London, England · November 6, 1940

On this date in 1940 a German Heinkel 111 was shot down and sank in the shal­lows off south­ern Eng­land. A water­logged X‑Geraet (“X‑device”) was recovered. The X‑Geraet played a role in the Battle of the Beams, a period early in the war when German bombers were equipped with increas­ingly accu­rate sys­tems of radio navi­ga­tion. Before the X‑Geraet there was the Knicke­bein (“Crooked Leg”), first tried out in 1939, which broad­cast guid­ance beams from occu­pied Europe that inter­sected at the point where bombs should be dropped on Brit­ish tar­gets. Ger­man bombers would fly into one beam, or set of radio waves, and “ride” it until the radio opera­tor started hearing the tones from the second beam on a second receiver. When the steady “on course” sound was heard from the second receiver, the bom­bardier dropped his pay­load. (The cross-beam develop­ment had its in­cep­tion in blind landings at air­ports in bad weather or at night.) Knicke­bein was used early in the Luft­waffe’s night-bombing cam­paign and was suc­ceeded by the four-beam X‑Geraet, which was simi­lar in con­cept, but it oper­ated at a much higher fre­quency and was used to greater effect. The Brit­ish city of Coven­try, with its 14th‑century Gothic cathe­dral, was its best-known victim, on Novem­ber 14–15, 1940. The Y‑Geraet was an improve­ment in that it used a single narrow beam from the ground station pointed over the tar­get. Day­time bombing could rarely achieve the accu­racy of these night­time bombing raids. By way of example, bombs dropped using the X‑Geraet were placed with­in 100 yards of the device’s cen­tering beam, good enough to hit a large fac­tory. The British scien­tific com­mu­nity fought back with a variety of its own increas­ingly effec­tive counter­measures involving jam­ming, “bending” or dis­torting the Ger­man navi­ga­tional beams, and tricking the Ger­mans to drop bombs away from their intended tar­gets. A raid on Britain’s second-largest city, Bir­ming­ham, on the night of Novem­ber 19, 1940, less than a week after the suc­cessful Coven­try raid, was dis­rupted by jam­ming. The British were slowly gaining the upper hand. But the Battle of the Beams and the Blitz on British cities really only ended when the Luft­waffe moved its bombers to the East­ern Front in May 1941, in pre­para­tion for Opera­tion Bar­b­arossa, the attack on the Soviet Union and the harbinger of Nazi Germany’s ruinous end.

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Battle of the Beams: Defeating Germany’s Radio Guidance Systems

Knickebein radio transmitter locations in EuropeFour-beam X-Geraet system for locating target

Left: Map showing Knickebein radio trans­mitters whose inter­secting beams were broad­cast from Schleswig-Hol­stein near the Danish border, at Kleve near the Dutch border, and at Loer­rach near the border with France and Swit­zer­land in south­western Ger­many. Following the fall of Nor­way, the Nether­lands, and France in April–June 1940, addi­tional Knicke­bein trans­mitters were installed in those coun­tries as well. The Knicke­bein was the pre­de­ces­sor to the more accurate X- and Y-Geraet systems, which required new, more sophisticated radio equipment.

Right: The X‑Geraet used a series of beams to locate the target, each beam named after a river. The guide beam “Weser” was intersected by a series of three very narrow single beams, the “Rhine,” “Oder,” and “Elbe.” The inter­secting beams accu­rately mea­sured dis­tances across the guide beam. The “Oder” and “Elbe” were spaced roughly 5 to 10 kilometers (3.1 to 6.2 miles) from the bomb release point along the line of “Weser.” The bombs were auto­ma­tically released on signal from the device. The British were able to defeat the auto­mated sys­tem by trans­mitting a false “Elbe,” so that the bombs dropped pre­maturely, miles short of their target.

The Secret War: Battle of the Beams (Electronic Warfare)