This contribution is somewhat aside from the flow of the discussion so far. Mr. Garfield has dealt with the general features of radio altimetry and I am going to describe only one version of a radio altimeter in a little more detail. What I have to say is also a progress report in continuation of an earlier note published in the Journal; at that time the possibility of obtaining altitude as additional information from an airborne doppler navigator was discussed, and it was concluded that an accuracy of ± 5 per cent ± 2 ft. over the range of 10–500 ft. was possible. It was emphasized that the work was in an investigational stage, and no specific design was described. Today, over five years later, I can report on the flight trials of an engineered equipment using these principles. The long gap is not a true measure of the total time from design study to a unit in production, but the hazards of either commercial or military supported development programmes can extend the time in a manner which is all too familiar to many people.