Time from NPL

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Time from NPL is located in Cumbria
Map showing the location of the Anthorn VLF transmitter within Cumbria.

Coordinates: 54°55′N 3°17′W / 54.91°N 3.28°W / 54.91; -3.28

The Time from NPL is a radio signal broadcast from the Anthorn VLF transmitter near Anthorn, Cumbria which serves as the United Kingdom's national time reference.[1] 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.[2] The service is provided by VT Communications under licence from the NPL and is funded by the Department for Innovation, Universities and Skills.[1]

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 1012, controlled by caesium atomic clocks at the radio station.[1]

Contents

[edit] History

A radio station at Rugby was first operated by the Post Office from 1926, with the call-sign GBR. From 19 December 1927, it broadcast a 15.8 kHz time signal from the Royal Observatory which could be received worldwide. It consisted of 306 pulses in the five minutes up to and including 10:00 and 18:00 GMT, with a longer pulse at the start of each minute. Frequency-shift keying was added in 1967, making the signal harder to use as a reference. Eventually, time signals from GBR were terminated in November 1986 and it is no longer used as a frequency reference.[3]

The MSF signals started in 1950, following the transmission pattern described below. They were originally intended to provide frequency references at 2.5, 5 and 10 MHz, originally only occasionally during the day. At first, there were announcements every fifteen minutes, beginning with the Morse code representation of "MSF MSF MSF", followed by speech "This is MSF, Rugby, England, transmitting ...".[3] 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.[3]

On 27 February 2007 the NPL started tests of the new time signal transmissions from Anthorn, latitude 54° 55' N, and longitude 3° 15' W.[4] This station has the callsign GBZ and is operated by VT Communications.

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.[5][6]

[edit] The 'MSF signal' and the 'Rugby clock'

From the time signal's inauguration in 1950 until April 1, 2007, it was transmitted from the Rugby VLF transmitter near Rugby, Warwickshire.[3][7] The transmitter's original location meant that the clock was referred to as "the Rugby clock".[8][9] Following its relocation, the NPL now formally calls the signal "The Time from NPL".[7]

'MSF' is not an abbreviation. The Rugby transmitter's callsign was MSF because 'M' is one of the ITU prefixes allocated to the United Kingdom. The letters 'SF' were randomly allocated. This resulted in the common terminology "the MSF signal", which is still used by the NPL.[3] It has been suggested that 'SF' was chosen to stand for "standard frequency", but the NPL has no evidence of this.[1]

[edit] 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 1012.[1] The signal strength is greater than 10 mV/m at 100 km and greater than 100 μV/m at 1000 km from the transmitter, and thus can be received at these strengths throughout the UK.[7] The signal can also be received and is widely used in northern and western Europe.[1]

Whilst at Rugby, the transmitter used 70 kW of mains power and generated 60 kW of radio frequency power. The 180m high T-aerial antenna was 500m across at its top. The vertical part of the antenna radiated the signal, meaning that the received strength was largely the same in all directions (it was approximately omnidirectional).[3]

[edit] Protocol

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.[10]

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 ±1 second of Greenwich Mean (solar) Time (GMT).

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, and bit B is transmitted from 200 to 300 ms. Carrier on represents a bit value of 0 and carrier off a value of 1. 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.

Consecutive bits from 01B–08B are set to 1 to indicate positive DUT1 values from +0.1s to +0.8s. (For example, bit 05B is set if DUT1 ≥ 0.5 s.) Consecutive bits from 09B–16B are set to 1 to indicate negative DUT1 values from −0.1s to −0.8s. (For example, bit 13B is set if DUT1 ≤ −0.5 s.)

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. In the event of a leap second, the minute consists of 59 or 61 seconds. 0 bits are inserted or deleted around second 17, and all time code bits marked with an asterisk (*) are moved by 1 second, so they retain their relationship to the beginning of the following minute rather than the previous one.[10]

Bits *17A–*51A encode the time of the following minute in binary-coded decimal, most significant bit first. Beginning with bit *17A comes 4 bits of tens of years, 4 bits of years, 1 bit of tens of months, 4 bits of months, 2 bits of tens of days, 4 bits of days, 3 bits of day of week (0=sunday), 2 bits of tens of hours, 4 bits of hours, 3 bits of tens of minutes, and 4 bits of minutes.

Bits *54B–*57B provide odd parity over the time code. The 4 parity bits cover years (8 bits), months and days (11 bits), day of week (3 bits), and time of day (13 bits) respectively.

In addition to the 500 ms carrier-off minute marker, 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.

[edit] Shortcomings of the current signal format

MSF does not broadcast any advance warning of an upcoming leap second. Therefore, unless a leap-second announcement is manually entered into a receiver in advance, it may take some time until an autonomous MSF receiver regains synchronization with UTC after a leap second (especially if there is no robust reception at the time of the leap second).

The time signal uses only a single bit (*58B) to distinguish between whether the broadcast time is GMT or BST.[10]

[edit] See also

[edit] References

  1. ^ a b c d e f "NPL Time Metrology". National Physical Laboratory, UK. http://www.npl.co.uk/time/msf/. Retrieved 6 April 2007. 
  2. ^ "MSF The Time from NPL". National Physical Laboratory, UK. http://www.clockco.co.uk/documents/msf.pdf. Retrieved 6 April 2007. 
  3. ^ a b c d e f "NPL T&F: FAQs". National Physical Laboratory, UK. http://www.npl.co.uk/server.php?show=nav.891. Retrieved 23 January 2008. 
  4. ^ Kelly Eve (28 February 2007). "Time Begins in Anthorn Village". News & Star (Cumbrian Newspapers Ltd). http://www.newsandstar.co.uk/news/viewarticle.aspx?id=471174. Retrieved 28 February 2007. 
  5. ^ Pallab Ghosh (10 May 2006). "Times changing for radio signal". BBC News. http://news.bbc.co.uk/1/hi/sci/tech/4756593.stm. Retrieved 28 March 2007. 
  6. ^ "Time change marks end of an era". BBC News. 25 March 2007. http://news.bbc.co.uk/1/hi/sci/tech/6483969.stm. Retrieved 28 March 2007. 
  7. ^ a b c "Time Has Moved". National Physical Laboratory, UK. http://www.npl.co.uk/time/msf/change.html. Retrieved 6 April 2007. 
  8. ^ "ARM+FPGA based Rugby clock". Computer Laboratory, University of Cambridge. http://www.cl.cam.ac.uk/teaching/2005/ECADArch/lab6.html. Retrieved 6 April 2007. 
  9. ^ "MSF 60 kHz: References for electronics projects". National Physical Laboratory, UK. http://www.npl.co.uk/time/msf/electronic_projects.html. Retrieved 6 April 2007. 
  10. ^ a b c "MSF 60 kHz Time and Date Code". NPL Time and Frequency Services. National Physical Laboratory. http://www.npl.co.uk/upload/pdf/MSF_Time_Date_Code.pdf. Retrieved 26 May 2008. 

[edit] External links

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