Around the coast of Britain stands a chain of radio sentinels probing the skies with short-wave beams to locate approaching enemy warplanes by night or day, in cloud, smoke or fog. Technical details of the radio detector are a major war secret, but its existence was made known when the British air chief marshal called for radio technicians to man the listening posts. Dr. Lee de Forest, American radio inventor, believes the beams are projected in a conical path toward the sea and sky from the top of 240-foot towers dotting the coastline. When a plane comes within that cone, the radio signals are reflected back to the tower and received by instruments installed on the tower at various altitudes and pointing in all directions. Within these receivers, with paraboloid reflectors and dipole antennas, the rebounding beams - radio echoes - register their intensity according to distance, direction and altitude. This information is relayed by wires to the instrument table at the base of the tower where a technician, wearing earphones, translates the data and sends it instantly to the antiaircraft command posts. Such a radio-echo alarm system would be a further development of the terrain clearance indicator announced in 1938 by the Bell Laboratories and United Air Lines. In that instrument the time elapsed in projecting a signal from plane to earth and receiving its echo was measured, and from that time the actual altitude of the plane above ground was computed instantly.