A device for the unambiguous measurement of the angle of roll of a projectile, comprises at least a radar equipped with means of processing and sending a signal to the casing of the projectile in at least one direction of incident polarization; a set of parallel grooves made on the casing, the depth of which is modulated dissymmetrically with respect to the axis of symmetry of the projectile; the axis of symmetry of the projectile not passing through the point of the antenna of the radar where the antenna beam is generated, the processing means analyzing, in reception, a signal back-scattered by the casing of the projectile, the signal being modulated as a function of the angle of roll of the projectile, the modulation having two maximum local values corresponding to two angular roll positions of the projectile such that the polarization E is parallel to the grooves, the processing means removing the 180.degree. ambiguity by comparing the levels of the local maximum values. The device can be applied especially to the correction of the paths of projectiles fired by a gun and when the correction requires knowledge of the roll position of the projectiles.