PARIS — One of the four European Galileo positioning, navigation and timing satellites stopped transmitting on two of its three channels May 27 because of a sudden loss of power, forcing ground teams to shut down the satellite after putting it in safe mode while examining possible causes, European government officials said.

These officials said the anomaly, which as of July 3 had not been resolved, occurred in a matter of seconds May 27 and shut down the E1 signal first. That signal, which transmits Galileo’s Open Service, re-established itself almost immediately. But as soon as it was back in service, the two other channels’ power dropped and did not recover. The full satellite then was shut down by ground teams.

European Space Agency officials, having looked in vain for a cause since the incident occurred, said July 3 they planned to switch the satellite back on the week of July 7 to determine its status. They hope then to be able to gather clues as to what happened.

They said whatever the cause of the problem, it was not related to the satellite’s onboard atomic clocks, which do not affect the transmission strength of the satellite. “We still don’t know what happened, but we know it had nothing to do with the clocks,” one official said.

Officials said that because the four In-Orbit Validation satellites — launched two at a time in October 2011 and October 2012 — differ in both onboard power technology and manufacturer, the preparations for the next Galileo launch, set for late August, have not been suspended and are not expected to be delayed as a result of the problem.

Officials said they have been unable to find any event external to the satellites or onboard that could explain such a precipitous drop in transmitting power. The E1 signal, which was the first to be affected, returned to full power within less than 30 seconds, they said. Then signals five and six went out. They said nothing like this has happened before on any of the spacecraft.

The four 700-kilogram In-Orbit Validation satellites were built by a consortium led by Airbus Defence and Space of Germany, and Thales Alenia Space of France and Italy. The satellites were designed to operate in medium Earth orbit for more than 12 years. 

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Peter B. de Selding was the Paris Bureau Chief for SpaceNews.