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Europe's Newest Global Positioning Satellites Are in the Wrong Positions

The ESA finds itself with an unfortunate salvage mission.
Image: Galileo In-Orbit Validation satellite/ESA

It's been a rough week for the European Space Agency. An anomaly with the upper-stage of a Soyuz rocket meant two Galileo satellites didn't quite make the trip into orbit. Mission scientists have been deep in problem solving mode ever since, trying to figure out how to salvage what they can of the mission.

The ESA's Galileo mission is Europe's own global navigation satellite program, akin to and interoperable with America's Global Positioning System (GPS) and Russia's Glonass. The European network transmits in dual frequencies, which means that it can give users real-time positions with an accuracy up to three feet. It's also an extremely reliable network designed to stay online through most natural disasters and adverse weather. It's even good enough to warn users within seconds if one of its satellites goes offline, making it a great network for critical applications like managing commercial transportation.

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The first pieces of the Galileo system were launched on October 21, 2011; these were the first two operational satellites designed to validate the concept. A second pair followed on October 12, 2012. Now, additional satellites are filling out the system to bring it to a properly operational level. Fully deployed, it will consist of some 30 satellites, 27 operational ones and three spare units, orbiting nearly 14,430 miles above the planet on a path inclined 56 degrees to the equator. (For comparison, the US GPS system uses 32 satellites, of which eight are tasked with making redundant measurements.)

But two ESA satellites didn't quite make it to that orbital height or inclination last week. On August 22, a Soyuz rocket launched normally and released the two satellites on schedule, but when the ESA started tracked the pair it was clear from telemetry that they hadn't gotten to where they were supposed to go. Rather than the planned circular, 18,580-mile orbit inclined 55 degrees from the equator, the satellites ended up in an elliptical orbit 16,280 miles at its highest point inclined 49.8 degrees from the equator. So: not great.

Image: 30-satellite Galileo constellation/ESA

Initial analysis found the problem was rooted in the rocket's Fregat upper-stage, a versatile and reasonably common Russian-built rocket component. And even though the orbit wasn't perfect, the ESA was at least in contact with the two satellites and had both fully under control. At no point did we risk annihilation as a species at the hand of these rogue satellites. What's more, although in the wrong positions, they were both in good health with their solar arrays deployed and systems at full power. ESA engineers, however, put both satellites into safe mode.

The Galileo team's attention then shifted to ways they might make the best of this bad situation; fixing an orbit isn't as easy as just firing some onboard thrusters to bump them into the right plane. All spacecraft launch with a fixed amount of fuel on board and this is fuel that, in the case of orbital missions, is usually reserved to maintain a set orbit after launch so the satellite can last as long as possible before its systems start shutting down. Using any onboard fuel reserves to adjust an orbit at the start of a mission essentially cuts the satellite's active lifetime way short.

The Galileo satellites are no exception. They have a small amount of simple and highly reliable monopropellant hydrazine on board. The purpose of this propellant system is to satisfy propulsion requirements of the nominal mission, basically nudging the satellites from time to time to keep them in the right orbit for as long as they remain functioning spacecraft. This propellant system uses eight thrusters clustered for redundancy in two independent branches that can be independently activated or isolated as needed. And the system as a whole can be used in blow-down mode or End Of Life mode to send the spacecraft back through the atmosphere.

But it's not clear whether there is enough propellant on board these Galileo spacecrafts to substantially change the orbit enough to correct the error and still have enough left to maintain the mission.

So a week later we're still in a sort of holding pattern. The latest from ESA says that the satellites remain in good working condition and flight controllers are ready to proceed to the next stage of the launch and early operations-phase activities. The team is also looking at ways to exploit the spacecraft to get as much use out of them as possible, and is starting to consider whether or not a recovery mission will be in the cards. Whatever happens, the space agency won't let these two spacecraft go down without a fight.