Hi Francois,
To get the positions and orientations of the MEG sensors relatively to the head of the subject, most MEG systems use head localization coils (3 for CTF and 4D, 4 for Elekta, 5 for Yokogawa, etc). I think you can find online documentation about this technique easily.
Do you to build my own sensors with coordinates and orientation? can you please share with me one online document to understand what you mean?
Thank you
Ousai