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. 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.

Single transmitter

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

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

A current list of times signal stations is published by the BIPM as an appendix to their annual report; the appendix includes coordinates of transmitter sites, operating schedules for stations, and the uncertainty of the carrier frequency of transmitters. ^[6] 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.)

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 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. ^{[7] [8]}

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. 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.

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.

Other broadcasts

When stratum is referred to it means NTP stratum, a traceable clock accuracy level.

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. 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.^[9]

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 BCD.

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 GPS-Glonass 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.

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.

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.^[10]

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.

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.

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

Very often radio clocks have bugs in their software relating to daylight saving time (DST). This often leads to clocks not updating themselves to the correct civil local time when DST transitions on to off or off to on. Clocks that interpret longwave stations tend to be the most affected due to very minimal low level software (often hand coded assembler, or compiled C) used to decode and display the time signal.

On some systems, notably WWVB, DST is much less an issue because part of the transmitted timecode contains a "DST is now in effect" flag. Therefore even if a change in law occurs which changes the transition days (the most common change), the transmitted timecode will be updated and the clock's software only needs to pay attention to this DST flag. The only change which would typically cause DST transition problems would be if the transition time were to change (e.g. 0200 hours local time).

See also

• Time signal
• Time transfer
• Network Time Protocol (NTP)
• Speaking clock
• Atomic clock
• Greenwich Time Signal
• Time from NPL
• Clock network
• Casio Wave Ceptor wristwatches

References

External links

• 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