GPS Time Server and its Accuracy from space

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The GPS network (Global Positioning System), is commonly known as a satellite navigation system. It however, actually relays a ultra-precise time signal from an onboard atomic clock.

It is this information that is received by satellite navigation devices that can then triangulate the position of the receiver by working out how long the signal has taken to arrive from various satellites.

These time signals, like all radio transmissions travel at the speed of light (which is close to 300,000km a second). It is therefore highly important that these devices are not just accurate to a second but to a millionth of a second otherwise the navigation system would be useless.

It is this timing information that can be utilized by a GPS time server as a base for network time. Although this timing information is not in a UTC format (Coordinated Universal Time), the World’s global timescale, it easily converted because of its origin from an atomic clock.

A GPS time server can receive the signal from a GPS aerial although this does need to have a good view of the sky as the satellites relay their transmissions via line-of-sight.
Using a dedicated GPS time server a computer network can be synchronised to within a few milliseconds of NTP (milli=1000th of a second) and provide security and authentication.

Following the increase use of GPS technology over the last few years, GPS time servers are now relatively inexpensive and are simple and straight forward systems to install.

Next Generation of Accurate Atomic Clocks Starts Ticking as NIST scientists unveil new strontium clock

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Those chronological pioneers at NIST have teamed up with the University of Colorado and have developed the world’s most accurate atomic clock to date. The strontium based clock is nearly twice as accurate as the current caesium clocks used to govern UTC (Coordinated Universal Time) as it loses just a second every 300 million years.

Strontium based atomic clocks are now being seen as the way forward in timekeeping as higher levels of accuracy are attainable that are just not possible with the caesium atom. Strontium clocks, like their predecessors work by harnessing the natural yet highly consistent vibration of atoms.

However, these new generations of clocks use laser beams and extremely low temperatures close to absolute zero to control the atoms and it is hoped it is a step forward to creating a perfectly precise clock.

This extreme accuracy may seem a step too far and unnecessary but the uses for such precision are many fold and when you consider the technologies that have been developed that are based on the first generation of atomic clocks such as GPS navigation, NTP server synchronisation and digital broadcasting a new world of exciting technology based on these new clocks could just be around the corner.

While currently the world’s global timescale, UTC, is based on the time told by a constellation of caesium clocks (and incidentally so is t he definition of a second as just over 9 billion caesium ticks), it is thought that when the Consultative Committee for Time and Frequency at the Bureau International des Poids et Mesures (BIPM) next meets it will discuss whether to make these next generation of atomic clocks the new standard.

However, strontium clocks are not the only method of highly precise time. Last year a quantum clock, also developed at NIST managed accuracy of 1 second in 1 billion years. However, this type of clock can’t be directly monitored and requires a more complex scheme to monitor the time.

Receiving the Time and Finding the Correct Time Source

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The Body Clock Natures Own NTP Server

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Developing new methods of telling the time accurately and precisely has developed to a new obsession amongst chronologists in the twenty first century. Since the development of the first atomic clocks in the 1950’s with millisecond accuracy the race was started with organisation such as the US’s NIST (National Institute for Standards and Time) and the UK’s NPL (National Physical Laboratory) developing increasingly accurate atomic clocks.

Atomic clocks are used as the time source for high technologies and applications such as satellite navigation and air traffic control, they are also the source for time signals used by NTP servers to synchronise computer networks.

An NTP server works by continually adjusting the computers system clock to ensure it matches the time relayed by the atomic clock. In doing this the NTP server can keep a computer network to within a few milliseconds of atomic clock controlled UTC (Coordinated Universal Time).

However, as remarkable this technology may seem it appears Mother Nature has already been doing the very same thing with our own body clocks.

The human body clock is only just being understood by medical science (the study of which is called Chronobiology) but what is known is that the body clock extremely important in the functioning of our day to day lives; it is also highly accurate and works in a very similar way to the NTP server.

Whilst a NTP time server receives a time signal from an atomic clock and adjust the system clocks on computers to match, our body clocks do the very same thing. The body clock runs in a circadian rhythm in other words a 24 hour clock. When the sun rises in the morning part of the brain that governs the body clock called the suprachiasmatic nucleus – which is located in the brain’s hypothalamus, automatically corrects for the sun’s movement.

In this way the human body clock adjusts for the darker winters and lighter months of the summer which is why you may find it more difficult to wake in the winter. The body clock adjusts itself every day to ensure it is synchronised to the rotation of the sun just as a NTP time server synchronises a computer’s system clock to ensure it is running accurately  to its timing source – the atomic clock.

NTP GPS Server Using Satellite Time Signals

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The railway timetable was only useful if people all used the same time scale. A train leaving at would be missed if a watch was five minutes slow so synchronisation of time became a new obsession.






Leap Second Errors and Configuration

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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.

It could be the last Leap Second tonight as there are calls to have it scrapped

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At midnight on tonight an extra second will be added as recommended by the International Earth rotation and Reference systems Service (IERS). That means for the last minute of 2008 there will 61 seconds.

Leap Seconds have been added nearly every year since the inception of UTC (Coordinated Universal Time) in the 1970’s. The extra second is added to ensure UTC keeps in synch with GMT (Greenwich Meantime or sometimes called UT1). GMT is the traditional 24 hour clock system where a day is defined as the rotation of the Earth which takes 86,400 seconds for a complete revolution.

Unfortunately the Earth can often be a little tardy in its spin and if the extra seconds were no added at the end of the year to compensate eventually the two systems (UTC and GMT) would drift apart. In a millennium the time difference would only be an hour but many argue to a have a time system that does no correspond to the movement of the heavens would be irrational and occupations such as farming and astronomy would be made more difficult.

However, not everybody sees it that way wit some arguing that as te entire world’s computer networks are synchronised to UTC using NTP servers then the fudging of the extra second causes untold amounts of trouble.

Now a group within the International Telecommunications Union, called has recommended abolishing the leap second. Group member Elisa Felicitas Arias, of the International Bureau of Weights and Measures in Paris, France, argues that a timescale that doesn’t need regular tweaking is essential in an increasingly interconnected world. What’s more, she says, ships and aircraft now navigate via GPS rather than the old time system. GPS runs on a version of atomic time.

Next year, member states of the ITU are due to vote on the proposal. If 70 per cent support the idea, an official decision will be made at the World Radio Conference in 2011. According to a report co-authored by Felicitas Arias, most member states support the idea. The UK, however, is against reworking its laws, which include the solar timescale Greenwich Mean Time. Without the UK abolition may be difficult, says Felicitas Arias.

“In theory, adding a second is as easy as flipping a switch; in practice, it rarely works that way,” says Dennis McCarthy of the US Naval Research Laboratory, which provides the time standard used by the US military. Most likely to be affected are IT systems that need precision of less than a second. In 1998 – two leap seconds ago – cellphone communications blacked out over part of the southern US. Different regions of service had slipped into slightly different times, preventing proper relaying of signals.

All quotes attributed to the BBC

Keeping Track of the Worlds Time and Difficulties in Synchronisation

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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.

MSF Technical Information

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The MSF transmission from Anthorn (latitude 54° 55′ N, longitude 3° 15′ W) is the principal means of disseminating the UK national standards of time and frequency which are maintained by the National Physical Laboratory. The effective monopole radiated power is 15 kW and the antenna is substantially omnidirectional. The signal strength is greater than 10 mV/m at 100 km and greater than 100 μV/m at 1000 km from the transmitter. The signal is widely used in northern and western Europe. The carrier frequency is maintained at 60 kHz to within 2 parts in 1012.

Simple on-off carrier modulation is used, the rise and fall times of the carrier are determined by the combination of antenna and transmitter. The timing of these edges is governed by the seconds and minutes of Coordinated Universal Time (UTC), which is always within a second of Greenwich Mean Time (GMT). Every UTC second is marked by an ‘off’ preceded by at least 500 ms of carrier, and this second marker is transmitted with an accuracy better than ±1 ms.

The first second of the minute begins with a period of 500 ms with the carrier off, to serve as a minute marker. The other 59 (or, exceptionally, 60 or 58) seconds of the minute always begin with at least 100 ms ‘off’ and end with at least 700 ms of carrier. Seconds 01-16 carry information for the current minute about the difference (DUT1) between astronomical time and atomic time, and the remaining seconds convey the time and date code. The time and date code information is always given in terms of UK clock time and date, which is UTC in winter and UTC+1h when Summer Time is in effect, and it relates to the minute following that in which it is transmitted.

Dedicated MSF NTP Server devices are available that can connect directly to the MSF transmission.

Information Courtesy of NPL