AN altitude meter for airplanes functioning on the principle of a radio echo from the ground was developed a number of years ago with the hope that this would become one of the ways in which radio would contribute to the art of aviation. After the results of this work were announced in 1928, the author, not being an aviator, did not feel justified in advocating the use of this method because the demand for such service should naturally come from the aviators. It is, therefore, an interesting indication that requests for further information on this subject-have come from the Institute of the Aeronautical Sciences. The radio echo altitude meter has a single tube radio receiver operated with an antenna which takes the form of a single wire trailing behind the plane. The receiver is so designed and adjusted that it radiates a wave towards the ground and simultaneously receives the echo reflection of this wave returning from the ground. The radiated and reflected wave combines into the characteristic pattern of a standing wave, with well defined maxima and minima which are directly indicated on an instrument dial. In the tests which were made and which will be described later, the first positive indication of approach to the ground was given at an altitude of 600 feet, and it was found that by the guidance of this indication it was possible to navigate over rolling country and steep hills and depend upon receiving a positive warning signal when the ground is approached within 600 feet. If it is desired the system can be readily redesigned by changing the wave length so that the first warning will be given at a level of 1,000 feet, which is likely to be more useful for commercial aviation. Such a design also has the advantage that the characteristic maxima and minima of the standing wave will occur an even 200 feet apart; thus making a convenient scale for judging ground distances. Fig. 1 shows the amplitude of the echo indications at altitudes between the ground and 1,000 feet. The successive maxima and minima increase in amplitude as the ground is approached. There is a maximum at 200 feet, a minimum at 400 feet; a maximum at 600 feet and a minimum at 800 feet, etc. The instrument thus distinguishes between the different levels by the amplitude of the swing of the needle. If it is assumed that a plane is navigating in a fog the pilot can get accurate information of the approach of the ground FIG. 1.