Electro-Magnetic-Articulography has been a well-established technology for many years The new AG500 System even allows the investigation of articulatory movements in three dimensions. Still, calibration is a crucial point to obtain reliable and accurate data. After a short glance of the mathematical background and a review of previous methods, the present Circal device is discussed. Due to its construction a basic calibration problem probably remains, since the Circal neglects sensor orientations outside of the x/y-plane. Depending on the sensors actual position and orientation, a suboptimal calibration can have a mild or dramatic influence on the position calculation, which might even fail. Several approaches are thinkable to overcome the calibration problem, three types are discussed, which can be characterized as mechanical, physical, and mathematical solution. Finally the actual work on a mathematical solution is briefly presented.