The Time from NPL is a radio signal broadcast from the Anthorn Radio Station near Anthorn, Cumbria, which serves as the United Kingdom's national time reference. The time signal is derived from three atomic clocks installed at the transmitter site, and is based on time standards maintained by the UK's National Physical Laboratory (NPL) in Teddington. The service is provided by Babcock International (which acquired former providers VT Communications), under contract to the NPL. It was funded by the former Department for Business, Innovation and Skills; NPL Management Limited (NPLML) was owned by the Department for Business, Energy and Industrial Strategy (BEIS), and NPL operated as a public corporation.
The signal, also known as the MSF signal (and formerly the Rugby clock), is broadcast at a highly accurate frequency of 60 kHz and can be received throughout the UK, and in much of northern and western Europe.
The signal's carrier frequency is maintained at 60 kHz to within 2 parts in 10<sup>12</sup>, controlled by caesium atomic clocks at the radio station. Eventually, time signals from GBR were terminated in November 1986 and it is no longer used as a frequency reference. From May 1953, the signal was broadcast 24 hours a day, but with regular five-minute stoppages to allow the reception of other signals. The 60 kHz signal finally became an uninterrupted 24-hour service in 1966, and the frequency references were discontinued in February 1988. The Anthorn station was operated by VT Communications from its opening in 2007 until 2010, when it was acquired by Babcock International.
The formal inauguration of the relocated facility took place on 1 April 2007, when the name of the service became "The Time from NPL" and the signal from Rugby was permanently switched off. The change in location and consequent change in signal strength can make some equipment designed to use the MSF signal fail to continue doing so. This is found more in domestic equipment not designed for optimum sensitivity and positioned haphazardly.
Currently, the signal from Anthorn is still usable as far as the Mediterranean, although it is weaker than Rugby.
The 'MSF signal' and the 'Rugby clock'
From the time signal's inauguration in 1950 until 1 April 2007 it was transmitted from Rugby Radio Station near Rugby, Warwickshire. Following its relocation in 2007 to Cumbria, the NPL now formally calls the signal "The Time from NPL". According to the NPL it seems the call sign was chosen so that it could be memorized as "Master Standard Frequency" but MSF was not intended to be an abbreviation.
Transmission and reception
The transmitted signal has an effective radiated power of 17 kW, on a frequency of 60 kHz to within 2 parts in 10<sup>12</sup>. The signal can also be received, and is widely used, in northern and western Europe.
Slow code
The signal is encoded as follows:
The MSF transmitter is switched off for brief intervals (on-off keying) near the beginning of each second to encode the current time and date. The rise and fall times of the 60 kHz carrier are determined by the combination of antenna and transmitter.
Each UTC second begins with 100 ms of 'off', preceded by at least 500 ms of carrier. The second marker is transmitted with an accuracy better than ±1 ms relative to Coordinated Universal Time (UTC), which is itself always within ±0.9 seconds of Universal Time (UT1) which is the mean solar time which would actually be observed at 0° longitude.
The first second of the minute, denoted second 00, 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', followed by two data bits of 100 ms each, and end with at least 700 ms of carrier.
- Bit A is transmitted from 100 to 200 ms after the second
- Bit B is transmitted from 200 to 300 ms after the second
Negative Polarity Bit Signalling
- <code>Carrier ON</code> represents a bit value of 0.
- <code>Carrier OFF</code> represents a bit value of 1.
If each second is considered as ten 100 ms pieces, the minute marker is transmitted as <code>1111100000</code>, while all other seconds are transmitted as <code>1AB0000000</code>.
Although two data bits are transmitted per second, the time code (as currently transmitted) has the property that only one of them is variable; non-zero B bits are only transmitted when the corresponding A bit has a fixed value. However, the official NPL documentation states that these “fixed value bits” 01A-16A, 17B-51B, and 52B and 59B are currently set at '0', but that they may be used in the future.
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 begins 43 seconds before the corresponding minute marker (second 17 of the previous minute, in the absence of leap seconds), and is always given in terms of UK civil time, which is UTC in winter and UTC+1h when Summer Time is in effect.
{|class="wikitable" style="text-align:center"
|+ MSF time code<br />Shaded bits are fixed
! Bit !! A !! B !! Meaning
|rowspan=21|
! Bit !! A !! B !! Meaning
|rowspan=21|
! Bit !! A !! B !!colspan=2| Meaning
|-
| 00 ||bgcolor=lightblue| 1 ||bgcolor=lightblue| 1 ||bgcolor=lightblue| Minute mark
| 20 || 10 ||bgcolor=lightgrey| 0 ||rowspan=5| Year<br />(00–99)
| 40 || 10 ||bgcolor=lightgrey| 0 ||rowspan=5 colspan=2| Hour<br />(00–23)
|-
| 01 ||bgcolor=lightgrey| 0 || +0.1 ||rowspan=8| DUT1<br />(0.1–0.8)<br />Unary encoding,<br />bit set if<br />DUT1 ≥ Weight
| 21 || 8 ||bgcolor=lightgrey| 0
| 41 || 8 ||bgcolor=lightgrey| 0
|-
| 02 ||bgcolor=lightgrey| 0 || +0.2
| 22 || 4 ||bgcolor=lightgrey| 0
| 42 || 4 ||bgcolor=lightgrey| 0
|-
| 03 ||bgcolor=lightgrey| 0 || +0.3
| 23 || 2 ||bgcolor=lightgrey| 0
| 43 || 2 ||bgcolor=lightgrey| 0
|-
| 04 ||bgcolor=lightgrey| 0 || +0.4
| 24 || 1 ||bgcolor=lightgrey| 0
| 44 || 1 ||bgcolor=lightgrey| 0
|-
| 05 ||bgcolor=lightgrey| 0 || +0.5
| 25 || 10 ||bgcolor=lightgrey| 0 ||rowspan=5| Month<br />(01–12)
| 45 || 40 ||bgcolor=lightgrey| 0 ||rowspan=7 colspan=2| Minute<br />(00–59)
|-
| 06 ||bgcolor=lightgrey| 0 || +0.6
| 26 || 8 ||bgcolor=lightgrey| 0
| 46 || 20 ||bgcolor=lightgrey| 0
|-
| 07 ||bgcolor=lightgrey| 0 || +0.7
| 27 || 4 ||bgcolor=lightgrey| 0
| 47 || 10 ||bgcolor=lightgrey| 0
|-
| 08 ||bgcolor=lightgrey| 0 || +0.8
| 28 || 2 ||bgcolor=lightgrey| 0
| 48 || 8 ||bgcolor=lightgrey| 0
|-
| 09 ||bgcolor=lightgrey| 0 || −0.1 ||rowspan=8| DUT1<br />(−0.1–−0.8)<br />Unary encoding,<br />bit set if<br />DUT1 ≤ Weight
| 29 || 1 ||bgcolor=lightgrey| 0
| 49 || 4 ||bgcolor=lightgrey| 0
|-
| 10 ||bgcolor=lightgrey| 0 || −0.2
| 30 || 20 ||bgcolor=lightgrey| 0 ||rowspan=6| Day of month<br />(01–31)
| 50 || 2 ||bgcolor=lightgrey| 0
|-
| 11 ||bgcolor=lightgrey| 0 || −0.3
| 31 || 10 ||bgcolor=lightgrey| 0
| 51 || 1 ||bgcolor=lightgrey| 0
|-
| 12 ||bgcolor=lightgrey| 0 || −0.4
| 32 || 8 ||bgcolor=lightgrey| 0
| 52 ||bgcolor=lightpink| 0 ||bgcolor=lightgrey| 0 ||bgcolor=lightpink colspan=2| Minute marker 01111110
|-
| 13 ||bgcolor=lightgrey| 0 || −0.5
| 33 || 4 ||bgcolor=lightgrey| 0
| 53 ||bgcolor=lightpink| 1 || STW ||colspan=2| Summer time warning.
|-
| 14 ||bgcolor=lightgrey| 0 || −0.6
| 34 || 2 ||bgcolor=lightgrey| 0
| 54 ||bgcolor=lightpink| 1 || P1 || Year (17A–24A) ||rowspan=4|Odd<br />parity<br />over
|-
| 15 ||bgcolor=lightgrey| 0 || −0.7
| 35 || 1 ||bgcolor=lightgrey| 0
| 55 ||bgcolor=lightpink| 1 || P2 || Day (25A–35A)
|-
| 16 ||bgcolor=lightgrey| 0 || −0.8
| 36 || 4 ||bgcolor=lightgrey| 0 ||rowspan=3| Day of week<br />Sunday=0<br />Saturday=6
| 56 ||bgcolor=lightpink| 1 || P3 || DOW (36A–38A)
|-
| 17 || 80 ||bgcolor=lightgrey| 0 ||rowspan=3| Year (00–99)
| 37 || 2 ||bgcolor=lightgrey| 0
| 57 ||bgcolor=lightpink| 1 || P4 || Time (39A–51A)
|-
| 18 || 40 ||bgcolor=lightgrey| 0
| 38 || 1 ||bgcolor=lightgrey| 0
| 58 ||bgcolor=lightpink| 1 || ST ||colspan=2| Summer time in effect.
|-
| 19 || 20 ||bgcolor=lightgrey| 0
| 39 || 20 ||bgcolor=lightgrey| 0 || Hour (00–23)
| 59 ||bgcolor=lightpink| 0 ||bgcolor=lightgrey| 0 ||bgcolor=lightgrey colspan=2| Unused, always 0.
|}
Consecutive bits from 01B–08B are set to 1 to indicate positive DUT1 values from +0.1s to +0.8s. For example, a DUT1 of +0.5 s is indicated by bits 01B–05B being set to 1 (e.g., 11111000 where bit 01B is the leftmost). Consecutive bits from 09B–16B are set to 1 to indicate negative DUT1 values from −0.1s to −0.8s. For example, bit 11B is set if DUT1 ≤ −0.3 s.
In case of a leap second, a zero bit is inserted between seconds 16 and 17.