Radio clock

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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. 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 multiple transmitters, like the Global Positioning System. Such systems may be used to set computer clocks or clocks meant for human readability, or for any purpose where accurate time is needed.

Contents 1 Single transmitter 1.1 Longwave and shortwave transmissions 1.1.1 List of time signal radio stations 1.1.2 Clock receivers 1.2 Other broadcasts 2 Multiple transmitters 2.1 GPS clocks 2.2 Galileo positioning system 2.3 LORAN 2.4 Astronomy timekeeping 3 See also 4 References 5 External links

[edit] Single transmitter

Radio clocks synchronized to terrestrial time signals can usually achieve an accuracy of around 1 millisecond relative to the time standard^{[citation needed]}, generally limited by uncertainties and variability in radio propagation.

[edit] Longwave and shortwave transmissions

Radio clocks depend on time signal of radio stations. These time standards specify:

• the broadcast frequency of the frequency standard
• the exact geographic location of each antenna, so the radio signal’s time of propagation can be estimated
• how the beginning of each second interval is determined
• how the signal is modulated to identify the current time

[edit] List of time signal radio stations

Frequency	Callsign	Country	Location	Aerial type	Power	Remarks
40 kHz	JJY	Japan	Mount Otakadoya, Fukushima	Capacitance hat, height 250 m	50 kW
60 kHz	GBZ	United Kingdom	Anthorn, Cumbria		17 kW
	JJY	Japan	Mount Hagane, Kyushu	Capacitance hat, height 200 m	50 kW
	WWVB	United States	Fort Collins, Colorado	Two capacitance hats, height 122 m	70 kW
66.66 kHz	RBU	Russia	Elektrougli, Moscow		10 kW
68.5 KHz	BPC	China	Xi'an
75 kHz	HBG	Switzerland	Prangins		20 kW	until 31 Dec 2011
77.5 kHz	DCF77	Germany	Mainflingen, Hesse	vertical omni-directional antennas with top-loading capacity, height 150 m [1]	50 kW
162 kHz	TDF	France	Allouis	Two guyed steel lattice masts, height 350 m, fed on the top	2000 kW
2.5 MHz	BPM	China	Xi'an
	WWV	United States	Fort Collins, Colorado		2.5 kW	BCD time code on 100 Hz sub-carrier
	WWVH	United States	Kekaha, Hawaii		5 kW
3.33 MHz	CHU	Canada	Ottawa, Ontario		3 kW	300 baud Bell 103 time code
5 MHz	BPM	China	Xi'an
	WWV	United States	Fort Collins, Colorado		10 kW	BCD time code on 100 Hz sub-carrier
	WWVH	United States	Kekaha, Hawaii		10 kW
7.85 MHz	CHU	Canada	Ottawa, Ontario		10 kW	300 baud Bell 103 time code
10 MHz	BPM	China	Xi'an
	WWV	United States	Fort Collins, Colorado		10 kW	BCD time code on 100 Hz sub-carrier
	WWVH	United States	Kekaha, Hawaii		10 kW
14.67 MHz	CHU	Canada	Ottawa, Ontario		3 kW	300 baud Bell 103 time code
15 MHz	BPM	China	Xi'an
	WWV	United States	Fort Collins, Colorado		10 kW	BCD time code on 100 Hz sub-carrier
	WWVH	United States	Kekaha, Hawaii		10 kW
20 MHz	WWV	United States	Fort Collins, Colorado		2.5 kW	BCD time code on 100 Hz sub-carrier

Many other countries can receive these signals (JJY can sometimes be received in Western Australia, Tasmania, and the Pacific Northwest of North America at night), but it 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 transit delay of approximately 1 ms for every 300 km the receiver is from the transmitter. When operating properly and correctly synchronized, better brands of radio clocks are normally accurate to the second. (Product advertising often claims higher accuracy, but for many or most users that is only a theoretical possibility.)^{[citation needed]}

A low frequency (LF) radio clock.

LF time signal receiver.

World's first radio clock wrist watch, Junghans Mega (analog model).

Citizen Atessa Eco-Drive ATV53-3023 analog-digital chronograph with 4 area Radio Controlled reception (North America, Europe, China, Japan).

[edit] Clock receivers

Many manufacturers and retailers sell radio clocks under the name "atomic clocks", but the clocks themselves are not atomic. Instead, they 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 Colorado, USA whenever propagation conditions permitted, automatically switching between the 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 frequency standard 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, US$400 preassembled, and was considered impressive at the time. Heath Company was granted a patent for their design. ^{[2] [3]}

In the 2000s radio-based "atomic clocks" became common in retail stores; As of 2010^[update] prices start at around US$15 in many countries^{[citation needed]}. 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 of similar accuracy to a non-radio-controlled quartz timepiece. Some clocks include an indicator to alert users to possible inaccuracy when synchronization has not been successful within the last 24 to 48 hours. In other cases, the indicator will indicate that synchronization has been achieved within the last few hours, and will go blank in the mid-morning.^{[citation needed]}

Modern radio clocks can be referenced to atomic clocks, and provide access to high-quality atomic-derived time over a wide area using inexpensive equipment. They are suitable for scientific or other work which does not require higher accuracy than they can provide.

[edit] Other broadcasts

Interval signals

Many analog broadcast stations also transmit a distinctive tone or tones at the precise top of every hour, derived from an official source. Most well known is the Greenwich Time Signal, transmitted on BBC radio since 1924. In the US, WTIC in Hartford, Connecticut has broadcast the Morse code letter "V" every hour, on the hour, since 1943.

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 time-code information, like the Radio France longwave transmitter on 162 kHz. Many digital radio and digital television schemes also include provisions for time-code transmission.

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.^[4]

FM Radio Data System (RDS)

RDS can send a clock signal with sub-second precision, but not all RDS networks or stations using RDS send accurate time signals.

Digital Radio Mondiale (DRM)

DRM is able to send a clock signal, but one not as precise as GPS-Glonass clock signals.

Mobile telephones

Some mobile telephone technologies, such as Qualcomm's CDMA, are designed to distribute high-quality standard time signals (referenced to GPS in the case of CDMA). CDMA clocks are increasingly popular for providing reference time to computer networks. Their precision is nearly as good as that of GPS clocks, but since the signal comes from a nearby cell phone base station rather than a distant satellite, CDMA clocks generally work better inside buildings. So in many cases, when a GPS reference clock would require installing an outdoor antenna, a CDMA clock can overcome this requirement.

Internet

the availability almost everywhere of connection to the Internet has superseded many applications formerly served by transmitted time signals. In particular, any computer equipment connected to the Internet has easy access to time via the Network Time Protocol (NTP). Unless special precautions are taken, time derived from the Internet and distributed over a computer network may have an error of a second or so.

[edit] Multiple transmitters

Multiple time sources may be combined to derive a more accurate time synchronization sources. This is what is done in satellite navigation systems such as the Global Positioning System. GPS, Galileo and GLONASS satellite navigation systems have a caesium or rubidium atomic clock on each satellite, referenced to a clock or clocks on the ground. Some navigation units can serve as local time standards, with a precision of about one microsecond (µs). The recent revival and enhancement of the terrestrial based radio navigation system, LORAN will provide another multiple source time distribution system.

[edit] GPS clocks

Many modern radio clocks use the Global Positioning System to provide more accurate time than can be obtained from these 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 (OXCO) 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.^[5]

[edit] Galileo positioning system

Using the Global Positioning System is dependent on the goodwill of the United States government for the operation of the GPS satellite constellation. This is not acceptable for many critical non-US civilian and military systems, although it may be acceptable for many civilian purposes, as it is assumed by most users that the civilian GPS signal would not be switched off except in the event of a global crisis of unprecedented proportions.

The planned establishment of the Galileo positioning system by the EU (expected to be fully operational in 2013) is intended to provide a second source of time for GPS-compatible clocks that are also equipped to receive and decode the Galileo signals.

[edit] LORAN

Renewed interest in LORAN applications and development has recently appeared as an augmentation to GPS and other GNSS systems. Enhanced LORAN, also known as eLORAN or E-LORAN, comprises an advancement in receiver design and transmission characteristics which increase the accuracy and usefulness of traditional LORAN to that comparable with unenhanced GPS. eLoran also includes additional pulses which can transmit auxiliary data such as DGPS corrections and UTC information. eLoran receivers now use "all in view" reception, incorporating signals from all stations in range.

[edit] Astronomy timekeeping

Although any GPS 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 on most consumer GPS units may not be as precise. This is because an inexpensive GPS unit typically has one CPU that is multitasking. The highest-priority task for the CPU is maintaining satellite lock. Updating the display is assigned a lower priority. Therefore, the displayed time of most consumer handheld GPS units will be accurate to about one half of a second — more than sufficient for most civil timekeeping purposes, but not for scientific applications such as astronomy.

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.

[edit] See also

• Time signal
• Time transfer
• Network time protocol
• Speaking clock
• Atomic clock
• Greenwich Time Signal
• Time from NPL
• Clock network
• Casio Wave Ceptor wristwatches

[edit] References

[edit] External links

Look up radio clock in Wiktionary, the free dictionary.

• IOTA Observers Manual This manual from the International Occultation Timing Association has very extensive details on methods of accurate time measurement for astronomical research purposes
• NPL list of Standard Time and Frequency Transmissions
• List of long- and short-wave time-stations and their transmission codes
• NIST website
• NRC Canada clock
• Greenwich Mean Time and world time
• German PTB clock
• UTC and TAI time service from BIPM, Paris
• NIST Internet Time Service (ITS): Set Your Computer Clock Via the Internet
• Informative site from a hobbyist who has built his own clock
• NTP Public Services Project
• NTP Project Development Website
• International Loran Association

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