to distinctive landmarks can be monitored thus the bulk store 10 contains prepared binary data assembled prior to the commencement of the flight relating to distinctive cross roads railway junctions lakes and rivers and coast line estuaries etc in a binary format depending upon the speed of the airborne vehicle the appropriate frames of information are extracted at the appropriate time and entered into the reference memory 9 after modification to allow for the orientation and height of the airborne vehicle as previously mentioned this stored data is then compared with the real time data entered into the scene memory 7 when a portion of the scene memory is found which corresponds with the pre stored data the correlation analyser indicates that the current position of the airborne vehicle has been determined any slight positional errors i e deviations from the predetermined path are compensated by the output of the system so as to slightly alter the direction speed or attitude of the airborne body to direct it towards the next designated reference scene this process continues possibly over many hundreds of miles as the airborne vehicle steadily approaches its predetermined destination the spacing apart of the locations of the reference scenes is of course chosen with regard to the degree of navigational drift which can occur in each case the size of.