The Occulter Position Sensor Emitters (OPSE) metrology sub-system for the PROBA-3 mission

The development and employment of metrology controlled diluted systems for astronomical applications in space represents a cornerstone to dramatically improve the observation limits. In this scenario, the ESA mission PROBA-3 is the first technology demonstration mission aimed at verifying and validating several metrological systems to obtain high accuracy formation flying (FF) of two satellites. The two spacecraft will be aligned and maintained in a FF, around the apogee of their geocentric orbit, at an average inter-satellite distance of 144m, with an accuracy down to sub-millimetre level. The scientific task of the mission, that is the FF validation tool, is the observation of the Sun's Corona at high spatial and temporal resolution within the field of view [1.08; 3.0]R
_⊙
. To this end, one satellite will house a coronagraph telescope, named ASPIICS, and the other one, the external occulter. In this paper, we review the metrology concept applied to the mission with a special focus on the Occulter Position Sensor Emitters (OPSE) metrology sub-system, a set of 3 monochromatic Led emitters positioned on opposite-Sun surface of the occulter and imaged at the ASPIICS's focal plane.