Galileo satellites atop Europeanized Russian Soyuz rocket. Credit: ESA/CNES/Arianespace

PONTE VEDRA, Florida — 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 reduced broadcast power on all four as a precautionary measure.

As they prepare for the planned Aug. 21 launch of the first of 22 fully operational Galileo spacecraft, agency officials said reducing power levels by 1.5 decibels on all four satellites will have no perceptible effect on Galileo system users.

Also unaffected will be the launch campaigns for the 22 Full Operational Capability (FOC) satellites that begin Aug. 21 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.

In a conference call with journalists, ESA’s director of navigation programs, Didier Faivre, said the agency had concluded negotiations with launch services provider Arianespace for the procurement of three Ariane 5 heavy-lift rockets, each to carry four 700-kilogram Galileo satellites into medium Earth orbit.

Faivre said the contract, to be signed later in the day, is valued at slightly more than 500 million euros ($681 million) for launches to occur between 2015 and 2017.

Galileo is designed as a constellation of 30 satellites, including in-orbit spares. The 22 FOC satellites ordered from OHB AG of Bremen, Germany, are expected to be in orbit by 2017. The 20-nation ESA must first validate the production rhythm and confirm, with Arianespace, launch slots aboard medium-lift Soyuz and heavy-lift Ariane 5 vehicles.

The four satellites needed to complete the 30-satellite constellation have not yet been ordered. Faivre said Galileo’s owner — the European Commission — has all the financing it needs to procure the final four satellites but that the commission and ESA are in no rush to conclude a contract.

With more than two years of satellite construction and launches already booked, ESA and the commission have time to determine whether any modifications are needed to the current Galileo design before ordering the final four satellites.

Europe has never before produced such a large constellation of spacecraft. Once completed by OHB, the spacecraft are sent for testing at ESA’s Estec technology center in Noordwijk, Netherlands, before being shipped to the French Guiana spaceport.

Faivre said ESA is struggling to fit the Galileo launches into an already jam-packed Arianespace manifest and that the dates for the 2015 missions had not yet been finalized. ESA and Arianespace have agreed that following the Aug. 21 launch, a Soyuz vehicle would deploy two more Galileo satellites sometime in December.

Galileo ground controllers shut down one of the four in-orbit satellites following a sudden loss of power in May. Since then they have been 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.

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 incident is 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 anomaly.

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.

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