The launch of the first satellites in Europe's version of GPS has been delayed.
The flight to orbit of the two Galileo spacecraft on a Russian Soyuz rocket from French Guiana was due to occur early on Thursday morning.
However, with just under three hours to the scheduled lift-off, the call was made to put back the launch by at least 24 hours.
The cause was an anomaly detected during the fuelling of the rocket.
This related to a leaking valve on the pipes feeding propellant to the vehicle's third stage. Engineers are now working to replace the valve.
"We don't yet know how long the delay will be but if the launch occurs on Friday, it will be at 07:30 [local time (10:30 GMT; 11:30 BST)]," said a spokesman for the European Space Agency. "We'll know more later today," Franco Bonacina told BBC News.
The mission will be the first for Soyuz in French Guiana. The rocket normally flies from northern Russia, and from Kazakhstan - the famous Baikonur Cosmodrome.
A new, half-a-billion-euro launch complex has been constructed for Soyuz in the South American jungle, about 10km up the coast from the launch pad of Europe's Ariane rockets.
By launching closer to the equator than its traditional pads, the Soyuz can get a bigger boost from the Earth's rotation, meaning it can put heavier payloads in orbit.
Long programmeThe launch, when it does happen, will see two satellites placed in a 23,222km-high orbit.
A full rollout to incorporate some 30 satellites will probably take most of the decade and cost European taxpayers well in excess of 5bn euros.
Compared with the Americans' current version of GPS, Galileo carries more precise atomic clocks - the heart of any sat-nav system. In theory, the data transmitted by Galileo should therefore be significantly better than its US counterpart. Whereas a position fixed by the publicly available GPS signal might have an error of about 10m, Galileo's errors should be on the scale of a metre or so.
EADS Astrium's James Hinds shows Pallab Ghosh inside a Galileo satellite
But the plan is to make both systems interoperable, meaning the biggest, most obvious benefit to users will simply be the fact that they can see more satellites in the sky.
So, as the decade progresses and the number of spacecraft in orbit increases, the performance of all sat-nav devices should improve. Fixes ought to be faster and more reliable, even in testing environments such as big cities where tall buildings will often obscure a receiver's view of the transmitting spacecraft.
Few people perhaps recognise the full extent of GPS usage today. Sat-nav is not just about drivers trying to find their way on unfamiliar roads - banks employ GPS time to stamp global financial transactions; and telecommunications and computer networks are synchronised on the "ticks" of the satellites' atomic clocks.
Galileo should have been operational by now but the project has run into myriad technical, commercial and political obstacles. Its biggest crisis occurred in 2007 when the public-private partnership set up to build and run the system collapsed. The project was very nearly abandoned at that point.
- Like Baikonur, Sinnamary has a large flame bowl under the pad
- A key difference is the mobile gantry, withdrawn prior to launch
- Soyuz receives a big boost by launching closer to the equator
- Rockets are brought to the pad along a 700m-long rail line
- The segments of a Soyuz are assembled in the MIK building
- Launch control is just 1km from the pad, in a secure bunker
- Other buildings on the 120ha site include propellant storage areas
Source: http://www.bbc.co.uk/go/rss/int/news/-/news/science-environment-15372540
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