Radio clock

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A radio clock

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. The picture shows a type of radio-controlled digital clock. Radio-controlled analog clocks are also available.

A radio-controlled clock consists of an antenna for receiving the RF time code signal, a receiving circuit to convert the time code RF signal into the simple (digital) time code, and a controller circuit to decode the time code bit stream(s) and to drive an output circuit, which could be an LCD in case of digital clocks or stepping motors in the case of analog clocks.

1 Operation
- 1.1 Terrestrial time signals
2 Loran clocks
3 GPS clocks
4 Other access
5 See also
6 External links

[edit] Operation

Radio clocks depend on time signal radio stations, which usually

refer the broadcast frequency to the frequency standard
broadcast 'pips' to identify the starts of second intervals
broadcast time codes as a way of identifying second intervals
publish the exact geographic location of each antenna, so the radio signal’s time of propagation can be estimated.

A variety of frequencies helps reception no matter what the ionospheric weather.

[edit] Terrestrial time signals

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.

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 40°40′49″N, 105°02′27″W ) at 2.5, 5, 10, 15 and 20 MHz at 2.5 kW to 10 kW,
- Shortwave radio station WWVH (a female voice, on Kauai near Kekaha, at about 21°59′16″N, 159°45′50″W) at 2.5, 5, 10, and 15 MHz at 2.5 kW to 10 kW.
German Broadcasts: A time signal from DCF77 (Mainflingen, an atomic clock near Frankfurt at about 50°01′N 9°00′E) 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 Rugby (which will be relocated to Anthorn 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 162kHz from a site near Allouis.

[edit] Loran clocks

Loran-C time signals may also be used for radio clock synchronization, by augmenting their highly accurate frequency transmissions with external measurements of the offsets of LORAN navigation signals against time standards.

[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. Because they compute the time and position simultaneously from readings from several sources, GPS clocks can automatically compensate for line-of-sight delay and many radio propagation defects, and can achieve sub-microsecond precision under ideal conditions. 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. The highest-quality GPS clocks have their own precision clocks — either atomic clocks or temperature-controlled crystal clock — so they can maintain accurate time during any interruption of GPS signals.

Note that 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 [1] has detailed technical information about precision timekeeping for the amateur astronomer.

GPS, Galileo and GLONASS: These 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).

However, GPS clocks are 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.

The radio frequencies are set by the clocks and are precise standards, useful for adjusting receivers.

[edit] Other access

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 still do provide highly accurate time beeps, such as WTIC (noted below), or WCBS (AM, 880 kHz) in New York City.

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 subaudible timecode information, like the Radio France longwave transmitter on 162 kHz. Many digital radio and digital television schemes also include provisions for timecode 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 Mondial (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.

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, including Apple Computer’s Mac OS X, 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).

Time signal stations