The Shadow Position Sensors (SPS) metrology subsystem on-board PROBA-3 mission : Design and performance

PROBA-3 is a two-spacecraft ESA mission carrying the space-based diluted coronagraph ASPIICS. The imaging instrument is hosted on the first spacecraft with the second acting as external occulter. In order to accomplish the payload's scientific tasks, PROBA-3 will ensure sub-millimeter reciprocal positioning of its two satellites by means of closedloop on-board metrology. The Shadow Position Sensors (SPS) sense the penumbra around the instrument aperture and return the 3-D displacement of the coronagraph satellite with respect to its nominal position by running a dedicated algorithm. In this paper we describe how the SPS works and the choices made in order to accomplish the mission objectives.