Inertial navigation accuracy enhancement

Apparatus for the improvement of inertial navigation system (INS) position/velocity comprises frame stores for storing sequential images from a video camera and a comparator for comparing pixels in predicted images, derived from earlier stored images as modified by INS linear and angular velocity outputs, with pixels in actual later stored images, to derive a velocity error correction signal which is used to correct the INS output. The video processing correction system described is passive and hence does not reveal the aircraft's whereabouts as would be the case with other positional correction systems using radar, nor does it rely on systems outside the control of the INS, e.g., GPS, nor is it reliant on a vast terrain feature database.