Abstract: An observation instrument located on a satellite is provided with light sources, whose radiation is reflected by the facets of at least one optical dihedron, whose shape is such that it deflects very slightly or not at all the reflected beam, even if it is displaced. Thus, the reflected beam, which is representative of the sighting axis of the instrument is reflected to a sensor, having a known orientation, in order to give the real direction of the axis. The invention has application to locations or tracking on the ground by the instrument.

Abstract: Angular position finding system for an observation instrument, possibly fitted with a sighting change mirror and which includes one or two reflecting mirrors and one or two alignment sensors. Light patterns are produced both on the instrument and on the alignment sensor and are reflected by the mirror to the alignment sensor where their positions are detected. The orientation of the mirror, and indirectly the orientation of the instrument, can be deducted. Furthermore, if the alignment sensor is equipped with a device for determining an absolute orientation reference, the orientation of the instrument with respect to this reference may also be known.