A radio-controlled clock (RCC) is a clock controlled by a radio, as opposed to a clock-radio, where the clock controls the radio. A RCC has a radio receiver built into it, tuned to a transmitter that transmits time signals. These time signals are decoded and used to regulate the clock.

MSF Rugby

In the UK the time signal transmitter is MSF Rugby, transmitting on a frequency of 60kHz with an estimated equivalent monopole radiated power (EMRP) of 15kW.

EMRP is an indication of the power radiated by the transmitter. Monopole indicates that it's an omni-directional antenna, as opposed to a dipole which is bi-directional.

Most developed countries have their own time signal transmitters, and those that do not are usually within reach of one that does. MSF is the station's call-sign and, despite many attempts to show otherwise, has no meaning except for the 'M', which shows that it is a UK station.

The frequency of 60kHz is in the very low frequency band that produces a wavelength of around five kilometres. This VLF gives the transmitter a very long range, approximately 1000 kilometres. This means that MSF Rugby can be received throughout the British Isles and in a sizeable chunk of western Europe.

VLF frequencies are propagated as ground waves that eliminate the vagaries of the ionosphere, where short wave or high frequency signals go. The National Physical Laboratory runs MSF Rugby under the auspices of British Telecom, and is funded by the UK government's Department of Trade and Industry.

The transmitter operates 24 hours a day, 365 days a year barring shutdowns for maintenance. MSF transmits not only time information but also date information, automatically correcting for British Summer Time or daylight-saving time elsewhere.

Atomic Clocks

An atomic clock using caesium as its timing source generates the time signals. This clock is found at Teddington in Middlesex. The second is the basic unit of time from which all other time related values are derived. The second is a SI Unit defined as follows by the Physics Laboratory of the United States National Institute of Standards and Technology:

The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.

Atomic clocks maintain Universal Co-ordinated Time (UCT). UCT is, for most practical purposes, the same as Greenwich Mean Time (GMT). GMT uses astronomical time. Because of the vagaries in the Earth's rotation and inaccuracies inherent in GMT, UCT and GMT will drift apart. When this difference exceeds one second, a leap second is added or subtracted as required from GMT to bring them both in line again. This second is added or subtracted at midnight the end of June or December.

Most leap seconds in recent years have been added because the Earth's rotation is slowing. This last time this occurred was on 31 December, 1998. A second was inserted then, making 1998 one second longer than 1999.

The caesium clock at Teddington has an accuracy of one second in 300,000 years. Some countries maintain reference clocks, to which all other atomic clocks refer. One of the most accurate is at the German National Standards Laboratory in Brunswick, which has an accuracy of one second in 4.5 million years. Experiments have been taking place aimed at increasing the accuracy of atomic clocks to one second in 10 billion years. Ten billion years is the approximate age of the Universe.

The Clocks

Radio-controlled clocks can be either mains-powered or battery-powered. Radio-controlled wrist watches are also available, but they are often a bit expensive. Most RCCs have a digital display, but analogue displays are available. Mains-powered clocks have their receivers on permanently, whereas battery-powered ones only switch the receiver on once every one or two hours. This is to save battery life, as the power drain when the receiver is on is far greater than that from the clock alone. During the intervening period, the clock acts like a normal quartz clock.

When the power is first applied, the clock display will take on a random set of numbers. It takes between one and two minutes for the clock to synchronise with the transmitter, depending on where in the sequence the transmitter is and the strength of the signal. Once the time is synchronised, the display will show the time and date depending on the facilities of the particular clock. Most radio-controlled clocks also have a built-in alarm.

As the timing source is taken from an atomic clock, you now have the most accurate time piece outside a laboratory. Propagation delays from Teddington to the clock in your living room are estimated to be around ten microseconds.

Radio-controlled clocks can be bought for around £20. An Internet search will reveal many suppliers.