Abstract: The three-axis angular position of a tetrahedral reflector which is mounted on a platform or element is monitored by first and second optical apparatus aimed at the reflector at equal angles on opposite sides of the axis of the reflective element thereby providing off-axis viewing of the reflective element for monitoring the angular movement of that element. The first optical apparatus is an autocollimator having a source of radiation for producing a beam of radiation which is applied to a beam splitter through a collimating lens which is aimed at the reflector. The return image from the reflector is passed through the beam splitter to a detector. The second optical apparatus may simply be a receiver including a second lens in optical alignment with a second detector with the lens being aimed at the tetrahedral reflector. The second optical apparatus may be in the form of an autocollimator of the same type as the first optical apparatus.