World interest in the development of a satisfactory fully automatic all weather landing system has increased considerably in the past year or so. Statistics on accidents reveal the disquieting fact that a very large percentage occur in the landing phase, either in poor visibility or in conditions where limited or misleading visual clues conspire to affect a pilot's judgment of the required vertical descent path. Factors such as the changed performance characteristics of large turbo-jet aircraft, higher approach and landing speeds and differences in the approach attitude of new aircraft all add to the pilot's difficulties during the landing operation.