Performance modelling of the fine lateral and longitudinal sensor (FLLS) for ESA's PROBA-3 mission

PROBA-3 is a European Space Agency (ESA) mission due to launch in 2020. The aim of the mission is two-fold: To study the solar corona, and to demonstrate formation flying technology. Two satellites will fly in formation, 150 m apart, to form a coronagraph instrument. The relative displacement between the two satellites must be known to within 300 μm to produce accurate coronagraph data. The instrument that will provide this high-accuracy measurement is FLLS-the Fine Lateral and Longitudinal Sensor. Built by Neptec UK and Neptec Design Group, FLLS uses a retro-reflected laser beam to monitor the position of one satellite with respect to the other. Phase measurements of the reflected signal are used to compute the longitudinal displacement between the two satellites. The lateral displacement is measured from the position of the returning beam on a CMOS sensor. FLLS is currently in preliminary testing, in collaboration with Surrey Space Centre. To support the testing, the performance of the lateral system of FLLS has been modelled. A computational model of the full optical system has been produced, and several calibration routines have been tested on the modelled beam images. The results of these routines have enabled predicted preliminary performance analysis of the FLLS instrument. The paper and presentation will provide an overview of the method, model, and results of the preliminary performance modelling of FLLS.