Time Synchronisation

Until 1967 the second was defined using the motion of the Earth which rotates once on its axis every 24 hours, and there are 3,600 seconds in that hour and 86,400 in 24.

That would be fine if the earth was punctual but in fact it is not. The Earth’s rotation rate changes every day by thousands of nanoseconds, and this is due in a large part to wind and waves spinning around the Earth and causing drag.

Over the course of thousands of days, these changes in the rate of rotation can result in the Earth’s spin getting out of synch with the high-precision atomic clocks that we use to keep the UTC system (Coordinated Universal Time) ticking over. For this reason the Earth’s rotation is monitored and timed using the far off flashes from a type of collapsed star called a quasar that flash with an ultra precise rhythm many millions of light years away. By monitoring the Earth’s spin against these far away objects it can be worked out how much the rotation has slowed.

Once a second of slowing has been built up, The International Earth Rotation Service (IERS), recommends a Leap Second to be added, usually at the end of the year.

Other complications arise when it comes to synchronising the Earth to one timescale. In 1905, Albert Einstein’s theory of relativity showed that there is no such thing as absolute time. Every clock, everywhere in the universe, ticks at a different rate. For GPS, this is an enormous issue because it turns out that the clocks on the satellites drift by almost 40,000 nanoseconds per day relative to the clocks on the ground because they are high above the Earth’s surface (and therefore in a weaker gravitational field) and are moving fast relative to the ground.

And as light can travel Forty-thousand feet in that time, you can see the problem. Einstein’s equations first written down in 1905 and 1915 are used to correct for this time-shift, allowing GPS to work, planes to navigate safely and GPS NTP servers to receive the correct time.

Be careful when counting down to the New Year this week as an extra second is to be added to the last minute of 2008. Leap Seconds are added to UTC (Coordinated Universal Time) the time used by most NTP Server systems and has been going on since its inception in 1972 with a total of 33 seconds having already been added to UTC since then.

However there are calls to make this leap second the last ever. The International Telecommunications Union (ITU) is proposing to abolish them in favour of adding a “leap hour” every 600 years.

They claim that the Leap Second creates confusion and can cause software crashes. They cite 1996 as an example, when computers at Associated Press Radio crashed causing them to broadcast the wrong programmes, and a 2003 bug caused some GPS units to show the time as 62:30.

However, there is stiff opposition to abolishing the Leap Second, among them astronomers, Robert Massey of the Royal Astronomical Society told the London Times: “It would be a change with profound cultural implications.  We’d be decoupling our clocks from what the Sun is telling us.”

The implications for astronomers would require expensive changes to the software of astronomical telescopes and it would become almost impossible for sailors to navigate by sextant. It would also mean the Britain would lose its role as the World’s timekeepers as the Greenwich Meridian (the position of the sun at midday) would gradually move south to France until the proposed leap hour would return it to its original position after 600 years.

Apart from the usual celebrations and revelry the end of December brought with the addition of another Leap Second to UTC time (Coordinated Universal Time).

UTC is the global timescale used by computer networks across the world ensuring that everybody is keeping the same time. Leap Seconds are added to UTC by the International Earth Rotation Service (IERS) in response to the slowing of the Earth’s rotation due to tidal forces and other anomalies. Failure to insert a leap second would mean that UTC would drift away from GMT (Greenwich Meantime) – often referred to as UT1. GMT is based on the position of the celestial bodies so at midday the sun is at its highest above the Greenwich Meridian.

If UTC and GMT were to drift apart it would make life difficult for people like astronomers and farmers and eventually night and day would drift (albeit in a thousand years or so).

Normally leap seconds are added to the very last minute of December 31 but occasionally if more than one is required in a year then is added in the summer.

Leap seconds, however, are controversial and can also cause problems if equipment isn’t designed with leap seconds in mind. For instance, the most recent leap second was added on 31 December and it caused database giant Oracle’s Cluster Ready Service to fail. It resulted in the system automatically rebooting itself on New Year.

Leap Seconds can also cause problems if networks are synchronised using Internet time sources or devices that require manual intervention.  Fortunately most dedicated NTP servers are designed with Leap Seconds in mind. These devices require no intervention and will automatically adjust the entire network to the correct time when there is a Leap Second.

A dedicated NTP server is not only self-adjusting requiring no manual intervention  but also they are highly accurate being stratum 1 servers (most Internet time sources are stratum 2 devices in other words devices that receive time signals from stratum 1 devices then reissue it) but they are also highly secure being external devices not required to be behind the firewall.

NTP is the standard protocol for synchronising networks and is used throughout the world to ensure computers are all running the same UTC (Coordinated Universal Time) time.

UTC was developed after the invention of atomic clocks to enable the global community to synchronise to the same time. It is based on Greenwich Meantime but accounts for the slowing of the Earth’s rotation by occasional adding ‘leap seconds’.

There are many ways to receive a UTC time source to synchronise a network to. Most commonly the internet is used as a timing source but inaccuracies, distance and lack of authentication means many internet time sources are virtually useless as a way of maintaining accurate time.

The most accurate and efficient way a of receiving UTC time is by using a dedicated GPS NTP server. These comprise of a GPS antenna, GPS receiver and NTP software.

These time servers receive the time code from GPS (global positioning system) satellites which contain atomic clocks, necessary to provide positioning information. Although the time is not broadcast as UTC, NTP software converts it.

Using a GPS NTP server means that accuracy to within a few hundred nanoseconds is possible using relatively low cost equipment. GPS NTP servers can also receive this time signal from literally anywhere in the world as long as the GPS antenna can get a good view of the sky.

The only disadvantages of a GPS NTP server is that the antenna is limited in its distance from the time server. Although high quality coaxial cable and a signal amplifier may help to achieve distances of up to 300 feet (100 metres).

Because of the advances in GPS technology over recent years, GPS NTP Servers are now more accurate and less expensive as they have ever been.

Time is one of the least understood aspects of our universe. We know it exists yet we have trouble grasping exactly what it is. Time can be viewed in two ways, it is a man made concept used as a tool to describe to explain the sequence of events, comparing the durations and intervals between them.

Time is one of the fundamental quantities which also includes distance, velocity, mass, momentum, energy, and weight and thanks to the work of Einstein and others we know time also makes up the very fabric of our Universe.

Here are ten facts you may or may not have known about time.

10. Time is not a constant; time is relative to different observers. The only constant in the Universe is the speed of light which means no matter how fast you are travelling the speed of light will remain the same although time will slow down.

9. Time can be described as a dimension and along with the other three dimensions we are aware of (up/down, left/right and forward/backward) forms a four dimensional ‘space-time’.

8. Time always moves forward yet many theoretical physicists believe that backwards time travel could be possible.

7. Gravity can warp space-time making time slow down the stronger the gravitational force. Experiments with atomic clocks show the higher above sea level they are (and therefore under less gravitational influence) the faster they run (although the difference is very small).

6. As the speed of light is the only constant in the Universe no matter how fast you travel, light will always seem to be the same speed, this is because time will slow down. A journey at close to the speed of light may seem like a few seconds for a traveler but to an observer it would have taken thousands of years.

5. Time has not always existed. Time started with the big bang and will end if the Universe does.

4. Time can be perceived differently by our brains depending on our activities. A boring day will ‘drag’ on whilst if we are enjoying ourselves time will seem to ‘fly’, this phenomenon is referred to as ‘temporal illusion’ by psychologists.

3. Time appears to accelerate the older we get. Some (including Stephen Hawking) suggest the reason for this is that when we are ten years old a year is a tenth of our whole life and seems a long time, yet for a sixty-year-old a year is just a 60th of their life and therefore perceived as a shorter period.

2. Some modern atomic clocks are so accurate they can lose less than a second in 400 million years.

1. A universal time scale has been developed called UTC (Coordinated Universal Time) which is based on the time told by atomic clocks but compensates for the minute slowing of the Earth’s rotation (caused by the gravity of the Moon) by adding Leap Seconds every year to prevent day from creeping into night (albeit in a millennia or two).

Thanks to atomic clocks and UTC time computer networks all over the world can receive a UTC time source over the Internet, via a national radio transmission or through the GPS network. A NTP server (Network Time Protocol) can synchronise all devices on a network to that time.