Europe is building its own satellite-navigation system called Galileo. BBC News looks at why such a network is deemed necessary when we already have the US Global Positioning System.

Galileo will be a global network of 30 satellites providing precise timing and location information to users on the ground and in the air. It is costing some 3.4bn euros (£2.3bn; $4bn) of public and private investment and represents the biggest space project yet undertaken in Europe.

Galileo’s first demonstrator spacecraft is being launched on 28 December; a second platform will follow in the New Year. They will trial the in-orbit technologies needed to run the system. These include atomic clocks, the heart of any global positioning system.

If all goes according to plan, a full constellation of Galileo satellites will be in operation by the end of 2010.

Why does Europe want Galileo?

On an important level, Galileo is a political project.

Galileo constellation (Esa)
A European Commission and European Space Agency project
30 satellites to be launched in batches by end of 2010
Will work alongside US GPS and Russian Glonass systems
Promises real-time positioning down to less than a metre
Guaranteed under all but most extreme circumstances
Suitable for safety-critical roles where lives depend on service

Like Airbus and the Ariane rocket programme, the new sat-nav system will assert Europe’s independence. It will give EU countries guaranteed access to a service that is currently provided by a foreign (US) power.

GPS is a military-run programme; its signals can be degraded or switched off. Yes, the service is free, but its continuity and quality come with no guarantees – which means it cannot be relied upon, certainly not for safety-of-life applications such as landing planes and controlling trains.

Galileo will be a civil system. It will be run by a private consortium and will offer guaranteed levels of service.

How will Galileo differ from GPS?

As brilliant as GPS is, its accuracy and availability can on occasions leave a lot to be desired, as anyone who has a receiver will know. Sometimes it can be very difficult to get a fix and the accuracy can drift out to 10m or more.

The new Galileo system will offer five service levels (see below) and bring a step-change in performance. Since the first GPS satellite was launched in the late 1970s, sat-nav technology has evolved enormously.

Galileo should offer greater accuracy – down to a metre and less; greater penetration – in urban centres, inside buildings, and under trees; and a faster fix.

The Galileo system will also come with an "integrity" component – it will be able to tell users if there are major errors that could compromise performance.

Users will also benefit enormously from the agreement between Europe and the US to make their sat-nav systems compatible and "interoperable". That is, future receivers will be able to get a fix using satellites from either constellation.

And when the US introduces the next generation of GPS, users will see a further jump in performance.

More here.