PCT No. PCT/SE78/00067 Sec. 371 Date July 2, 1979 Sec. 102(e) Date June 29, 1979 PCT Filed July 2, 1979 PCT Pub. No. W079/00260 PCT Pub. Date May 17, 1979A correlation follower comprising an image sensor (1) adjustable sideways and in height with a limited field of view, which is cyclically scanned by the sensor the output signal of which reflects the image content within the field of view and a video correlator (3) for controlling the alignment of the sensor in dependance of the output signal of the image sensor.The video correlator (3) has two addressable memories (12, 14) the one of which is a real time memory (12), i.e. in this memory a section of the field of view is stored for each cycle. The other memory is a reference image memory (14) and is updated with the content of the real time memory (12). During the correlation the contents of the memories are displaced in relation to each other and an error signal, corresponding to the position of displacement for which maximum correlation is achieved, is made to control, through said control circuits, the alignment of the sensor.To make the tracking process insensitive to disturbancies and image elements which appear momentarily in the field of view, for each scanning cycle only a part of the positions of the reference image memory (14) are updated by selecting the addresses to said positions randomly or according to a predetermined rule of selection. By that the positions which are updated during one cycle in positions that are distributed over the entire area of the memory.