Category: Time Synchronisation

Types of Atomic Clock Receivers

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MSF atomic clock receiver

The controlling radio signal for the National Physical Laboratory‘s atomic clock is transmitted on the MSF 60kHz signal via the transmitter at , CumbriaAnthorn, operated by British Telecom. This radio atomic clock time signal should have a range of some 1,500 km or 937.5 miles. All of the British Isles are of course within this radius.
The National Physical Laboratory’s role as keeper of the national time standards is to ensure that the UK time-scale agrees with Co-ordinated Universal Time (UTC) to the highest levels of accuracy and to make that time available across the UK. As an example, the MSF (MSF being the three-letter call sign to identify the source of the signal) radio broadcast provides the time signal for, electronic share trading, the clocks at most railway stations and for BT’s speaking clock.

DCF atomic clock receiver

The controlling radio signal for the German clock is transmitted via long wave from the DCF 77kHz transmitter at Mainflinger, near Dieburg, some 25 km south east of Frankfurt – the transmitter of German National Time Standards. It is similar in operation to the Cumbria transmitter, however there are two antennas (radio masts) so the radio atomic clock time signal can be maintained at all times.

Long wave is the preferred radio frequency for transmitting radio atomic clock time code binary signals as it performs most consistently in the stable lower part of the ionosphere. This is because the long wave signal carrying the time code to your timepiece travels in two ways; directly and indirectly. Between 700 km (437.5 miles) to 900 km (562.5 miles) of each transmitter the carrier wave can travel directly to the timepiece. The radio signal also reaches the timepiece via being bounced off the underside of the ionosphere. During the hours of daylight a part of the ionosphere called the “D layer” at an altitude of some 70 km (43.75 miles) is responsible for reflecting the long wave radio signal. During the hours of darkness when the sun’s radiation is not acting from outside the atmosphere, this layer rises to an altitude of some 90 km (56.25 miles) becoming the “E layer” in the process. Simple trigonometry will show that signals thus reflected will travel further.

A large part of the European Union area is covered by this transmitter facilitating reception for those who travel widely in Europe. The German clock is set on Central European Time – one hour ahead of U.K. time, following an inter-governmental decision, from the 22nd October, 1995, U.K. time will always be 1 hour less than European Time with both the U.K. and mainland Europe advancing and retarding clocks at the same “time”.

WVVB atomic clock receiver

A radio atomic clock system is available in North America set up and operated by NIST – the National Institute of Standards and Technology, located in Fort Collins, Colorado.

WWVB  has high transmitter power (50,000 watts), a very efficient antenna and an extremely low frequency (60,000 Hz). For comparison, a typical AM radio station broadcasts at a frequency of 1,000,000 Hz. The combination of high power and low frequency gives the radio waves from MSF a lot of bounce, and this single station can therefore cover the entire continental United States plus much of Canada and Central America.

The radio atomic clock time codes are sent from WWVB using one of the simplest systems possible, and at a very low data rate of one bit per second. The 60,000 Hz signal is always transmitted, but every second it is significantly reduced in power for a period of 0.2, 0.5 or 0.8 seconds:

• 0.2 seconds of reduced power means a binary zero • 0.5 seconds of reduced power is a binary one. • 0.8 seconds of reduced power is a separator.

The time code is sent in BCD (Binary Coded Decimal) and indicates minutes, hours, day of the year and year, along with information about daylight savings time and leap years. The time is transmitted using 53 bits and 7 separators, and therefore takes 60 seconds to transmit.

A clock or watch can contain an extremely small and relatively simple radio atomic clock antenna and receiver to decode the information in the signal and set the atomic clock time accurately. All that you have to do is set the time zone, and the atomic clock will display the correct time.

Features of Network Time Protocol

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NTP is reliant on a reference clock and all clocks on the NTP network are synchronised to that time. It is therefore imperative that the reference clock is as accurate as possible. The most accurate timepieces are atomic clocks. These large physics lab devices can maintain accurate time over millions of years without losing a second.

An NTP server will receive the time from an atomic clock either from across the internet, the GPS network or radio transmissions. In using a atomic clock as a reference an NTP network will be accurate to within a few milliseconds of the world’s global timescale UTC (Coordinated Universal Time).

NTP is a hierarchical system. The closer a device is to the reference clock the higher on the NTP strata it is. An atomic clock reference clock is a stratum 0 device and a NTP server that receives the time from it is a stratum 1 device, clients of the NTP server are stratum 2 devices and so on.

Because of this hierarchical system, devices lower down the strata can also be used as a reference which allows huge networks to operate while connected to just one NTP time server.

NTP is a protocol that is fault tolerant. NTP watches out for errors and can process multiple time sources and the protocol will automatically select the best.   Even when a reference clock is temporarily unavailable, NTP can use past measurements to estimate the current time..

Finding the Time

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Finding out what the time is, is something we all take for granted. Clocks are everywhere and a glance at a wristwatch, clock tower, computer screen or even a microwave will tell us what the time is. However, telling the time has not always been that easy.

Clocks didn’t arrive until the middle ages and their accuracy was incredibly poor. True time telling accuracy didn’t arrive until after the arrival of the electronic clock in the nineteenth century. However, many of the modern technologies and applications that we take for granted in the modern world such as satellite navigation, air traffic control and internet trading require a precision and accuracy that far exceeds an electronic clock.

Atomic clocks are by far the most accurate time telling devices. They are so accurate that the world’s global timescale that is based on them (Coordinated Universal Time) has to be occasionally adjusted to account for the slowing of the Earth’s rotation. These adjustments take the form of additional seconds known as leap seconds.

Atomic clock accuracy is so precise that not even a second of time is lost in over a million years whilst an electronic clock by comparison will lose a second in a week.

But is this accuracy really necessary? When you look at technologies such as global positioning then the answer is yes. Satellite navigation systems like GPS work by triangulating time signals generated by atomic clocks onboard the satellites. As these signals are transmitted at the speed of light they travel nearly 100,000 k m each second. Any inaccuracy in the clock by even a thousandth of a second could see the positioning information out by miles.

Computer networks that have to communicate with each other across the globe have to ensure they are running not just accurate time but also are synchronised with each other. Any transactions conducted on networks without synchronisation can result in all sorts of errors.

Fort his reason computer networks use NTP (Network Time Protocol) and network time servers often referred to as an NTP server. These devices receive a timing signal from an atomic clock and distribute it amongst a network in doing so a network is ensured to be as accurate and precise as possible.

Receiving the Time and Finding the Correct Time Source

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So you have decided to synchronize your network to UTC (Coordinated Universal Time), you have a time server that utilizes NTP (Network Time Protocol) now the only thing to decide on is where to receive the time from.

NTP servers do not generate time they simply receive a secure signal from an atomic clock but it is this constant checking of the time that keeps the NTP server accurate and in turn the network that it is synchronizing.

Receiving an atomic clock time signal is where the NTP server comes into its own. There are many sources of UTC time across the Internet but these are not recommended for any corporate use or for whenever security is an issue as internet sources of UTC are external to the firewall and can compromise security – we will discuss this in more detail in future posts.

Commonly, there are two types of time server. There are those that receive an atomic clock source of UTC time from long wave radio broadcasts or those that use the GPS network (Global Positioning System) as a source.

The long wave radio transmissions are broadcast by several national physics laboratories. The most common signals are the USA’s WWVB (broadcast by NIST – National Institute for Standards and Time), the UK’s MSF (broadcast by the UK National Physical Laboratory) and the German DCF signal (Broadcast by the German National Physics Laboratory).

Not every country produces these time signals and the signals are vulnerable to interference from topography. However, in the USA the WWVB signal is receivable in most areas of North America (including Canada) although the signal strength will vary depending on local geography such as mountains etc.

The GPS signal on the other hand is available literally everywhere on the planet as along as the GPS antenna attached to the GPS NTP server can have a clear view of the sky.

Both systems are a truly reliable and accurate method of UTC time and using either will allow synchronization of a computer network to within a few milliseconds of UTC.

Difficulties in telling the time!

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Precision in telling the time has never been as important as it is now. Ultra precise atomic clocks are the foundation for many of the technologies and innovations of the twentieth century. The internet, satellite navigation, air traffic control and global banking all just a few of the applications that is reliant on particularly accurate timekeeping.

The problem we have faced in the modern age is that our understanding exactly of what time is has changed tremendously over the last century. Previously it was thought that time was constant, unchanging and that we travelled forward in time at the same rate.

Measuring the passing of time was straight forward too. Each day, governed by the revolution of the Earth was divided into 24 equal amounts – the hour.  However, after the discoveries of Einstein during the last century, it was soon discovered time was not at all constant and could vary for different observers as speed and even gravity can slow it down.

As our timekeeping became more precise another problem became apparent and that was the age old method of keeping track of the time, by using the Earth’s rotation, was not an accurate method.

Because of the Moon’s gravitational influence on our oceans, the Earth’s spin is sporadic, sometimes falling short of the 24 hour day and sometimes running longer.

Atomic clocks were developed to try to keep time as precise as possible. They work by using the unchanging oscillations of an atom’s electron as they change orbit. This ‘ticking’ of an atom occurs over nine billion times a second in caesium atoms which makes them an ideal basis for a clock.

This ultra precise atomic clock time (known officially as International Atomic Time – TAI) is the basis for the world’s official timescale, although because of the need to keep the timescale in parallel with the rotation of the Earth (important when dealing with extra terrestrial bodies such as astronomical objects or even satellites) addition seconds, known as leap second, are added to TAI, this altered timescale is known as UTC – Coordinated Universal Time.

UTC is the timescale used by businesses, industry and governments all around the world. As it is governed by atomic clocks it means the entire world can communicate using the same timescale, governed by the ultra-precise atomic clocks. Computer networks all over the world receive this time using NTP servers (Network Time Protocol) ensuring that everybody has the same time to within a few milliseconds.

How to Install and Configure a NTP Server

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Network Time Protocol (NTP) is one of the Internet’s oldest protocols still utilised. Invented by Dr David Mills from the University of Delaware it has been in use since 1985. NTP is a protocol designed to synchronize the clocks on computers and networks across the Internet or Local Area Networks (LANs).

NTP (version 4) can maintain time over the public Internet to within 10 milliseconds (1/100th of a second) and can perform even better over LANs with accuracies of 200 microseconds (1/5000th of a second) under ideal conditions.

NTP works within the TCP/IP suite and relies on UDP, a less complex form of NTP exists called Simple Network Time Protocol (SNTP) that does not require the storing of information about previous communications, needed by NTP. It is used in some devices and applications where high accuracy timing is not as important.

Time synchronisation with NTP is relatively simple, it synchronises time with reference to a reliable clock source. This source could be relative (a computer’s internal clock or the time on a wrist-watch) or absolute (A UTC – Universal Coordinated Time – clock source that is accurate as is humanely possible).

Atomic clocks are the most absolute time-keeping devices. They work on the principle that the atom, caesium-133, has an exact number of cycles of radiation every second (9,192,631,770). This has proved so accurate the International System of Units (SI) has now defined the second as the duration of 9,192,631,770 cycles of radiation of the caesium-133 atom.

However, atomic clocks are extremely expensive and are generally only to be found in large-scale physics laboratories. However, NTP can synchronise networks to an atomic clock by using either the Global Positioning System (GPS) or a specialist radio transmission.

The most widely used is the GPS system which consists of a number of satellites providing accurate positioning and location information. Each GPS satellite can only do this by utilising an atomic clock which in turn can be can be used as a timing reference.

A typical GPS receiver can provide timing information to within a few nanoseconds of UTC as long as there is an antenna situated with a good view of the sky.

There are also a number of national time and frequency radio transmissions that can be used to synchronise a NTP server. In Britain the signal (called MSF) is broadcast by the National Physics Laboratory in Cumbria which serves as the United Kingdom’s national time reference, there are also similar systems in Colorado, US (WWVB) and in Frankfurt, Germany (DCF-77). These signals provides UTC time to an accuracy of 100 microseconds, however, the radio signal has a finite range and is vulnerable to interference.

The distance from the reference clock is known as the stratum levels and they exist to prevent cycles in the NTP. Stratum 0, are devices such as atomic clocks connected directly to a computer. Stratum 1, are computers attached to stratum 0 devices, while Stratum 2 are computers that send NTP requests to Stratum 1 servers. NTP can support up to 256 strata.

All Microsoft Windows versions since 2000 include the Windows Time Service (w32time.exe) which has the ability to synchronise the computer clock to an NTP server (or an SNTP server – a simplified version of NTP) Many LINUX and UNIX based operating systems also have a version of NTP but the source code is free to download (current version 4.2.4) at the NTP website (ntp.org).

It is strongly recommended by Microsoft and others, that external based timing should be used rather than Internet based, as these can’t be authenticated. Specialist NTP time servers are available that can synchronise time on networks using either the MSF (or equivalent) or GPS signal.

Synchronising Computer Networks to an Atomic Clock

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Atomic clocks are well-known for being accurate. Most people may never have seen one but are probably aware that atomic clocks keep highly precise time. In fact modern atomic clock will keep accurate time and not lose a second in one hundred million years.

This amount of precision may seem overkill but a multitude of modern technologies rely on atomic clocks and require such a high level of precision. A perfect example is the satellite navigation systems now found in most auto cars. GPS is reliant on atomic clocks because the satellite signals used in triangulation travel at the speed of light which in a single second can cover nearly 100,000 km.

So it can be seen how some modern technologies rely on this ultra precise timekeeping from atomic clocks but their use doesn’t stop there. Atomic clocks govern the world’s global timescale UTC (Coordinated Universal Time) and they can also be used to synchronise computer networks too.

It may seem extreme to use this nanosecond precision to synchronise computer networks too but as many time sensitive transactions are conducted across the internet with such trades as the stock exchange where prices can fall or rise each and every second it can be seen why atomic clocks are used.

To receive the time from an atomic clock a dedicated NTP server is the most secure and accurate method. These devices receive a time signal broadcast by either atomic clocks from national physics laboratories or direct from the atomic clocks onboard GPS satellites.

By using a dedicated NTP server a computer network will be more secure and as it is synchronised to UTC (the global timescale) it will in effect be synchronised with every other computer network using a NTP server.

NTP GPS Server Using Satellite Time Signals

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The NTP GPS server is a dedicated device that uses the time signal from the GPS (Global Positioning System) network. GPS is now a common tool for motorists with satellite navigation devices fitted to most new cars. But GPS is far more than just an aid for positioning, at the very heart of the GPS network is the atomic clocks that are inside each GPS satellite.

The GPS system works by transmitting the time from these clocks along with the position and velocity of the satellite. A satellite navigation receiver will work out when it receives this time how long it took to arrive and therefore how far the signal travelled. Using three or more of these signals the satellite navigation device can work out exactly where it is.

GPS can only do this because of the atomic clocks that it uses to transmit the time signals. These time signals travel, like all radio signals, at the speed of light so an inaccuracy of just 1 millisecond (1/1000 of a second) could result in the satellite navigation being nearly 300 kilometres out.

Because these clocks have to be so accurate, they make an ideal source of time for a NTP time server. NTP (Network Time Protocol) is the software that distributes the time from the time server to the network. GPS time and UTC (Coordinated Universal Time) the civil timescale is not quite the same thing but are base don the same timescale so NTP has no trouble converting it. Using a dedicated NTP GPS server a network can be realistically synchronised to within a few milliseconds of UTC

The GPS clock is another term often given to a GPS time server. The GPS network consists of 21 active satellites (and a few spare) 10,000 miles in orbit above the Earth and each satellite circles the Earth twice a day. Designed for satellite navigation, A GPS receiver needs at least three satellites to maintain a position. However, in the case of a GPS clock just one satellite is required making it far easier to obtain a reliable signal.

Each satellite continuously transmits its own position and a time code. The time code is generated by an onboard atomic clock and is highly accurate, it has to be as this information is used by the GPS receiver to triangulate a position and if it was just half a second out the Sat Nav  unit would be inaccurate by thousands of miles.

The Importance of the Atomic Clock

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Most people have vaguely heard of the atomic clock and presume they know what one is but very few people know just how important atomic clocks are for the running of our day to day lives in the twenty first century.

There are so many technologies that are reliant on atomic clocks and without many of the tasks we take for granted would be impossible. Air traffic control, satellite navigation and internet trading are just a few of the applications that are reliant on the ultra precise chronometry of an atomic clock.

Exactly what an atomic clock is, is often misunderstood. In simple terms an atomic clock is a device that uses the oscillations of atoms at different energy states to count ticks between seconds. Currently caesium is the preferred atom because it has over 9 billion ticks every second and because these oscillations never change it makes them a highly accurate method of keeping time.

Atomic clocks despite what many people claim are only ever found in large scale physics laboratories such as NPL (UK National Physical Laboratory) and NIST (US National Institute of Standards and Time). Often people suggest they have an atomic clock that controls their computer network or that they have an atomic clock on their wall. This is not true and what people are referring to is that they have a clock or time server that receives the time from an atomic clock.

Devices like the NTP time server often receive atomic clock signals form places such as NIST or NPL via long wave radio. Another method for receiving time from atomic clocks is using the GPS network (Global Positioning System).

The GPS network and satellite navigation are in fact a good example of why atomic clock synchonization is much needed with such high level of accuracy. Modern atomic clocks such as those found at NIST, NPL and inside orbiting GPS satellites are accurate to within a second every 100 million years or so. This accuracy is crucial when you examine how something like a cars GPS satellite navigation system works.

A GPS system works by triangulating the time signals sent from three or more separate GPS satellites and their onboard atomic clocks. Because these signals travel at the speed of light (nearly 100,000km a second) an inaccuracy of even one whole millisecond could put the navigational information out by 100 kilometres.

This high level of accuracy is also required for technologies such as air traffic control ensuring our crowded skies remain safe and is even critical for many Internet transactions such as trading in derivatives where the value can rise and fall every second.

Network Time Server Dual Signals

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A network time server (commonly referred to as the NTP time server after the protocol used in synchronisation – Network Time Protocol) is a device that receives a single time signal and distributes it to all devices on a network.

Network time servers are preferred as a synchronisation tool rather than the much simpler internet time servers because they are far more secure. Using the internet as a basis for time information would mean using a source outside the firewall which could allow malicious users to take advantage.

Network time servers on the other hand work inside the firewall by receiving source of UTC time (Coordinated Universal Time) from either the GPS network or specialist radio transmissions broadcast from national physics laboratories.

Both of these signals are incredibly accurate and secure with both methods providing millisecond accuracy to UTC. However, there are downsides to both systems. The radio signals broadcast by nation time and frequency laboratories are susceptible to interference and locality, while the GPS signal, although available literally everywhere on the globe can occasional be lost too (often due to bad weather interfering with the line-of-sight GPS signals.

For computer networks where high levels of accuracy are imperative, dual systems are often incorporated. These network time servers receive the time signal from both the GPS network and the radio transmissions and select an average for even more accuracy.  However, the real advantage of using a dual system is that if one signal fails, for what ever the reason, the network will not have to rely on the inaccurate system clocks as the other method of receiving UTC time should still be operational.