The field of the invention is that of optical systems for detecting the posture of a mobile object in space. The system comprises an electro-optical fixed device of known orientation comprising a first point emission source, a telecentric emission/reception optic and a photosensitive matrix sensor. An assembly comprising an optical cubic wedge is disposed on the mobile object. The input face of the cubic wedge comprises a mask in the shape of a parallelogram, each side of the parallelogram comprising a geometric marking making it possible to identify it, the image of the mask projected on the photosensitive matrix sensor, by reflection on the faces of the cubic wedge, being the intersection of the projection of the mask and of the projection of its image inverted with respect to the centeR of the cubic wedge. Analysis of this image makes it possible to determine the orientation of the cubic wedge.