Time Synchronisation

Accurate time using Atomic Clocks is available across the entire globe using the GPS Atomic Clock time signal transmitted from a constellation of satellites above the Earth and a NTP GPS server which provides the ability to synchronize the time on computers and other electrical equipment.

The GPS signal is operated by the US Airforce (51st Space Wing) and was originally intended solely for military use but after an airliner was accidentally shot down during the cold war it was released for civilian use too.

GPS is currently the world’s only Global and Navigational Satellite System although the Europe Galileo project is expected to be completed by 2012.

GPS is based on a constellation of between 24 and 32 Medium Earth Orbit satellites. These satellite all house an onboard atomic clock. Atomic clocks are the most accurate timekeeping devices keeping time to within a few hundred nanoseconds.

A GPS receiver calculates its position on Earth by timing the signals sent by the GPS satellites. Each satellite continually transmits the time as told by its atomic clock along with information about the location of the satellite that sent it. The receiver uses the arrival time of each message to measure the distance to each satellite. By using the distance from four or more satellites the GPS receiver can work out via triangulation exactly where it is in the world.

Dedicated NTP GPS server (Network Time Protocol) are tuned to receive the GPS time signals are available. These devices connect o a computer network like any other server only these receive the timing signal and distribute it to other machines on the network using NTP (Network Time Protocol).

NTP GPS server is a time server that uses the protocol NTP (Network Time Protocol) receives its authoritative time source from the GPS satellite network (Global Positioning System).

Other varieties of time server exist most if not all dedicated devices use NTP as it is universally excepted as the most superlative time protocol, there are however numerous ways a network time server (whether NTP or not) can receive a time signal.

The internet is a common source for timing information although very few administrators who have a dedicated time server device would use the Internet. This is because external time sources across the Internet are the wrong side of a uses firewall and therefore pose a potential security risk not-to-mention many Internet timing sources are not very accurate and also most are stratum 2 devices and therefore further away from the original time source than a stratum 1 dedicated NTP server (stratum 0 devices are atomic clocks).

The other methods a dedicated NTP server (often called network time server) can receive a timing signal is via GPS or radio transmissions that are broadcast from national physics laboratories.

Radio signal are a good source of UTC time (coordinated universal time) as they are maintained and run by organisations such NPL (National Physical Lab. UK) or NIST (National Institute for Standards and Time – US). However, these long wave transmissions are not broadcast in every country and the signals are susceptible to local interference and topography.

On the other hand a dedicated NTP GPS server receives its time signal from the GPS satellite system (each satellite contains an atomic clock). These signals offer excellent accuracy and furthermore are available everywhere on the planet where there is a clear view of the sky.

A NTP GPS server is one of the most stable and reliable ways to receive a timing signal, they are also the simple and easy to install. However, there are some common problems that some users come across in the installation of their NTP GPS Server.

By far the most common occurrence with a NTP GPS server a Is when no GPS signal is being received. There are normally several possible reasons why a NTP GPS server is failing to receive a signal.

The GPS antenna should be correctly aligned with a 180 degree view of the sky. It should also be away from sources of interference such as television aerials or rooftop generators.

The GPS antenna should also be correctly plugged in and the cable length form the antenna to the NTP GPS Server should not exceed 300 metres unless the signal is amplified.

While it takes only a few minutes for the NTP GPS antenna to locate a satellite to ensure a continuous and reliable link with the satellite the NTP GPS server should be left on for at least 48 hours to ensure stability.

A NTP GPS server is one of the most stable and reliable ways to receive a timing signal, they are also the simple and easy to install. However, there are some common problems that some users come across in the installation of their NTP GPS Server.

By far the most common occurrence with a NTP GPS server a Is when no GPS signal is being received. There are normally several possible reasons why a NTP GPS server is failing to receive a signal.

The GPS antenna should be correctly aligned with a 180 degree view of the sky. It should also be away from sources of interference such as television aerials or rooftop generators.

The GPS antenna should also be correctly plugged in and the cable length form the antenna to the NTP GPS Server should not exceed 300 metres unless the signal is amplified.

While it takes only a few minutes for the NTP GPS antenna to locate a satellite to ensure a continuous and reliable link with the satellite the NTP GPS server should be left on for at least 48 hours to ensure stability.

Most dedicated NTP GPS servers are simple and easy to install, most manufacturers include guidance and full software with their devices to limited potential difficulties in installation.

There are however, a few important aspects about installing a NTP GPS server that are often overlooked by some technicians.

The GPS signal works by line-of-sight, that is to say that the satellite has to be visible to the antenna. Obviously as GPS satellites are-in orbit that means that the antenna has to have a good view of the sky. It is also important to keep the antenna away from sources of interference such as satellite dishes or generators.

The cable length of the antenna is also important. The maximum cable length without any amplification device is 1000 metres

Whilst most NTP GPS servers can locate a satellite in a matter of minutes In order to maintain a stable satellite fix, most NTP GPS servers need to be left on for at least 48 hours to ensure stability.

Configuration of a NTP GPS server can be done over the Internet in browser window, alternatively some dedicated devices come with LCD screen and external configuration controls.

What is NTP?

NTP is a protocol developed over 25 years ago to deal with the problems of time synchronisation across the Internet. Network Time Protocol distributes a single time source to all computers and devices by using an algorithm to inspect the drift of each device and then correcting the time.

While NTP can distribute any time reference it was designed in particular to synchronise computer networks to UTC (Coordinated Universal Time). UTC is a global time scale used by computer networks around the world. It is based on the time told by atomic clocks and is therefore highly accurate.

What is GPS?

The Global Positioning System (GPS) is currently the world’s only global navigational satellite system (GNSS) and is controlled by the American military. GPS is most commonly thought of as a navigational tool but as the principles of GPS navigation involve an atomic clock onboard each GPS satellite transmitting its time, this information can be used by NTP as a highly accurate source of UTC.

What does a NTP GPS server do?

A NTP GPS server is a dedicated computer server that receives a GPS timing signal via a GPS antenna and then distributes it amongst the network.  NTP GPS servers can provide accuracy to within a few milliseconds of UTC time.

Just as your computer is only as good as the software it is running (and in most cases the operator too), a time server is only as useful as the timing source it uses.

Accuracy is of course the fundamental feature in time servers and Network Time Protocol (NTP) has been designed to ensure the highest accuracy possible is always obtainable.

Unfortunately many administrators opt to use an Internet timing source to run their time server with. Unfortunately several studies of Internet timing references discovered that nearly two thirds were inaccurate by over several seconds (almost an eternity in the world of NTP) and those that weren’t were often too far away from a client to provide reliable accuracy.

Probably the most accurate and widely used method of retrieving a reliable time reference is to use a dedicated GPS Time Server.

The Global Positioning System is ideal to provide an accurate timing reference as each global positioning satellite contains an atomic clock that beams timing information directly to Earth. As these satellites are in a relatively low and known orbit the distance the signal travels can be compensated for by the GPS time server ensuring a high level of accuracy.

Another benefit is that a GPS time server will always be able to receive a signal no matter where in the world it is situated as long as the GPS antenna can have a clear view of the sky, enabling it to communicate with a satellite.

A GPS time server are also relatively inexpensive and advances in GPS technology means the costs are perennially falling. There is a downside to a GPS server in that to ensure the antenna can spot a satellite it needs to be situated  on certain buildings this can be expensive, impractical or even impossible as the maximum cable length should never exceed 100 metres.