Star-based defocus computing technique for PLEIADES-HR satellites

PLEIADES-HR is an earth observing system developed by the French National Space Agency, CNES. It consists of two satellites launched on December 2011 (PHR-1A) and December 2012 (PHR-1B). Each satellite is designed to provide optical 70 cm resolution panchromatic and 2.80m colored images to civilian and defense users. During commissioning period of these satellites, thanks to their extreme agility, new calibration methods have been tested based on the observation of celestial bodies, and stars in particular. It has then been made possible to perform MTF and defocus measurement (in order to refocus), geometrical bias computation, focal plane assessment, absolute calibration, ghost images localization, micro-vibrations measurement, etc… This article deals with the problem of satellite refocusing. By using images of stars, the problem can be considered as a phase diversity inverse problem. Significant evolution has been brought to the previous method developed during the commissioning period in order to improve accuracy and reduce operating constraints of the method.