Kammhuber Line

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A map of part of the Kammhuber Line stolen by a Belgian agent and passed-on to the British in 1942. The 'belt' and nightfighter 'boxes' are shown

The Kammhuber Line was the name given to the German night air defense system established in July 1940 by Colonel Josef Kammhuber.

The first version of the Line consisted of a series of radar stations with overlapping coverage, layered three deep from Denmark to the middle of France, each covering a zone about 32km long (north-south) and 20km wide (east-west). Each control center was known as a Himmelbett zone, consisting of a Freya radar with a range of about 100km, a "master searchlight" directed by the radar, and a number of manually directed searchlights spread through the cell. Each cell was also assigned one primary and one backup night fighter. The fighter used was usually a Dornier Do 17, Junkers Ju 88 or Messerschmitt Bf 110.

RAF bombers flying into Germany or France would have to cross the line at some point, and the Freya radar operators would direct the master searchlight to illuminate the plane. Once this had happened other manually-controlled searchlights would also pick up the plane, and the night fighters would be directed to intercept the now-lit bomber. However, demands by the Bürgermeisters in Germany led to the recall of the searchlights to the major cities.

Later versions of the Himmelbett added two Würzburg radars, with a range of about 30km. Unlike the early-warning Freya, Würzburg's were accurate (and complex) tracking radars. One would be locked onto the night fighter as soon as it entered the cell. After the Freya picked up a target the second Würzburg would lock onto it, thereby allowing controllers in the Himmelbett center to get continual readings on the positions of both planes, controlling them to a visual interception. To aid in this, a number of the night fighers were fitted with a short-range infrared device known as Spanner, but these proved almost useless in practice. Later additions added the short-range Lichtenstein radar to the aircraft, allowing them to detect the aircraft once the operators had directed them into the general area.

British intelligence soon discovered the nature of the Kammhuber Line and started studying ways to defeat it. At the time Bomber Command sent in their planes one at a time in order to force the defenses to be spread as far apart as possible, meaning that any one aircraft would have to deal with little concentrated flak. However this also meant the Himmelbett centers were only dealing with perhaps one or two planes at a time, making their job much easier.

At the urging of R.V. Jones, Bomber Command reorganized their attacks into streams of bombers, carefully positioned so the stream would fly right down the middle of a cell. Data provided to the British scientists allowed them to calculate that the bomber stream would overwhelm the six potential interceptions per hour that the German "Tame Boar" (Zahme Sau) night fighters could manage in any one Himmelbett zone. It was then a matter of calculating the statistical loss from collisions against the statistical loss from night fighters to calculate how close the bombers should fly to minimise RAF losses. The introduction of Gee allowed the RAF bombers to fly by a common route and at the same speed to and from the target, each aircraft being allotted a height band and a time slot in a bomber stream to minimize the risk of collision. The first use of the bomber stream was the first 1,000 bomber raid against Cologne on the night of 30/31 May 1942^[1]. This tactic was extremely effective, leading to fighting between Kammhuber and Erhard Milch, his boss.

Although the success rate of the Line dropped, the network of radars and plotting stations continued to prove their worth. Now when a raid started, night fighters from any base within range would be directed into the stream, where it was hoped they would be able to find aircraft with their own radars. At the same time a massive building program started to add hundreds of additional Würzburgs to the system, although the infrastructure needed was extensive. The boxes were initially the radius of the Würzburg radars, about 22 miles, but more powerful radar later on made the boxes up to 100 miles across. Eventually, the line of boxes would be several deep, especially around larger towns and the Ruhr valley. Once again the system started to score increasing successes against the British raids.

The British were ready for this development, however, and as soon as the rates started to improve they added 'windowing' cover. By dropping chaff from a number of "lead" bombers, the German radar operators saw what appeared to be a stream entering their box, each packet of chaff appearing to be a bomber on their displays. Night fighters would then be sent to attack this stream, only to find empty space. Just as the fighters reached the chaff stream, the "real" stream would appear hundreds of miles away, too far to be attacked. The first time this was used was during the firestorm attack on Hamburg, (Operation Gomorrah), and proved spectacularly effective. The Germans radar operators eventually learned to spot the lead bombers at the edge of the windowing, making it less effective.