GPS Time Server – GPS Facts

From military hardware to GPS time servers

The Global Positioning System was designed and built by the US military in the late 1970’s, although it didn’t achieve initial operational capability until 1993. GPS was originally designed as a military only system but in 1983 after a USSR aircraft shot down a Korean airliner that had accidentally strayed into Soviet airspace, the then President of the United States, Ronald Reagan, vowed the system would be available for civilian use once completed.

Full Operational Capability was declared by the now named NAVSTAR GPS in April 1995 and in 1996 to fulfil his predecessors promise the U.S. President Bill Clinton issued a policy directive declaring GPS to be a dual-use system; one for US military use and one system for civilians.

Currently GPS is the World’s only GNSS (Global Navigational Satellite System) although the Russian GLONASS system that was operational during the Cold War but has since fallen into disrepair is being repaired and a European GNSS known as Galileo is expected to be operational by 2012, other systems developed by China and India are also being developed.

GPS is primarily a navigational system that transmits precise timing information via an onboard atomic clock. It is this information and the satellites location that is used to triangulate positioning. However, the GPS signal can also be used by a GPS time server as a timing source.

GPS Time Server – Receiving Time from Space

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 Server – NAVSTAR

NAVSTAR GPS is not an acronym although many people assume it is. NAVSTAR is the original name given to the US military’s Global Navigational Satellite System (GNSS).

Currently NAVSTAR is the world’s only fully functioning GNSS, although the European Galileo system is expected to be operational in a few years and the former Russian GLONASS system is being put back into commission after it fell into disrepair following the collapse of the Soviet Union.

NAVSTAR is controlled and run by the US Air Force’s 50th Space Wing but has been available for civilian use since the late 1980’s when a Korean passenger airliner was accidently shot down.

NAVSTAR is most commonly used as a positioning and navigational tool but the information it transmits is basic timing data that is generated by an onboard atomic clock.

It is these ultra precise timing signals that a satellite navigation system will use to work out positioning but they can also be utilised by a GPS time server as a source of UTC (coordinated universal time). Strictly speaking the time signal from NAVSTAR is not in UTC format but is kept very close to UTC (within 1 microseconds).

Using NAVSTAR GPS as a timing reference means that a UTC time source can be utilised safely, securely and reliably from anywhere on the globe.

GPS Time Server – A Revolution in Chronology

A GPS time server is often called a GPS clock although this is a misnomer. In fact a GPS time server simply relays the time from an external source around a network it doesn’t keep time itself.

The GPS time server receives its timing signal from the GPS network (Global Positioning System) and has revolutionised the way computers are able to keep track of time.

The GPS network is controlled by the US military and it was due to a military accident involving an airliner that the US government allowed the GPS network to be utilised by civilians around the globe.

The GPS system works by relaying precise time information which can be calculated by a GPS receiver to work out distances. This precise timing information is generated by an onboard atomic clock in the satellite and it is this atomic clock that a GPS time server receives the time from.

Without GPS the only other alternatives to receiving a time signal that relays UTC time (coordinated universal time) would be to use the national time and frequency radio broadcasts transmitted by several national physics laboratories, although these are not available everywhere. Alternatively the only other solution is to use the Internet which means using a device external to your firewall and also a time server that cannot be proven accurate.

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.