Abstract: An autocollimating assembly is provided for the tracking or position-sensing sub-system of a stellar navigational system, the sub-system being mounted on the stable platform of an inertial measuring unit. The autocollimating assembly, in conjunction with mirrors mounted on the internal surface of the spherical case of the inertial measuring unit permits accelerometer bias, scale factor and alignment measurements to be made, as well as gyro drift and alignment measurements, and sensor alignment for scale factor measurements, with the sub-system installed in the space vehicle. The autocollimating assembly of the invention includes a pattern of point light sources which generate a plurality of light beams which are collimated by the lens system of the sub-system, the beams being reflected back from one of the mirrors on the internal surface of the case of the inertial measuring unit. The reflected light is focused onto the surface of the vidicon or solid state stellar sensor in the sub-system. The angle .theta.