Updated Dec. 13 at 10:03 a.m. Eastern.
WASHINGTON — Arianespace performed its eleventh and final launch of the year today, sending four Galileo European navigation satellites into medium Earth orbit.
The launch, which took place at 1:36 p.m. Eastern from the Guiana Space Centre in French Guiana, brings the Galileo constellation to 22 satellites, though four are not currently in use.
Arianespace used an upgraded version of the Ariane 5, called “Evolution Storable” or Ariane 5 ES, modified further to support the deployment of Galileo, including enhancements for a nearly four hour long “ballistic” or non-propulsive transport phase of the mission.
Only one other Ariane 5, also an ES variant, has been used for Galileo. The rest all used Soyuz rockets to launch pairs at a time.
The European Commission has one more Ariane 5 launch of another four Galileo satellites planned for July 2018 to complete the constellation, ensuring complete availability of the European Global Navigation Satellite System (GNSS).
No more clock issues
The four new satellites all sport refurbished atomic clocks to prevent a repeat of the malfunctions on older satellites. Paul Verhoef, the European Space Agency’s director of navigation, said investigations into the clock problems identified the causes, and additional corrective steps followed “in order to make sure that we keep the clocks alive as long as we can.”
“We are very confident that these measures are very good,” he said during a Dec. 11 call with reporters, adding that though there are still risks, confidence abounds that the fixed clocks will work as planned. “For us the issue of the clocks are behind us.”
Each Galileo satellite carries four atomic clocks, of which not all are identical to prevent the risk of single-point failure. Verhoef said the current constellation, which began service in December last year, is “doing very well,” despite previous clock issues. The next four Galileo satellites will also use refurbished clocks, he said.
Though Galileo is designed to bring autonomous GNSS capabilities to Europe, the constellation is most accurate when paired with the U.S. Global Positioning System (GPS). Verhoef said the combination of GPS and Galileo “will allow an accuracy of around 30 centimeters,” but declined to state the accuracy of Galileo as a standalone system.
European GNSS Agency (GSA) Galileo service program manager Rodrigo da Costa called Galileo’s standalone accuracy “theoretical” because the majority of users (smartphones and other devices) will have access to several constellations.
“It’s not mutually exclusive — multi-constellation and having a fully autonomous system,” added Christoph Kautz, deputy head of unit, Galileo and the European Geostationary Navigation Overlay Service, EGNOS – Applications, Security, International Relations, at the European Commission. “We are following different goals. One is the strategic goal of being autonomous, and that we will achieve, and then we have also the goal to provide benefits to the user, and this we provide interoperability, for example, with GPS.”
While Galileo officials coordinated with other GNSS systems such as Russia’s Glonass and China’s Beidou, Kautz said the European Commission forged a unique international agreement with the U.S. for interoperability between Galileo and GPS.
Welcoming the wayward back
Two Galileo satellites launched into an incorrect orbit by a Soyuz in 2014 will likely be reintegrated into the constellation next year, Verhoef said, despite being in an elliptical orbit. Da Costa said the two satellites have undergone “extensive testing” the past few months, and can still contribute to the European navigation system.
“We will of course choose the right time to bring them into operational mode such that they can be of advantage for the users,” Da Costa said.
Verhoef said the two elliptical satellites have the same 12-year life expectancy as the other Galileo satellites.
“After the extensive testing in orbit, we haven’t detected any significant degradation of the electronic components or the solar array cells, despite the fact that they have been through the Van Allen radiation belts more than the other satellites,” said Javier Benedicto, ESA’s Galileo program manager.
However, because adjusting their orbits required the use of additional propellant, they will not deorbit the same as others in the constellation. Benedicto said ESA will need to design a unique graveyard orbit for them.
The two elliptical satellites will bring the total number of operational Galileo satellites to 24. That number would have been 26, but one of the first Galileo satellites has an antenna problem, and ESA chose to put one satellite in a spare mode, Verhoef said.
Galileo contractors are already poised to start on 12 “Batch 3” satellites, for which Bremen-based OHB Systems AG continues as the prime contractor and Surrey Satellite Technology Limited in the U.K. stays as payload provider. Incumbent supplier Ruag Space will also continue in providing control and data units. The first four of those satellites are scheduled to launch two at a time on Ariane 62 rockets in 2020 and 2021.