ESA's Proba-3 Mission Demonstrates Breakthrough in Precision Formation Flying

ESA's Proba-3 Mission Demonstrates Breakthrough in Precision Formation Flying
by Erica Marchand
Paris, France (SPX) May 14, 2025

For the first time, the European Space Agency's Proba-3 mission has achieved precise formation flying, maintaining millimetre-level alignment between two spacecraft in orbit without ground control for several hours.

The mission's two satellites, the Coronagraph and the Occulter, achieved a 150-metre separation in orbit, effectively acting as a single giant instrument. This alignment was the result of extensive engineering work, including efforts at the European Space Security and Education Centre in Redu, Belgium, where ESA engineers and industrial partners calibrated and tested the spacecraft.

The spacecraft maintain their relative positions using a sophisticated Visual Based System, including wide-angle and narrow-angle cameras on the Occulter that track LED markers on the Coronagraph. This innovative approach allows the satellites to autonomously manage their positions during each orbit, a significant advancement in satellite coordination.

Raphael Rougeot, a Proba-3 systems engineer, explains, "The formation flying is performed when the spacecraft are more than 50,000 km above Earth, where gravitational forces are weaker, reducing the propellant needed to maintain alignment." The ultimate aim is to position the two spacecraft so that the 1.4-m disc on the Occulter casts a precise 5-cm shadow onto the Coronagraph, enabling unprecedented study of the Sun's corona.

Achieving this required two key innovations. The first was the integration of the Fine Lateral and Longitudinal Sensor (FLLS), a laser-based instrument that provides millimetre-level positioning accuracy. Jorg Versluys, Proba-3 payloads manager, describes it as "a laser beam fired from the Occulter, reflected back by the Coronagraph's retroreflector, and detected to provide precise relative positioning."

The second breakthrough involved the use of a shadow position sensor, which relies on light intensity measurements around the Coronagraph aperture to ensure it remains in the shadow cast by the Occulter.

Damien Galano, Proba-3 project manager, highlights the precision achieved: "We are talking about millimetric accuracy in range and sub-millimetric in lateral position."

The mission, managed by ESA and led by Spain's Sener, includes contributions from 29 companies across 14 countries, including GMV and Airbus Defence and Space in Spain, and Redwire Space and Spacebel in Belgium. The Coronagraph instrument was developed by Belgium's Centre Spatial de Liege (CSL), with science data processing managed by the Royal Observatory of Belgium. Proba-3 was launched on 5 December 2024 from the Satish Dhawan Space Centre in Sriharikota, India.