Measuring Time
The daily rotation of the Earth, the Sun and the Moon provide us with our basic daily, monthly and yearly astronomical 'clocks'. Over time of course all sorts of mechanical clocks have been invented to subdivide the day [1,2]. Often clocks were only used to divide up the daylight hours partly because of the wide spread use of sundials and partly because that's when people most needed to know the time. In Renaissance Italy there were 24 hours in the day but, instead of starting the day at midnight as we do, they began the day half an hour after sunset: thus the 24th hour was the last hour of daytime [3].
Navigation and time measurement
The real step-challenge to time keeping however came as empires expanded and accurate time determination was essential for safe navigation. This story has been wonderfully told in a two-part CH4 TV series (a must see DVD) based on Dava Sobels great book 'Longitude' [4,5]. The series focuses on the heroic efforts of John Harrison developing his wonderful clocks; a life's work that cracked the longitude (time keeping) problem at sea.
A year at the NPL - My Life as a Time Lord
In 1988 I had a very happy year working at the National Physical Laboratory (NPL, Teddington UK) as part of a progressive 4 year Physics degree at Surrey University. The NPL is the nation's national measurement institute and is a world leader in accurate standards [6]. I worked in the Time and Frequency Section of the NPL transferring time around the world and so I am legitimately able to say I was a 'Time Lord' for a year!
I was fortunate to work with Brian Swaby and Dennis Suitcliffe. Dennis was in the last year of a very distinguished career at the NPL. In the 1960's he worked with Essen on the first atomic clocks - clocks having unprecedented stability accurate to better than 1 second in 100,000 years [7,8] (see p56-57 of ref. 1 for a lab photo of Suitcliffe and Essen).
My duties included making a daily comparison of the NPL's atomic clocks (Teddington, UK) with other clocks around the world. We did this using the newly commissioned GPS satellite system using the radio signal from the satellites as a 'flyby' reference to compare the clocks at other locations. I also built a custom 'stopwatch' calibrated at various temperatures which had a stability of better than 1/100 second / day. I also designed various different types of receiver to see which would perform best for picking up the NPL's MSF signal transmitted from Rugby on 60kHz.
Bushy House at the NPL
Our main labs at the NPL were very near Bushy House, an impressive old building which at the time was a museum full of amazing clocks and scientific apparatus. I remember being shown a 6' piece of Zeppelin girder which looked like it would weigh a few kg but I was amazed to find I could pick it up with my little finger! It must have been made of a very interesting early alloy. In the basement of Bushy House were temperature controlled rooms that were used to test out equipment for prolonged periods at set temperatures (e.g. 0°C, 20°C and 40°C). These rooms were constantly maintained at fixed temperatures and had massive oak doors - they were the perfect cross between high-tech modern research and the old style of 'Victorian' type engineering. I was told later that as a 'cost cutting' exercise, someone sanctioned the turning-off of the 24 hour heating and cooling equipment causing the solid oak doors to swell up and distort ....
Outside the temperature controlled rooms previous experimenters and researchers had laid out table-upon-table of components, pieces of equipment and test apparatus. They obviously felt that this was preferred to storing everything away, as it's all too easy with people moving from one job to another, to lose track of equipment. It was a complete delight for me; I could explore and experiment to my heart's content - which I did.
The stability of various clocks
A detailed study of clocks has to differentiate between short and long term stabilities [1,2]. A wrist watch on a persons wrist is to a certain extent temperature stabilised by their body temperature. It may however be designed to include a short drift associated with having a few hours (at room temperature) on the bedside table. The regular daily variations in temperature throughout the day may cause short term variations but these tend to average-out over the day. A watch can therefore keep quite good time over the year. Even with a relatively poor short term stability they can have quite good long term stability (because the varying factors are in themselves quite regular).
The early mechanical clocks were accurate to a few minutes a day. Harrison's clocks were better by at least an order of magnitude. Providing a stability of better than a few seconds a day, even on rolling ships and in bad weather. Quartz oscillators in the 1940 were good to a second a month and a temperature controlled ('ovened') crystal is good to perhaps 1 second a year [6].
To get better than that you need an 'atomic clock'. A microwave radio source is 'locked' onto a particular electronic transition of metal atoms such as in a gas of Rubidium (Rb) or Caesium (Cs). The technique provides incredible stability. Once the microwave frequency is divided down to say 1 pulse per second the time reference is very stable indeed. Cs atomic clocks are accurate to better than a second in 300,000 years [2,6]. The GPS satellite system also uses atomic clocks but at less accurate time to the user of ca. 1 second in 1000 years or so.
Recently a technique called the caesium fountain and also masers (the microwave equivalent of the laser) have provided even greater stabilities perhaps approaching 100 times better than the standard Cs clocks. Finally some scientists have proposed using the precise timing of the turning of pulsars - neutron stars - as galactic reference points. The rotation modulated radio waves emitted from these stars have estimated stabilities a million times better than the best atomic clocks.
Note sometimes stabilities are given in terms of parts per million say, so for example a crystal oscillator may have a stability of 1 part in 105, this is 1 part in 100,000 which means that in 100000 seconds it will vary by 1 second. 100000 seconds is 1667 minutes, which is 28 hours. So if you used that oscillator for a clock it would be accurate to about a second a day.
Who needs such stability in a clock?
Such stabilities are useful for all sorts of scientific experiments of course but actually they are also important more generally in society. The GPS devices such as Sat-Navs use precise timings to calculate distances and altitudes and velocities for navigation. However all sorts of financial transactions (say from one stock exchange to another) need to be done precisely so that moment to moment interest rates, exchange rates and stock values can be agreed upon for the many billion-of-pound transactions going on all the time.
The idea of a radio clock
Rather than each of us having a very accurate clock its obviously a great idea to send the accurate time signals over the radio. The idea of a stable radio time signal is not just about great stabilities though, it's also about having a clock that adjusts itself and so never has to be set or adjusted by the user. It's a clock that can be relied upon to always tell the correct time. The clock will automatically adjusting for summer (BST) and winter (UTC*) times but also account for the odd leap second that must be slipped in every now and again to precise align with a calculated 'average day' [2,9].
* UTC stands for 'Universal Time, Coordinated' which is equivalent to the 'old' GMT.
MSF - radio time signal
In 1950's the UK started a time service on 60 kHz transmission from Rugby, roughly at the center of the UK [9,10]. Apparently the letters MSF do not represent 'Standard Frequency' but were simply a random three letter allocation back then. MSF transmits a slow CW time code second by second over each minute by dropping the carrier every second. A 'fast code' containing all the time information was also transmitted at the start of each minuet but this service was removed in 1998 [2,10]. We will explain the way the time data is encoded on the MSF signal in part II.
Visiting Rugby in the late 1980's
Part of my duties at the NPL was to accompany Brian Swaby to Rugby, with a portable atomic clock, to measure the rate of the Rugby clock directly. I remember driving up with Brian and being mesmerised by the massive masts (every radio amateurs dream!) .... disappearing into the low clouds. Four 820 foot masts, part of a series of 12 masts originally put up in 1920's the largest masts ever erected at that time) held the antenna. Each tower was supported on an enormous granite insulator the size of a small house.
One story they told me was that there were lifts going up within the masts which were used for routine inspections of the antennas and structural integrity of the masts. Apparently a few of the masts had slight twists part of the way up so that, while arising up the mast, the lift would at some point produce a heart stooping screech, come momentarily to a stop, before shunting on up again to the top - not for the faint hearted!
Inside the Rugby buildings you felt like an ant inside an old valve radio. There were very large Litz coils perhaps 15' to 20' in diameter, huge variometers and banks of transmitting amplifiers. The MSF signal was created by a huge thermionic valve about a yard high if my memory has not exaggerated it over the years.
Giving a talk about MSF to the Worthing radio club recently I meet someone who used to live near the MSF transmitter in the 1950's. He used the local high power MSF transmission to his advantage. He was an enthusiastic radio builder and he told me that he used a long wire and rectifier / smother circuit to rectify the MSF carrier so that he could use the DC from it to power his transistor radios - he made a 'battery free' radio powered radio! [10].
MSF moves: Rugby to Anthorn
In 2007 the Rugby station was closed and a new MSF transmitter was established in Anthorn in northern England [2,9]. The tall masts at Rugby were demolished one by one. If you want too you can go online to YouTube and see them going down - quite sad really [11].
Other LF time signals
Other countries around the world have LF time coded signals [12], most are single bit codes while the UK is a two bit code (see part II).
These include:
MSF 60 kHz (2 bit code, 17kW ERP, Caesium standard, UK)
WWVB 60 kHz (1 bit code, 50kW ERP, Caesium standard, Colorado, USA)
JJY 60 kHz (1 bit code, 50kW ERP, Caesium standard, Japan)
DCF77 77.5 kHz (50kW ERP, Caesium standard, Germany)
(Note: the 2.5, 5 and 10MHz short wave time standard signals were discontinued in 1988)
Finally
Next issue we look at the MSF signal in more detail and explain how the time signal is encoded onto the 60kHz signal.
References and links
[1] There are many books about clocks and timekeeping, one rather nice collection is:
Time, Catalogue Edited by A J Turner, Nieuwke Kerk, Amsterdam, 1990. ISBN 90 12 06863 0
[2] For the Time and frequency section of the NPL and also for the MSF data sheet (PDF)
[3] Lucrezia Borgia: life, love and death in Renaissance Italy, Sarah Bradford, Penguin, 2004. ISBN 0 141 01413 X
[4] Dava Sobel, The Illustrated Longitude, Harper Collins, 1999 ISBN 0 00 105337 X
[5] Longitude, DVD, Granada Film Productions, CH4 TV series, 2002.
[6] The little big book of Metrology, NPL, 2008. ISBN 978 0 946754 52 6. Also see: www.npl.co.uk
[7] L Essen and atomic clocks, wiki URL
[8] atomic clocks wiki
[9] wiki MSF history
[10] Public lecture to the Worthing and district Amateur Radio club: My life as a time lord, J. P. Hare, May 5th 2010.
[11] YouTube clips of 820' Rugby masts coming down
YouTube clips of 820' Rugby masts coming down
[12] LF time coded signals around the world
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