Angular drag-free control and fine satellite-to-satellite pointing for the Next Generation Gravity Missions

The paper presents the design and some simulated results of the attitude control of a two-satellite formation under study by the European Space Agency for the Next Generation Gravity Mission. The formation spacecrafts, with a distance between them of more than 200 km and orbiting around the Earth at about 300 km altitude, must align their axis to the satellite-to-satellite line with a microradian accuracy (pointing control). This is made possible by specific optical sensors accompanying the inter-satellite laser interferometer. Such sensors allow each satellite to autonomously align after an acquisition procedure. Pointing control is constrained by the angular drag-free control, which, imposed by Earth gravimetry, must zero the spacecraft angular acceleration vector below 0.01 microradian/s² in the science bandwidth. This is made possible by ultrafine accelerometers of the GOCE-class, whose measurements must be coordinated with attitude sensors to meet drag-free and pointing requirements. Embedded Model Control shows how coordination can be implemented around the embedded model of the spacecraft attitude and of the formation frame quaternion. Evidence and discussion about some critical requirements is also included.