Science correspondent Jonathan Amos looks over the IOV payloads
The Galileo satellite-navigation system is coming.
After all the wrangling, the delays and the furore over cost, Europe's version of GPS is finally starting to take shape.
In an industrial cleanroom in southern England, engineers are reaching key milestones in their preparation of four satellites.
It will be these In-Orbit Validation (IOV) models which will prove the Galileo concept.
Due for launch in pairs in late 2010 and early-2011, the "pathfinders" will form a mini-constellation in the sky.
They will transmit the navigation signals that demonstrate the European system can become a reality.
The rest of the network should then follow soon afterwards. Galileo will eventually comprise some 30 satellites, to inform and guide users the world over.
"Even with just a few Galileo spacecraft in orbit - if you had a GPS and Galileo-compatible receiver, you would begin to see a difference, simply by virtue of having more satellites in the sky," explained Dr Mike Healy from EADS Astrium.
The company's Portsmouth cleanroom is responsible for assembling the IOV payloads.
Its engineers have been sent sat-nav components from across Europe. This equipment is being installed in 1.5m-by-1.5m-by-3m boxes.
On a level
The critical elements of an IOV include its two passive hydrogen-maser atomic clocks, the ultra-precise time-pieces on which Galileo's performance depends.
GALILEO UNDER CONSTRUCTION
A European Commission and European Space Agency project
Up to 32 satellites to be launched in batches in coming years
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
"The main thing is maintaining their accuracy," said Peter Hollands, the payload project manager.
"You have to protect them from magnetic fields, from shock and vibration, and from thermal excursions. So the variation in temperature of the clocks on the IOVs is controlled to within about a degree [Celsius] to keep them stable."
The payloads must also carry signal generation units. These complex processing centres take the "ticks" from the clocks and turn them into the all-important navigation signal. That signal then goes through amplifiers before being transmitted to users on the ground via an antenna.
All of these elements have to be integrated and tested.
The first complete payload is due to go out the door in a matter of weeks.
The box will be sent to Thales Alenia Space in Rome, Italy, where it will be attached to the main spacecraft bus, incorporating a propulsion system, avionics and solar panels, etc, before being shipped to Europe's spaceport in French Guiana.
A Russian Soyuz rocket has been entrusted with the deployment in orbit.
Galileo will work alongside GPS. It is expected to improve substantially the availability and accuracy of timing and navigation signals delivered from space.
Users should get quicker, more reliable fixes and be able to locate their positions with an error of one metre compared with the current GPS-only error of several metres.
Of course, Europe's single biggest space services project should have been operational by now.
Political and financial uncertainty have hindered progress; the collapse in 2007 of the private consortium invited to build and run the network came very close to delivering a knock-out punch.
But there have been technical challenges, also. European industry has had to develop some novel equipment for Galileo, and getting them all to interact seamlessly and robustly at a systems level has taken time.
That there had been slippage on such a major endeavour should not have been a surprise, argued Dr Healy.
"When we make telecoms satellites, they are essentially a variation on a theme and we can now do them to a very consistent timescale," he explained.
"But when we have to do significant developments, as in the case of Galileo, these spacecraft are bound to take longer and there will be a degree of uncertainty on timescales.
"Remember, Europe has never done navigation before."
This week sees the deadline for the consortia competing to build the remaining Galileo satellites to submit their final prices to the project's leaders, the European Commission (EC) and the European Space Agency (Esa).
Contracts for up to 22 Full Operational Capability (FOC) spacecraft are expected to be awarded before the year's end.
EADS Astrium and Thales Alenia Space may have built the IOVs but they are not guaranteed to get the next batch. Certainly, not all of them.
There has been wide speculation in Europe that the EC and Esa will hand some of the FOC order to a rival consortium led by OHB of Germany.
Whatever the outcome, payload integration will be undertaken in the UK - if not in Astrium's Portsmouth cleanroom then at Surrey Satellite Technology Limited (SSTL) in Guildford.
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