Note: First published in The Aeroplane, September 23, 1949, pp. 441-2. Facsimile is reprinted in Ascent to Orbit (1984). Versions in other books are abridged and have different prefatory notes, but the piece is essentially the same. For semi-fictional treatment of the same subject see Clarke's only non-sf novel, Glide Path (1963).

Synopsis: An account of Clarke's wartime experience when he worked in Cornwall with ''a bunch of wild young scientists and engineers'' on the so called GCA (Ground Controlled Approach), a radar system designed to ''talk aircraft down, rather than shoot them down.''