Time Synchronisation

The global positioning system has been around since the 1980’s. It was designed and built by the United States Military who wanted an accurate positioning system for battlefield situations. However, following the accidental shooting down or a Korean airliner, the then US president (Ronald Reagan) agreed that the system should be allowed to be used by civilians as a way of preventing such a disaster from occurring again.

From then on the system has broadcast in to two frequencies L2 for the US Military and L1 for civilian use. The system works by using ultra precise atomic clocks that are on board each satellite. The GPS transmission is a timecode produced from this clock combined with information such as the position and velocity of the satellite. This information is then picked up by the satellite navigation receiver that calculates how long the message took to reach it and therefore how far from the satellite it is.

By using triangulation (use of three of these signals) the exact position on Earth of the GPS receiver can be ascertained. Because the speed of the transmissions, like all radio signals, travels at the speed of light it is highly important that the GPS clocks are ultra-precise. Just one second of inaccuracy is enough to make the navigational unit inaccurate to over 100,000 miles as light can travel such vast distances in such a short space of time.

Because GPS clocks have such a high level of accuracy it means they also have another use. The GPS signal, being available anywhere on the planet, is a highly efficient means of getting a time signal to synchronise a computer network too. A dedicated GPS time server will receive the GPS signal then convert the atomic time signal from it (known as GPS time) and convert it to UTC (Coordinated Universal Time) which is simple to do as both timescales are based on International Atomic Time (TAI) and the only difference being GPS time does not account for leap seconds meaning it is ‘exactly’ 15 seconds faster.

A GPS time server will most likely use the protocol NTP (Network Time Protocol) to distribute the time to a network. NTP is by far the most commonly used network time protocol and is installed in most dedicated time servers and a version is also included in most Windows and Linux operating systems.

In an age of atomic clocks and the NTP server time keeping is now more accurate then ever with ever increasing precision having allowed many of the technologies and systems we now take for granted.

Whilst timekeeping has always been a preoccupation of mankind, it has only been in the last few decades that true accuracy has been possible thanks to the advent of the atomic clock.

Before atomic time, electrical oscillators like those found in the average digital watch were the most accurate measure of time and whilst electronic clocks like these are far more precise than their predecessors – the mechanical clocks, they can still drift by up to a second a week.

But why does time need to be so precise, after all, how important can a second be? In the day-to-day running of our lives a second isn’t that important and electronic clocks (and even mechanical ones) provide adequate timekeeping for our needs.

In our day-to-day lives a second makes little difference but in many modern applications a second can be an age.

Modern satellite navigation is one example. These devices can pinpoint a location anywhere on earth to within a few metres. Yet they can only do this because of the ultra-precise nature of the atomic clocks that control the system as the time signal sent from the navigation satellites travels at the speed of light which is nearly 300,000 km a second.

As light can travel such a vast distance in a second any atomic clock governing a satellite navigation system that was just one second out it would the positioning would be inaccurate by thousands of miles, rendering the positioning system useless.

There are many other technologies that require similar accuracy and also many of the ways we trade and communicate. Stocks and shares fluctuate up and down every second and global trade requires that everybody all over the world has to communicate using the same time.

Most computer networks are controlled by using a NTP server (Network Time Protocol). These devices allow computer networks to all use the same atomic clock based timescale UTC (coordinated universal time). By utilising UTC via a NTP server, computer networks can be synchronised to within a few milliseconds of each other.

Time synchronisation is now a critical aspect of network management enabling time sensitive applications to be conducted from across the globe. Without correct synchronisation computer systems would be unable to communicate with each other and transactions such as seat reservation, Internet auctions and online banking would be impossible.

For effective time synchronisation the global timescale UTC (Coordinated Universal Time) is a prerequisite. While a computer network can be synchronised to any single time source, UTC is employed by computer networks all over the world. By synchronising to a UTC time source a computer network can therefore be synchronised to every other computer network across the globe that also use UTC as their time source.

Receiving a reliable UTC time source is not as easy as it sounds. Many network administrators opt to use a UTC Internet time source. Whilst many of these time sources are accurate enough, they can be too far away to provide reliability and there are plenty of Internet time sources that are vastly inaccurate.

Another reason why Internet time sources should not be used as a source of time synchronisation is because an Internet time source is outside of a firewall and leaving a gap in the firewall to receive timing information can leave a system open to abuse.

So that UTC time can be opted as a civil time throughout the world several national physics laboratories broadcast a UTC timing signal that can be received and utilised as a network time source. Unfortunately, however, these time signals are not available in every country and even in those areas where a signal exists; they can be quite often obstructed by interference and local topography.

Another method for receiving a source of UTC time is to use the GPS satellite network. Strictly speaking the Global Positioning System (GPS ) does not relay UTC but it is a time based on International Atomic Time (TAI) with a predefined offset. A GPS NTP clock can simply convert the GPS time into UTC for synchronisation purposes.

The main advantage of using GPS is that a GPS signal is available anywhere on the planet providing that there is a clear view of the sky above (GPS transmissions are broadcast via line-of-sight) so UTC synchronisation can be conducted anywhere.

Often administrators will boast of their network’s timing accuracy by lamenting that it is all controlled by an atomic clock. Whilst indirectly this may be true what they are referring to is a network time server that receives time from an atomic clock.

Atomic clocks are ridiculously expensive but are by far the most accurate chronometers possible. In fact they are so accurate the international timescale UTC (coordinated universal time) has to have seconds known as ‘leap seconds’ added because the Earth’s rotation is not as precise as our clocks.

Because an atomic clock is so precise they make the ideal sources for UTC time on computer networks; making possible such time sensitive transactions as global trading in stocks and shares, airline reservation and even Internet auctions.

Atomic clocks are known as stratum 0 servers. This is because they sit at the top of the NTP (network time protocol) hierarchical tree. Servers that receive a timing signal; from these stratum 0 atomic clocks are known as stratum 1 devices. It is these stratum 1 devices that are network time servers, used to synchronise a network to.

Machines that use a network time server also have a place on the NTP stratum tree as a stratum 2 server. Computers and devices do not just have to get timing information from a stratum 1 time server but also stratum 2 servers. They can also synchronise with each other.

Network time servers receive the atomic clock timing signal from either a specialist radio transmission in (certain countries only) or from the GPS network. Atomic time signals are available over the internet but these stratum 1 servers are notoriously inaccurate.

How do you set the time on your watch? From the speaking clock, the chimes of Big Ben perhaps but have you wondered where they get the time from?

There is no master clock that the world relies on but there is a global timescale called UTC (Coordinated Universal Time). UTC was developed after the development of atomic clocks. It is based on a combination of International Atomic Time (TAI – the time told by these clocks) and Greenwich Meantime (GMT).

While TAI is incredibly accurate it is actually too accurate as the Earths rotation slows due to the effect of the moon’s gravity. It compensates for this by adding Leap Seconds to keep in similar to GMT, otherwise night would creep into day.

UTC is governed by a constellation of atomic clocks all over the world to ensure even more accuracy (and to prevent any political squabbles).

A NTP Time Server is a device that can receive UTC time through either a radio transmission (in certain countries) or the GPS network. NTP (Network Time Protocol) is designed to synchronise all devices on a network to the time received by the NTP time server.

A NTP time server allows networks all over the world to communciate safely synchronised to the exact same time.

UTC and NTP time servers allow the whole world to communicate within the same timescale. Without them, many of the applications and processes we do online would be impossible such as Internet trading, the stock exchange, buying airline tickets and even sending and receiving email.

Atomic clocks use an atomic resonance frequency standard as their timekeeping element and are by far the most accurate chronometers possible with the latest Strontium based atomic clocks boasting a precision of a less than a second lost in several hundred million years.

The clocks maintain a continuous and stable time scale called International Atomic Time (TAI). However, for civil time, another time scale, Coordinated Universal Time (UTC)which  is derived from TAI, but synchronized using leap seconds to UTC, to keep it based on the rotation of the Earth.

UTC is a global timescale that is commonly used to synchronise the clocks on computer networks allowing machines from across the globe to communicate together and conduct time sensitive applications.

Unfortunately atomic clocks are highly expensive pieces of equipment and are generally only to be found in high technology physics laboratories or onboard satellites. However, several national physics laboratories broadcast the time told by their atomic clocks via a long wave radio transmission.

These signals are commonly picked up and utilized by radio controlled wall and desk clocks and by NTP time servers (Network Time Protocol).

The transmissions from the national standards agencies maintain an accuracy of 10-9 seconds per day (approximately 1 part in 1014). MSF is the signal broadcast by National Physical Laboratory in, Anthorn, Cumbria. Other countries boast their own signals the most common being the DCF77 transmission broadcast from Mainflingen near Frankfurt, Germany and the USA’s WWVB signal broadcast from Fort Collins, Colorado.

All these times signals work in a similar way. At the start of each second the strength of the signal is either reduced by between 6 and 10 dB

This article explains the origins and workings of atomic clocks and how they are used to synchronise computer networks all over the world using NTP servers.

In conventional electronic clocks time is kept by running an electrical current through an oscillator which produces a repetitive electrical signal this is then governed by a quartz crystal to keep precision. These crystal oscillators are far more accurate than mechanical clocks but will still drift, perhaps over a second a week.

For day-to-day use crystal oscillators are a fine way to keep track of time; in the everyday running of our lives, a second makes very little difference, however, as light or radio waves can travel 300,000 miles in a second, some high technologies such as satellite navigation or global communication, require far more accuracy to be possible.

Atomic clocks are a timekeeping device that uses the known atomic resonance frequency of an atom to keep time. The first truly accurate atomic clock was built in 1955 at the National Physical Laboratory in the UK and was based on the caesium atom -133 which oscillates at exactly 9,192,631,770 every second.

This oscillation is actually a repetitive signal from the microwave radiation emitted by electrons in an atom when they change energy levels. Much of an atomic clock is designed to create the correct state to cause and augment oscillations.
Although other atoms can be used, the oscillation (9,192,631,770 a second) of the caesium -133 atom is now accepted by the International System of Units as being the definition of one second.

Atomic clocks are generally very large and constitute many highly technical apparatus such as vacuums and require whole teams of scientists to maintain and monitor the clocks. Much of which goes into compensating for  unwanted side-effects such as frequencies of other atoms in the clock and even gravitational dilation (where according to Einstein’s theory clocks at different heights run differently because of the differences in the gravitational field)  This makes atomic clocks highly expensive.

Fortunately many large scale national physical laboratories transmit radio time signals from their atomic clocks which can be used to synchronise standard crystal oscillators too.

Atomic clocks are also the basis of GPS (Global Positioning System) as each satellite contains an atomic clock as accurate time is integral for positioning (a position anywhere is made up of a direction, a velocity and time).
GPS signals can also be used to capture a time signal. This is now the most common way computer networks retain accurate time which is also essential in many communications and applications.

Most computer networks use a NTP server (Network Time Protocol) to synchonise their devices to an atomic time signal received via the GPS network.

A universal timescale, UTC (Coordinated universal Time), has been developed based on the time told by atomic clocks, TAI (International Atomic Time). UTC accounts for the slowing of the Earths rotation by adding leap seconds to TAI so as to prevent the gradual drift of night into day (although that would take 40,000 years or so) and allows the whole world to communicate using the same timescale.

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.

We have all heard of a leap year – that extra day added to the calendar every four years. It may give us a longer February but it is also essential in keeping our calendars and seasons accurate. If the extra day is not added to a leap year then eventually (admittedly after over a century) the Winter will begin in July and the summer will start around Christmas (and vice – versa in the southern hemisphere) because the Earth takes an extra six hours longer than the 365 days of a year to circle the sun.

A leap year may be a bit of a fudge but the alternative would be to have a quarter day at the end of the year which would of course throw our days and nights out of sync with each other (and could you imagine just having a six hour day – some of us struggle to get things done in 24!).

We have of course always measured time in relation to the movement of the Earth – a day being an entire revolution, a year an orbit of the sun. However, as our way of measuring time became more and more accurate it soon became apparent that there were more irregularities in the Earth’s rotation than just the extra six hours in a year.

GMT (Greenwich Mean Time) was developed because there was a need for a time scale where the mean position of the sun at noon, averaged throughout the year, is above the Greenwich Meridian (zero longitude) and daylight saving hours are added or taken away depending on the time of year.

However, in 1955 the first atomic clock went into operation following the discovery of the stability of the caesium-133 atom which vibrated at an exact rate (9,192,631,770 a second). Impressed with this accuracy, The International System of Units of Measurement (SI) decided that a second should be defined as this number of oscillations of the caesium-133 atom.

Following the SI second a time scale called International Atomic Time (TAI – from the French Temp Atomique International) which was a simple count, in seconds, for the 24 hours of our day. Conversely as TAI is not related to the movement of the Earth, it was soon discovered that TAI and atomic clocks were far more stable and reliable than the Earth itself (in fact an atomic clock is 1,000,000 times more accurate than the Earths rotation).

Generally the Earth is continually slowing in its rotation (although, inexplicably, every now-and-then it seems to speed up) so TAI is of little use for those that wish their clocks to be in step with the Earth (astronomers being by far the most vocal of these).

So another time scale was developed called Coordinated Universal Time (UTC – again from the French – Temp Universel Coordonne). This was based on atomic time (TAI) but small adjustment are made to keep it in step with GMT (which incidentally is now commonly referred to as UT1 or depending on time zone UT+1 UT+2 UT+3 etc)

UTC is adjusted by the insertion of extra seconds, called leap seconds, as necessary to keep it within a second of GMT (or UT1). It is possible a second may have to be removed in the future but that hasn’t happened as yet. UTC is essential in modern industry and technology where computers are synchronised to UTC time, usually through a NTP server (Network Time Protocol) – to allow international time sensitive transactions.

A leap second is normally inserted at the end of December in the last hour (although occasionally it has been done in June, March and September). The decision as to whether a leap second is required is taken by the Earth Orientation Centre of the International Earth Rotation and Reference Systems Service (IERS), who monitor the Earth’s rotation and suggest the adjustment about six months in advance.

When a leap second is added there becomes 61 seconds in that final minute of the year. The familiar ‘six pips’ radio signal gains an extra pip and even London’s famous Big Ben is held back a second before it bongs (but not an extra bong as they are meant to represent the hours)

There have been 33 leap seconds added to UTC since 1972 (although the first ten were added retrospectively) but as the Earth’s rotation is continuing to slow it is estimated that over the next millennia or two leap seconds will have to be added each month.