The Bombing War: Europe 1939-1945

The German Air Force leadership had a persistently exaggerated view of just how accurate their bombing could be under such circumstances. Maps issued to bomber crew had very precise target zones marked in a series of cross-hatched blue rectangles or rhomboids; the precise targets (a gasometer, a power station) were indicated by a small solid circle, while open red circles indicated the presence of decoy sites.¹²⁵ They were expected to find these targets and not to waste their bombs. Instruction to crews in January 1941 identified inaccuracy as the principal problem confronting the force: ‘too many bombs have fallen on open ground far away from the target area ordered’.¹²⁶ The RAF calculated that only between 10 per cent and 30 per cent of aircraft actually found their target (though on a fine moonlit night an estimated 47 per cent found Coventry).¹²⁷ For this there were many explanations. Crews arriving at the bomber bases in the winter of 1940–41 were less experienced than many of the crews lost in combat or accident. At the training schools bomber crew were taught to fly using electronic aids to within one degree of accuracy, but by September the British understood the German beam system, which had never worked perfectly, and were beginning a programme of countermeasures to jam the signals and confuse the pilots.¹²⁸ Although this did not usually prevent German bombers from finding a target city, it was sometimes the wrong one. A raid on the Rolls-Royce works in Derby in May 1941 ended up bombing the nearby city of Nottingham.¹²⁹

The ability of British scientific intelligence, then its infancy, to grasp the nature of the beam system and to effect countermeasures was entirely predictable. The RAF had itself been using the German blind-landing Lorenz system since the 1930s and was familiar with its basic principles. Prisoner interrogations in the spring of 1940 alerted the British to the existence of blind-bombing technology. In June, Churchill’s scientific adviser, Frederick Lindemann, was convinced of the threat posed by the beams by a young Oxford scientist, R. V. Jones, who had been recruited by the Air Ministry as a scientific intelligence expert. Churchill himself insisted on action. An RAF unit, No. 80 Wing, was set up to research countermeasures and on 21 June 1940 the beam frequencies were finally detected. Under the codename ‘Headache’ a high-level research programme began to identify the source of the beams and to find ways of interfering with the signals. Transmitters were set up in southern and central England which could spoil the quality of the German Knickebein radio pulses, though they could not eliminate them altogether. Known as ‘aspirins’ (to cure the headache), there were 15 in place by October, 58 by summer 1941, 68 by the end of the year.¹³⁰ Additional jamming stations were used for the X- and Y-Gerät, which by spring 1941 proved capable of completely misleading the pathfinder forces. By May only one-quarter of German aircraft were getting the bomb-drop signal.¹³¹

Once it became evident that the beams could be distorted or interrupted, crews were advised to avoid complete reliance on the Knickebein system and to resort to using visible markers, including woods, railways, water landmarks (estuaries, rivers, coastline), but not roads, because the roadmaps were known to be out of date.¹³² They were encouraged to imitate the British practice of Koppelnavigation, coupling electronic and visual methods to find the route and the target. All units were asked to choose the most experienced crews for each operation as ‘illuminators’ (Beleuchter) to guide less experienced crews, and to help them avoid British decoy fires which by the end of 1940 stretched across the English countryside.¹³³ Over the following months reliance on electronic navigation and the pathfinders of Kampfgruppe 100 declined and crews had to resort to careful plotting and observation. This was a difficult task, as RAF crews were finding over Germany, and it made the German bomber force as a whole a much blunter instrument than it had been at the start of the campaign. At night, aircraft could achieve a relatively high concentration of bombs on a target area, but not on a precise target. German airmen shot down over Britain and held at the Trent Park interrogation centre in north London were overheard by hidden microphones to admit that the beams were no longer trusted.¹³⁴ They complained among themselves that bombing accuracy was a difficult thing to achieve, demanding high levels of competence. The tension between expectation and reality is well illustrated by the following recorded exchange between a German Air Force major and a lieutenant: