Battle of the Beams

The Battle of the Beams was a period early in the Second World War when bombers of the German Air Force (Luftwaffe) used a number of increasingly accurate systems of radio navigation for night bombing in the United Kingdom.

British scientific intelligence at the Air Ministry fought back with a variety of their own increasingly effective means, involving jamming and deception signals.

For bombing, the Luftwaffe built huge versions of the antennas to provide much greater accuracy at long range, named Knickebein and X-Gerät.

These were used during the early stages of the Blitz with great effect, in one case laying a strip of bombs down the centerline of a factory deep in England.

Tipped off about the system's operation by pre-war military intelligence, the British responded by sending their own Morse code signals so that the aircraft crew believed they were always properly centred in the beam while they flew wildly off course.

The Germans eventually abandoned the entire concept of radio navigation over the UK, concluding the British would continue to successfully jam it.

Before the start of the war on 1 September 1939, Lufthansa and the German aircraft industry invested heavily in the development of commercial aviation, and in systems and methods that would improve safety and reliability.

Frederick Lindemann, leading scientific adviser to the government, argued that any such system would not be able to follow the curvature of the Earth, although T. S. Eckersley of the Marconi company had said it could.

The RAF lacked equipment capable of detecting 30–33 MHz Lorenz signals, so they purchased an American Hallicrafters S-27 amateur radio receiver[14] from a shop in Lisle Street, London.

[13] The crew were not told specifics, and were simply ordered to search for radio signals around 30 MHz having Lorenz characteristics and, if they found any, to determine their bearing.

It was subsequently realised that the argument over whether the beams would bend around the Earth was entirely academic, as the transmitters were more or less in line-of-sight to high altitude bombers.

The Butt Report proved this to be wrong: aerial reconnaissance returned photographs of the RAF bombing raids, showing that they were rarely, if ever, anywhere near their targets.

Later, local radio transmitters broadcast an extra "dot signal" at low power on nights where raids were expected.

[17] The German practice of turning on the beams long before the bombers reached the target area aided the British efforts.

Avro Ansons fitted with receivers would be flown around the country to find the beams' location to be reported to nearby broadcasters.

Eventually, the beams could be inclined by a controlled amount which enabled the British to fool the Germans into dropping their bombs where they wanted them.

Plendl had been working for some time to produce a much more accurate version of the same basic concept, which was eventually delivered as X-Gerät (X-Apparatus).

The beams were so narrow that bombers could not find them without help, so a low-power wide-beam version of Knickebein was set up at the same station to act as a guide.

As the bomber followed the Weser beam and reached Rhine, the radio operator heard a brief signal and set up his equipment.

The system was first tested on 20 December 1939 when a bomber from KGr 100 flown by Oberleutnant Hermann Schmidt flew over London at 7,000m (23,000 ft).

Initial defences against the system were deployed in a similar fashion to Knickebein in an attempt to disrupt the Coventry raid but proved to be a failure.

[26] The mystery was eventually revealed after an X-Gerät-equipped Heinkel He 111 crashed on 6 November 1940 on the English coast at West Bay, Bridport.

[27] On examination, it was learned that a new instrument was being used that automatically decoded the dots and dashes and moved a pointer on a display in the cockpit in front of the pilot.

Since the final stages of the release were automatic, the clock would reverse prematurely and drop the bombs kilometres short of the target.

[30] As the British slowly gained the upper hand in the Battle of the Beams, they started considering what the next German system would entail.

He concluded that it might work on the basis described by the anti-Nazi German mathematician and physicist Hans Mayer, who while visiting Norway had provided a large amount of information in what is now known as the Oslo Report.

Coupled with the direction of the beam (adjusted for a maximum return signal), the bomber's position could be established with considerable accuracy.

The bombers did not have to track the beam, instead the ground controllers could calculate it and then give radio instructions to the pilot to correct the flight path.

The gradually increasing power conditioned the Germans such they did not realise that the system was being interfered with, but believed that it suffered several inherent defects.

The Luftwaffe, finally realising that the British had been deploying countermeasures from the very first day that the system was used operationally, completely lost faith in electronic navigation aids as the British had predicted, and did not deploy any further system against Great Britain,[36] although by this time Hitler's attention was turning towards Eastern Europe.