Patent ID: 7945309

Claim:
Position-sensing apparatus, comprising: a set of radiators, which are adapted to be placed at respective positions in a vicinity of a body of a subject and to generate electromagnetic energy fields; a position sensor, which is adapted to placed in the body of the subject and to generate sensor signals responsive to the energy fields; one or more reference elements, which are adapted to be placed at fixed respective positions in a vicinity of the sensor and to generate reference signals responsive to the energy fields; and a control unit, which is adapted to: determine and store in memory, for each of the reference elements, respective undistorted reference parameters as undistorted magnetic field values, responsive to the positions of the reference elements, receive the sensor signals and the reference signals, calculate a reference element error for each reference element, responsive to an interaction of a metal article with the energy fields, responsive to the stored undistorted magnetic field values of the reference element, and responsive to the reference signal generated by the reference element, and calculate a position of the sensor, responsive to the sensor signals and the reference element errors.