A catheter-based tracking system includes one or morefield generators and a processor. The one or more fieldgenerators, located adjacent to a Magnetic ResonanceImagining (MRI) system, are configured to apply AlternatingCurrent (AC) magnetic fields. The processor is configuredto receive signals, which are produced responsively to theAC magnetic fields in a single axis sensor (SAS) that isfitted at a distal end of a catheter inserted into an organof a patient, to calculate, based on the signals receivedfrom the SAS, a direction of the distal end, to receivefrom a Hall effect sensor that is fitted at the distal endof the catheter a sensed component of a Direct Current (DC)magnetic field of the magnetic resonance imagining (MRI)system, and, based on the calculated direction and thesensed component of the DC magnetic field, to calculate aroll-angle of the distal end inside the organ.
