Position sensing using electric dipole fields

A medical apparatus, comprising:

• a reference probe, comprising:
• a flexible insertion tube having a circumferential surface extending along a longitudinal axis, a distal end configured to be inserted a body cavity, the distal end having a distal tip;
• at least three pairs of mutually isolated electrodes fixedly attached linearly along the circumferential surface of the flexible insertion tube:
• i] a first pair of the at least three pairs of mutually isolated electrodes being a pair of ring electrodes proximal the distal tip and separated by a first fixed distance along the longitudinal axis to be a first dipole having a dipole moment directed along the longitudinal axis,
• ii] a second pair of the at least three pairs of mutually isolated electrodes being radially offset from each other by 180° along a second pair axis perpendicular to the longitudinal axis to be a second dipole having a dipole moment directed along the second pair axis, and
• iii] a third pair of the at least three pairs of mutually isolated electrodes being radially offset from each other by 180° along a third pair axis perpendicular to the longitudinal axis and orthogonal to the second pair axis to be a third dipole having a dipole moment directed along the third pair axis, the second pair of mutually isolated electrodes and the third pair of mutually isolated electrodes being positioned directly on the surface of the distal end between the first pair of mutually isolated ring electrodes, wherein each electrode of each of the at least three pairs of mutually isolated electrodes comprising source electrodes for generating an electrical field;
• and
• a position sensor fixedly located in the distal end;
• a supplementary probe having an electrode fixed thereto;
• and
• a processor, configured to inject respective alternating currents into at least one pair of the at least three pairs of mutually isolated electrodes to generate the electrical field therefrom, to measure a potential generated at the electrode of the supplementary probe in response to the electrical field, and to determine a location of the electrode on the supplementary probe with respect to a reference probe frame of reference and to then calculate the location of the electrode on the supplementary probe in the frame of reference using a location vector relating to a position of the position sensor.