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

The paper presents the design and the 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, distant more than 200 km and Earth orbiting at about 300 km altitude, must align their axis to the satellite-to-satellite line (SSL) with a microradian accuracy (pointing control). This is made possible by specific optical sensors accompanying the inter-satellite laser interferometer, capable of materializing the SSL. Such sensors allow each satellite to autonomously align after an acquisition procedure. Pointing control is severely constrained by the angular drag-free control, which is imposed by Earth gravimetry (science), and must zero the spacecraft angular acceleration vector below 0.01 microradian/s2 in the science bandwidth. This is made possible by the ultrafine accelerometers whose measurements must be coordinated with attitude sensors to meet drag-free and pointing requirements. Embedded Model Control shows how naturally coordination can be implemented around the embedded model of the spacecraft attitude and of the formation frame quaternion.