PROBA-3 formation-flying metrology: algorithms for the shadow position sensor system

Abstract

PROBA-3 ESA's mission aims at demonstrating the possibility and the capacity to carry out a space mission in which two spacecrafts fly in formation and maintain a fixed configuration. In particular, these two satellites-the Coronagraph Spacecraft (CSC) and the Occulter Spacecraft (OSC)will form a 150-meters externally occulted coronagraph for the purpose of observing the faint solar corona, close to the solar limbi.e. 1.05 solar radii from the Sun's center (R). The first satellite will host the ASPIICS (Association de Satellites Pour l'Imagerie et l'Interférométrie de la Couronne Solaire) coronagraph as primary payload. These features give to the PROBA-3 mission the characteristics of both, a technological and a scientific mission. Several metrology systems have been implemented in order to keep the formation-flying configuration. Among them, the Shadow Position Sensors (SPSs) assembly. The SPSs are designed to verify the sun-pointing alignment between the Coronagraph pupil entrance centre and the umbra cone generated by the Occulter Disk. The accurate alignment between the spacecrafts is required for observations of the solar corona as much close to the limb as 1.05 R.The metrological system based on the SPSs is composed of two sets of four micro arrays of Silicon Photomultipliers (SiPMs) located on the coronagraph pupil plane and acquiring data related to the intensity of the penumbra illumination level to retrieve the spacecrafts relative position. We developed and tested a dedicated algorithm for retrieving the satellites position with respect to the Sun. Starting from the measurements of the penumbra profile in four different spots and applying a suitable logic, the algorithm evaluates the spacecraft tri-dimensional relative position. In particular, during the observational phase, when the two satellites will be at 150 meters of distance, the algorithm will compute the relative position around the ideal aligned position with an accuracy of 500μm within the lateral plane and 500 mm for the longitudinal measurement. This work describes the formation flying algorithm based on the SPS measurements. In particular, the implementation logic and the formulae are described together with the results of the algorithm testing.

Feasibility study of an interferometric Small-Sat to study exoplanets

“…The next one, PROBA-3 is planned to be launched soon. 27 The size of all PROBA satellites is around 1 m 3 . With deployable arms, one can assume to achieve a baseline length up to 5 m.…”

“…In fact, formation flying was already partially demonstrated by the PRISMA mission 26 and will be improved by PROBA-3. 27,28 In Ref. 29, we have presented our first results.…”

Laboratory testbed for the calibration and the validation of the shadow position sensor subsystem of the PROBA3 ESA mission

“…The double gain processing, high gain (HG) ad low gain (LG), is needed to cover the very large dynamic range of the expected illumination (from umbra to full Sun illumination) and to satisfy the accuracy requirements that demands for a high sensitivity to very small flux variations. The digital readouts are transferred to the on-board software, where a dedicated metrology algorithm processes the SPS measurements to calculate the FF alignment [10] by comparing the readouts differences between opposite diodes. To convert the measured irradiances into the displacement of the coronagraph with respect to the umbra, the algorithm requires an accurate knowledge of the light distribution on the SPS plane.…”

Exoplanet detection yield of a space-based Bracewell interferometer from small to medium satellites

“…Composed of two spacecraft, these satellites will maintain formation autonomously to submillimeter precision in the lateral plane and millimeter precision in the longitudinal plane at distances of 150 m or more. 25 It is an important step toward a large interferometric mission since formation flying is necessary to increase the baseline length up to a few hundred meters. 10 The size of all PROBA satellites is around 1 m 3 .…”