By Jonathan Amos
Science reporter, BBC News
The Giove-B demonstrator was launched earlier this year
A test spacecraft for Europe's future sat-nav system is performing well after being taken offline for two weeks, the European Space Agency (Esa) says.
The Giove-B satellite was forced into a "safe mode" in which only essential power systems were kept running.
Esa engineers say the mode was adopted when the onboard computer registered an anomaly, but reports that it was caused by space radiation are speculative.
Project manager Javier Benedicto said such glitches were not unexpected.
The purpose of a test campaign was intended precisely to pick up any technical issues before the main satellite constellation was launched, he added.
"This is one of the reasons you fly such spacecraft," he told BBC News.
"We carry on Giove-B two independent radiation monitoring instruments, and there is analysis ongoing to make the correlation between the measurements taken by those instruments and the time when the event was detected; and therefore we are not able to draw any conclusions to that extent for the time being."
Giove-B contains the technologies that will be incorporated into the Galileo network when it becomes operational.
These include the atomic clocks which provide the precise timing that underpins all sat-nav applications.
Giove-B went into safe mode on 9 September. This is standard procedure for a spacecraft when an anomaly arises. The satellite halts its mission activities and concentrates on keeping its batteries topped up by ensuring its solar panels are properly aligned with the Sun.
Engineers put the spacecraft back in a stable, active mode on 24 September.
GALILEO UNDER CONSTRUCTION
A European Commission and European Space Agency project
30 satellites to be launched in batches by end of 2013
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
"We brought the satellite into nominal operations. All the functions are working well. The cause is under investigation. We have lots of data, lots of telemetry and we are currently investigating that," Mr Benedicto explained.
Giove-B, and its sister demonstrator Giove-A, are the first European missions to fly in the high-radiation environment of a medium-Earth orbit (MEO), at an altitude of 24,000km.
This region of space is known to have a high intensity of fast moving particles that can disrupt spacecraft electronics; and one of the objectives of the test satellites is to get a better understanding of the space environment in MEO.
But Mr Benedicto said the Giove mission so far had not seen any unexpected conditions.
"The MEO is particularly severe in terms of radiation, however we have taken the necessary precautions on this satellite to sustain that," he explained.
"This could have been a possibility but there is no evidence that this caused that parameter to go into that mode. Therefore we cannot confirm that, and we cannot draw any conclusions in that respect either."
Giove-A was unaffected by the event.
The demonstrators will be followed by 30 operational satellites and their systems will all be protected, so far as is possible, from space radiation.
Galileo is envisaged as a rival to - but technologically complementary with - the American GPS (Global Positioning System).
It is touted as a key high-technology venture for the EU.
Galileo is designed to improve substantially the availability and accuracy of sat-nav signals.
These signals already play a fundamental role - not just in navigation, but also in electricity distribution, in synchronising computer networks, and in stamping financial transactions.
Galileo's improved clocks are expected to deepen and extend this role.
The better penetration and guarantees of service promised by Galileo should also give many more entrepreneurs the confidence to build business plans around sat-nav, say supporters.
It is supposed to become fully operational by 2013.
Europe has already spent more than 1.5bn euros on the project. At the end of last year, ministers approved a further 3.4bn euros for the period 2007-2013 - although these funds cover not only Galileo but an associated programme known as Egnos, which is an "overlay" system of control and correction that improves the current GPS service received in Europe.