A system includes multiple electrically-conductive channels and a processor. The processor is configured to receive, over the electrically-conductive channels, (i) respective first electric currents from a probe, which is within a body of a patient, via a plurality of first electrodes, which are attached to skin of the patient at a region of the body, and (ii) a second electric current from the probe via a second electrode, which is attached to the skin and is connected to one of the channels. The processor is further configured to ascertain respective first electric-current values of the first electric currents and a second electric-current value of the second electric current, and to calculate a position of the probe between the region and the second electrode, based on the first electric-current values and the second electric-current value. Other embodiments are also described.