An actuator for imparting steering movement to a tiller of a propulsion unit of a marine vessel comprises an absolute position sensor operable to sense a steering position and a relative position sensor operable to sense a position of the motor. A steering control unit calibrates the relative position sensor based on a signal of the absolute position sensor. Calibration of the relative position sensor based on a signal of the absolute position sensor initializes an accumulative position which accumulates a relative position as the actuator moves over time.