Galileo Satellite Left in Bad Orbit Reaches Operating Position
LUXEMBOURG — One of two Galileo navigation satellites placed into a useless orbit in August following a malfunction of its Soyuz-Fregat launcher has been successfully moved into a higher orbit with sufficient fuel to operate for 12 years, the European Space Agency said.
The second will be similarly moved by late December, and pending tests of their positioning, navigation and timing payloads, the two spacecraft are likely to be suitable for inclusion into the future Galileo navigation constellation, said Didier Faivre, ESA’s director for navigation.
Faivre said the first of the two already has sent signals to Galileo terminal users worldwide from the new orbit. While some adjustment to the Galileo network’s ground infrastructure may be needed, he said there is no reason both cannot fulfill most of the role they originally had been assigned.
Addressing reporters during the 20-nation ESA’s ministerial council meeting, where Galileo was not on the agenda, Faivre said the end result is the best possible scenario given where the satellites were dropped off by the Soyuz rocket’s Fregat upper stage.
The satellites were built to operate in a circular orbit some 23,000 kilometers in altitude. They are the first of 22 Galileo Full Operational Capability satellites, built by OHB AG of Bremen, Germany.
The Fregat malfunction, later determined to have been caused by a design oversight that allowed Fregat builders to place hydrazine fuel lines too close to super-cold helium lines, left the satellites in an orbit with a 26,000-kilometer apogee and a 13,800-kilometer perigee.
They could not perform a navigation function from that orbit, in part because they were so close to Earth that their sensors could not see a horizon and correctly determine their position. A second issue was the daily exposure to the Van Allen radiation belts.
Faivre said the priority had been to use the satellites’ ample propellant — 65 kilograms per satellite — to raise the perigee. This was done for the first satellite starting in late October and involved 10 firings of its onboard thrusters.
On arrival at the new orbit, the satellite had 15 kilograms of fuel left, enough for 12 years of operations given the stability of the orbit and the fact that the Galileo constellation does not require regular orbital maneuvering.
As the first Galileo satellites to be delivered by OHB, the two spacecraft are being put through thorough testing to resolve any issues before other spacecraft join them in orbit. Faivre said despite the exposure, over more than a month, to the high-radiation environment, the first satellite has shown no effects of radiation damage.
ESA and the European Commission, the executive arm of the 28-nation European Union and Galileo’s owner, are debating whether to return to the Soyuz-Fregat rocket for a launch in early 2015 or plan to launch four more Galileo spacecraft on a heavy-lift Ariane 5 rocket.
ESA has ordered three Ariane 5 rockets, each to carry four satellites, for the Galileo constellation, in addition to four more Soyuz vehicles. It is likely that further Galileo spacecraft will be ordered in 2015 as the constellation was originally designed to comprise 30 satellites in medium-Earth orbit.
Four Galileo In-Orbit Validation (IOV) satellites were launched in 2011 and 2012 aboard Soyuz rockets. Since then, one has been taken out of service following a problem that rendered it unable to use two frequencies.
A long investigation into the failure on the IOV-4 satellite is continuing, Faivre said. It now appears it may have been caused by a defective antenna.
All four IOV satellites use the same antenna design — the OHB-built satellites use a different design — but Faivre said indications are that the other three IOV spacecraft may not have the same problem. As a precaution, the three others have been in reduced-power mode while the investigation is ongoing.
“We are confident we can use them in operational mode within the specifications” of power supply, Faivre said. “What happened on this IOV-4 satellite is odd. We have part of an antenna [of the same design] being tested in our labs now.
“One of the possible root causes links the problem with the power emitted by the antenna. When we know more we’ll decide what to do with the other three. Since this event occurred in May and June, no more issues have arisen,” Faivre said.
While the E5 and E6 frequency bands are definitively out of service on the IOV-4 satellite, the spacecraft is still capable of broadcasting in the E1 Open Service band, which is used both by Galileo and by the U.S. GPS navigation system, and in the search-and-rescue band, Faivre said.