Fine lateral and longitudinal sensor (FLLS) on-board ESA’S PROBA-3 mission

PROBA-3 is a European Space Agency (ESA) mission to study the Sun’s corona, and is the world’s first precision formation flying mission. The mission will comprise a pair of satellites separated by 150 m, whose relative displacement must be monitored to within 300 µm in order to produce an accurate coronagraph. This measurement is provided by FLLS – the Fine Lateral and Longitudinal Sensor – being designed and built by Neptec UK and Neptec Design Group Canada. FLLS uses a retro-reflected laser beam to monitor the position of the occulter-disc satellite with respect to the coronagraph satellite. Phase measurements of the reflected beam are used to determine the longitudinal displacement between the two satellites – up to 250 m – while the motion of the returning beam on a CMOS sensor measures the lateral displacement. This system is being designed in collaboration with Surrey Space Centre, and presents exciting challenges in performance testing and ground-based calibration over its full operating range. The completed FLLS system will be suitable for any type of mission requiring accurate displacement measurements. This could be between a constellation of satellites observing the Earth, or within a science mission monitoring instrumentation positions. FLLS could allow large-scale structures to be deployed and maintained in space, monitoring structural distortion before, during, and after deployment, and providing in-flight corrections to data collection. Examples of such future applications include in-orbit observatories, positioning of telecommunication satellite antennas, and deployable mechanisms on lunar or Martian missions. The paper and anticipated presentation will provide a comprehensive overview of FLLS including technical designs, calibration and performance test plans, as well as the envisaged scope for future applications.