Abstract: For fixing the position of a moving vehicle, typically an aircraft, a map of a strip of ground along the theoretical path of the vehicle is prepared. The map has two parts. The transparency i at each point of one part is in direct relation with the value of a predetermined parameter representative of the ground at that point, typically the altitude. The transparency at each point in the other part is in inverse relation of the parameter. During movement of the vehicle, the successive actual values of the parameter are measured and stored over a distance p. Such values are used for modulating light fluxes which are directed along a plurality of bands each representative of distance p, in succession. A correlation factor is determined for each band based on integration of light fluxes received by sensors through the bands in succession. The location of the band on the map which corresponds to the best correlation is determined and used for fixing the position.