Explanation: An autopilot functions as a closed-loop control system to maintain a desired course. It needs to know the actual position of the rudder to compare it against the commanded position. A **rudder feedback unit** provides this crucial information, indicating how far the rudder has turned. This feedback allows the autopilot to precisely adjust the rudder, correct for overshoots, and counteract external forces like waves, ensuring the vessel stays on course. GPS provides navigation data, telling the autopilot *where* to go, but not *how* to physically control the steering mechanism or its current state. Engine RPM data is for propulsion, entirely unrelated to steering. SSAS (Ship Security Alert System) is a security device and has no role in navigation or steering control.