This paper describes the design of a robust automatic guidance system for a combine harvester. The automatic steering system controls the harvester based on the measured position of the swath on the field. The swath is detected using a laser scanner. The translation and rotation of the machine are taken into account to recalculate the swath's relative position every time step. With the help of a model of the harvester (derived in this paper), the desired wheel position is calculated in function of the desired bearing change. The desired wheel position is then transmitted over the CAN network of the harvester to the actuator.