The present invention relates to a system comprising: multiple sets of current injecting electrodes configured to be secured to one or more stable locations relative to an organ in a patients body, the multiple sets of current injecting electrodes comprising electrodes selected from the group consisting of current injecting electrodes that are mounted a first catheter placed inside the body and body-surface electrodes a second catheter that includes one or more measuring electrodes a third catheter that includes one or more additional electrodes and a processing system. The processing system is configured to determine expected signal measurements of the field inside the organ using a non-interpolation based calculation, the expected potentials being based on signals generated by current injecting electrodes that are mounted a first catheter placed inside the body or body-surface electrodes and measured by electrodes on a second catheter and determine a position of each of the one or more additional electrodes on a third catheter in the organ based on the measurements made by the additional electrodes and the determined expected signal measurements of the field.
