Time from NPL videos and info

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]

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 16 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]

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.^[7]^[3] 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]

Transmission and reception

The transmitted signal has an effective radiated power of 15 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]

Protocol

The MSF time and date code format is summarised by the following principle: Simple on-off carrier modulation is used, the rise and fall times of the 60 kHz carrier are determined by the combination of antenna and transmitter.^[10] 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, 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' and end with at least 700 ms of carrier, and carry two binary-coded decimal (BCD) data bits. Bit A is transmitted 100 ms after the second marker and bit B 200 ms after the second marker, and both are 100 ms in duration. 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.

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 identifiers are used as follows: 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).^[10]

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 (UTC + 0 h) or BST (UTC + 1 h).^[10]