ESA GNC Conference Papers Repository

BIOMASS is one candidate for the ESA 7th Earth Explorer mission. It is a space-borne radar mission operating at P-Band intended to tackle some major scientific questions concerning the global forest biomass distribution and annual changes, including the interaction with the carbon cycle. To achieve its objectives, the spacecraft will fly in a dawn-dusk sun-synchronous orbit at an altitude of approximately 600 km, with a Synthetic Aperture Radar (SAR) looking sideways at approximately 25 degrees off-nadir towards the dark side of the orbit. A Reflector Antenna concept has been selected for the P-band antenna; the large size and flexibility of which influences the AOCS architecture, performance requirements, equipment selection and development requirements. In its deployed state, the fundamental frequency of the antenna is currently predicted to be around 0.14 Hz. Considering AOCS, the moment of inertia of a large antenna mounted on a lightweight bus, together with the disturbance torques generated by the atmospheric drag and gravity gradient in low Earth orbit have a significant impact. Furthermore, the potential coupling of two major flexible structures (antenna and solar array) connected to the main body of the spacecraft and the need to avoid exciting the flexible modes during the mission requires detailed analysis of the dynamics of the whole system. Since the appendage dynamics characteristics are potentially challenging for a classical control law, a robust control design (Hinfinity synthesis) has been developed in parallel as a form of risk mitigation. The robust controller has been obtained in the frame of the parallel study to the BIOMASS phase A study, which uses BIOMASS as a benchmark design case: "Modern Attitude Control for EO Satellites with Large Flexible Elements". The paper presents the AOCS controller design challenges and solutions considering the BIOMASS mission definition including pointing/stability requirements, appendage dynamics characteristics, environmental disturbance torques, and sensors and actuators definitions. Additionally, a comparison is made between the Classical and the Robust Designs; both of which are confirmed to satisfy the AOCS requirements. Although the Robust design generally exhibits better performance/robustness characteristics compared to the Classical design, the Robust Design is retained as a means of risk mitigation against potential future evolution of the requirements/Spacecraft whilst the satisfactory Classical Design is selected as the baseline for BIOMASS Normal Mode.