Abstract: A means for the measurement of the alignment error between a plurality of ordinate reference frames and a relatively absolute reference frame determines all three degrees of freedom utilizing a single beam of light with relatively narrow angular extent. The measurement means has a light source, collimator, a plurality of reflecting folding mirrors and a pair of retroreflectors rigidly fixed to each coordinate reference frame. As the collimated light beam strikes a folding mirror it is reflected toward the corresponding coordinate reference frame where it is reflected back by the retroreflectors. The angles of tilt, twist and rotation at which it is reflected are representative of the misalignment of the coordinate reference frame. The folding mirror then reflects the retroreflected beam back toward the source, where the beams are detected.