The invention relates to a system for determining electrical characteristics like electrical potentials on a surface of a heart (5). An esophageal electrode structure (6) measures electrical characteristics within an esophagus and a position determination unit (34) determines the position of the esophageal electrode structure within the esophagus and the position of the surface of the heart (5). The electrical characteristics on the surface of the heart (5) are then determined based on the measured electrical characteristics and based on the determined positions of the esophageal electrode structure (6) and the surface of the heart (5). Since for measuring the electrical characteristics the esophageal electrode structure (6) is used, the electrical characteristics can be measured within the esophagus and thus close to the surface of the heart (5), thereby allowing for an improved accuracy of determining the electrical characteristics on the surface of the heart (5).
