Time Synchronisation

When we take a glance at our watches or the office clock we often take for granted that the time we are given is correct. We may notice if our watches are ten minutes fast or slow but take little heed if they are a second or two out.

Yet for thousands of years mankind has strode to get ever increasingly accurate clocks the benefits of which are plentiful today in our age of satellite navigation, NTP servers, the Internet and global communications.

To understand how accurate time can be measured it is first important to understand the concept of time itself. Time as it has been measured on Earth for millennia is a different concept to time itself which as Einstein informed us was part of the fabric of the universe itself in what he described as a four dimensional space-time.

Yet we have historically measured time based not on the passing of time itself but the rotation of our planet in relation to the Sun and the Moon. A day is divided into 24 equal parts (hours) each of which is divided into 60 minutes and the minute is divided into 60 seconds.

However, it has now been realised that measuring time this way can not be considered accurate as the Earth’s rotation varies from day to day. All sorts of variable such as tidal forces, hurricanes, solar winds and even the amount of snow at the poles effects the speed of the Earth’s rotation. In fact when the dinosaurs first started roaming the Earth, the length of a day as we measure it now would have only been 22 hours.

We now base our timekeeping on the transition of atoms with a second based on 9,192,631,770 periods of the radiation emitted by the hyperfine transition of a unionized caesium atom in the ground state. Whilst this may sound complicated it really is just an atomic ‘tick’ that never alters and therefore can provide a highly accurate reference to base our time on.

Atomic clocks use this atomic resonance and can keep time that is so accurate a second isn’t lost in even a billion years. Modern technologies all take advantage of this precision enabling many of the communications and global trade we benefit from today with the utilisation of satellite navigation, NTP servers and air traffic control changing the way we live our lives.

Atomic clocks are the pinnacle of time keeping devices. Modern atomic clocks can keep time to such accuracy that in 100,000,000 years (100 million) they do not lose even a second in time. Because of this high level of accuracy, atomic clocks are the basis for the world’s timescale.

To allow global communication and time sensitive transactions such as the buying of stacks and shares a global timescale, based on the time told by atomic clocks, was developed in 1972. This timescale, Coordinated Universal Time (UTC) is governed and controlled by the International Bureau of weights and Measures (BIPM) who use a constellation of over 230 atomic clocks from 65 laboratories all over the world to ensure high levels of accuracy.

Atomic clocks are based on the fundamental properties of the atom, known as quantum mechanics.  Quantum mechanics suggest that an electron (negatively charged particle) that orbits an atom’s nucleus can exist in different levels or orbit planes depending if they absorb or release the correct amount of energy. Once an electron has absorbed or released enough energy in can ‘jump’ to another level, this is known as a quantum jump.

The frequency between these two energy states is what is used to keep time. Most atomic clocks are based on the caesium atom which has 9,192,631,770 periods of radiation corresponding to the transition between the two levels. Because of the accuracy of caesium clocks the BIPM now considers a second to be defined as 9,192,631,770 cycles of the caesium atom.

Atomic clocks are used in thousands of different applications where precise timing is essential. Satellite communication, air traffic control, internet trading and GPS all require atomic clocks to keep time. Atomic clocks can also be used as a method of synchronising computer networks.

A computer network using a NTP time server can use either a radio transmission or the signals broadcast by GPS satellites (Global Positioning System) as a timing source. The NTP program (or daemon) will then ensure all devices on that network will be synchronised to the time as told by the atomic clock.

By using a NTP server synchronised to an atomic clock, a computer network can run the identical coordinated universal time as other networks allowing time sensitive transactions to be conducted from across the globe.

Keeping track of time is something most people take for granted, yet the science of timekeeping has a long and fascinating history.

Keeping track of time was always based on the relationship between the Earth, Moon and Sun. The first timekeeping devices are thought to be monuments like Stone Henge in the UK that would recognise the winter or summer solstice allowing early man to calculate when to plant crops.

Dividing the day up into hours and being able to keep track of them has proved more difficult to civilisations.

The first timing devices were sundials, obelisks and water clocks but it wasn’t until the development of mechanical clocks in the middle-ages that time-telling started to become more accurate.

Mechanical clocks continued to develop until the turn of the twentieth century when they were bettered in accuracy by electronic oscillators that would use the resonance of a crystal (often quartz) to keep a stable time.

While electronic clocks provided accuracy to within a second a day, the atomic clock that uses the resonance of an atom (in most cases caesium -133) and was developed in the 1950’s demonstrated millisecond accuracy – not losing a second in several thousands of years.

Now atomic clocks are approaching nano-second accuracy (one second every billion years) with new developments like strontium. The atomic clock has also made horologists realise that basing a time system on the movement of the Earth and celestial bodies is unreliable as the Earth slows and speeds up.

UTC (coordinated universal time) was developed to combat this by adding leap seconds to keep atomic time in line with GMT (Greenwich Meantime). Now computer networks all over the world can synchronise to UTC and atomic clocks by using a time server.

A time server will receive a time from an atomic clock source and synchronise an entire network to this time. Without time servers, atomic clocks and UTC, technologies such as satellite communication, the Internet and global trading would be near impossible.

Many network administrators boast that there networks are perfectly synchronised because they have an atomic clock as an NTP server. In actual fact as atomic clocks cost several millions of pounds and are quite vast in size it is doubtful the average server room contains such a timepiece.

What in fact they are referring to is that they have an NTP server that receives a timing source from an atomic clock. However, just because atomic clocks are the most accurate chronometers in the world, accurate to a few nanoseconds (billionth of a second) it doesn’t necessarily mean that a network using one as a timing source is receiving the same sort of accuracy

Atomic clocks work on the principle that certain atoms (in most atomic clocks the caesium -133 atom) oscillates at an exact frequency at certain energy levels. In the case of the caesium atom it resonates at exactly 9,192,631,770 every second.  Because of this exact resonance, atomic clocks lose less than a second in millions of years. In fact, the resonance of the caesium atom is so precise that the International System of Units has defined the second as exactly that number of oscillations of the caesium atom.

NTP servers can receive the time from an atomic clock through several sources. Obviously the Internet contains thousands of timing servers, some of which are hooked up to an atomic clock, others however, can be over ten seconds out of sync.

Furthermore, using an Internet timing source can leave a system open to abuse as the timing references cannot be authenticated. Also, the distance from a host, client and server can make dramatic differences in the accuracy.

The most accurate and effective way of receiving a timing source from an atomic clock is to use the national time and frequency broadcast that several country’s national physics laboratories transmit. Alternatively the American GPS (Global Positioning System) transmits the time from its own satellite’s atomic clocks. both methods can provide perfect synchronisation and accuracy to within a few milliseconds.

UTC (Coordinated Universal Time) is the global civil timescale used by millions of people, businesses and authorities across the globe. UTC is based on the time told by caesium atomic clocks. These clocks are the most reliably accurate chronometers on Earth, able to maintain accurate time for several millions of years whilst neither losing nor gaining a second.

Unfortunately caesium clocks are far too expensive and delicate pieces of machinery to make it practical for us all to have one but fortunately the time that they tell is transmitted by several countries. These nation’s national physics laboratories tend to broadcast the UTC time from these clocks by long-wave.

In the UK the 60 kHz transmission is broadcast by the National Physical Laboratory from a transmitter in Anthorn in Cumbria (it was based in Rugby until 2007). NPL constantly maintain the transmissions and assess its accuracy. Whilst the MSF signal is a British based transmission is possible to receive the signal in some parts of northern Europe and Scandinavia.

However, in mainland Europe, the strongest time and frequency signal is the German transmission broadcast from Frankfurt in Germany. This signal known as the DCF is controlled and maintained by the German National Physics Laboratory. While Switzerland also has its own time and frequency signal, the German DCF signal is by far the most widely used in Europe.

In the USA a similar system is maintained by NIST (National Institute for Standards and Time) and is broadcast from Fort Collins, Colorado. This signal is known as WWVB and is available in most parts of Northern America (including Canada).

Japan maintains its own timing broadcast (JJY) also which is popular in the south pacific and several other countries (such as France) maintain their own signals too although these tend to have only minor coverage.

All these times signals operate in a similar fashion. The strength of the signal is either reduced by between 6 and 10 dB or switched off for a specific amount of time before being restored at the start of each second. The amount of time the signal is reduced indicates a stream of binary numbers with positioning markers.
The signals operate on a 60 kHz frequency and carry a time and date code which relays the following information in binary format: Year, month, day of month,  day of week,  hour,  minute,  DUT1 (the difference between UTC and UT1 which is based on the Earths rotation). The signals also relay information about local time such as British Summer Time.

The worst part of a power cut is running around the house setting all the clocks and timers back to the correct time, it can take ages and you will always forget one, however, as long as you have a wrist watch it should be quite easy to get your clocks all telling the same time. But what time is your wrist watch set too and who regulates that time?

Complete precision and accuracy in time telling is not essential for our day-to-day lives and neither is synchronisation, our computer may be a few minutes slower than our wall clock but it will make little difference when we send an email.

However, what if the person we sent the email to has a computer clock that is even slower? They may end up sending a reply before they have technically received it. Computers are easily fooled if timestamps run backwards – remember the millennium bug!

For this reason it is important for computers, particularly those that deal with time sensitive or financial applications, to be telling the same time; otherwise global stocks could be bought whilst already sold-out or an airline seat, already purchased could be bought again by a buyer with a slower computer clock.

The regulation of time didn’t start until after the development of atomic clocks when the oscillation of the caesium atom became the standard definition of a second (9,192,631,770 a second).

The time told by these atomic clocks was so accurate a new timescale was developed called International Atomic Time (TAI). However, it was discovered that the traditional method of telling time, based on the revolution of the Earth (ie 24 hours in a day) and this new timescale soon became out of sync with each other as the gravity from the moon alters the revolution of the Earth, slowing it down.

This difference in the Earths spin is only minute but enough people argued (mainly astronomers) that if it was not compensated for, night would eventually creep into day (albeit in many thousands of years) and it would be difficult to keep track of the celestial bodies.

A compromise was called for and the new timescale, Universal Coordinated Time (UTC) was developed that accounted for the slowing of the Earth’s spin by adding leap seconds every year or so.

UTC has meant that modern technologies and applications such as the Global Positioning System, satellite communication, live television broadcasts and global trading have become possible.

Computer networks can receive UTC time and keep all their devices synchronised to it by using a NTP server (Network Time Protocol). NTP servers can receive UTC time from an atomic clock source via the Internet, a national radio transmission or through the GPS network.