A radio clock or radio-controlled clock (RCC), and often colloquially (and incorrectly) referred to as an "atomic clock", is a type of quartz clock or watch that is automatically synchronized to a time code transmitted by a radio transmitter connected to a time standard 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 systems such as Global Positioning System. 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 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 bands. Systems using dedicated time signal stations can achieve accuracy of a few tens of milliseconds.
Satellite navigation 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 radio support, ranging from watches with basic control of functionality via a mobile app to full smartwatches obtain time information from a connected phone, with no need to receive time signal broadcasts.
Single transmitter Radio clocks synchronized to a terrestrial time signal 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. 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 + List of radio time signal stations Frequency Callsign Country Authority Location Aerial type Power Remarks RJH69 BelarusVNIIFTRI Vileyka Triple umbrella antenna3 umbrella antennas, fixed on 3 guyed tubular masts, insulated against ground with a height of and 15 guyed lattice masts with a height of This is Beta time signal. — official signal specification. The signal is transmitted in non-overlapping time:02:00–02:20 UTC RAB9904:00–04:25 UTC RJH8606:00–06:20 UTC RAB9907:00–07:25 UTC RJH6908:00–08:25 UTC RJH9009:00–09:25 UTC RJH7710:00–10:25 UTC RJH8611:00–11:20 UTC RJH63 RJH77 RussiaVNIIFTRI Arkhangelsk Triple umbrella antenna3 umbrella antennas, fixed on 18 guyed lattice masts, height of central masts: 305 metres RJH63 RussiaVNIIFTRI Krasnodar Umbrella antennaumbrella antenna, fixed on 13 guyed lattice masts, height of central mast: RJH90 RussiaVNIIFTRI Nizhny Novgorod Triple umbrella antenna3 umbrella antennas, fixed on 3 guyed tubular masts, insulated against ground with a height of and 15 guyed lattice masts with a height of RJH86in air RJH66 KyrgyzstanVNIIFTRI Bishkek Triple umbrella antenna3 umbrella antennas, fixed on 18 guyed lattice masts, height of central masts: RAB99 RussiaVNIIFTRI Khabarovsk Umbrella antennaumbrella antenna, fixed on 18 guyed lattice masts arranged in 3 rows, height of central masts: JJY JapanNICT Mount Otakadoya, Fukushima Capacitance hat, height Located near Fukushima RTZ RussiaVNIIFTRI Irkutsk Umbrella antenna PM time code JJY JapanNICT Mount Hagane, Kyushu Capacitance hat, height Located on Kyūshū Island MSF United KingdomNPL Anthorn, Cumbria Triple T-antenna3 T-antennas, spun above ground between two high guyed grounded masts in a distance of Range up to WWVB United StatesNIST Near Fort Collins, Colorado Two capacitance hats, height Received through most of mainland U.S. RBU RussiaVNIIFTRI Taldom, Moscow 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 ChinaNTSC Shangqiu, Henan 4 guyed masts, arranged in a square 21 hours per day, with a 3-hour break from 05:00–08:00 (China Standard Time) daily (21:00–24:00 UTC) HBG SwitzerlandMETAS Prangins T-antennaT-antenna spun between two tall, grounded free-standing lattice towers in a distance of Discontinued as of 1 January 2012 DCF77 GermanyPTB Mainflingen, Hessen 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 Taiwan Zhongli T-antennaT-antenna spun between two telecommunication towers in a distance of Frequency for radio navigation system BPL ChinaNTSC Pucheng, Shaanxi Single guyed lattice steel mast Loran-C compatible format signal on air from 05:30 to 13:30 UTC, with a reception radius up to RNS-E RussiaVNIIFTRI Karachev (44 km from Bryansk) 5 guyed masts CHAYKA compatible format signal04:00–10:00 UTC and 14:00–18:00 UTC RNS-V RussiaVNIIFTRI Alexandrovsk-Sakhalinsky Single guyed mast CHAYKA compatible format signal23:00–05:00 UTC and 11:00–17:00 UTC Frequency for radio teleswitch system GermanyPTB Mainflingen T-antenna EFR radio teleswitch — in Germantime signal only (no reference frequency)FSK ± 170 Hz 200 baud HungaryPTB Lakihegy Tower Single guyed mast GermanyPTB Burg bei Magdeburg Single guyed mast Frequency for AM-broadcasting ALS162 France Allouis 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 since 1988, before 200 kHz United KingdomNPL Droitwich T-aerialDroitwich uses a T-aerial suspended between two (699') guyed steel lattice radio masts, which stand apart. Additional transmitters is at Burghead and Westerglen. The time signal is transmitted by phase modulation. Polskie Radio Poland Solec Kujawski Guyed mast Phase-modulated time signal BPM ChinaNTSC Pucheng, Shaanxi (BCD time code on 125 Hz sub-carrier not yet activated) 07:30–01:00 UTC WWV United StatesNIST Near Fort Collins, Colorado Broadband monopole Binary-coded decimal (BCD) time code on sub-carrier WWVH United StatesNIST Kekaha, Hawaii CHU CanadaNRC Ottawa, Ontario 300 baud Bell 103 time code. Discontinued effective 22 June 2026. RWM RussiaVNIIFTRI Taldom, Moscow CW (, ) BPM ChinaNTSC Pucheng, Shaanxi BCD time code on 125 Hz sub-carrier.00:00–24:00 UTC HLA KRISS Daejeon WWV United StatesNIST Near Fort Collins, Colorado Broadband monopole Time signal article says 2.5 kW BCD time code on sub-carrier WWVH United StatesNIST Kekaha, Hawaii YVTO Venezuela Caracas CHU CanadaNRC Ottawa, Ontario Bell 103 time code RWM RussiaVNIIFTRI Taldom, Moscow CW (, ) BPM ChinaNTSC Pucheng, Shaanxi (BCD time code on 125 Hz sub-carrier not yet activated)00:00–24:00 UTC LOL ArgentinaSHN Buenos Aires Observatorio Naval Buenos Aires WWV United StatesNIST Near Fort Collins, Colorado Broadband monopole BCD time code on sub-carrier WWVH United StatesNIST Kekaha, Hawaii PPE Brazil Rio de Janeiro, RJ Horizontal half-wavelength dipole Maintained by National Observatory (Brazil) CHU CanadaNRC Ottawa, Ontario 300 baud Bell 103 time code RWM RussiaVNIIFTRI Taldom, Moscow CW (, ) BPM ChinaNTSC Pucheng, Shaanxi (BCD time code on 125 Hz sub-carrier not yet activated)01:00–09:00 UTC WWV United StatesNIST Near Fort Collins, Colorado Broadband monopole BCD time code on sub-carrier WWVH United StatesNIST Kekaha, Hawaii WWV United StatesNIST Near Fort Collins, Colorado Broadband monopole BCD time code on sub-carrier WWV United StatesNIST Near Fort Collins, Colorado Broadband monopole Schedule: variable (experimental broadcast) MIKES FinlandMIKES Espoo, Finland 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 31 July 2018."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 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 in late 1983. Their model GC-1000 "Most Accurate Clock" received shortwave time signals from radio station WWV in Fort Collins, Colorado. 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. 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 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 a patent for its design.
By 1990, engineers from German watchmaker Junghans had miniaturized this technology to fit into the case of a digital wristwatch. The following year the analog version Junghans Mega with hands was launched.
In the 2000s, radio-based "atomic clocks" became common in retail stores; as of 2010 prices start at around US$15 in many countries." Radio controlled clock £19.95 Clocks may have other features such as indoor thermometers and weather station 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.