Counter-measures
British intelligence soon discovered the nature of the Kammhuber Line and started studying ways to defeat it. At the time RAF Bomber Command sent in their planes one at a time to force the defenses to be spread as far apart as possible, meaning that any one aircraft had to deal with little concentrated flak. This also meant the Himmelbett centres 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 - the so-called Bomber stream, carefully positioned so the stream flew down the middle of a single 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 a 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 radio navigation in 1942 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. 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 were directed into the stream, where it was hoped they would be able to find aircraft with their radar. At the same time a massive building program started to add hundreds of 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 was 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, and as soon as the rates started to improve - for the Germans - they introduced "Window". Dropping strips of foil 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 were then sent to attack this stream, only to find empty space. Just as the fighters reached the false stream, the "real" stream appeared hundreds of miles away, too far to be attacked. The first time this was used was during Operation Gomorrah (a week-long bombing campaign against Hamburg) and proved spectacularly effective. The German radar operators eventually learned to spot the lead bombers at the edge of the windowing, making it less effective. The British had held back from introducing Window for over a year lest the technique be adopted by the Germans and used against British cities.
A more sophisticated method for blinding the German radar was "Mandrel", a jamming signal broadcast from aircraft accompanying the bomber stream or later certain bombers themselves. This progressed into jamming techniques against individual German radar types and spoofing radars to see bomber streams that weren't there. The British also attacked the communications between ground stations and fighters, broadcasting false directions in authentic accents over the radio.
One other element was long-range nightfighters operating against the German nightfighters, using a system called "Serrate" to home in on the German nightfighter radar signals. At least three squadrons equipped with Bristol Beaufighter and de Havilland Mosquito were part of No. 100 Group RAF supporting Bomber Command with electronic countermeasures.
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