A system and method for localizing medical instruments during cardiovascular medical procedures is described. One embodiment comprises an electromagnetic field generator； an antenna reference instrument adapted to be introduced into the heart of a subject and including at least one electromagnetic sensor and at least one electrode； at least one roving instrument adapted to be introduced into the thorax cavity of the subject and including at least one electrode； and a control unit configured to determine position coordinates of the antenna reference instrument based on an electromagnetic signal from the electromagnetic field generator sensed by the electromagnetic sensor, measure an electrical-potential difference between the electrode of the antenna reference instrument and the electrode of the roving instrument, and calibrate the measured electrical-potential difference using the determined position coordinates of the antenna reference instrument to determine position coordinates of the roving instrument.
