A surgical tracking system (20) includes a wireless position sensor (70), which is adapted to be implanted in a bone of a subject, and responsively to externally-applied magnetic fields within a working volume of the tracking system, to generate and transmit sensor signals indicative of coordinates of the position sensor within the bone. A plurality of field generator coils (32) are adapted to generate the magnetic fields so as to define the working volume. The field generators are fixed in predetermined locations to a reference structure (40), which is movable relative to the subject in order to position the working volume so as to intercept the bone. A system controller (36) is coupled to receive and process the sensor signals so as to determine the coordinates of the position sensor within the bone. <；IMAGE>；
