A system for extending the tracking volume in a probe tracking system, the system comprising: a probe having one or more probe electrodes (50A, 50B, 50C) which are configured to inject current into a body of a patient; multiple electrically-conductive channels; a plurality of first electrodes, each first electrode connected to a respective electrically-conductive channel of the multiple electrically-conductive channels; a second electrode connected to one of the plurality of first electrodes and thus to the respective electrically-conductive channel, and a processor, configured to: receive, over the electrically-conductive channels, (i) respective first electric currents from the one or more probe electrodes (55A, 55B, 55C), via the plurality of first electrodes, and (ii) a second electric current from the one or more probe electrodes (55A, 55B, 55C) via the second electrode, ascertain respective first electric-current values of the first electric currents and a second electric-current value of the second electric current, and calculate a position of the probe between a region of the body including at least part of a thorax of the patient and the second electrode, based on the first electric-current values and the second electric-current value when the first electrodes are attached to the skin at the thorax of the patient and when the second electrode is attached to the skin of the patient at a position such that an insertion point of the probe is between the second electrode and the plurality of first electrodes. a probe having one or more probe electrodes (50A, 50B, 50C) which are configured to inject current into a body of a patient; multiple electrically-conductive channels; a plurality of first electrodes, each first electrode connected to a respective electrically-conductive channel of the multiple electrically-conductive channels; a second electrode connected to one of the plurality of first electrodes and thus to the respective electrically-conductive channel, and a processor, configured to: receive, over the electrically-conductive channels, (i) respective first electric currents from the one or more probe electrodes (55A, 55B, 55C), via the plurality of first electrodes, and (ii) a second electric current from the one or more probe electrodes (55A, 55B, 55C) via the second electrode, ascertain respective first electric-current values of the first electric currents and a second electric-current value of the second electric current, and calculate a position of the probe between a region of the body including at least part of a thorax of the patient and the second electrode, based on the first electric-current values and the second electric-current value when the first electrodes are attached to the skin at the thorax of the patient and when the second electrode is attached to the skin of the patient at a position such that an insertion point of the probe is between the second electrode and the plurality of first electrodes. receive, over the electrically-conductive channels, (i) respective first electric currents from the one or more probe electrodes (55A, 55B, 55C), via the plurality of first electrodes, and (ii) a second electric current from the one or more probe electrodes (55A, 55B, 55C) via the second electrode, (i) respective first electric currents from the one or more probe electrodes (55A, 55B, 55C), via the plurality of first electrodes, and (ii) a second electric current from the one or more probe electrodes (55A, 55B, 55C) via the second electrode, ascertain respective first electric-current values of the first electric currents and a second electric-current value of the second electric current, and calculate a position of the probe between a region of the body including at least part of a thorax of the patient and the second electrode, based on the first electric-current values and the second electric-current value when the first electrodes are attached to the skin at the thorax of the patient and when the second electrode is attached to the skin of the patient at a position such that an insertion point of the probe is between the second electrode and the plurality of first electrodes.