GPS - Time Keeping in the Satellite Age

We are all used to the Sat Nav 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.

Currently the American Military run the Global Positioning System (GPS) which at present is the world's only fully operational Global Navigation Satellite System (GNSS). However, the Europeans have started their own GNSS system called Galileo which is expected to be fully operational by 2013. Russia and China are also working on their own systems too.

This will mean more accurate direction finding for all of us as the differing GNSS are expected to be interoperable. However, there is more to GNSS than just direction finding.

GNSS systems work by using satellites several thousand miles above the Earth's that beam positional information down to a GNSS receiver (like a sat nav).

However 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.

As a consequence of having this accurate time sources, GPS and the new breed of GNSS systems can all be used to receive an absolute or UTC (Universal Coordinated Time) time source.

This time source is currently used by computer networks running a NTP time server to synchronise all machines and devices to the same time.

Having a time signal broadcast all over the world means the entire globe can synchronise to the exact same time, vital in many computer applications.

When the new GNSS systems are fully functional timing accuracy will also increase along side accuracy in direction finding.

About the Author

Richard N Williams is a technical author and a specialist in the telecommunications and network time synchronisation industries. For more information about NTP servers please visit the Galsys homepage.

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