L12: Two new Galileo satellites on their way to duty

3, 2, 1, lift-off… This sequence often reminds us of astronauts departing into space, but with humanity increasingly supported by robotics and all kinds of computing, more and more often our semi-automated inventions are the ones being launched into space.

Just over a week ago, after months of training (known as qualification in terms of satellite preparations), two of our Galileo constellation satellites departed successfully on April 28th at 00:34:54.177, which put the satellites at usual launch stress in extreme thermal, vacuum, vibration and sound conditions, formerly qualified in the test chambers back at the facilities in ESTEC ( ESA Technology ), Netherlands.

After 3:19 hours of ascension, the satellites were released from the launcher dispenser, free-floating at an injection orbit (not their final operation orbit yet) with masterful accuracy. Bidding farewell to the upper stage and eager to start their boot sequence, both satellites provided telemetry to the LEOP control centre (GCC-D in Oberpfaffenhofen, Germany) at 03:54 UTC on Sunday, April 28th, 2024. For several minutes, the ground control teams observed the autonomous initialization sequence, holding their breath during the firing of thermal knives (fuse-like systems cut by temperature) that would free the folded solar panels. Although the batteries are fully charged at lift-off, the satellite requires solar illumination over the full extension of its panels to keep charging the batteries. This will ensure enough power over its lifetime for all subsystems to operate.

With both +Y and -Y axis panels deployed, the control team declared the achievement of the holding point at 04:17 UTC. However, the breathing part was not achieved at GCC yet: the solar panels still needed to rotate towards the Sun to maximize the power towards the batteries. At 04:48 UTC, the solar panels finished rotating, charging batteries at full capacity, and the LEOP team declared the breathing point. Time to celebrate on Earth!

Back in space, the two satellites were flying close to each other and spinning slowly, typical after release from a launcher, waiting for the teams on the ground to start their controlling role. EUSPA and ESA teams, side by side with their GSOp operations and OHB SE satellite providers, were carrying all kinds of operations ranging from measuring distances from ground stations to the satellites or adapting to signal variations due to spinning, to observing and mapping the telemetry of each satellite subsystem - giving the green light to the control team in the ground to command the satellite and start downloading buffered TM from the satellite boot-up.

As hours pass, the teams start testing the internal reaction wheels in the satellite, a pyramid of rapidly rotating discs that keep the momentum of the satellite, avoiding random movement caused by external conditions, and later control the satellite's rotation without the use of thrusting and precious propellant. Running up in steps of 50 rad/s up to a maximum velocity of 500 rad/s, and then back down to its standard velocity at 200 rad/s, the operators carefully commissioned the reaction wheels. In parallel, the redundant transmission and reception subsystem was tested, to ensure that the satellite will remain controllable.

Et voilà, after 2 days the satellite is ready to stop spinning and look at the Earth, but the Moon is in view, and the optimal conditions for locking on Earth require the Moon to hide behind Earth. Once the moment for each satellite arrives, the satellite slews to fix an axis towards the Sun, and starts rotating and scanning for Earth’s borders with its squared set of four infrared Earth sensors (IRES). Once each sensor has borders in view, and the same amount of Earth visibility, the satellite is locked on its nadir on Earth and stops rotating. Now the satellite is perfectly centred, not spinning anymore, and ready to transmit towards the Earth with optimal signal conditions.

Things are far from finished on the ground, and it's time for the satellites to say farewell to each other, as they operate in different positions in orbit. Once again, each satellite is rotated, once at a time, in order to align the thrusters to the orbital travel direction, and with drift start thrusting, the first major use of propellant, each satellite was sent towards one of the 16 possible slots in orbit, where we operate our satellites to provide navigation to our users on Earth.

Our teams have been meticulously dedicated to making a success out of each step, ensuring that each of the critical platform subsystems are working nominally.  Now, we can look forward to 2 months of carefully testing the satellite payload and its clocks, with further manoeuvres to finely allocate the satellite in its reached position, and looking forward to providing more and more navigation signals to our users on Earth.

Stay tuned, as our teams are working to bring more navigation signals and better availability. Don’t hesitate to visit our GSC website, for more information once these satellites are ready for usage, and to learn more about the technicalities of this #EUSpace project, and all the service updates and new opportunities for our private users, governments and industry in Europe and beyond.