Radio clock
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A radio 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.
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[edit] 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.
[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
Time signals that can be used as references for radio clocks include:
- U.S. NIST Broadcasts:
- Longwave radio station WWVB at 60 kHz (binary coded decimal only) at 50 kW
- Shortwave radio station WWV (a male voice, Fort Collins, Colorado, about 100 km north of Denver at approximately ) at 2.5, 5, 10, 15 and 20 MHz at 2.5 kW to 10 kW. This voice signal is available by telephone at 1-303-499-7111.
- Shortwave radio station WWVH (a female voice, on Kauai near Kekaha, Hawaii, at about ) at 2.5, 5, 10, and 15 MHz at 2.5 kW to 10 kW
- German Broadcasts: A time signal from DCF77 (Mainflingen, a transmitter near Frankfurt at 50 kW at about ) can be received on 77.5 kHz to a range of about 2000 km
- Canadian Broadcasts: The official time can be obtained by tuning to radio station CHU (Ottawa, Ontario) at 3.33, 7.335 and 14.67 MHz, with FSK digital time data sent once per minute at 300 baud
- UK Broadcasts: A time signal from MSF, an atomic clock near Anthorn (which was relocated from Rugby on 2007-04-01) can be received on 60 kHz
- The JJY radio stations in Japan on 40/60 kHz
- The BPM radio station in Xi'an, China at 2.5, 5, 10 and 15 MHz
- Swiss Broadcasts: The legal Swiss time can be picked up from the HBG longwave transmitter in Prangins on 75 kHz. The time code is compatible with that of the German DCF-77 transmitter.
- French Broadcasts: Station TDF transmits timecodes on 162 kHz by phase modulation of the Allouis longwave broadcasting station.
There are a number of longwave radio transmitters around the world. In particular, DCF77 (Germany), HPG (Switzerland), JJY (Japan), NPL or MSF (United Kingdom), TDF (France), WWVB (United States). Many other countries can receive these signals (JJY can sometimes be received even in Western Australia and Tasmania 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.)
[edit] Clock receivers
One of the first radio clocks was offered by Heathkit around 1980. It received signals from the NIST clock in Colorado in the early morning hours, synchronized to it, and kept time between synchronizations with a quartz-crystal clock with an LED display. It sold for less than $100 and was considered impressive at the time. In the 2000s, such clocks had become common. Simple units can be purchased in the US at most electronics or discount stores for $20 to $50 and often feature wireless outdoor and indoor thermometers.
Modern radio clocks can be referenced to atomic clocks, and provide a means of accessing high-quality atomic-derived time over a wide area using inexpensive equipment. However, radio clocks are not appropriate for high-precision scientific work. Many retailers sell radio clocks under the name "atomic clocks", but the clocks themselves are not inherently atomic.
Typical radio "atomic clocks" require placement in a location with a relatively unobstructed atmospheric path to the transmitter, perform synchronization once a day during the nighttime, and need fair to good atmospheric conditions to successfully update the time. The device that keeps track of the time between updates, or in their absence, is usually a cheap and relatively inaccurate quartz-crystal clock, since it is thought that an expensive precise time keeper is not necessary with automatic atomic clock updates. The clock may 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.
[edit] Other broadcasts
- News radio: One method to access standard time is to listen to the news on radio. National radio news programs set their clocks to the transmissions from the standards departments of their respective countries. In the era when national broadcasting networks operated over point-to-point terrestrial microwave links, the time announcements were very precise. Today, however, satellite and digital networks often have latencies on the order of a second. In places where a car radio can receive more than one station broadcasting the same national news program, when switching between them one often either misses part of a word or hears part of the same word twice due to such variations. Also, once upon a time every radio station had a local full-time engineer who took considerable pride in keeping their clocks accurate; today many stations do not care as much about such details. Some stations, such as WTIC (noted below) and WCBS (AM, 880 kHz), still do provide highly accurate time beeps. But HD Radio broadcasts and analog simulcasts of HD Radio broadcasts have a delay of up to 15 seconds, and stations carrying network news broadcasts may run them, along with locally originated programs, through a delay system. Recently, WINS (AM, 1010 kHz) in New York City moved to HD broadcasting, so its time signal is now only approximate.
- 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.
- 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.
[edit] Other indirect sources
While not strictly radio sources, these sources of time signals are indirectly synchronized to primary radio sources:
- Network Time Protocol (NTP): The Network Time Protocol (NTP) is a protocol for synchronizing the clocks of computer systems over data networks such as the Internet, and has been in use since before 1985. It is designed particularly to resist the effects of variable latency, such as on the Internet. In practice, NTP is usually precise to within a few tens of milliseconds when used over the Internet. Many computer operating systems set their clocks automatically using NTP. For operating systems lacking this functionality, third-party NTP client software is usually available.
- Web sites: Some time references are available through Web sites. Time referenced to the U.S. NIST/USNO and French BIPM atomic clocks are available to the public on their Web sites (see below) with a time-of day display precise to within about 300 ms, depending on the round-trip travel time of IP packets between the client system and the server. Both NIST and BIPM use applets to provide this service: the applet running in your web browser exchanges packets with their server; both also display precision estimates based on network latency. On the dates when civil time changes, time-related sites on the Internet are often very slow to respond due to heavy usage; it is therefore wise to check one's clocks a day or two before the seasonal time change will occur.
- Telephone: The U.S. clocks are also available by phone at +1 (303) 499-7111 (WWV), +1 (808) 335-4363 (WWVH), or +1 (202) 762-1401, +1 (202) 762-1069, and +1 (719) 567-6742 (USNO). Canadian clocks are available by phone at +1 (613) 745-1576 (English) and +1 (613) 745-9426 (French). See also Speaking clock.
[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 over several periods of these phenomena. No GPS receiver directly computes time or frequency, rather they use GPS to discipline an oscillator; a quartz crystal in low-end navigation receivers, 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 so that after a day or so of operation it will know its position to within a few meters. Once it has averaged its position, it can then determine accurate time even if it can only pick up signals from one or two satellites.
[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 2010) 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, while updating the display gets a lower priority. Therefore, the displayed time of most consumer handheld GPS units will be accurate to around half a second — more than sufficient accuracy 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
- Clock network
[edit] 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
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