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Nightfighting in World War Two
By Lee Brimmicombe-Wood

Part 4. Boxes, Rooms and Four Post Beds

THE STORY SO FAR: The development of air intercept (AI) radar was one of the final pieces of the jigsaw for the night fighter commanders. The trick now was to integrate it into a system of ground control.

By mid-1942 the Germans already had a number of systems of night fighter control. First was the Henaja system of illuminated fighting using searchlights (see Part 1). This was being enhanced by the use of the new Würzburg radar, which had the precision to direct a searchlight beam onto a bomber. Each searchlight zone had three Würzburgs positioned at the front, middle and back, able to detect bombers as they entered and exited the zone. However, few bombers were shot down as they left because RAF pilots began to dive as they transited the zone, accelerating past the hazard at low altitude.

The second system was an evolution of the Freya-AN system for directing individual fighters by radar (see Part 2). This grew into a line of 'dark' night fighting zones, or Dunaja, positioned in front of the coastal radar chain. The Freya radar was not precise enough for effective ground control, but the new 'Giant' Würzburg (Würzburg-Riese) had both accuracy and range enough for the job.

With the arrival of Würzburg-Riese in the autumn of 1941 Josef Kammhuber was able to overhaul his nightfighting organization. The searchlight belts were widened and now Dunaja zones 36km in depth, combining Freya and Würzburg, were placed directly in front of the Henaja belt. A further piece of the jigsaw was the Seeburg plotting table, derived from ideas developed by the operational units. This was a 1:50,000 map of the interception zone onto which points of light, representing bomber and fighter units, were projected. This information, updated from the radars, gave a controller a picture of the battle that was clear enough for him to direct a nightfighter to its prey.

ILLUSTRATION: A Belgian patriot (a Brussels jeweller) supplied the Allies with this drawing of the Seeburg table, showing how light pointers beneath the table could be rotated to plot an aircraft's location on the map.

This system of dark and light fighting zones was to prove tremendously complex and Kammhuber experimented with narrowing and widening the searchlight belt. It wasn't until the spring of 1942 that many of the bugs were worked out and the system began to work effectively.

However, Kammhuber's XII Fliegerkorps was faced with a new setback. The Gauleiters of regions most frequently raided by the RAF petitioned Hitler, claiming that the vast searchlight resources devoted to the 'Kammhuber Line' were a luxury and would be better employed in support of the flak defending their towns and cities. From March to July 1942 the searchlight belt was gradually withdrawn on Hitler's orders.

The decision to withdraw the searchlights shocked and angered the night-flyers, but gave birth to two new systems of control. The 'combined' fighting zone, or Konaja, was a response to the loss of the searchlights and the lack of success at intercepting fast, four-engined bombers as they dived through the illuminated belts. By reasoning that bombers had to maintain a fixed course and height when approaching a target, Kammhuber tried to combine flak, the withdrawn searchlights and fighters over a number of target cities. There were problems with coordinating the fighters with flak (which would tend to shoot at everything and disobey orders to cease fire at the approach of friendly fighters) and Konaja at best enjoyed modest success.

The other system was a further development of Dunaja radar-directed 'dark fighting'. The withdrawal of the searchlight belt left the radars in position. At each station one 'Red Giant' Würzburg-Riese would acquire an enemy, while a second 'Green Giant' tracked a nightfighter. Both positions were transmitted back to a Seeburg plotting table from which a controller would guide the interception. This procedure became known as the Himmelbett ('Four Post Bed') method. Himmelbett was not without its disadvantages. As with Freya-AN only one fighter could be controlled at a time, which meant that each box-like Himmelbett zone, or Raum, could easily be overwhelmed by a mass raid. Though Würzburg-Riese was a more precise radar than Freya, interception was still a matter of luck.

PHOTO: This surviving example of the FuMG 65 Würzburg-Riese is preserved at Douvres. As can be seen it was, indeed, a 'Giant'. It used the electronics and 50cm wavelength of the small Würzburg, but achieved greater range by increasing the size of the dish from 3.0 to 7.4m diameter. It was able to search out to 60km, one and a half times the range of the original Würzburg, though 80km was claimed. The exceptional accuracy of the Würzburg in azimuth (±0.2 degrees) was achieved by rotating the dipole to create a conical form of lobe switching. As with many radars of this period, height-finding was less accurate, achieving ±4-16 degrees. With various modifications and improvements it remained in service as the standard ground control radar for the remainder of the war.

The turning point came with the deployment of Lichtenstein BC AI radar aboard aircraft. Production of sets had delayed deployment and it wasn't until mid-1942 that these radars reached units in significant numbers. After initial suspicion (some guinea-pig pilots at II/NJG 1 regarded the sets as 'new-fangled rubbish') successes meant they were quickly embraced by the nightfighter crews. With airborne radar able to provide terminal guidance to the bombers, the success rate shot up so sharply that pilots soon forgot the loss of the searchlights.

In Britain, the RAF was already ahead in the field of AI radar and Ground Controlled Intercept (GCI). With the revolutionary map-like PPI radar display (see Part 2) controllers had the means to effectively guide Beaufighter and Blenheim fighters to the point where their AI Mk IV could pick up bomber targets. As with the Himmelbett system, stations in the GCI network could guide just one aircraft at a time and the system could easily become saturated. To counter this possibility, Air Marshal Sir William Sholto Douglas of the RAF and General Sir Frederick Pile of AA Command devised the Smack system of searchlight control in late 1941.

Smack repositioned searchlights into a series of 'Fighter Boxes' deployed ahead of the Ack-Ack gun line. Box size varied with the target being protected, but ideally it had a depth of 32 miles and a width of 14 miles. At the centre of the box a vertically-pointing searchlight provided a beacon around which a fighter could orbit. The first 12 miles of the box was called the 'Indicating Zone' whose purpose was to use lights to point the general direction of the enemy's approach. Behind this was the 'Killing Zone', where searchlights were dense enough to continuously illuminate an enemy for a visual kill. Of course, AI interceptions were also permitted in these zones and if fighters were able to make AI contact they could call a radio codeword to douse the searchlights. It was not unusual for a pilot to complete his interception before reaching the end of the Killer Zone; should he not be successful he would break off before entering the gun defended area and leave the bomber to the Ack-Ack guns.

ILLUSTRATION: The Smack system resembled Kammhuber's efforts to integrate dark fighting and searchlights before it was undone by the Gauleiters. It was flexible in that it could supplement the GCI network. Searchlights in the Indicator Zones could point nightfighters in the direction of bombers so that they could complete interceptions without ground control.

Smack was fully implemented by early 1942, and in concert with the GCI network and the superlative new Mosquito nightfighter remained the principal means of night interception for the remainder of the war. First bloodied during the 'Baedecker Raids' of 1942, the system would reach the peak of effectiveness in January 1944 during the 'baby Blitz', Operation Steinbock, when it took an enormous toll on the attacking Luftwaffe.

But that was in the future, and the air defences of Great Britain never had to face the massive raids that, in 1943, were to smash the Kammhuber Line.

Next: Bomber streams, Wild Boars and Tame Boars.

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