Proba-3 mission advances with autonomous orbital alignment

Proba-3 mission advances with autonomous orbital alignment
by Erica Marchand
Paris, France (SPX) Mar 30, 2025

Two ESA satellites have achieved a major milestone in their bid to fly in perfect unison, as part of the European Space Agency's Proba-3 mission. This week, the twin spacecraft succeeded in autonomously holding their formation while aligned with the Sun, remaining locked in position for several hours without input from ground control.

Inside the mission control center at ESA's European Space Security and Education Centre (ESEC) in Redu, Belgium, engineers monitored streams of telemetry, code, and real-time diagnostics from the orbiting pair. With focused attention, the Proba-3 flight team issued commands in carefully timed sequences to guide the spacecraft through critical maneuvers.

Proba-3 represents the first mission designed explicitly for ultra-precise formation flying. The two spacecraft-the Coronagraph and the Occulter-operate as a coordinated system, separated by 150 meters in their most accurate configuration. This arrangement mimics a single, large-scale space observatory.

During each orbit, the Occulter's 1.4-meter disc casts a pinpoint 5-centimeter shadow onto the Coronagraph's optical instrument. This shadowing allows the Coronagraph to observe the Sun's faint corona by blocking the intense solar light.

Although other missions use coordinated satellite formations, Proba-3 is the first to achieve and maintain millimeter-level precision in positioning for extended durations, all without external control.

Earlier in the week, ESA's Redu team began executing the first operational phase of this complex maneuvering. Through ground-initiated thrust commands, the spacecraft closed their separation from 600 meters to just 144 meters. Simultaneously, they aligned into the required orientation for the Coronagraph to enter the shadow zone.

From that point, onboard systems took over. The spacecraft assumed full autonomy in managing their formation.

"First, we used GPS information to determine the precise location of the two satellites in space. Then we commanded the thrusters to eject small amounts of propellant to get the spacecraft as close as possible to the desired formation, to about 144 metres apart," explained Proba-3 mission manager Damien Galano.

"Once the on-board autonomy was activated, the spacecraft measured and controlled their relative positioning using the Visual Based System, which consists of a wide-angle camera on the Occulter tracking a set of flashing LED lights on the Coronagraph, supplemented by a narrow-angle camera that enables a more precise positioning."

Proba-3 systems engineer Teodor Bozhanov added: "From here on, the spacecraft were maintaining their position autonomously, using the intersatellite link to exchange vital positioning information with each other."

"Through the link, the Occulter spacecraft can also send instructions to its partner. If the positioning software detects a misalignment, the propulsion system can make small adjustments to get the two aligned again. At this stage, we were not interfering, only monitoring."

"For the Coronagraph spacecraft to move into the shadow cast by the Occulter, the system uses a set of shadow-detecting sensors that are located around the coronagraph instrument. This allows the satellites to stay in one line with the Sun."

According to systems engineer Esther Bastida Pertegaz, the mission's success yielded a significant trove of performance data: "Over the past two days, we collected a wealth of data which we will now use to finetune the systems. In the coming weeks, we will do more testing to achieve the desired precision, making Proba-3 the world's first-ever precision formation flying mission."

Noelia Peinado of ESA's General Support Technology Programme (GSTP), which developed the onboard positioning systems, emphasized the novelty of the achievement: "The key words here are 'precise' and 'autonomous'."

Proba-3's cutting-edge hardware and software were developed as technology demonstrators to validate their performance for future space missions.

"It is the combination of all these instruments and software working together that make Proba-3 unique. A decade ago, none of these technologies were available - now we are about to accomplish what no one has before, enabling many more ambitious missions to come."