A terrain clearance indicator

There is described a radio altimeter that gives continuously on the plane a measurement of the separation between the plane and the earth's surface or projections therefrom. There is projected from the plane and reflected from the earth back to it a very short radio wave, the frequency of which is continuously swung back and forth. The returned wave is thereby made to differ from the outgoing wave in frequency by an amount that is proportional to the echo path; and the difference or “beat” frequency is indicated on a frequency meter calibrated in feet of separation. The paper outlines some of the early efforts in this field, some of the technical problems involved, the theory of the system and the practical experimental results that have been obtained.