PONTE VEDRA, Fla. — The European Space Agency on Aug. 20 said it has still not determined what caused a sudden power drop in May aboard one of the four in-orbit Galileo positioning, navigation and timing satellites and that it has dropped broadcast power on all four as a precautionary measure.

Agency officials said reducing power levels by 1.5 decibels on all four satellites will have no perceptible effect for Galileo system users.

Also unaffected will be the launch campaigns for the 22 Full Operational Capability (FOC) satellites that began Aug. 22 with the launch of two satellites aboard a Europeanized Russian Soyuz rocket from Europe’s Guiana Space Center spaceport on the northeast coast of South America.

Galileo ground controllers shut down one of the four in-orbit satellites following a sudden loss of power in May. Since then they have searching for a root cause of the incident, without success.

Javier Benedicto, ESA’s Galileo program manager, said during the briefing that Galileo teams have examined more than 40 different failure scenarios in the search for a cause, with no firm results thus far. The investigation has included asking satellite component builders to reproduce certain components for testing. It has also included tilting the affected satellite to assess radiation pattern changes on the spacecraft’s L-band antenna, he said.

Benedicto said the investigating team concluded that maintaining high broadcast power might exacerbate the problem, so all four satellites’ output was dropped by 1.5 decibels.

The May power loss was on the satellite named FM-4. An apparently unrelated power drop occurred in mid-2013 on the FM-3 satellite, Benedicto said, resulting in a 2-decibel decrease in power on one of the satellite’s signals.

The investigation into that early incident is also ongoing, Benedicto said. No root cause has been found, and the inquiry has now been merged with the search for a cause for the FM-4 failure.

Another anomaly, also apparently unrelated to the others, occurred on the FM-1 satellite. This was subsequently tied to a failure on the satellite’s solid-state power amplifier. Benedicto said that in the coming weeks ground teams would switch power to the backup system.

So far, Benedicto said, there appears to be no common thread that ties together the various problems, but it could be that there is a relationship between the anomalies on FM-2 and FM-4.

“We backed off 1.5 dB [decibels] on all four spacecraft about a month ago,” Benedicto said. “This was done as a precautionary measure while we are continuing to investigate, before we come to any conclusions. This is not causing any degradation of mission performance, as this was a margin we had after [the satellites’] launch. It would not be perceivable by users at this point.”

Benedicto said he remained confident that despite coming up empty-handed after more than three months of work, the ground team ultimately will determine the cause. The satellites are designed to operate for 12 years and ESA expects them to reach that target, he said.

He conceded that it is unlikely that full power on two of the FM-4 satellite’s frequency bands, called E5 and E6, will ever be restored. Broadcasts from the E1 signal, which includes Public Regulated Service for government and military users, can be restored at any time following the current test campaign.

“All of this takes time,” Benedicto said. “Reaching a conclusion is crucial for us to be confident in the performance of all four IOV spacecraft for the coming years.”

Peter B. de Selding was the Paris bureau chief for SpaceNews.