Time Synchronisation

Currently there is only one Global Navigation Satellite System (GNSS) the NAVSTAR GPS which has been open for civilian use since the late 1980’s.

Most commonly, the GPS system is thought to provide navigational information allowing drivers, sailors and pilots to pinpoint their position anywhere in the world.

In fact, the only information beamed from a GPS satellite is the time which is generated by the satellites internal atomic clock. This timing signal is so accurate that a GPS receiver can use the signal from three satellites and pinpoint the location to within a few metres by working out how long each precise signal took to arrive.

Currently a GPS NTP server can use this timing information to synchronise entire computer networks to providing accuracy to within a few milliseconds.

However, the European Union is currently working on Europe’s own Global Navigation Satellite System called Galileo, which will rival the GPS network by providing its own timing and positioning information.

However, Galileo is designed to be interoperable with GPS meaning that a current GPS NTP server will be able to receive both signals, although some software adjustments may have to be made.

This interoperability will provide increased accuracy and may make national time and frequency radio broadcasts obsolete as they will not be able to produce a comparable accuracy.

Furthermore, Russia, China and India are currently planning their own GNSS systems which may provide even more accuracy. GPS has already revolutionised the way the world works not only by allowing precise positioning but also enabling entire globe to synchronise to the same timescale using a GPS NTP server. It is expected that even more advances in technology will emerge once the next generation of GNSS begin their transmissions.

GPS time servers are network time servers that receive a timing signal from the GPS network and distribute it amongst all devices on a network ensuring that the entire network is synchronised.

GPS is an ideal time source as a GPS signal is available anywhere on the globe. GPS stands for Global Positioning System, the GPS network is owned by the US military and controlled and run by the US air force (space wing). It is however, since the late 1980’s been opened up to the world’s civilian population as tool to aid navigation.

The GPS network is actually a constellation of 32 satellites that orbit the Earth, they do not actually provide positioning information (GPS receivers do that) but transmit from their onboard atomic clocks a timing signal.

This timing signal is what is used to work out a global position by triangulating 3-4 timing signals a receiver can work out how far and therefore the position you are from a satellite. In essence then, a global positioning satellite is just an orbiting clock and it is this information that is broadcast that can be picked up by a GPS time server and distributed amongst a network.

Whilst strictly speaking GPS time is not the same as the global timescale UTC (coordinated universal time), a GPS time server will automatically convert the time format into UTC.

A GPS time server can provide unbridled accuracy with networks able to maintain accuracy to within a few milliseconds of UTC.

GPS time servers have revolutionised the world of synchronisation. There advantages over other forms of timing references is many fold but as there are over 30 GPS satellites one will always be in range of a GPS receiver.

A GPS time server (Global Positioning System) bought fifteen years ago would have cost somewhere in the reason of £8,000-£10,000 ($15,000-$17,000) but thanks to the growing use of the technology the price of GPS receivers has plummeted and they can be bought for as little as £350 ($600). His has made GPS the dominate form of timing references for time synchronisation.

A GPS time server will come in several forms, some are designed to be fitted into standard server racks, these rack-mountable GPS time server will take up one or two standard U spaces depending on server type.

Other GPS time servers are smaller and more discrete, ideal to be located outside a server room. While GPS time servers offer unrivalled accuracy and a signal is available literally everywhere on the planet it does have one down-side in that a GPS antenna has to have a clear view of the sky. This means that the antenna has to be situated on a roof of a building otherwise there is a possibility that the signal will be lost.

In selecting a NTP time server the things to think about  are where the device will be located and which timing reference would be most suitable for the applications it is required for. There are benefits and drawbacks to both the radio broadcasts and the GPS timing signals that are commonly used in these servers.

Not every country transmits a national time and frequency broadcast which means if a time server is to be located outside of the US, Japan, Germany, the UK or France it is doubtful that a signal  will be received (although some of these broadcasts can be received in neighbouring states).

The radio frequency is also susceptible to atmospheric interference and can be blocked by local topography.

GPS on the other hand can be received anywhere in the world it is also slightly  more accurate – a typical GPS receiver can provide timing information to within a few nanoseconds of UTC (Coordinated Universal Time) while national time and frequency broadcasts are accurate to 1 – 20 milliseconds.

However, using a radio broadcast as a timing reference is that it is possible to receive it from inside a building (although basements and rooms with metal furnishings can cause interference). The drawback of GPS is that the antenna needs to be situated on the roof to have a clear view of the sky to be able to find and receive the signal broadcast from the satellites (although it is possible sometimes to receive a signal through window).

The American GPS system is currently revolutionising the way we navigate. Once a secretive Cold War military weapon it now adorns the dashboards of one in three cars.

But GPS is much more than a handy navigational tool. The constellation of 24 satellites each contain some of the world’s most accurate chronometers in the shape of atomic clocks. These provide such accuracy in timing that a million years could pass and not even a second would have been gained or lost

These clocks are what enable us to pinpoint our location on Earth as a GPS receiver can workout how long the timing signal took to reach it and therefore how far away from the satellite it is. Using three or four satellites means an exact location can be pin-pointed by triangulation. Atomic clocks need to be used as just one seconds inaccuracy could mean a location could be hundreds of thousands of miles out because of the vast distances radio signals can travel in that time.

These timing signals can also be utilised to provide extremely accurate synchronisation for computer networks by receiving the timing signal via a GPS antenna connected to a GPS time server. A GPS time server uses NTP (Network Time Protocol) to synchronise machines on a network.

Because of the accuracy of the atomic clocks a GPS Time Server can obtain an accuracy to within a few hundred nanoseconds of UTC time (a nano is one billionth of a second)

UTC (Coordinated Universal Time) is a global timescale developed after the invention of the atomic clock. It is a standardised time scale base on Greenwich Meantime but allows for the minute slowing of the Earth’s rotation (caused by the Moon’s gravity).

A GPS time server can also receive a timing signal from anywhere in the world (as long as it can get a clear view of the sky)