This was the German original concept for a night-fighter network and tactics in which each target (a British bomber) needed the attentions of two Freya radars (one to track the bomber and the other to track the intercepting night-fighter), with a ground controller vectoring in the fighter until its own Würzburg short-range radar picked up the target bomber (April 1941/May 1945).
A chain of such stations ran round the perimeter of Germany from the North Sea to the Swiss frontier.
The ‘Himmelbett’ system was the operational core of the ‘Kammhuber Line’, as the British called it and which was the night air-defence system ordered in July 1940 and activated in April 1941 by Generalmajor (later General) Josef Kammhuber and his XII Fliegerkorps which was, initially, a formation to co-ordinate Flak, searchlight and radar units. At first the line comprised a series of radar stations with overlapping coverage, layered three deep from Denmark to the middle of France, each covering a zone about 18.5 miles (30 km) deep and 12.5 miles (20 km) wide. Each control centre was known as a ‘Himmelbett’ zone, consisting of a Freya radar with a range of about 62 miles (100 km), 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 back-up night-fighter (usually Dornier Do 17, Junkers Ju 88 or Messerschmitt Bf 110 twin-engined machines).
This ground-controlled interception tactic had been preceded by the use of Messerschmitt Bf 109 single-engined fighters, without radar, which were home on bombers illuminated by searchlights in the ‘Helle Nachtjagd’ (illuminated night fighting) tactic.
Any British night bomber flying into Germany or France had to cross the line at some point, at which time the Freya radar operators directed the master searchlight to illuminate the bomber. Once this had happened other manually controlled searchlights also picked up the plane, and the night-fighters were directed to intercept the illuminated bomber. However, the demands of Bürgermeisters within Germany meant that many of the searchlights were recalled to bolster the defence of major German cities, and this reduced the capability of the ‘Kammhuber Line’.
Later versions of the ‘Himmelbett’ system added two Würzburg radars, each with a range of about 18.5 miles (30 km). Unlike the early warning Freya radar, the Würzburg radar was able to track accurately. One radar was locked onto the night-fighter as soon as it entered the cell, and as soon as the Freya radar picked up a target the second Würzburg locked onto it. All position reports were sent to the ‘Himmelbett’ control centre, which allowed operators in the ‘Himmelbett’ centre to plot the movement of both aircraft. A control officer then used radio to direct the night-fighter to a position in which it could sight the bomber visually and then make its attack.
To aid interception a number of the night-fighters were fitted with a short-range IR device known as ‘Spanner’, but this proved almost useless. Later the short-range Lichtenstein radar was added to the aircraft, allowing them to detect aircraft once the operators had directed them into the area, making searchlights largely redundant.
The system had reached maturity by 1942, and then remained essentially unaltered until the end of the war.
British intelligence soon established the nature of the ‘Kammhuber Line’ and started to investigate ways in which it could be defeated. At the time RAF Bomber Command sent its aircraft singly to divide the defences, meaning that any one bomber had to deal with little concentrated Flak. But this also meant that each ‘Himmelbett’ centre had to deal with only one or two aircraft at a time, much easing the German task. Bomber Command then reorganised its attacks into bomber streams, carefully positioned so the stream flew down the middle of a cell.
Data provided to British scientists allowed them to calculate that the bomber stream would overwhelm the six potential interceptions per hour that the ‘Zahme Sau’ (tame boar) 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 the ‘Gee’ radio navigation aid allowed the British 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 minimise the risk of collision.
The first use of the bomber stream was ‘Millennium’, the first ‘Thousand-Bomber’ raid against Köln on the night of 30/31 May 1942. The new British tactic was very effective, and there developed a major difference of opinion between Kammhuber and Generalfeldmarschall Erhard Milch, the air inspector general, about how best to counter the British tactic. Although the success rate of the ‘Kammhuber Line’ declined, the network of radars and plotting stations continued to prove its value. When a raid started, night-fighters from every base within range were directed into the stream, where it was hoped they would be able to find aircraft with their own radar. At the same time a massive building programme started to add hundreds of Würzburg radars to the system. The boxes were initially sized to the radius of the Würzburg radars, about 22 miles (35 km), but the later advent of more powerful radars allowed the boxes to be resized to a width of 100 miles (160 km). The line of boxes was eventually several deep, especially around larger cities and in 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 German success rate started to improve the British introduced chaff, designated as ‘Window’ at the time. After chaff had been dropped by a number of ‘lead’ bombers, the German radar operators saw what appeared to be a stream entering their box, each packet of chaff appearing on their screens to be a bomber. Night-fighters were then sent to attack this stream, only to find empty space. Just as the fighters reached the chaff stream, the real stream appeared hundreds of miles away, too far distant to be attacked.
The first time this new British tactic was employed was during the ‘Gomorrah’ firestorm attack on Hamburg in August 1943, and proved very effective. The German radar operators eventually learned to spot the lead bombers at the edge of the ‘Window’ cloud, making it less effective.