An application controller for an application unit, for marking substances comprises a radiation sensor, by means of which the relative position of the application unit relative to a reference plane may be determined. The relative position of the application unit can be determined by means for recording the orientation of the application unit controller relative to the reference plane from the association of position with orientation information, such that control instructions, in particular for positional correction may be generated by the application unit controller. The position of the application unit can be adjusted by an actuator device according to the control instructions.