GPS Time Server – Utilising Satellite Navigation as a Timing Source

Satellite navigation has been around for the last few decades courtesy of the American military controlled GPS system (Global Positioning System). The GPS network is currently the World’s only GNSS (global navigation satellite system, although Europe, Russia and China are developing their own.

GNSS systems like GPS are based on the same principle. A constellation of satellites orbiting the Earth each have onboard an atomic clock. These clocks are accurate to within a few microseconds and it is this time-telling accuracy that allows satellite navigation possible.

The GPS satellite continually transmits its location along with the time told by its atomic clock. It is this information that is received by a satellite navigation unit that works out how long the message took to arrive from the satellite and therefore the distance away from it. By using four or more satellites a GPS receiver can triangulate its exact position.

Atomic clocks have to be used onboard the satellites as even tiny inaccuracies in time could cause huge errors in navigation. The signals from the satellites travel at the speed of light which can manage 300,000 km every second so even a millisecond out would make the satellite navigation inaccurate by hundreds of kilometres.

Because of its accuracy, this timing signal broadcast from the onboard atomic clocks can be utilised as a source of UTC time by a GPS time server. A GPS antenna connected to a dedicated time server can synchronise a computer network to within a few milliseconds of UTC time.

GPS Time Server Troubleshooting Tips

A GPS time server is probably the most secure, accurate and reliable method of receiving a UTC time signal (Coordinated Universal Time). However, some problems do occur but fortunately the solutions are relatively straightforward.

One of the most common causes for problems with a GPS time server is in failure to receive a signal. While the GPS transmission is extremely impervious to weather interference and as most GPS time servers can receive a signal from between one and twelve different satellites, the transmission is very robust. If a signal is not being received then the first point of call is the antenna.

A GPS time server antenna needs to have a clear view of the sky this is because the signal from the satellites works by direct line-of-sight. While the signal is robust avoid placing the transmitter near to a satellite dish or other broadcast aerial as this could interfere with the signal. Also be aware of foliage which can creep over the antenna and block the signal. Some GPS antennas can work through a window, however the window must not have any coating on the glass such as E-glass and there must still be a direct line of sight from the window to the satellite.

The cable connecting the antenna can also cause problems. Ensure there are no breaks and that the cable is not too long for the specifications of the antenna. Too much cable and the signal may not have enough strength to make the distance.

A much rarer problem for a GPS time server would be trouble with the receiver. Fortunately most dedicated GPS time servers have a reset and test mode. If you are confident that the antenna is in fully working and operational order then reset the receiver and run the test mode.

If all the above fails it is possible that the unit is faulty and should be returned to your manufacture or supplier.

GPS Time Server Basics

The GPS (Globally Positioning System) has been commercially available since the mid-1980’s and has revolutionised the way people navigate in aeroplanes, ships and now cars. GPS is currently the world’s only Global and Navigational Satellite System although the Europe Galileo project is expected to be completed by 2012.

Originally built for the United States Military, it was following a major air disaster in 1985 the then President of the USA, Ronald Regan, decided to make the military positing system available for civilians to prevent future tragedies.

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.

As the onboard atomic clocks are so accurate the GPS time signals can be used by NTP time servers. The time signal can be received via a time server fitted with a GPS antenna. This time signal can then be distributed across a network. The advantage of GPS as a timing reference is that the signal is literally available anywhere in the world so long as the antenna can get a good view of the sky.

GPS Time Server and NTP (Network Time Protocol)

We are all used to Satellite Navigation by now. More and more people are installing those little black boxes into their cars and throwing away their old paper road maps. The advantages of satellite navigation are many fold – from constant updates keeping the maps current to being able to pin point your location miles from any landmarks or road signs but GPS has more uses than merely triangulating a position for direction finding, it can be utilized to provide time and frequency information worldwide.

Since the early 1990’s the Global Positioning System (GPS) has been the worlds’ only fully functioning Global Navigational Satellite System (GNSS). Run by the American military, GPS (sometimes referred to as NAVSTAR) has allowed accurate timing and location finding all over the world.

To accurately pinpoint a location, all GNSS systems require an absolute time source, that is a time source as accurate as humanely possible such as that from an atomic clock. Without knowing exactly what the time is a GNSS satellite would not be able to accurately pin point a location (as the Earth, satellites and people are all moving about a location can only be defined by a position and time). Because of the distance of the satellites away from the Earth, even an inaccuracy of a second or two could mean a sat nav’s location could be miles out.

For this reason each satellite has a highly accurate atomic clock onboard which can also be used by NTP (Network Time Protocol) servers to synchronise computer networks. GPS is an ideal time and frequency source because it can provide highly accurate time anywhere in the world using relatively cheap components.

A GPS receiver decodes the signal sent from the GPS antenna to a computer readable protocol which can be utilised by most time servers and operating systems including, Windows, LINUX and UNIX.

The GPS receiver also outputs a precise pulse every second that GPS NTP servers and computer time servers may utilise to provide ultra-precise timing. The pulse-per-second timing on most receivers is accurate to within 0.001 of a second of UTC (Coordinated Universal Time or Temps Universel Coordonné).

GPS is ideal in providing NTP time servers or stand-alone computers with a highly accurate external reference for synchronisation. Even with relatively low cost equipment, accuracy of hundred nanoseconds (a nanosecond = a billionth of a second) can be reasonably achieved using GPS as an external reference.

In 2002, the European Space Agency and European Union agreed to build Europe’s own GNSS called Galileo. To compete with the new and more advanced GNSS technologies the GPS programme is currently being upgraded and it is expected that when Galileo begins relaying signals both systems will become interoperable allowing even more accuracy in timing and positioning.