Positioning device with dynamic metal immunity

A position detecting apparatus (18) comprising: a set of radiators (34), which are adapted to be placed at respective positions in proximity to the body of a subject (24) and to generate electromagnetic energy fields; a position sensor (20) which is adapted to be located in the body of a subject (24) and generating sensor signals in response to the energy fields; one or more reference elements (22) which are adapted pair being located in respective positions in proximity to the sensor (20) and to generate reference signals responsive to the energy fields; and a control unit (50) which is adapted to: determine, couple each reference element (22), baselines without respective distortion in response to the positions of the reference elements, receive the sensor signals and reference signals. calculating the error element torque reference each reference element (22) in response to the interaction of a metal article (40, 41) with the energy fields, in response to the reference parameter undistorted reference element ( 22), and in response to the reference signal generated by the reference element, and calculate the reference position sensor (20), in response to the sensor signals and errors of the reference element, characterized in that at least one the one or more reference elements (22) is adapted to be located in a fixed position known relative to the set of radiators (34) during operation of the apparatus (18), which means that the position sensor (20) is located in the subject (24).