Antonianna, Lisa, Kimberley and Tijmen – those are the names of the four Galileo satellites launched on 17 November 2016, at precisely 14:06 CET, for the first time with a specially adapted version of the European heavyweight carrier Ariane 5 from the European spaceport in French Guiana. Until now, a Soyuz rocket had brought the more than 700-kilogram navigation satellites to their orbits at an altitude of 23,222 kilometres – two at a time. However, the more powerful Ariane 5 can transport four satellites at a time. This means that 18 satellites will now be in orbit and the European Commission can now provide the first Galileo services. "This includes the open service with which citizens can, for example, use navigation devices to determine their position more precisely than ever before. A limited search and rescue service should also be available," explains René Kleeßen, Galileo programme manager for the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) Space Administration. However, commercial services have yet to begin. Only when 30 satellites orbit Earth in 2020 – 24 operational and six as possible backups – will the Galileo system be fully functional.
New launch capabilities keep Galileo expansion on track
To ensure that Europe's satellite navigation system progresses rapidly, two further Ariane 5ES launches are planned. "This is significantly accelerating the expansion of the system and ensuring that we can now meet the scheduled date – an important step as manufacturers worldwide are waiting to install their new chipsets into smartphones and navigation receivers and bring them into the market. The start pistol has now been fired for the 18-satellite constellation," Kleeßen praises the new launch capabilities of Ariane 5. Here, they have reverted to the 5ES rocket version fitted with the re-ignitable EPS upper stage and its Aestus engine – built by Airbus Safran Launchers (ASL) in Ottobrunn. "These re-ignitions are important, as the Ariane 5ES, unlike its sister ECA, will deploy the satellites this time in a circular orbit, thus making a further ignition at the furthest point of the flight path itself," explains Denis Regenbrecht, who is responsible for the Ariane programme at the DLR Space Administration. After the first 11-minute ignition and the subsequent, more than three-hour coasting flight phase, Aestus was switched on again for six and a half minutes. At the end of this flight phase, the satellites were released at an altitude of 22,900 kilometres. The Galileo satellites will use their own propulsion to reach their target orbit while the EPS upper stage falls into a graveyard orbit after emptying its tanks and reducing the pressure.
Put through its paces
In order to ensure these manoeuvres go smoothly, the Aestus engine was tested for the current flight in the European Space Agency (ESA) P4.2 high-altitude test rig at the DLR site in Lampoldshausen with a total of eight ignition tests and a long duration test, followed by a re- ignition. Since the engine was newly produced for the Galileo launches, a so-called 'Ariane Research and Technology Accompaniment' (ARTA) campaign ensured this flight acceptance. For this purpose, a total of 115 ignition tests were carried out, in addition to a test profile with a long heat flow and a brief re-ignition after a simulated, propulsion-less free-floating phase.
Finally, four further long duration tests followed with four re-ignitions. "We have really tested this engine thoroughly for its new mission before it fulfils its new task for the Galileo programme," explains Stefan Schlechtriem, Director of the DLR Institute for Space Propulsion in Lampoldshausen.
Further Ariane conversions
The Ariane 5ES has already been successfully used five times to supply the International Space Station (ISS) with the Automated Transfer Vehicle (ATV). But this European space transporter weighed in at 20 tons – a real heavyweight. By contrast, the four Galileo satellites each weigh only about 700 kilograms each. This difference in weight resulted in major conversion measures. It was therefore decided in 2012 to modify this Ariane version so that it could carry four Galileo satellites and, in addition to the Soyuz rocket, be a further possibility for launch.
However, these changes posed a major technical challenge for the engineers. "It was really difficult to prove that the Galileo satellites would be able to withstand the launch loads in a four- pack on the flight. For this, they had to undergo additional qualification tests," explains Regenbrecht.
Four instead of two – a challenge for the ground station?
Although the entry of four satellites into their target orbits with Ariane is no more difficult than two with Soyuz, the Galileo ground team at the DLR site in Oberpfaffenhofen must simultaneously accomplish double the workload. "As with all past Galileo launches, DLR GfR is responsible for the commissioning of the main components and the final, high precision positioning of the satellites. In this case, we are dealing with four at a time. This is new and means parallel operations for which our expert teams are optimally prepared in the control rooms due to their routine," emphasizes Walter Päffgen, Managing Director of the DLR subsidiary DLR GfR mbH.
Galileo's future with Ariane 6
In the future too, new Galileo satellites will have to start replacing their predecessors. From 2020 onwards, the light A62 version of the new European Ariane 6 carrier rocket will be able to take over this important task. It is designed for these launches and can bring two Galileo satellites to their target orbits without further modification. "We need the quadruple launches for the rapid expansion of the system. In the case of later replacement of the satellites, however, this is no longer appropriate since the new satellites would always be released at exactly the same orbit altitude. It is rather unlikely that four satellites at the same orbit altitude will fail simultaneously," emphasizes Kleeßen.