MSF time signal

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The MSF time signal is a broadcast from the Rugby VLF transmitter near Rugby, Warwickshire based on time standards maintained by the British National Physical Laboratory. The transmitted signal has an effective radiated power of 15 kW, on a frequency of 60 kHz, the same frequency used by WWVB.

MSF is a radio callsign, and not an abbreviation. 'M' is one of the prefixes allocated to the United Kingdom, and the letters 'SF' were randomly allocated. However, Post Office staff that operated the station in 1951 insist that the name derives from the Modulated Standard Frequency scientific reference transmissions started in that year.

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Protocol

Signal is transmitted at one bit per second, where a long pulse (200 ms) is 1 and a short pulse (100 ms) is 0. The entire message is 53 bits long, and starts on the 17th second of each minute. Each field is coded in binary, with the most significant bit earliest. DUT1 is sent in seconds one through sixteen inclusive past the minute, by using two short pulses per second for each tenth of a second of the value of DUT1, and encoding the sign of DUT1 by putting the double pulses in seconds one to eight for positive, and seconds nine to sixteen for negative.

Field	Length	Starting in the	Range
Year	8 bits	17th second	00-99
Month	5 bits	25th second	1 (January) - 12 (December)
Day	6 bits	30th second	1-31
Weekday	3 bits	36th second	0 (Sunday) - 6 (Saturday)
Hour	6 bits	39th second	0-23
Minute	7 bits	45th second	0-59
Magic number	8 bits	52nd second	= 126 (01111110 in binary)

Notes

Centuries are not represented in this transmission
The 1's in the magic number can contain additional information, by encoding 0 as 200 ms 0, and 1 as 300 ms:
- Bits 54-57 are checksums
- Bit 58 is 1 during British Summer Time and 0 during Greenwich Mean Time
The number of seconds in a minute can be between 59 and 62, so a leap second will typically not be noticed until at least 17 seconds after it happens
Consumer clocks typically update once an hour from this signal, and use a standard quartz crystal to keep time between updates or when the signal is unavailable.