National Physical Laboratory time signal

The National Physical Laboratory 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.

The transmitter's callsign is MSF. It is 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.

Early in 2007 the NPL will start new time signal transmissions from the Anthorn VLF transmitter, Cumbria, latitude 54° 55' N, and longitude 3° 15' W. This station currently has the callsign GBZ and is operated by VT Communications. The signals will have the same power and frequency as before: effective radiated power of 15 kW, at 60 kHz. The formal inauguration of the relocated facility will be 1 April 2007, when the name of the service will change to "The time from NPL" and the signal from Rugby will be switched off.

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.

See also

*Greenwich Time Signal

External links

* http://www.npl.co.uk/time/msf/
*The Official History of Rugby Radio Station

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