DESIGN OF LOW-BANDWIDTH ATTITUDE CONTROLLERS FOR TIME-OPTIMAL TRANSIENT BEHAVIOUR

High-performance pointing missions are characterized by stringent attitude jitter requirements, typically specified in the frequency domain. These requirements are often fulfilled by low-bandwidth linear control laws which are chosen for their ease of design, implementation, and verification. However, low-bandwidth attitude controllers exhibit poor transient behaviour for both planned mode transitions and unplanned onboard disturbances. This paper introduces a simple method of linear controller design which delivers low-bandwidth steady-state behaviour coupled with time-optimal transient behaviour. The time-optimal transients are achieved using the eigenmode initialization, a linear control technique which ensures that any planned or unplanned transients decay along the fastest eigenmodes of the controller-plant closed loop. These "active modes" are designed to meet system constraints (e.g., actuator authority) while the remaining modes are left to the control engineer as a trade between transient sensitivity and steady-state performance. Following a theoretical derivation, two eigenmode controllers are designed for a challenging AOCS mode transition from a legacy Airbus high-performance pointing mission. The linear controllers and lightweight initialization routine are implemented in flight-like code in closed-loop with high-fidelity flight-verified models for actuator commanding and measurement processing. Simulation results demonstrate the controller's ability to deliver low-jitter steady-state behaviour alongside time-optimal transients, aligning well with theoretical predictions for reduced transient time and maximal use of available actuation authority.