Luftwaffe Units. At approximately the same time the Luftwaffe E-Stelle, in conjunc-tion with the firms of Dr. Hell, Berling, and Hagenuk, Kiel, commenced a similar research and development designed to produce an optical unit for use in rivers against bridges. Toward the close of the war they had completed the Forelle unit, which is discussed la-ter and which was used with some success by the Naval sabotage group (KdK).