu ...

such as an atomic clock. Such a clock may be synchronized to the time sent by a single transmitter, such as many national or regional time transmitters, or may use the multiple transmitters used by

satellite navigation A satellite navigation or satnav system is a system that uses satellites to provide autonomous geopositioning. A satellite navigation system with global coverage is termed global navigation satellite system (GNSS). , four global systems are ope ...

systems such as

Global Positioning System The Global Positioning System (GPS) is a satellite-based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is one of the global navigation satellite systems (GNSS) that provide ge ...

. Such systems may be used to automatically set clocks or for any purpose where accurate time is needed. Radio clocks may include any feature available for a clock, such as alarm function, display of ambient temperature and humidity, broadcast radio reception, etc. One common style of radio-controlled clock uses time signals transmitted by dedicated terrestrial

longwave In radio, longwave (also spelled long wave or long-wave and commonly abbreviated LW) is the part of the radio spectrum with wavelengths longer than what was originally called the medium-wave (MW) broadcasting band. The term is historic, dati ...

radio transmitters, which emit a time code that can be demodulated and displayed by the radio controlled clock. The radio controlled clock will contain an accurate time base oscillator to maintain timekeeping if the radio signal is momentarily unavailable. Other radio controlled clocks use the time signals transmitted by dedicated transmitters in the

shortwave Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (app ...

bands. Systems using dedicated time signal stations can achieve accuracy of a few tens of milliseconds. GPS satellite receivers also internally generate accurate time information from the satellite signals. Dedicated GPS timing receivers are accurate to better than 1 microsecond; however, general-purpose or consumer grade GPS may have an offset of up to one second between the internally calculated time, which is much more accurate than 1 second, and the time displayed on the screen. Other broadcast services may include timekeeping information of varying accuracy within their signals. Timepieces with

Bluetooth Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is li ...

radio support, ranging from watches with basic control of functionality via a

mobile app A mobile application or app is a computer program or software application designed to run on a mobile device such as a smartphone, phone, tablet computer, tablet, or smartwatch, watch. Mobile applications often stand in contrast to desktop appli ...

to full

smartwatch A smartwatch is a portable wearable computer that resembles a wristwatch. Most modern smartwatches are operated via a touchscreen, and rely on mobile apps that run on a connected device (such as a smartphone) in order to provide core functions. ...

es obtain time information from a connected

phone A telephone, colloquially referred to as a phone, is a telecommunications device that enables two or more users to conduct a conversation when they are too far apart to be easily heard directly. A telephone converts sound, typically and most ...

, with no need to receive time signal broadcasts.

Single transmitter

Radio clocks synchronized to a terrestrial

time signal A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day. Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks; however, au ...

can usually achieve an accuracy within a hundredth of a second relative to the time standard, generally limited by uncertainties and variability in

radio propagation Radio propagation is the behavior of radio waves as they travel, or are wave propagation, propagated, from one point to another in vacuum, or into various parts of the atmosphere. As a form of electromagnetic radiation, like light waves, radio w ...

. Some timekeepers, particularly watches such as some Casio Wave Ceptors which are more likely than desk clocks to be used when travelling, can synchronise to any one of several different time signals transmitted in different regions.

Longwave and shortwave transmissions

Radio clocks depend on coded time signals from radio stations. The stations vary in broadcast frequency, in geographic location, and in how the signal is modulated to identify the current time. In general, each station has its own format for the time code.

List of radio time signal stations

, , Triple T-antenna3 T-antennas, spun above ground between two high guyed grounded masts in a distance of , , , , Range up to , - ,

WWVB WWVB is a longwave time signal radio station near Fort Collins, Colorado, and is operated by the National Institute of Standards and Technology (NIST). Most radio clock, radio-controlled clocks in North America use WWVB's transmissions to set th ...

, ,

NIST The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical s ...

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Fort Collins, Colorado Fort Collins is a List of municipalities in Colorado#Home rule municipality, home rule municipality in Larimer County, Colorado, United States, and its county seat. The population was 169,810 at the 2020 United States census, 2020 census, an i ...

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VNIIFTRI All-Russian Scientific Research Institute for Physical-Engineering and Radiotechnical Metrology (VNIIFTRI; ) was founded in 1955. It is located in the settlement of Mendeleyevo of Solnechnogorsky District, Moscow Oblast, Russia (), and also it in ...

, ,

Taldom Taldom () is a types of inhabited localities in Russia, town in Moscow Oblast, Russia, located north of Moscow, on a suburban railway connecting Moscow to Savyolovo. Population: It was previously known as ''Taldom'' (until 1918), ''Leninsk'' ( ...

Moscow Moscow is the Capital city, capital and List of cities and towns in Russia by population, largest city of Russia, standing on the Moskva (river), Moskva River in Central Russia. It has a population estimated at over 13 million residents with ...

} , , Umbrella antennaumbrella antenna, fixed on a high central tower insulated against ground and five high lattice masts insulated against ground in a distance of from the central tower , , , , PM time code , - , , , BPC , ,

NTSC NTSC (from National Television System Committee) is the first American standard for analog television, published and adopted in 1941. In 1961, it was assigned the designation System M. It is also known as EIA standard 170. In 1953, a second ...

, ,

Shangqiu Shangqiu ( zh, ), Postal romanization, alternately romanized as Shangkiu, is a city in eastern Henan province, Central China. It borders Kaifeng to the northwest, Zhoukou to the southwest, and the provinces of Shandong and Anhui to the northeast ...

Henan Henan; alternatively Honan is a province in Central China. Henan is home to many heritage sites, including Yinxu, the ruins of the final capital of the Shang dynasty () and the Shaolin Temple. Four of the historical capitals of China, Lu ...

, , 4 guyed masts, arranged in a square , , , , 21 hours per day, with a 3-hour break from 05:00–08:00 (

China Standard Time The time in China follows a single standard UTC offset, time offset of UTC+08:00, where Beijing is located, even though the country spans five geographical time zones. It is the largest sovereign nation in the world that officially observes only ...

) daily (21:00–24:00 UTC) , - , , , ~~HBG~~ , , Switzerland Switzerland, officially the Swiss Confederation, is a landlocked country located in west-central Europe. It is bordered by Italy to the south, France to the west, Germany to the north, and Austria and Liechtenstein to the east. Switzerland ...

~~METAS~~ , , Prangins Prangins () is a Municipalities of Switzerland, municipality in the district of Nyon (district), Nyon in the Cantons of Switzerland, canton of Vaud in Switzerland. It is located on Lake Geneva. History Prangins is first mentioned around 1135-85 a ...

, , ~~T-antenna~~T-antenna spun between two tall, grounded free-standing lattice towers in a distance of , , , , Discontinued as of 1 January 2012 , - , rowspan=2 , ,

DCF77 DCF77 is a German longwave time signal and standard-frequency radio station. It started service as a standard-frequency station on 1 January 1959. In June 1973, date and time information was added. Its primary and backup transmitter are ...

, ,
PTB , ,

Mainflingen Mainflingen is a village and part (''Ortsteil'') of the municipality Mainhausen, in the district of Offenbach in Hesse with about 4000 inhabitants. It is known for its time signal transmitter, DCF77, which controls almost all radio clocks in Wes ...

Hessen Hesse or Hessen ( ), officially the State of Hesse (), is a state in Germany. Its capital city is Wiesbaden, and the largest urban area is Frankfurt, which is also the country's principal financial centre. Two other major historic cities are Da ...

, , Vertical omni-directional antennas with top-loading capacity, height , , , , Located southeast of Frankfurt am Main with a range of up to Dennis D. McCarthy, P. Kenneth Seidelmann ''Time: From Earth Rotation to Atomic Physics'' Wiley-VCH, 2009 page 257 090917 compuphase.com , - , BSF , , , , Zhongli
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, , Pucheng,

Shaanxi Shaanxi is a Provinces of China, province in north Northwestern China. It borders the province-level divisions of Inner Mongolia to the north; Shanxi and Henan to the east; Hubei, Chongqing, and Sichuan to the south; and Gansu and Ningxia to t ...

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Bryansk Bryansk (, ) is a types of inhabited localities in Russia, city and the administrative center of Bryansk Oblast, Russia, situated on the Desna (river), Desna River, southwest of Moscow. It has a population of 379,152 at the 2021 census. Bryans ...

, , 5 guyed masts , , , ,

CHAYKA Chaika or Chayka, the word for gull in several Slavic languages, may refer to: People * Chaika (surname), a Slavic surname (including a list of people with the name) * Chaika or Chayka, the call sign for astronaut Valentina Tereshkova Places ...

compatible format signal
04:00–10:00 UTC and 14:00–18:00 UTC , - , RNS-V , ,

, , Alexandrovsk-Sakhalinsky
, , Single guyed mast , , , ,

compatible format signal
23:00–05:00 UTC and 11:00–17:00 UTC , - , bgcolor=lightgrey , Frequency for radio teleswitch system, , DCF49 , ,
PTB , ,

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time signal only (no reference frequency)
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PTB , , Lakihegy
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PTB , ,

Burg bei Magdeburg Burg (; also known as Burg bei Magdeburg to distinguish from other places with the same name) is a town of about 22,400 inhabitants on the Elbe–Havel Canal in northeastern Germany, northeast of Magdeburg. It is the capital of the Jerichower L ...

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, ,

Allouis Allouis () is a commune in the Cher department of the Centre-Val de Loire region of France in the valley of the river Yèvre, about northwest of Bourges. Population Radio transmitter Since 1939, the town of Allouis has housed the central ...

, , Two guyed steel lattice masts, height , fed on the top , , , , AM-broadcasting transmitter, located south of Paris with a range of up to , using PM with encoding similar to DCF77and requiring a more complex receiver for demodulating time signal , - , bgcolor=silver , since 1988, before 200 kHz , , , ,
NPL , ,

Droitwich Droitwich Spa (often abbreviated to Droitwich ) is an historic spa town in the Wychavon district in northern Worcestershire, England, on the River Salwarpe. It is located approximately south-west of Birmingham and north-east of Worcester. ...

, , T-aerialDroitwich uses a T-aerial suspended between two (699') guyed steel lattice

radio masts Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connected to ...

, which stand apart. , , , , Additional transmitters is at

Burghead Burghead ( or ''The Broch'', ) is a small town in Moray, Scotland, about north-west of Elgin, Moray, Elgin. The town is mainly built on a peninsula that projects north-westward into the Moray Firth, surrounding it by water on three sides. Peopl ...

and Westerglen. The time signal is transmitted by phase modulation. , - , bgcolor=PaleGreen rowspan=3 , , BPM , ,

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07:30–01:00 UTC , - , WWV , ,

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WWVH WWVH is the callsign of the U.S. National Institute of Standards and Technology's shortwave radio time signal station located at the Pacific Missile Range Facility, Pacific Missile Range Facility Barking Sands, in Kekaha, Hawaii, Kekaha, on the ...

, ,

Kekaha, Hawaii Kekaha (literally, "the place" in Hawaiian) is a census-designated place (CDP) in Kauai County, Hawaii, United States. The population was 3,715 at the 2020 census, up from 3,175 at the 2000 census. History For most of the 20th century, the ...

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CHU Chu or CHU may refer to: Chinese history * Chu (state) (c. 1030 BC–223 BC), a state during the Zhou dynasty * Western Chu (206 BC–202 BC), a state founded and ruled by Xiang Yu * Chu Kingdom (Han dynasty) (201 BC–70 AD), a kingdom of the H ...

, ,
NRC , ,

Ottawa, Ontario Ottawa is the capital city of Canada. It is located in the southern Ontario, southern portion of the province of Ontario, at the confluence of the Ottawa River and the Rideau River. Ottawa borders Gatineau, Gatineau, Quebec, and forms the cor ...

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, , , , , , BCD time code on 125 Hz sub-carrier.
00:00–24:00 UTC , - , HLA , ,

KRISS Kriss might refer to: People * Kriss (surname) * Kriss (given name) Organizations * Korea Research Institute of Standards and Science * KRISS, a U.S. firearms company, who developed the ''Vector'' and ''KARD''; see List of firearms Other uses ...

, ,

Daejeon Daejeon (; ) is South Korea's list of cities in South Korea, fifth-largest metropolis, with a population of nearly 1.5 million. Located in a central lowland valley between the Sobaek Mountains and the Geum River, the city is known both as a ...

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YVTO YVTO is the callsign of the official time signal from the Juan Manuel Cagigal Naval Observatory in Caracas, Venezuela. The content of YVTO's signal, which is a continuous 1 kW amplitude modulated carrier wave at 5.000 MHz, is much simpler t ...

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Caracas Caracas ( , ), officially Santiago de León de Caracas (CCS), is the capital and largest city of Venezuela, and the center of the Metropolitan Region of Caracas (or Greater Caracas). Caracas is located along the Guaire River in the northern p ...

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LOL LOL, or lol, is an initialism for laughing out loud, and a popular element of Internet slang, which can be used to indicate amusement, irony, or double meanings. It was first used almost exclusively on Usenet, but has since become widesprea ...

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Buenos Aires Buenos Aires, controlled by the government of the Autonomous City of Buenos Aires, is the Capital city, capital and largest city of Argentina. It is located on the southwest of the Río de la Plata. Buenos Aires is classified as an Alpha− glob ...

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Rio de Janeiro, RJ Rio de Janeiro, or simply Rio, is the capital of the Rio de Janeiro (state), state of Rio de Janeiro. It is the List of cities in Brazil by population, second-most-populous city in Brazil (after São Paulo) and the Largest cities in the America ...

, , Horizontal half-wavelength dipole , , , , Maintained by

National Observatory (Brazil) The National Observatory ( Portuguese: ''Observatório Nacional'' or ''ON'') is an institution localized in Rio de Janeiro, Brazil. Founded by Pedro I on October 15, 1827, it is one of the oldest scientific institutions in the country. Its init ...

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, , Broadband monopole , , , , Schedule: variable (experimental broadcast) , - , MIKES Mikes may refer to: * Centre for Metrology and Accreditation, MIKES, Centre for Metrology and Accreditation, Finland, a research organisation * Mikes (restaurant), a Canadian restaurant chain * Mikes (surname), a surname * Mikes, the Hungarian nam ...
, ,
MIKES Mikes may refer to: * Centre for Metrology and Accreditation, MIKES, Centre for Metrology and Accreditation, Finland, a research organisation * Mikes (restaurant), a Canadian restaurant chain * Mikes (surname), a surname * Mikes, the Hungarian nam ...
, , Espoo, Finland Espoo (, ; ) is a city in Finland. It is located to the west of the capital, Helsinki, in southern Uusimaa. The population is approximately . It is the most populous Municipalities of Finland, municipality in Finland. Espoo is part of the Helsi ...

, , ~~λ/4 sloper antenna~~ , , Reproduces a QSL letter from MIKES with technical details. , , ~~1 kHz amplitude modulation similar to DCF77.~~
As of 2017 the transmission is discontinued until further notice.BIPM Annual Report on Time Activities �
Time Signals
, retrieved 2018 July 31.
"MIKES has a transmitter for time code and precise 25 MHz frequency for those near the Helsinki metropolitan area who need precise time and frequency." Descriptions Many other countries can receive these signals (

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

can sometimes be received in New Zealand, Western Australia, Tasmania, Southeast Asia, parts of Western Europe and the Pacific Northwest of North America at night), but success depends on the time of day, atmospheric conditions, and interference from intervening buildings. Reception is generally better if the clock is placed near a window facing the transmitter. There is also a propagation delay of approximately for every the receiver is from the transmitter.

Clock receivers

A number of manufacturers and retailers sell radio clocks that receive coded time signals from a radio station, which, in turn, derives the time from a true atomic clock. One of the first radio clocks was offered by

Heathkit Heathkit is the brand name of kits and other electronic products produced and marketed by the Heath Company. The products over the decades have included electronic test equipment, high fidelity home audio equipment, television receivers, amateu ...

in late 1983. Their model GC-1000 "Most Accurate Clock" received shortwave time signals from radio station WWV in

. It automatically switched between WWV's 5, 10, and 15 MHz frequencies to find the strongest signal as conditions changed through the day and year. It kept time during periods of poor reception with a quartz-crystal

oscillator Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...

. This oscillator was disciplined, meaning that the microprocessor-based clock used the highly accurate time signal received from WWV to trim the crystal oscillator. The timekeeping between updates was thus considerably more accurate than the crystal alone could have achieved. Time down to the tenth of a second was shown on an

LED A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresp ...

display. The GC-1000 originally sold for US$250 in kit form and US$400 preassembled, and was considered impressive at the time. Heath Company was granted
patent
for its design. By 1990, engineers from German watchmaker

Junghans Junghans Uhren GmbH is a German watch and clock manufacturer. The company is located in the district of Rottweil, in the town of Schramberg, Baden-Württemberg, southwest Germany. In 1903, Junghans was the largest watch factory in the world an ...

had miniaturized this technology to fit into the case of a digital wristwatch. The following year the analog version

Junghans MEGA The Junghans Mega was "the world's first radio-controlled wristwatch with hands" JUNGHANS UHREN GmbH, Schramberg (analog watch) in 1991. It was produced by the German watchmaker Junghans, who had already introduced a digital watch called Mega 1 ...

with hands was launched. In the 2000s (decade) radio-based "atomic clocks" became common in retail stores; as of 2010 prices start at around US$15 in many countries. Clocks may have other features such as indoor thermometers and

weather station A weather station is a facility, either on land or sea, with instruments and equipment for measuring atmosphere of Earth, atmospheric conditions to provide information for weather forecasting, weather forecasts and to study the weather and clima ...

functionality. These use signals transmitted by the appropriate transmitter for the country in which they are to be used. Depending upon signal strength they may require placement in a location with a relatively unobstructed path to the transmitter and need fair to good atmospheric conditions to successfully update the time. Inexpensive clocks keep track of the time between updates, or in their absence, with a non-disciplined quartz-crystal clock, with the accuracy typical of non-radio-controlled quartz timepieces. Some clocks include indicators to alert users to possible inaccuracy when synchronization has not been recently successful. The United States

National Institute of Standards and Technology The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into Outline of p ...

(NIST) has published guidelines recommending that radio clock movements keep time between synchronizations to within ±0.5 seconds to keep time correct when rounded to the nearest second."How Accurate is a Radio Controlled Clock?"
by Michael Lombardi (2010). Some of these movements can keep time between synchronizations to within ±0.2 seconds by synchronizing more than once spread over a day. Timepieces with

radio support, ranging from watches with basic control of functionality via a

to full

es obtain time information from a connected

, with no need to receive time signal broadcasts.

Other broadcasts

; Attached to other broadcast stations: Broadcast stations in many countries have carriers precisely synchronized to a standard phase and frequency, such as the

BBC Radio 4 BBC Radio 4 is a British national radio station owned and operated by the BBC. The station replaced the BBC Home Service on 30 September 1967 and broadcasts a wide variety of spoken-word programmes from the BBC's headquarters at Broadcasti ...

service on 198 kHz, and some also transmit sub-audible or even inaudible time-code information, like the

Radio France Radio France () is the French national public radio broadcaster. Stations Radio France offers seven national networks: *France Inter — Radio France's "generalist media, generalist" station, featuring entertaining and informative talk mixed wi ...

longwave transmitter on 162 kHz. Attached time signal systems generally use audible tones or phase modulation of the carrier wave. ;

Teletext Teletext, or broadcast teletext, is a standard for displaying text and rudimentary graphics on suitably equipped television sets. Teletext sends data in the broadcast signal, hidden in the invisible vertical blanking interval area at the to ...

(TTX): Digital text pages embedded in television video also provide accurate time. Many modern TV sets and VCRs with TTX decoders can obtain accurate time from Teletext and set the internal clock. However, the TTX time can vary up to 5 minutes. 100506 digitalspy.co.uk Many

digital radio Digital radio is the use of digital technology to transmit or receive across the radio spectrum. Digital transmission by radio waves includes digital broadcasting, and especially digital audio radio services. This should not be confused with In ...

and

digital television Digital television (DTV) is the transmission of television signals using Digital signal, digital encoding, in contrast to the earlier analog television technology which used analog signals. At the time of its development it was considered an ...

schemes also include provisions for time-code transmission. ; Digital Terrestrial Television : The DVB and ATSC standards have 2 packet types that send time and date information to the receiver. Digital television systems can equal GPS stratum 2 accuracy (with short term clock discipline) and stratum 1 (with long term clock discipline) provided the transmitter site (or network) supports that level of functionality. ; VHF FM Radio Data System (RDS): RDS can send a clock signal with sub-second precision but with an accuracy no greater than 100 ms and with no indication of clock stratum. Not all RDS networks or stations using RDS send accurate time signals. The time stamp format for this technology is Modified Julian Date (MJD) plus UTC hours, UTC minutes and a local time offset. ; L-band and VHF

Digital Audio Broadcasting Digital Audio Broadcasting (DAB) is a digital radio international standard, standard for broadcasting digital audio radio services in many countries around the world, defined, supported, marketed and promoted by the WorldDAB organisation. T ...

: DAB systems provide a time signal that has a precision equal to or better than Digital Radio Mondiale (DRM) but like FM RDS do not indicate clock stratum. DAB systems can equal GPS stratum 2 accuracy (short term clock discipline) and stratum 1 (long term clock discipline) provided the transmitter site (or network) supports that level of functionality. The time stamp format for this technology is BCD. ; Digital Radio Mondiale (DRM): DRM is able to send a clock signal, but one not as precise as

navigation satellite A satellite navigation or satnav system is a system that uses satellites to provide autonomous geopositioning. A satellite navigation system with global coverage is termed global navigation satellite system (GNSS). , four global systems are op ...

clock signals. DRM timestamps received via shortwave (or multiple hop mediumwave) can be up to 200 ms off due to path delay. The time stamp format for this technology is BCD.

Gallery

Low cost DCF77 receiver.jpg, LF time signal receiver Junghans Mega.jpg, World's first radio clock wrist watch,

Junghans Mega The Junghans Mega was "the world's first radio-controlled wristwatch with hands" JUNGHANS UHREN GmbH, Schramberg (analog watch) in 1991. It was produced by the German watchmaker Junghans, who had already introduced a digital watch called Mega 1 ...

(analog model) File:Funkuhr.JPG, Radio controlled analog

wall clock A clock or chronometer is a device that measures and displays time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month, a ...

File:Bahnhof Kinding (Altmühltal), Bahnsteiguhr 2007.jpg, The DCF77 time signal is used by organizations like the

Deutsche Bahn (, ; abbreviated as DB or DB AG ) is the national railway company of Germany, and a state-owned enterprise under the control of the German government. Headquartered in the Bahntower in Berlin, it is a joint-stock company ( AG). DB was fou ...

railway company to synchronize their

station clock A station clock is a clock at a railway station that provides a standard indication of time to both passengers and railway staff. A railway station will often have several station clocks. They can be found in a clock tower, in the booking hall o ...

Multiple transmitters

A radio clock receiver may combine multiple time sources to improve its accuracy. This is what is done in

satellite navigation systems A satellite navigation or satnav system is a system that uses satellites to provide autonomous geopositioning. A satellite navigation system with global coverage is termed global navigation satellite system (GNSS). , four global systems are op ...

such as the

Galileo Galileo di Vincenzo Bonaiuti de' Galilei (15 February 1564 – 8 January 1642), commonly referred to as Galileo Galilei ( , , ) or mononymously as Galileo, was an Italian astronomer, physicist and engineer, sometimes described as a poly ...

, and

GLONASS GLONASS (, ; ) is a Russian satellite navigation system operating as part of a radionavigation-satellite service. It provides an alternative to Global Positioning System (GPS) and is the second navigational system in operation with global cove ...

Satellite navigation system A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scientif ...

s have one or more caesium, rubidium or hydrogen maser atomic clocks on each satellite, referenced to a clock or clocks on the ground. Dedicated timing receivers can serve as local time standards, with a precision better than 50 ns. The recent revival and enhancement of

LORAN LORAN (Long Range Navigation) was a hyperbolic navigation, hyperbolic radio navigation system developed in the United States during World War II. It was similar to the UK's Gee (navigation), Gee system but operated at lower frequencies in order ...

, a land-based radio navigation system, will provide another multiple source time distribution system.

GPS clocks

Many modern radio clocks use

systems such as

to provide more accurate time than can be obtained from terrestrial radio stations. These ''GPS clocks'' combine time estimates from multiple satellite atomic clocks with error estimates maintained by a network of ground stations. Due to effects inherent in radio propagation and ionospheric spread and delay, GPS timing requires averaging of these phenomena over several periods. No GPS receiver directly computes time or frequency, rather they use GPS to discipline an oscillator that may range from a quartz crystal in a low-end navigation receiver, through oven-controlled

crystal oscillators A crystal oscillator is an electronic oscillator circuit that uses a piezoelectric crystal as a frequency-selective element. The oscillator frequency is often used to keep track of time, as in quartz wristwatches, to provide a stable clock ...

(OCXO) in specialized units, to atomic oscillators (

rubidium Rubidium is a chemical element; it has Symbol (chemistry), symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have ...

) in some receivers used for

synchronization in telecommunications Many services running on modern digital telecommunications networks require accurate synchronization for correct operation. For example, if telephone exchanges are not synchronized, then bit slips will occur and degrade performance. Telecommunic ...

. For this reason, these devices are technically referred to as GPS-disciplined oscillators. GPS units intended primarily for time measurement as opposed to navigation can be set to assume the antenna position is fixed. In this mode, the device will average its position fixes. After approximately a day of operation, it will know its position to within a few meters. Once it has averaged its position, it can determine accurate time even if it can pick up signals from only one or two satellites. GPS clocks provide the precise time needed for

synchrophasor A phasor measurement unit (PMU) is a device used to estimate the magnitude and phase angle of an electrical phasor quantity (such as voltage or current) in the electricity grid using a common time source for synchronization. Time synchronization ...

measurement of voltage and current on the commercial power grid to determine the health of the system.

Astronomy timekeeping

Although any

receiver that is performing its primary navigational function must have an internal time reference accurate to a small fraction of a second, the displayed time is often not as precise as the internal clock. Most inexpensive navigation receivers have one

CPU A central processing unit (CPU), also called a central processor, main processor, or just processor, is the primary processor in a given computer. Its electronic circuitry executes instructions of a computer program, such as arithmetic, log ...

that is multitasking. The highest-priority task for the CPU is maintaining satellite lock—not updating the display. Multicore CPUs for navigation systems can only be found on high end products. For serious precision timekeeping, a more specialized GPS device is needed. Some amateur astronomers, most notably those who time

grazing lunar occultation __NOTOC__ A grazing lunar occultation (also lunar grazing occultation, lunar graze, or just graze) is a lunar occultation in which as the occulted star disappears and reappears intermittently on the edge of the Moon. A team of many observers can ...

events when the moon blocks the light from stars and planets, require the highest precision available for persons working outside large research institutions. The Web site of the International Occultation Timing Association has detailed technical information about precision timekeeping for the amateur astronomer.

Daylight saving time

Various formats listed above include a flag indicating the status of

daylight saving time Daylight saving time (DST), also referred to as daylight savings time, daylight time (Daylight saving time in the United States, United States and Daylight saving time in Canada, Canada), or summer time (British Summer Time, United Kingdom, ...

(DST) in the home country of the transmitter. This signal is typically used by clocks to adjust the displayed time to meet user expectations.

References

External links

IOTA Observers Manual
This manual from the ''International Occultation Timing Association'' has very extensive details on methods of accurate time measurement.
NIST website: WWVB Radio Controlled Clocks

NTP Project Development Website
{{Authority control Clocks Watches

Clock A clock or chronometer is a device that measures and displays time. The clock is one of the oldest Invention, human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month, a ...

Single transmitter

Longwave and shortwave transmissions

List of radio time signal stations

Clock receivers

Other broadcasts

Gallery

Multiple transmitters

GPS clocks

Astronomy timekeeping

Daylight saving time

See also

References

External links