Robust Attitude Control for Space Target Pointing in Proximity Operations

The space community has recently put considerable research efforts into innovative technological solutions enabling autonomous rendezvous of a chaser spacecraft toward a non-cooperative target for future Active Debris Removal (ADR) or On-Orbit Servicing (OOS) missions. Such in-orbit operations involve complex technical challenges, especially regarding the design of the chaser Guidance, Navigation and Control (GNC) system. For instance, it must be designed to provide relative state estimates with strict accuracy requirements. To achieve this goal, the GNC system must operate in Target Pointing Mode (TPM), meaning that the relative navigation sensor suite shall be continuously pointed toward the target, while the chaser follows complex relative trajectories. This work addresses such attitude control problem assuming that the chaser is equipped with a scanning LIDAR as a relative navigation sensor and a set of reaction wheels as attitude actuators. The proposed approach relies on a Proportional-Integral- Derivative (PID) scheme including a feed-forward term to follow the rate of variation of the target line of sight and an additional term to counteract the gyroscopic effects from actuators and the spacecraft. The attitude control performance is assessed in terms of pointing accuracy and control effort by means of numerical simulations carried out within a high-fidelity virtual environment. The simulator reproduces the operation of a complete GNC architecture by including a LIDAR point cloud generator, a state-of-the-art relative navigation algorithm, and a realistic model of the reaction wheels’ operation. Finally, a robust stability analysis of the designed controller is done within a Linear Fractional Transformation (LFT) framework by means of the µ-analysis tools, accounting for environment, sensing, and actuation uncertainties.