How do you design an attitude control system for a high-agility Earth observation satellite?

High-agility requirements: Rapid retargeting (degrees per second slew rates), Precise pointing (arcsecond stability), and Quick settling for image quality. ACS design process: Requirements flowdown - Image quality to jitter, slew time to torque capability, pointing accuracy to sensor accuracy; Actuator sizing - Reaction wheels sized for momentum storage and torque, CMGs considered for highest agility, Thruster backup for wheel failures; Sensor suite - Star trackers for accuracy (multiple for coverage), gyros for rate sensing during slews, and fine guidance sensors for arcsecond pointing; Control algorithm design - Eigenaxis slew optimization, feedforward and feedback control, and jitter suppression (isolators, active damping). Challenges: Structural flexibility excitation during slews, Wheel-induced disturbances, and Thermal snap from rapid attitude changes. Validation: Hardware-in-loop simulation, On-orbit characterization and calibration.