Radio clock

File:Atomic clock.jpg

A modern LF radio-controlled clock

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.

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 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
Template:Ntsh 25 kHz	RJH69	Belarus VNIIFTRI	Vileyka	Triple umbrella antenna		This is Beta time signal. The signal is transmitted in non-overlapping time: 02:00–02:20 UTC RAB99 04:00–04:25 UTC RJH86 06:00–06:20 UTC RAB99 07:00–07:25 UTC RJH69 08:00–08:25 UTC RJH90 09:00–09:25 UTC RJH77 10:00–10:25 UTC RJH86 11:00–11:20 UTC RJH63
	RJH77	Russia VNIIFTRI	Arkhangelsk	Triple umbrella antenna
	RJH63	Russia VNIIFTRI	Krasnodar	Umbrella antenna
	RJH90	Russia VNIIFTRI	Nizhny Novgorod	Triple umbrella antenna
	RJH86	Kyrgyzstan VNIIFTRI	Bishkek	Triple umbrella antenna
	RAB99	Russia VNIIFTRI	Khabarovsk	Umbrella antenna
Template:Ntsh 40 kHz	JJY	Japan NICT	Mount Otakadoya, Fukushima	Capacitance hat, height Template:Cvt		Located near Fukushima
Template:Ntsh 50 kHz	RTZ	Russia VNIIFTRI	Irkutsk	Umbrella antenna		PM time code
Template:Ntsh 60 kHz	JJY	Japan NICT	Mount Hagane, Kyushu	Capacitance hat, height Template:Cvt		Located on Kyūshū Island
	MSF	United Kingdom NPL	Anthorn, Cumbria	Triple T-antenna		Range up to Template:Cvt
	WWVB	United States NIST	Near Fort Collins, Colorado	Two capacitance hats, height Template:Cvt		Received through most of mainland U.S.
Template:Ntsh 66.66 kHz	RBU	Russia VNIIFTRI	Taldom, Moscow	Umbrella antenna		PM time code
Template:Ntsh 68.5 kHz	BPC	China NTSC	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)
Template:Ntsh 75 kHz	HBG	Template:Flagdeco Switzerland METAS	Prangins	T-antenna		Discontinued as of 1 January 2012
Template:Ntsh 77.5 kHz	DCF77	Germany PTB	Mainflingen, Hessen	Vertical omni-directional antennas with top-loading capacity, height		Located southeast of Frankfurt am Main with a range of up to Template:Cvt
	BSF	Taiwan	Zhongli	T-antenna
Template:Ntsh 100 kHz	BPL	China NTSC	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 Template:Cvt
	RNS-E	Russia VNIIFTRI	Bryansk	5 guyed masts		CHAYKA compatible format signal 04:00–10:00 UTC and 14:00–18:00 UTC
	RNS-V	Russia VNIIFTRI	Alexandrovsk-Sakhalinsky	Single guyed mast		CHAYKA compatible format signal 23:00–05:00 UTC and 11:00–17:00 UTC
Template:Ntsh 129.1 kHz	DCF49	Germany PTB	Mainflingen	T-antenna		EFR radio teleswitch time signal only (no reference frequency) FSK ± 170 Hz 200 baud
Template:Ntsh 135.6 kHz	HGA22	Hungary PTB	Lakihegy	Single guyed mast
Template:Ntsh 139 kHz	DCF39	Germany PTB	Burg bei Magdeburg	Single guyed mast
Template:Ntsh 162 kHz	ALS162	France Template:Ill	Allouis	Two guyed steel lattice masts, height Template:Cvt, fed on the top		AM-broadcasting transmitter, located Template:Cvt south of Paris with a range of up to Template:Cvt, using PM with encoding similar to DCF77
Template:Ntsh 198 kHz	BBC Radio 4	United Kingdom NPL	Droitwich	T-aerial		Additional (50 kW) transmitters is at Burghead and Westerglen. The time signal is transmitted by 25-bit/s phase modulation.
Template:Ntsh 225 kHz	Polskie Radio	Poland	Solec Kujawski	Guyed mast	1000 kW	Phase-modulated time signal
Template:Ntsh 2.5 MHz	BPM	China NTSC	Pucheng, Shaanxi			(BCD time code on 125 Hz sub-carrier not yet activated) 07:30–01:00 UTC
	WWV	United States NIST	Near Fort Collins, Colorado	Broadband monopole		Binary-coded decimal (BCD) time code on 100 Hz sub-carrier
	WWVH	United States NIST	Kekaha, Hawaii
Template:Ntsh 3.33 MHz	CHU	Canada NRC	Ottawa, Ontario			300 baud Bell 103 time code
Template:Ntsh 4.996 MHz	RWM	Russia VNIIFTRI	Taldom, Moscow			CW (, )
Template:Ntsh 5 MHz	BPM	China NTSC	Pucheng, Shaanxi			BCD time code on 125 Hz sub-carrier. 00:00–24:00 UTC
	HLA	Template:Flagu KRISS	Daejeon
	WWV	United States NIST	Near Fort Collins, Colorado	Broadband monopole		BCD time code on 100 Hz sub-carrier
	WWVH	United States NIST	Kekaha, Hawaii
	YVTO	Venezuela	Caracas
Template:Ntsh 7.85 MHz	CHU	Canada NRC	Ottawa, Ontario			300 baud Bell 103 time code
Template:Ntsh 9.996 MHz	RWM	Russia VNIIFTRI	Taldom, Moscow			CW (, )
Template:Ntsh 10 MHz	BPM	China NTSC	Pucheng, Shaanxi			(BCD time code on 125 Hz sub-carrier not yet activated) 00:00–24:00 UTC
	LOL	Argentina SHN	Buenos AiresTemplate:Refn			Observatorio Naval Buenos Aires
	WWV	United States NIST	Near Fort Collins, Colorado	Broadband monopole		BCD time code on 100 Hz sub-carrier
	WWVH	United States NIST	Kekaha, Hawaii
	PPE	Brazil	Rio de Janeiro, RJ	Horizontal half-wavelength dipole		Maintained by National Observatory (Brazil)
Template:Ntsh 14.67 MHz	CHU	Canada NRC	Ottawa, Ontario			300 baud Bell 103 time code
Template:Ntsh 14.996 MHz	RWM	Russia VNIIFTRI	Taldom, Moscow			CW (, )
Template:Ntsh 15 MHz	BPM	China NTSC	Pucheng, Shaanxi			(BCD time code on 125 Hz sub-carrier not yet activated) 01:00–09:00 UTC
	WWV	United States NIST	Near Fort Collins, Colorado	Broadband monopole		BCD time code on 100 Hz sub-carrier
	WWVH	United States NIST	Kekaha, Hawaii
Template:Ntsh 20 MHz	WWV	United States NIST	Near Fort Collins, Colorado	Broadband monopole		BCD time code on 100 Hz sub-carrier
Template:Ntsh 25 MHz	WWV	United States NIST	Near Fort Collins, Colorado	Broadband monopole		Schedule: variable (experimental broadcast)
	MIKES	Finland MIKES	Espoo, Finland	λ/4 sloper antenna		1 kHz amplitude modulation similar to DCF77. As of 2017 the transmission is discontinued until further notice. "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 1 ms for every Template:Cvt 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 Template:Webarchive 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. 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.

The United States National Institute of Standards and Technology (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. 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 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.

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 longwave service on 198 kHz, and some also transmit sub-audible or even inaudible time-code information, like the Radio France longwave transmitter on 162 kHz. Attached time signal systems generally use audible tones or phase modulation of the carrier wave.

Teletext (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.

Many digital radio and digital television 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

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 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

• LF time signal receiver
  LF time signal receiver
• World's first radio clock wrist watch, Junghans Mega (analog model)
  World's first radio clock wrist watch, Junghans Mega (analog model)
• Radio controlled analog wall clock
  Radio controlled analog wall clock
• The DCF77 time signal is used by organizations like the Deutsche Bahn railway company to synchronize their station clocks
  The DCF77 time signal is used by organizations like the Deutsche Bahn railway company to synchronize their station clocks

Multiple transmitters

A radio clock receiver may combine multiple time sources to improve its accuracy. This is what is done in satellite navigation systems such as the Global Positioning System, Galileo, and GLONASS. Satellite navigation systems 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, a land-based radio navigation system, will provide another multiple source time distribution system.

GPS clocks

Many modern radio clocks use satellite navigation systems such as Global Positioning System 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 (OCXO) in specialized units, to atomic oscillators (rubidium) in some receivers used for synchronization in telecommunications. 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 measurement of voltage and current on the commercial power grid to determine the health of the system.

Astronomy timekeeping

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

See also

• Casio Wave Ceptor
• Clock network
• Speaking clock
• Standard frequency and time signal service
• Time from NPL
• Time and frequency transfer
• Time synchronization in North America

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