A three-dimensional alternating current electromagnetic field tracking system (2) for determining the position and orientation of an object, wherein the system (2) comprises an electromagnetic field transmitter (4) comprising three coils (22, 22', 22") configured to generate three magnetic fields (Hx, Hy, Hz), one or more electromagnetic field receivers (12) each comprising three measurement coils and a control unit (46). The electromagnetic field transmitter ( 4) comprises at least one offset coil (8, 8', 8") arranged to generate an addition magnetic field (Hadd). The offset coil (8, 8', 8") is offset relative to the coils (22, 22', 22") of the electromagnetic field transmitter (4). The control unit (46) is configured to calibrate the system (2) on the basis of the addition magnetic field (Hadd).
