An Apparatus for determining the disposition of an object (22) relative to a reference frame includes at least one field generator (26), which generates an electromagnetic field in a vicinity of the object and at least one transducer (12), which is fixed to the object. The at least one transducer (12) vibrates at a predetermined vibrational frequency and emits energy, responsive to an interaction of the electromagnetic field therewith. One or more detectors (34,36,38) in a vicinity of the object are also utilized to detect the energy emitted by the transducer (12) and generate signals in response thereto. A signal processor (30) is also included for receiving and processing the detector signals to determine coordinates of the object (22). <；IMAGE>；
