National Physical Laboratory time signal

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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 signal strength is greater than 10 mV/m at 100 km and greater than 100 μV/m at 1000 km from the transmitter. The signal is widely used in northern and western Europe. The carrier frequency is maintained at 60 kHz to within 2 parts in 10¹².

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.

There will be a series of tests in the run up to the changeover, information on these is available from the National Physical Laboratory.

[edit] Protocol

The MSF time and date code format is summarised in the diagrams above. Simple on-off carrier modulation is used, the rise and fall times of the carrier are determined by the combination of antenna and transmitter. The timing of these edges is governed by the seconds and minutes of Coordinated Universal Time (UTC), which is always within a second of Greenwich Mean Time (GMT). Every UTC second is marked by an 'off' preceded by at least 500 ms of carrier, and this second marker is transmitted with an accuracy better than ±1 ms.

The first second of the minute begins with a period of 500 ms with the carrier off, to serve as a minute marker. The other 59 (or, exceptionally, 60 or 58) seconds of the minute always begin with at least 100 ms 'off' and end with at least 700 ms of carrier. Seconds 01-16 carry information for the current minute about the difference (DUT1) between atomic and astronomical time, and the remaining seconds convey the time and date code. The time and date code information is always given in terms of UK clock time and date, which is UTC in winter and UTC+1h when Summer Time is in effect, and it relates to the minute following that in which it is transmitted.

Bits 17B-*51B inclusive, and bits 01A-*16A inclusive, are currently set to '0', but may be used in the future. Bits *52B and *59B are currently set at '0' but they may be used in the future.

Minute Identifier Bits *53A, *54A, *55A, *56A, *57A and *58A are all set permanently at '1', and are always preceded by bit *52A at '0', and followed by bit *59A at '0'. This sequence 01111110 never appears elsewhere in bit A, so it uniquely identifies the following second 00 minute marker.

- *In minutes lengthened or shortened by a positive or negative leap second all these numbers are correspondingly increased or decreased by one (i.e. during these 61- or 59- second minutes the position of the time and date code is shifted by one second relative to the start of that minute).