A system for position tracking includes one or more field generators, which are arranged to generate one or more respective position-varying magnetic fields. A transponder includes a resonant circuit having a resonance frequency and including a field-responsive element, which is operative to vary the resonance frequency responsively to the one or more magnetic fields. A position tracker is arranged to remotely sense the resonance frequency of the resonant circuit and to determine a position of the transponder responsively to the sensed resonance frequency.