A medical apparatus, including a reference probehaving a flexible insertion tube with a distal end forinsertion into a body cavity, a pair of isolatedelectrodes fixedly attached to the distal end, and aposition sensor fixedly located in the distal end. Theapparatus also includes a supplementary probe having anelectrode fixed thereto. The apparatus further includes aprocessor, configured to inject respective alternatingcurrents into the pair of isolated electrodes so as togenerate an electrical field therefrom, to measure apotential generated at the electrode of the supplementaryprobe in response to the electrical field, and toevaluate a location of the supplementary probe withrespect to the reference probe in response to themeasured potential and in response to a position of thedistal end indicated by the position sensor.
