When asked about his choice to devote himself to research, Sébastien Fernandez responds with humility: “I do not have training as a theoretician. I wanted to continue doing engineering while being close to researchers, physicists, and fundamental physics. » At 31, this ISAE-Supaero graduate swapped the optical systems of the industrial company Sodern for a three-year research allowance. A doctoral student affiliated with the Time and Space Laboratory of the Paris Observatory, he works in the CNES premises in Toulouse. His thesis is co-funded by the French space agency and the Defense Innovation Agency of the Ministry of the Armed Forces (AID).
“I already had a little history with CNES”
In 2022, when Sébastien decided to get started, it was quite natural that he explored the CNES calls for contributions platform. “I already had a little history with CNES, because I did two internships there,” he explains. Indeed, in 2017, Sébastien is responsible for the optical instrument of the EyeSat nanosatellite. A year later, he flew to the Guiana Space Center and discovered the satellite preparation campaigns.
Fascinated by space optics and the observation of the Earth and the Universe, he found the appropriate research subject. “The challenge of my thesis is to compare the beating frequency of two optical clocks approximately 100 km apart. I am building a laser communication system that connects them, via an airborne relay,” explains Sébastien Fernandez.
Embarked aboard a helium balloon flying 300 meters above the ground, its system retransmits a laser whose frequency is tuned to one of the clocks. “It is a very concrete application of Einstein’s theory of relativity which predicts that the flow of time varies according to the altitude of the clock which measures it. » Noting this variation in rhythm at different points on the globe will make it possible to map the height of the ground and the relief of a given area, with unequaled centimeter precision.
Prepare the continuation of the Pharao project
His work foreshadows the launch into orbit of the generation of optical space clocks that will take over from Pharao. Since April 2025, this clock has been sailing aboard the International Space Station. Its frequency is constantly compared with ground reference clocks. “The laser link on which I am working will make it possible to further refine the already exceptional measurement of this variation in rhythm,” he says. On the Aire-sur-l’Adour site in Landes, the researcher took advantage of the CNES facilities to test his balloon. In Toulouse, two astronomical domes were made available to him to experiment with his optical link. “I have a lot of support and advice from the opticians around me and my two supervisors. »
As the writing of his manuscript draws to a close, the doctoral student takes stock of his three years of thesis. “We have a lot of resources and freedom of mind to test things. I learned to work in a very open framework, without knowing exactly where it will lead,” he observes. Research as a space for experiments, far from the formal specifications specific to the industry. “This requires rigor, because we must be able to report on our progress. This is an opportunity to learn and develop a very valuable work methodology. » After his defense, Sébastien Fernandez would like to continue his career as an engineer within CNES.
