By Jonathan Amos
BBC News science reporter
A few more weeks of testing and the box will be closed on one of the most important satellite payloads Europe is ever likely to send into space.
Currently sitting in a cleanroom in Portsmouth in southern England, the container holds the critical technologies on which a new multi-billion euro industry will be built.
If all the payload's components work as designed, a constellation of similar boxes will be ordered up and flown into orbit to complete the Galileo satellite-navigation network - Europe's answer to GPS, the US Global Positioning System.
Except, Galileo will be more than just a copycat. Its next-generation technologies promise a step change in accuracy and reliability for location-based services.
Where GPS signals currently struggle to penetrate our high-rise cities, Galileo will bring performance improvements that should see sat-nav receivers get a fix in even the deepest "urban canyons".
And that should spark an innovation stampede to put sat-nav into many more mobile devices. At least, that is the hope of the European Commission and the European Space Agency which are investing more money into Galileo than any previous space project.
The Portsmouth payload is currently being prepared by EADS-Astrium.
The company's engineers have taken delivery of pre-built components from all over the continent. Their job has been to integrate these disparate elements into a working whole.
GALILEO TEST BED SATELLITES
Surrey Satellite Technology Ltd
Contract worth 27.9m euros
Mass on lift-off of 400kg
Will complete frequency filings if launched first
To test space environment and key technologies
Galileo Industries consortium
Contract worth 72.3m euros
Mass on lift-off of 525kg
Will complete frequency filings if launched first
To test space environment and key technologies, including first flight of hydrogen maser clock
"There is a real level of experimentation attached to this test satellite - to put together all the technologies that were developed over the last few years across Europe and to demonstrate that they actually work," explains Gerrit Beyer, from the Business Development Navigation section of EADS-Astrium.
"Most of the elements come to us as customer-furnished items and it is at the point of integration that problems can arise. When you put elements together you can find they do not talk to each other properly - and we have to fix minor bugs. It's all about interface and performance checks."
First and foremost, there are the atomic clocks that provide the precise timing reference required to work out latitude, longitude and altitude.
As well as two rubidium atomic clocks, the payload will carry a hydrogen maser clock, which has a stability that is better than one nanosecond per day.
Its precision is greater than the rubidium devices and is one of the key design features that gives Galileo its enhanced performance.
Then there is the signal generation unit, a complex processing centre that produces the all-important navigation signal.
This signal goes through an amplification system and is converted to the three frequency bands over which Galileo will work.
Finally, a planar array antenna sits on the side of the box ready to transmit the signal to Earth.
In the Astrium cleanroom, the payload is attached to a moveable work platform and is surrounded by a bank of computer monitors. Each step of the integration is accompanied by another round of testing.
The clocks' temperature is carefully controlled to ensure there is not more than a one degree deviation from the optimum.
And the cleanroom regime ensures also the payload is protected from electrostatic, magnetic, and particulate damage.
"The preparation time for this spacecraft will be under 30 months from kick-off to launch - which is incredibly fast. What we're doing is more like a completely new scientific satellite and the timescales involved would usually run to many years," says Gordon Robertson, the payload engineering manager.
THE GALILEO PROJECT
Europe's own global satellite navigation system
30 satellites in three medium-Earth orbits (23,600km)
Will work alongside US GPS and Russian Glonass systems
Promises real-time positioning down to under a metre
Performance improvements should see sat-nav expand into many more mobile devices
Suitable for safety-critical systems - can run trains, guide cars and land planes
"The last few weeks have been very intensive. We've achieved so much in such a short period of time that we are wary of something unexpected happening.
"We're well aware that if we get a failure now or some mystery behaviour that we cannot isolate, it could delay us for many, many weeks which is costly and also puts at risk the slated launch schedule."
Soon, the box will be despatched to Rome, where engineers from Alenia Spazio will bolt it to a spacecraft chassis ready for launch from Kazakhstan on a Soyuz rocket.
Known by the uninspiring name GSTB-V2/B, the satellite will sit in a medium-Earth orbit (MEO), at 23,600km (14,600 miles) above the Earth.
This will be a first for Europe - it has previously only flown spacecraft in low or high (geostationary) orbits.
And that brings another complication. Satellites in MEO face particular radiation challenges and one of the major tasks of the test satellite will be to report back on the space environment.
A lot may seem to riding on GSTB-V2/B - but the Galileo project is spreading the risk.
Another Galileo Test Bed Satellite (GSTB-V2/A) is being built by Surrey Satellite Technology Limited (SSTL) at Guildford, a little under 80km (50 miles) down the road from Portsmouth.
Although not as big as the spacecraft being produced by Astrium and its partners in the Galileo Industries consortium, the Surrey test bed will nonetheless get to run the rule over a number of the key technologies.
It may even be the first to ride the Soyuz that has been booked for December. If that does happen, the Surrey satellite will perform the essential task of filing the first frequencies.
SSTL's test bed may yet get the nod on Galileo Industries' satellite
The International Telecommunication Union, which oversees the radio spectrum, has told Europe it must start using the space allocated for Galileo by mid-2006 or risk losing it.
"It's crucial we make sure there's a European satellite up there transmitting navigation signals because that will then secure our ownership of the frequencies," explained Professor Sir Martin Sweeting, SSTL's chief executive.
His company was chosen to produce a parallel satellite because of the expertise it has developed in putting together smallish spacecraft in tight timeframes.
The European Space Agency will not say publicly which of the two test beds will fly first, but either way it reflects well on the state of the British space industry that it should be asked to prepare both payloads.
"Galileo is an incredibly important system for Europe and the UK is playing a very large role," said Professor Sweeting. "It's always good if you've got some competition on these big projects, and having some friendly rivalry sharpens your approach.
"It's good for us and Astrium - and it's also extremely good for the European tax-payer because it does mean you are more likely to get an efficient and cost-effective approach."
Gordon Robertson added: "When you step back from the nitty-gritty of the day-to-day work, you are aware that this is one of Europe's greatest space projects. And it's also about having to work so closely with companies all over Europe.
"What we're doing may be a large chunk of it but it's very much dependent on what people are doing in Germany, France, Italy and elsewhere.
"We're teleconferencing every day, we're flying to meetings, sending e-mails - and pulling all that together is a challenge in itself."