How To Sync Your PCs Time To The Radio Atomic Clock Time Signal

By: Dave Evans

The MSF radio time broadcast is a long-wave radio transmission of highly accurate time. The signal can be received throughout the whole of the UK and much of North Western Europe. With the additional of a low-cost radio receiver computers and computer networks can utilise the signal for precise timing. This article describes how the MSF radio time signal can be used by computers and NTP servers to provide a precise time reference.

The time signal was broadcast from Rugby in the East Midlands. It was maintained by originally maintained by British Telecom. The radio transmitter has recently been relocated to Cumbia, North East England. VT communications was awardedIt is now maintained.

The signal is a long-wave radio signal broadcast at 60kHz. The time signal can generally be received throughout the UK and also most of Western Europe. The radio signal can generally be received indoors. However, reception problems can be caused by metal structures, electrically noise equipment or if the antenna is located below ground level.

The time signal is synchronised with precise atomic clocks located at the British National Physics Laboratory.

Local time and date information is broadcast continuously, repeated each minute. The time information is transmitted as on-off carrier modulated, pulse-width encoded data signals. A series of 59 data bits make up time information and is transmitted as one pulse per second. The data transmitted consists of the current time and date, leap second indicator, daylight saving time indicator and parity bits.

The signal breaks for a five hundred millisecond interval to indicate the begining of the minute. The remaining 59 data bits are pulses of 100, 200 or 300 milliseconds of signal off followed by an on period to complete the second.

Time information is broadcast in Binary Coded Decimal (BCD) format and is encoded as described below.

Bits 1 to 16 are used to convey information about the difference between atomic and astronomical time (DUT1).

Bits 17 to 24 provide the BCD encoded year in the range 0 to 99.

Bits 25 to 29 provide BCD encoded month of year information in the range 1 to 12.

Bits 30 to 35 provide BCD encoded day of the month information in the range 1 to 31.

Bits 36 to 38 provide BCD encoded hour in the range 0 to 23.

Bits 45 to 51 provide BCD encoded minutes in the range 0 to 59. Bit 58 is used as an indicator to specify daylight saving time, BST or GMT.

Software decodes for the MSF radio time broadcast is provided within the standard NTP server distribution for Linux. The NTP server software distribution has also been ported to Microsoft Windows operating systems. NTP is a standard way of synchronising time on computers and computer networks.

A number of MSF radio receivers are detailed on various web sites. AlsoComputer Technology Articles, low-cost radio time code receivers that can be connected to a PC serial port are available from a number of commercial vendors. These allow the time signal to be received and decoded by a standard PC.

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