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Euro space agency's Galileo satellites stricken by mystery clock failures

One in eight have stopped, admits org's director general

Europe's GPS-alike system Galileo is suffering a number of unexplained clock failures on its satellites, the European Space Agency has admitted.

Each of the 18 Galileo satellites currently in orbit carries four maser*-based atomic clocks. Two clocks are hydrogen-based, while the other pair use rubidium. The ESA says the clocks are accurate to within 1.8 nanoseconds over 12 hours.

Of the 72 Galileo clocks orbiting the Earth, nine have failed. No single satellite appears to have lost all its clocks, though the BBC noted that one has suffered two clock failures.

Only one aboard each spacecraft needs to be operational for Galileo to function as designed.

“Unfortunately we have right now the situation that we have failed rubidium clocks as well as failed hydrogen maser clocks,” ESA director general Jan Woerner told a press conference in Paris this morning.

Although one hydrogen clock in orbit had reportedly been restarted, this leaves five more in a failed state. Three of the nine failures occurred on the rubidium clocks.

A page from the ESA website dating back to 2001 identifies the original makers of the rubidium clocks as the Swiss Cantonal Observatory of Neuchatel, which appears to no longer be a functioning scientific institute.

ESA techies in the Netherlands are scratching their heads over the clock failures. The agency is wondering whether to continue launching new satellites with the clocks aboard, or to halt launches until the problem has been traced and cured. The first Galileo clocks were launched into orbit in 2006 aboard the GIOVA-A satellite, according to the ESA, with a rubidium clock winning the honour of being the first one activated by Galileo engineers.

According to the Beeb, India’s space agency uses the same clocks and has not experienced the same problems.

Galileo needs 24 satellites, operating as a constellation, to function. Thanks to attrition in service, spare sats are required at regular intervals. The clocks aboard the Galileo birds are regularly synchronised with ground-based reference atomic clocks, and the ESA says the reference system is accurate to within 28 billionths of a second. ®


* Maser: “microwave amplification by stimulated emission of radiation”. A device that emits microwave radiation in a very precise manner, similar to how a laser emits light. The ESA says: “If you force atoms to jump from one particular energy state to another, it will radiate an associated microwave signal at an extremely stable frequency.” Stanford University has a more detailed explanation of the physics behind it all.

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