The second payload is just weeks behind the first in the schedule
Europe's Galileo satellite-navigation system has passed a major milestone in its development.
The payload for the first spacecraft in the operational constellation leaves its Portsmouth, UK, factory on Monday for final preparations in Italy.
Proto-flight Model 1 includes all the equipment needed to generate and transmit location and timing data to users on the ground and in the air.
The final works and testing in Rome will get it ready for launch next year.
"We should be ready for an April flight and this is a very significant step towards that achievement," said Dr Mike Healy, the head of navigation at EADS Astrium, the company contracted to integrate the components of the payload.
Proto-flight Model 1 will be part of the In-Orbit Validation (IOV) exercise that initiates the EU project.
Four spacecraft will be flown in a mini-constellation in the sky to prove the European system can deliver the promised performance.
The rest of the satellites, known as the Full Operational Capability (FOC) spacecraft, will then follow soon afterwards. An initial 14 FOC spacecraft have already been ordered, with many more expected.
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 to within one metre compared with the current GPS-only error of several metres.
How does sat-nav work? Professor Cathryn Mitchell from the University of Bath explains
The 1.5m-by-1.5m-by-3m box leaving Portsmouth represents the "brains" of a Galileo satellite.
It includes critical features such as the atomic clocks, the signal generation units, amplifiers and antennas. It is the part of the satellite that sends the "where" and "when" information to receivers in cars and mobile phones.
Model 1 is being despatched to Thales Alenia Space in Rome, Italy, where it will be attached to the main spacecraft bus, or chassis, which incorporates a propulsion system, avionics and solar panels, etc - the elements needed to maintain the satellite in orbit.
Flight Model 2 is just a few weeks behind Model 1 in terms of its development stage. The current schedule has the pair launching together on a Soyuz rocket in early 2011.
GALILEO UNDER CONSTRUCTION
A European Commission and European Space Agency project
At least 22 satellites to launch 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 Models have yet to have their security units fully installed. These electronics boxes are integral to the encrypted and highly restricted sat-nav service Galileo will eventually provide to government agencies.
There is also uncertainty surrounding the units designed to provide a search and rescue (SaR) service. The SaR transponders are a Chinese contribution to the Galileo project.
However, the SaR units may not fly if Brussels and Beijing fail to settle their ongoing technical and strategic differences over the development of Galileo and the Asian nation's own Compass/Bediou system.
Asked to clarify the present situation to BBC News, the European Commission declined to comment, citing "conformity with the programme security instructions". A spokesman for the EC said these instructions meant technical details about the Galileo satellites' configuration could not be disclosed.
If engineers have problems incorporating the security units, or are asked to remove and replace the SaR units, this could push the launch of Models 1 and 2 deeper into 2011.
Meanwhile, the annual European Satellite Navigation Competition (ESNC) has got under way.
Organised through 21 centres across the world, this competition seeks novel uses for location, navigation and timing data delivered from space.
Past winners in the UK region have ranged from systems to track sailors or marine workers lost at sea, to multi-player games that transplant virtual worlds on to real world locations using mobile phones.
The British sector of the ESNC is being run by the Grace institute in Nottingham, which has been set up to be a centre of excellence in sat-nav research, engineering and applications.
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