HIGH POINTING ACCURACY WITH THE PICARD MICRO SATELLITE.

Abstract

PICARD is a major scientific mission leaded by CNES which will contribute, through the observation of the Sun, to the identification of the Sun influence of the global Earth climate changes. The PICARD Attitude and Orbit Control Subsystem, discussed in this paper, is developed for CNES by ALCATEL ALENIA SPACE.The PICARD platform including the AOCS is based on the MYRIADE micro-satellite platform developed by the CNES, with the necessary adaptations to cope with the very severe pointing and stability requirements imposed by the payload.With respect to the generic MYRIADE AOCS, a dedicated Fine Pointing Mode has been introduced in addition to the Normal Mode. This Fine Pointing is based on a Sun Ecartometry Sensor developed specifically for that purpose and installed within the payload. The 2-axes information of the SES are fused with the output of a “classical” autonomous Star Tracker for providing a 3-axes attitude introduced in the control algorithms.On top of the Fine Pointing Mode, the ultimate pointing performance of the payload line of sight is achieve through a dedicated high-frequency control stage part of the telescope itself. This control stage is based on sun image detectors integrated in the focal plane of the telescope and on local piezoelectric actuators which act directly on the telescope primary mirror.The paper will present first the scientific mission objectives and the AOCS architecture. It will then focus on the specific satellite Fine Pointing Mode developed for PICARD, with a fine presentation of the Sun Ecartometry Sensor and the required data processing algorithms. Finally it will present the high frequency upper control stage which is decoupled from the platform attitude control and is integrated to the payload telescope.