The World in Perfect Synchronization

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Synchronization is something we are familiar with everyday of our lives. From driving down the highway to walking crowded street; we automatically adapt our behaviour to synchronize with those around us. We drive in the same direction or walk the same thoroughfares as other commuters as failing to do so would make our journey a lot more difficult (and dangerous).

When it comes to timing, synchronisation is even more important. Even in our day to day dealings we expect a reasonable amount of synchronisation from people. When a meeting starts at 10am we expect everybody to be there within a few minutes.

However, when it comes to computer transactions across a network, accuracy in synchronisation becomes even more important where accuracy to a few seconds is too inadequate and synchronisation to the millisecond becomes essential.

Computers use time for every transaction and process they do and you only have to think back to the furore caused by the millennium bug to appreciate the importance computer’s place on time. When there is not precise enough synchronisation then all sorts of errors and problems can occur, particularly with time sensitive transactions.

Its not just transactions that can fail without adequate synchronisation but time stamps are used in computer log files so if something goes wrong or if a malicious user has invaded (which is very easy to do without adequate synchronisation) it can take a long time to discover what went wrong and even longer to fix the problems.

A lack of synchronisation can also have other effects such as data loss or failed retrieval it can also leave a company defenceless in any potential legal argument as a badly or unsynchronised network can be impossible to audit.

Millisecond synchronisation is however, not the headache many administrators assume it is going to be. Many opt to take advantage of many of the online timeservers that are available on the internet but in doing so can generate more problems than it solves such as having to leave the UDP port open in the firewall (to allow the timing information through) not-to-mention no guaranteed level of accuracy from the public time server.

A better and simpler solution is to use a dedicated network time server that uses the protocol NTP (Network Time Protocol). A NTP time server will plug straight into a network and use the GPS (Global Positioning System) or specialist radio transmissions to receive the time direct from an atomic clock and distribute it amongst the network.

Common GPS Queries

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Is the GPS time signal the same as the GPS positioning signal?

Yes. The signals that are broadcast by GPS satellites contain time information and the position of the satellite it came from (and its velocity). The timing information is generated by an onboard caesium atomic clock. It is this information used by satellite navigation devices (sat navs) that enables global positioning. Sat Navs use these signals from multiple satellites to triangulate a position.

How accurate is GPS positioning?

Because the time signal generated by GPS comes from an atomic clock it is accurate to within 16 nanoseconds (16 billionths of a second). As light travels nearly 186 000 miles in a second this equates to around 16 feet (5+metres) which means a GPS positioning system is usually accurate to this much.

Is GPS time the same as UTC?

No. GPS time, like UTC (Coordinated Universal Time)is based on International Atomic Time (TAI) – the time told by atomic clocks. However as the GPS system was developed several decades ago it is now 14 seconds (and soon to be 15) behind UTC because it has missed out on the Leap Seconds added to UTC to calibrate for the Earth’s slowing rotation.

How can I use GPS as a source of UTC then?

Fortunately a GPS time server will convert GPS to the current UTC time, which as od 1 January 2009 will mean it has to add exactly 15 seconds.

Keeping Accurate Time and The Importance of a Network Time Server

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A network time server can be one of the most crucial devices on a computer network as timestamps are vital for most computer applications from sending and email to debugging a network.

Tiny inaccuracies in a timestamp can cause havoc on a network, from emails arriving before they have technically been sent, to leaving an entire system vulnerable to security threats and even fraud.

However, a network time server is only as good as the time source that it synchronises to. Many network administrators opt to receive a timing code from the Internet, however, many Internet time sources are wholly inaccurate and often too far away from a client to provide any real accuracy.

Furthermore, Internet based time sources can’t be authenticated. Authentication is  a security measure used by NTP (Network Time Protocol which controls the network time server) to ensure the time server is exactly what it says it is).

To ensure accurate time is kept it is vital to select a time source that is both secure and accurate. There are two methods which can ensure a millisecond accuracy toUTC (coordinated universal time – a global timescale based on the time told by atomic clocks).

The first is to use a specialist national time and frequency transmission broadcast in several countries including the UK, USA, Germany, France and Japan. Unfortunately these broadcasts can’t be picked up everywhere but the second method is to use the timing signal broadcast by the GPS network which is available literally everywhere on the face of the planet.

A network time server will use this timing code and synchronise an entire network to it using NTP which is why they are often referred to as a NTP server or NTP time server. NTP continually adjusts the network’s clocks ensuring there is no drift.

Choosing the Right Time Signal for Your Network

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Computer network synchronisation is essential in the modern world. Many of the world’s computer networks are all synchronised to the same global timescale UTC (Coordinated Universal Time).

To govern synchronisation the protocol NTP (Network Time Protocol) is used in most cases as it is able to reliably synchronise a network to a few milliseconds off UTC time.

However, the accuracy of time synchronisation is solely dependent on the accuracy of whatever time reference is selected for NTP to distribute and here lies one of the fundamental errors made in synchronising computer networks.

Many network administrators rely on Internet time references as a source of UTC time, however, apart from the security risks they pose (being as they are on the wrong side of a network firewall) but also their accuracy can not be guaranteed and recent studies have found less than half of them providing any useful accuracies at all.

For a secure, accurate and reliable method of UTC there really are just two choices. Utilise the time signal from the GPS network or rely on the long wave transmissions broadcast by national physics laboratories such as NPL and NIST.

To select which method is best then the only factor to consider is the location of the NTP server that is to receive the time signal.

GPS is the most flexible in that the signal is available literally everywhere on the planet but the only downside to the signal is that a GPS antenna has to be situated on the roof as it needs a clear view of the sky. This may prove problematic if the time server is located in the lower floors of a sky scraper but on the whole most users of GPS time signals find that they are very reliable and incredibly accurate.

If GPS is impractical then the national time and frequencies provide an equally accurate and secure method of UTC time. These longwave signals are not broadcast by every country however, although the US WWVB signal broadcast by NIST in Colorado is available in most of North America including Canada.

There are various versions of this signal broadcast throughout Europe including the German DCF and the UK MSF which prove to be the most reliable and popular. These signals can often be picked up outside the nation’s borders too although it must be noted long wave transmissions are vulnerable to local interference and topography.

For complete peace of mind, dual system NTP servers that receive signals from both the GPS and national physics laboratories are available although they tend to be a little more expensive than single systems although utilising more than one time signal makes them doubly reliable.

Atomic Clocks Explained

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Is an Atomic Clock Radioactive?

An atomic clock keeps time better than any other clock. They even keep time better than the rotation of the Earth and the movement of the stars. Without the atomic clock, GPS navigation would be impossible, the Internet would not synchronise, and the position of the planets would not be known with enough accuracy for space probes and landers to be launched and monitored.

An atomic clock is not radioactive, it doesn’t rely on atomic decay. Rather, an atomic clock has an oscillating mass and a spring, just like ordinary clocks.

The big difference between a standard clock in your home and an atomic clock is that the oscillation in an atomic clock is between the nucleus of an atom and the surrounding electrons. This oscillation is not exactly a parallel to the balance wheel and hairspring of a clockwork watch, but the fact is that both use oscillations to keep track of passing time. The oscillation frequencies within the atom are determined by the mass of the nucleus and the gravity and electrostatic “spring” between the positive charge on the nucleus and the electron cloud surrounding it.

What Are The Types of Atomic Clock?

Today, though there are different types of atomic clock, the principle behind all of them remains the same. The major difference is associated with the element used and the means of detecting when the energy level changes. The various types of atomic clock include:

The Cesium atomic clock employs a beam of cesium atoms. The clock separates cesium atoms of different energy levels by magnetic field.

The Hydrogen atomic clock maintains hydrogen atoms at the required energy level in a container with walls of a special material so that the atoms don’t lose their higher energy state too quickly.

The Rubidium atomic clock, the simplest and most compact of all, use a glass cell of rubidium gas that changes its absorption of light at the optical rubidium frequency when the surrounding microwave frequency is just right.

The most accurate commercial atomic clock available today uses the cesium atom and the normal magnetic fields and detectors. In addition, the cesium atoms are stopped from zipping back and forth by laser beams, reducing small changes in frequency due to the Doppler effect.

When Was The Atomic Clock Invented? atomic clock

In 1945, Columbia University physics professor Isidor Rabi suggested that a clock could be made from a technique he developed in the 1930s called atomic beam magnetic resonance. By 1949, the National Bureau of Standards (NBS, now the National Institute of Standards and Technology, NIST) announced the world’s first atomic clock using the ammonia molecule as the source of vibrations, and by 1952 it announced the first atomic clock using cesium atoms as the vibration source, NBS-1.

In 1955, the National Physical Laboratory (NPL) in England built the first cesium-beam atomic clock used as a calibration source. Over the next decade, more advanced forms of the atomic clocks were created. In 1967, the 13th General Conference on Weights and Measures defined the SI second on the basis of vibrations of the cesium atom; the world’s time keeping system no longer had an astronomical basis at that point! NBS-4, the world’s most stable cesium atomic clock, was completed in 1968, and was used into the 1990s as part of the NPL time system.

In 1999, NPL-F1 began operation with an uncertainty of 1.7 parts in 10 to the 15th power, or accuracy to about one second in 20 million years, making it the most accurate atomic clock ever made (a distinction shared with a similar standard in Paris).

How Is Atomic Clock Time Measured?

The correct frequency for the particular cesium resonance is now defined by international agreement as 9,192,631,770 Hz so that when divided by this number the output is exactly 1 Hz, or 1 cycle per second.

The long-term accuracy achievable by modern cesium atomic clock (the most common type) is better than one second per one million years. The Hydrogen atomic clock shows a better short-term (one week) accuracy, approximately 10 times the accuracy of a cesium atomic clock. Therefore, the atomic clock has increased the accuracy of time measurement about one million times in comparison with the measurements carried out by means of astronomical techniques.

Synchonising to an Atomic Clock

The simplest way to synchonise to an atomic clock is to use a dedicated NTP server. These devices will receive either the GPS ataomic clock signal or radio waves from places like NIST or NPL.

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.

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.