Giove-B, a test satellite for Europe's Galileo system, has sent its first navigation signals back to Earth.
The European Space Agency (Esa) says the transmission is an "historic step", showing that Galileo will be able to work alongside the US GPS system.
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The 30-strong Galileo fleet is intended to be fully operational by 2013.
The remainder of the satellites will closely resemble Giove-B, which was launched atop a Russian Soyuz rocket on 26 April.
The half-tonne, 2.4x1x1m box assembled by EADS Astrium and Thales Alenia Space is the second demonstrator satellite to go into orbit following the launch of Giove-A in 2005.
More sophisticated than its predecessor, it will test key technologies such as the atomic clocks that provide the precise timing underpinning all sat-nav applications.
Despite an early glitch in the system used for orienting the satellite in space, the orbiter now appears to be working well.
Under offer
"Now with Giove-B broadcasting its highly accurate signal in space, we have a true representation of what Galileo will offer," said the system's project manager Javier Benedicto.
The offer, he said, would include "the most advanced satellite positioning services, while ensuring compatibility and interoperability with GPS".
Engineers are now assessing the signal coming down to Earth.
They will be looking for any indications that it is affected by its passage through the atmosphere, and checking that it does not interfere with any systems that use neighbouring bands of the electromagnetic spectrum.
A fundamental focus in coming days will be the in-orbit behaviour of Giove-B's passive hydrogen maser clock.
It is the most stable clock ever put in permanent orbit, and is designed to keep time with an accuracy of better than one nanosecond (billionth of a second) in 24 hours.
As well as this and its two other atomic clocks, the demonstrator will test the generation of signals across the full spectrum that Galileo intends to use for its five sat-nav services.
The Galileo programme itself has seen its timeline slip on a number of occasions, and has come close to being abandoned.
Europe has already spent 1.6bn euros ($2.5bn; Ј1.3bn) on the project, and ministers have warned that the additional 3.4bn euros ($5.3bn; Ј2.7bn) recently approved for sat-nav investments will be the limit on expenditure.
Galileo is envisaged as being technologically complementary to GPS, and is touted as a key high-technology venture for the EU.
Esa says that compared with using GPS alone, the combined systems "will provide higher accuracy in challenging environments where multipath and interference are present, and deeper penetration for indoor navigation".
(BBC)
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