Calibration of ISA accelerometer sensing axes for the BepiColombo mission

Abstract

ISA is a three-axis accelerometer embarked on-board BepiColombo, a joint ESA/JAXA space mission launched on the 19^th of October 2018 at 22.45 (local time) and devoted to put two spacecraft (MPO and MMO) around Mercury (2025), for a deeper investigation of the planet and its environment. ISA aims at measuring, directly and not by modelling, first time for a planet other than Earth, the non-gravitational perturbations the MPO will undergo during its stay around Mercury. Such data will be used within the frame of the Radio-Science Experiments to reconstruct, a posteriori and with very high accuracy, the true gravitational orbit of the spacecraft and, at last, the gravity field of Mercury. Three single accelerometers, each one characterised by its own sensing axis, i.e. the direction along which it is sensitive to accelerations, collect the non-gravitational perturbations encountered by the MPO. The arrangement of sensing axes, whose knowledge is essential to reconstruct the sensed accelerations, was measured with very high accuracy by the Experimental Gravitation group at IAPS/INAF in Rome, which manages ISA and holds the PI-ship. The group developed a calibration procedure to identify on-ground the direction of ISA sensing axes with respect to an optical reference frame. An experimental facility was designed and assembled to accomplish the measurement. In November 2015, the calibration was performed on the ISA Flight Model, currently in cruise phase towards Mercury. Results of the measurement campaign are reported and prove the determination of ISA sensing axes with an accuracy within 20-30 arcseconds per single axis, well below the 100 arcseconds required by the Radio-Science needs.