[en] ASDEX Upgrade tries to match the physical requirements of a poloidal field divertor to technical reactor needs. The plasma configuration fits present day reactor concepts (INTOR, ITER) on smaller scale. The aims of the AUG include two technical problems: Control of the non-circular bulk plasma configuration and control of the plasma boundary or divertor configuration respectively. For these purposes an exact measurement and adjustment of magnetic fields is required. Axial symmetry can be assumed for all plasma equilibria with a toroidal field component B_φ and two poloidal field components B_r and B_z. The measurements of field components in a vertical plane containing the axis are sufficient. Axissymmetry may be destroyed by eddy currents in metallic structure and stable or unstable plasma disturbances. Their investigations require measurements also in planes of different toroidal angles. Mode analysis of plasma disturbances can be detected by a selected numbers of signals on a poloidal circumference and on a toroidal circumference. With this so called Mirnov mode analysis, a determination of the poloidal mode number 1