Page last updated at 12:14 GMT, Wednesday, 30 April 2008 13:14 UK

Giove-B 'healthy' despite glitch

Giove-B (Esa)
The satellite achieved its correct orbit

A demonstrator satellite for Europe's proposed Galileo satellite navigation system is "in good health" despite a glitch shortly after launch.

Controllers were unable to use their preferred method for turning the spacecraft and instead had to fire the thruster engines - which wastes fuel.

However, they have now been able to correct the problem by uploading a software patch to the spacecraft.

The Giove-B satellite was launched atop a Russian Soyuz rocket on 26 April.

The demonstrator will test key technologies which will eventually be built into the 30 operational platforms forming the Galileo network.

These include the atomic clocks which provide the precise timing underpinning all sat-nav applications.

A European Space Agency (Esa) official told the BBC the satellite's commissioning phase was now progressing well and that the satellite was expected to send its first signals either next week or the week after.

Pointing at the Sun

After being carried to the correct orbit by the rocket's Fregat stage, the satellite's first task was to deploy its solar arrays and point them towards the Sun. This allows Giove-B to begin charging its batteries.

The preferred method for turning the spacecraft to achieve "Sun-pointing" is to use four gyroscope-like wheels driven by electric motors. Altering the speed of rotation of these "reaction wheels" allows the satellite to be rotated in space.

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

There is only limited time in which to do this, since the spacecraft has to operate on battery power from just before lift off.

But controllers at Italy's Fucino ground station found the reaction wheels were slow to respond, meaning they were unable to achieve stable Sun-pointing within the specified time period.

Instead, about five hours after launch, controllers placed the spacecraft in "safe mode" and used the small thruster engines to rotate Giove-B, using up some of the satellite's finite store of propellant.

"This happened at the beginning of the mission, after separation, and it caused some worries. But we have experienced similar situations with other satellites in the past," said Dominique Detain, a spokesman for Esa.

He explained that the satellite was otherwise in excellent health: "We are commissioning the satellite step-by-step, but so far so good," he said.

Giove-B - a 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.

On the first mission, technicians had to work round the clock to get the satellite commissioned in order to hit a deadline for claiming the frequencies Galileo will use to transmit its signals to receivers on the ground.

Good timekeeping

This time around, there is no such urgency. The second mission flies a spacecraft which is, to a large degree, a template for the 30 operational platforms that will follow.

A fundamental focus for Giove-B will be the in-orbit behaviour of its 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 its atomic clocks (Giove-B carries three), the latest demonstrator will test the generation of signals across the full spectrum 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; 1.3bn) on the project, and ministers have warned that the additional 3.4bn euros ($5.3bn; 2.7bn) 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.

It is designed to improve substantially the availability and accuracy of timing signals delivered from space.

Full capability has been set for the end of 2013.

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A guide to the main features on Giove-B




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