What is a "Radio Controlled" clock?

The simple answer is as follows: A radio controlled clock picks up on a signal sent through the airwaves, which carries information about the time. This signal tells the clock what time it is to the exact second and the clock adjusts its time accordingly. This means there's no need to change the time when the seasons change.

More Detailed Answer

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.

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.

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.

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.

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 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.