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