An aircraft navigation system has a digital map store of terrain features or man-made features in the region over which the aircraft is flying. Infra-red television cameras view the surrounding of the aircraft. Their outputs are supplied to processors in which the camera outputs are compared with a library of features transformed according to the camera viewing angle. Information about those features identified is supplied to a correlator which correlates the features against the map store to identify their location in the map and estimates the aircraft position as an output to a navigation computer. The system also indicates aircraft attitude which may be independent of known-features, from observation of the horizon. Position information can also be provided by dead reckoning from a known initial position by monitoring the change in positional relationship of a feature from the initial position to that at a later position.