A member of the rho-theta family of air navigation systems which define an aircraft's position by its distance and bearing to a single beacon. Such systems inherently answer the navigator's questions of "in what direction" and "how far," without additional computation, and are of particular value when the beacon has to be placed on a ship, oil-drilling rig, or small island. The main weakness of such systems is that bearing errors cause spatial error to increase with distance from the beacon. Major attention is therefore given to the reduction of bearing errors.