Four Galileo satellites are on one Ariane 5

11 Nov 2016 Europe’s next four Galileo navigation satellites are together on their dispenser atop the Ariane 5 launcher due to launch them next Thursday from Europe’s Spaceport in French Guiana.

Having been attached to their dispenser as a combined ‘upper composite’, the four 715 kg satellites were transported to the final assembly building on 31 October.

The next step saw them put on top of the upper stage of their customised launcher. Finally, on 3 November, the quartet was enclosed within a protective fairing – the last time they were seen by human eyes – to protect them from the onrushing atmosphere during ascent. In the following week, the rocket was prepared for takeoff, including filling its cryogenic upper stage, in preparation for the move to the launch pad on Tuesday, 15 November.

In development since 2012, this new Ariane variant has evolved from the version used to place ESA’s 20 t Automated Transfer Vehicle vehicle into low orbit. The new launcher has to carry a lighter payload but needs to take it up to the much higher altitude of 22 900 km. The target orbit is actually 300 km under the Galileo constellation’s final working altitude. This leaves Ariane’s upper stage in a stable ‘graveyard orbit’, while the four Galileos manoeuvre themselves up to their final set height.

The decision to proceed with this launch was taken after in-depth analysis of two recent anomalies occurring in rubidium atomic clocks aboard Galileo satellites in orbit. Highly accurate timing is core to satellite navigation. Each Galileo carries four atomic clocks to ensure quadruple redundancy of the timing subsystem: two Passive Hydrogen Maser clocks plus two Rubidium Atomic Frequency Standard (RAFS) clock.

Investigations jointly conducted by ESA and industry point to a short circuit in these two RAFS units. The ultimate objective of the fourfold redundancy on each satellite is to ensure operations over their planned lifetime. Taking into account the redundancy and considering that the upcoming launch will increase the constellation’s overall robustness,it has been decided to maintain the mid-November launch date.

Once this latest flight is complete, there will be 18 Galileo satellites in orbit – the single largest increase of any navigation satellite constellation from a single launch.

This will mark the first time that ESA deploys four satellites simultaneously. Usually, simply shepherding a spacecraft through the first critical days in orbit is a demanding enough task. A combined team from ESA and France’s CNES space agency based in Toulouse, France, will make contact, establish control and then see the quartet through its initial, critical, activities.

Two further Ariane 5 flights are planned for Galileo during the next two years, one each for the constellation’s remaining orbital planes.