ATHENA Space Telescope: Line of Sight Control with a Hexapod in the Loop

Sammanfattning: The Advanced Telescope for High-Energy Astrophysics, Athena, is an x-ray telescope with a 2000 kg mirror module mounted on a six degree of freedom hexapod mechanism. It is currently assessed in a phase A feasibility study as L-class mission in the European Space Agency’s Cosmic Vision 2015-2025 plan with a launch foreseen in 2028. The total mass of the spacecraft is approximately 8000 kg, which is mainly distributed to the mirror module on the one side and to the focal plane module on the other side of the telescope tube. Such a design is without precedent in any European mission and imposes several challenges on analysis and design of the complex line of sight pointing control system with the moving mirror module in the loop. Not only does the moving mirror mass lead to time-variant parameter uncertainties of the system (inertia), but also does the hexapod motion complicate the guidance algorithms and induce complex disturbances onto the SC attitude dynamics. These challenges have been approached in this thesis in three steps. First, a Matlab®/Simulink® library has been built up, including all components required to model a hexapod in pointing control simulations. This Hexapod Simulation Library includes a complete hexapod kinematic model, a simplified hexapod actuator model, a spacecraft attitude dynamic model with the hexapod in the loop, state determination algorithms with the hexapod in the loop as well as online guidance algorithms for combined spacecraft and hexapod maneuvers. Second, different operational scenarios have been designed and analyzed for comparison. Third, closed-loop pointing control simulations for a representative reference case study similar to the Athena spacecraft have been performed for first feasibility analysis and performance comparison of the different operational scenarios. With these simulations, first, the time-variant parameter uncertainties and complex disturbance torques caused by the moving mirror mass have been characterized. Thereby, the disturbance noise induced by hexapod actuator step quantization has been identified as a potential design driver that needs to be analyzed further in early phases of the project. Second, feasibility of a baseline pointing control concept has been shown, i.e. performing hexapod and spacecraft maneuvers sequentially in time. And third, possible improvements to the baseline concept have been analyzed providing up to 27% faster transition time between two observations for the simulated scenario by performing hexapod and spacecraft maneuvers simultaneously and applying a path optimized spacecraft trajectory.