Apparatus is provided for tracking an object in a body of a patient in the presence of an interfering article. A set of one or more radiators are adapted to generate an energy field at a fundamental frequency in a vicinity of the object. A position sensor, fixed to the object, is adapted to generate a signal responsive to the energy field. A control unit has access to a database of one or more harmonic frequency patterns associated with one or more respective specific types of interfering articles. The control unit is adapted to (a) receive the signal, (b) detect a pattern of harmonic frequencies of the fundamental frequency present in the signal responsive to an interaction of the interfering article with the energy field, (c) compare the pattern to harmonic frequency patterns stored in the database, (d) identify the interfering article responsive to the comparison, (e) correct the signal responsive to the identification of the interfering article and a magnitude of one or more of the harmonic frequencies in the detected pattern, and (f) determine position coordinates of the object responsive to the corrected signal.