Highly-accurate and reliable satellite navigation in Europe is coming a step closer with the inauguration of the first main control centre of a new continent-wide system.
By Ivan Noble
BBC News Online science staff
The Egnos (European Geostationary Navigation Overlay Service) facility is being inaugurated in Langen, Germany, on Friday.
The system should go into full operation in 2004.
It works by adding extra information to existing signals from the two military-run services currently in use - the United States' GPS and Russia's Glonass - to provide users with positional data accurate to within two metres.
"This signal is so-called European-made, even though it is based on GPS," the European Space Agency's Dominique Detain told BBC News Online.
Egnos also provides integrity information, warning users if there is a problem with the GPS system and instructing them not to rely upon it.
One of Egnos' aims is to provide a service accurate for civil aircraft control.
Manufacturers of GPS equipment are already working on Egnos-compatible receivers.
GPS systems tell their users where they are in (or above) the world by telling them their latitude, longitude and altitude.
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They derive the information from a network of satellites operated by the US military.
In the past, the US deliberately degraded the signal its satellites provided to non-military users to make civilian GPS systems much less accurate.
Egnos is dependent on GPS and if the Americans decided to resume degrading their GPS signals, Egnos would also deliver much poorer accuracy, although it would warn its users very rapidly that this was the case.
Europe's own GPS
Egnos is the first stage of a two-stage project to create a European satellite-based navigation system.
The second stage is Galileo, an independent network of 30 satellites which is scheduled to begin full operation in 2008.
Galileo will provide a service independent of the American and Russian systems but compatible with them.
The Galileo-compatible receivers will still be able to pick up GPS signals, giving them a much better chance of working well in built-up areas where high buildings can easily obscure line-of-sight to the satellites.
"The real advantage is when you get the two systems working together," aerospace specialist Astrium's Dr Michael Healy told BBC News Online.
Egnos is a joint project of the European Space Agency, the European Commission and the European Organisation for the Safety of Air Navigation, Eurocontrol.