PROBA3 is a space mission of the European Space Agency (ESA) having the main objective to verifying and validating several metrology and autonomous control systems for a formation flying of two independent satellites. PROBA-3 will operate in a high elliptical orbit, with perigee at 600 km and apogee at 60000 km.

Once the formation is acquired, the two satellites build a giant externally occulted coronagraph, named ASPIICS, with the telescope on one satellite and the external occulter on the other one, placed at the inter-satellite distance of 144m. The formation is maintained over 6 hours long transit across the apogee and during this period several resizing (relative approach and departure) and retargeting (off-pointing and repointing) operations are performed in order to confirm the effectiveness and efficiency of the metrology control systems and algorithms.

The observation of the Sun’s Corona in the field of view [1.08; 3.00] RSun is the scientific tool to confirm the alignment of the formation.

Thanks to the occultation geometry that allows to minimize the diffraction, PROBA3 is expected to return multi-band images of the Corona at high spatial and temporal resolution, very close to the solar limb as never done before.

Once the operations are completed, the formation breaks and the two satellites reach a relative safety position in order to prepare for the transit at the perigee, and then being launched to the next orbit.


INAF – Astrophysical Observatory of Turin is responsible for the design, development, manufacturing, qualification and acceptance tests of the SPS – Shadow Position Sensors metrological system, one of the most accurate instruments on board the PROBA3 mission.

The SPS consists of a series of 8 SiPM (Silicon PhotoMultiplier) assembled, together with the control electronics, within a mechanical flange specially designed to be installed in front of the ASPIICS telescope. The SPS will be exposed to the coronal radiation through a set of pinholes placed on a circumference concentric to the telescope’s entrance pupil. During the operations, when the telescope’s door will be opened, the SPS will be able to monitor the symmetry of the shadow generated by the occulter on the ASPIICS’s pupil plane and, thanks to a specialized algorithm, to measure with a high accuracy (<0,5mm) any displacement of the shadow with respect to the telescope reference system. From this measurement, the algorithm will determine the relative displacement of the two satellites and will return to the on-board software the necessary correction to bring the formation back to its nominal configuration.

The involvement of INAF-OATo includes:

– The realization of the mechanical parts;

– Software development, testing and validation;

– Contribution to the design and debugging of the electronics;

– Qualification and acceptance test of the whole system;

– Contribution to the implementation of the data analysis pipeline for the Science Operation Center.

INAF-OATO is also involved in the development of the control software for another metrological system: OPSE- Occulter Position SEnsors. The OPSE is a system based on a triad of LEDs positioned at the center of the occulter surface facing the telescope, which are observed together with the Corona and that will also control the relative movement of the two satellites. INAF also supports the testing and validation activities for the OPSE.


Beginning of 2023


Silvano Fineschi Email: silvano.fineschi at

Davide Loreggia Email: davide.loreggia at