The MSF-60 radio atomic clock time signal provides an accurate time reference broadcast throughout the British Isles. The time signal can be used as an accurate time reference to synchronise time on computers and computer networks. This article describes how the MSF-60 time signal can be decoded and used to synchronise computers and NTP time server systems.

The MSF radio time signal was, until recently, broadcast from Rugby, Warwickshire, where it was maintained by BT Radio Engineering. The transmitter has since been relocated to Anthorn, Cumbria, where it is now maintained by VT Communications.

The signal is a long-wave radio signal broadcast at 60kHz. The broadcast can be satisfactorily received throughout the British Isles and much of North-West 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 transmission is synchronised with highly accurate atomic clocks based at the National Physics Laboratory (NPL). 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 carrier is turned off for a 500-millisecond period to indicate the start of each minute. The other 59 seconds signify data bits and consist of between 100 and 300 milliseconds carrier off and at least 700 milliseconds of carrier on.

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 indicates British SummerWinter time (GMTBST).

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. Also a number of commercial vendors offer small low-cost radio receivers that can be interfaced to a PC to receive the MSF time signal.