1. A position and orientation system for sensing the position and orientation of a mobile object that is moveable in an environment comprising: a control system; multiple reference targets located in random positions within the environment to define a local reference frame, the positions of the reference targets being unknown to the control system at start-up of the control system; an optical tracking device mounted to the mobile object comprising one or more direction sensors that are arranged to detect visible reference targets and generate direction information relating to the direction of the visible reference targets relative to the optical tracking device, the direction information being continuously updated as the mobile object moves within the environment; and wherein the control system is arranged to operate the optical tracking device, receive the direction information, and process the direction information to: initially auto-calibrate at start-up to determine the 3D positions of the reference targets relative to each other within the local reference frame; and then subsequently generate updated position and orientation information relating to the current position and orientation of the mobile object in the local reference frame during movement of the mobile object based on the continuously updated direction information and the determined 3D positions of the reference targets at start-up.