By Jonathan Amos
BBC News science reporter
Europe is building its own satellite-navigation system called Galileo. BBC News looks at why such a network is deemed necessary when there is already the US Global Positioning System (GPS).
WHAT IS GALILEO?
Galileo will be a global network of at least 22 satellites providing precise timing and location information to users on the ground and in the air. Its development will cost billions of euros and it will become operational by the end of 2013. The investment makes Galileo one of the biggest space projects initiated in Europe.
Galileo's first demonstrator spacecraft was launched on 28 December, 2005, and a second platform in April 2008. The job of these two spacecraft was to trial the in-orbit technologies needed to run Galileo. These include atomic clocks, the heart of any sat-nav system.
WHY DOES EUROPE WANT GALILEO?
GALILEO UNDER CONSTRUCTION
A European Commission and European Space Agency project
At least 22 satellites to be launched in batches
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
On an important level, Galileo is a political project.
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 was initiated by the US military; and although it is highly unlikely its signals would be degraded or switched off in Europe, the Americans as owners have ultimate control. 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 (not without an augmentation service).
Galileo is intended to be a civil system. Eventually, it is likely 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 and bring, its designers say, 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; and 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.
Sat-nav systems determine a position by measuring the distances to a number of known locations - the spacecraft constellation in orbit
In practice, a sat-nav receiver will capture atomic-clock time signals sent from the satellites and convert them into the respective distances
A sat-nav device will use the data sent from at least four satellites to get the very best estimate of its position - whether on the ground or in the sky
The whole system is monitored from the ground to ensure satellite clocks do not drift and give out timings that might mislead the user
WHAT WILL GALILEO BE USED FOR?
Most people know sat-nav from its in-car application; but there is more. Much more.
The central component of sat-nav is precise timing. The atomic clocks flown on the spacecraft keep near-perfect time, equivalent to the theoretical loss of just billionths of a second over 24 hours.
THE SAT-NAV FUTURE
Navigation for navigation's sake will not drive applications
Uptake pushed forward by services that add value to data
Huge potential for internet-linked services run off mobiles
E.g. finding a restaurant, and directing you to nearest ATM
Multimedia delivered to tourists' mobiles as they walk around
'Guardian angel' services will locate separated children
Possibilities are endless; mobile firms already brainstorming
Database and billing companies planning for large markets
This precision timing already plays a fundamental but often neglected role in electricity distribution, the functioning of e-mail and the internet, and in the security of financial transactions.
Galileo's improved clocks - their precision is 10 times better than current space-qualified clocks - will deepen and extend this role.
The better penetration, accuracy and guarantees of service should also give many more entrepreneurs the confidence to build business plans around sat-nav.
With sat-nav capability increasingly incorporated into mobile devices, there is likely to be an explosion in new applications. Many of these will be quite novel and unexpected uses for sat-nav.
Nonetheless, the transport sector will obviously be a big beneficiary. Industry will derive major efficiency gains through better management of supply chains and haulage fleets.
Galileo will deliver the tools national governments need to introduce wide-scale road charging.
Galileo will also underpin Europe's new air-traffic control system. The single European sky initiative will overhaul current technologies used to keep planes at safe separations, and will allow pilots to fly their own routes and altitudes.
IS GALILEO WORTH THE COST?
There are many who have had deep reservations about the cost of Galileo from the outset - and, in particular, the uncertainties that exist about what the precise end-cost will be.
This prompted one sceptic to dub Galileo the "Common Agricultural Policy of the sky".
As first envisaged, it was going to cost EU taxpayers no more than 1.8bn euros. It will now cost them more than 5bn euros [PDF], according to the European Court of Auditors in its 2009 Galileo report. The European Commission itself talks about a figure of 7bn euros to complete the network.
There is also an intense debate about the true scale of the revenue opportunities available. Who will want to pay for Galileo-enhanced services and how much will they be prepared to pay?
GPS was built at considerable cost by the US taxpayer but the returns for the American economy mean that investment has been repaid many times over. Early GPS entrepreneurs are now dollar billionaires, but how much room is left in the sat-nav market for others?
Also, the progress of the project has hardly inspired confidence. The private consortium of aerospace and telecom companies selected to build and operate Galileo collapsed in 2007. Infighting and political meddling were blamed.
Galileo has been bedevilled by delays and cost overruns. A group of UK MPs said that Galileo provided "a textbook example of how not to run large-scale infrastructure projects".
All that said, the European Commission is adamant that the potential benefits are huge. Even if the value of the future global sat-nav market has been overstated (at 450bn euros annually from 2025 in one analysis), the returns to the EU economy demand member states press ahead with Galileo, the EC believes.
IS GALILEO A "BIG BROTHER" OR "SPY IN THE SKY?"
Galileo, like GPS, is a passive system; it cannot of itself track individuals. Just because someone carries an active receiver does not mean their every move can be followed.
This only becomes possible once positional information is forwarded to a third party.
Some individuals will choose to do this; one can imagine a mobile phone service that alerts friends when you are in their area.
Companies will "tag" high-value deliveries that report their position so that customers have a better idea of when their goods will arrive.
And governments will, in certain circumstances, insist this information is forwarded to collection centres. A good example would be road pricing, where a vehicle's movements built-up from sat-nav data are passed to roadside beacons or reported over cellular phone networks.
GALILEO WILL HAVE FIVE SERVICES
OPEN ACCESS NAVIGATION
This will be 'free to air' and for use by the mass market; Simple timing and positioning down to 1m
Encrypted; High accuracy at the cm scale; Guaranteed service for which service providers will charge fees
SAFETY OF LIFE NAVIGATION
Open service; For applications where guaranteed accuracy is essential; Integrity messages will warn of errors
PUBLIC REGULATED NAVIGATION
Encrypted; Continuous availability even in time of crisis; Government agencies will be main users
SEARCH AND RESCUE
System will pick up distress beacon locations; Feasible to send feedback, confirming help is on its way