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. The period ended when the Wehrmacht moved their forces to the East in May 1941, in preparation for the attack on the Soviet Union.[1] The idea of beam radio navigation was developed during the 1930s, initially as a blind landing aid. The basic concept is to produce two directional radio signals that are aimed slightly to the left and right of a runway's midline. Radio operators in the aircraft listen for these signals and determine which of the two beams they are flying in. This is normally accomplished by sending Morse code signals into the two beams, to identify right and left. 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 became convinced the British had somehow learned to bend radio signals. When the problem became widespread, the Germans introduced a new system that worked on different principles, the Y-Gerät. Having guessed the nature of this system from a passing mention, the British had already deployed countermeasures that rendered the system useless almost as soon as it was used. The Germans eventually abandoned the entire concept of radio navigation over the UK, concluding the British would continue to successfully jam it. The Lorenz beam and its two lobes. The "equisignal" area in the centre grows narrower, and more accurate, as the aircraft approaches the runway. 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. Considerable effort went into blind-landing aids which allowed aircraft to approach an airport at night or in foul weather. The primary system developed for this was the Lorenz system, developed by Johannes Plendl, which was in the process of being widely deployed on large civilian and military aircraft.[2] The Lorenz system worked by feeding a special three-element antenna system with a modulated radio signal. The signal was fed to the centre dipole, which had a slightly longer reflector element on either side set slightly back. A switch rapidly alternated the opened midpoint connection of each reflector in turn, sending the beam slightly to the left and then slightly to the right of the centreline of the runway. The beams widened as they spread from the antennas, so there was an area directly off the runway approach where the two signals overlapped. The switch was timed so it spent longer on the right side of the antenna than the left. An aircraft approaching the airport would tune one of their radios to the Lorenz frequency. If the crew was on the left side of the centreline, they would hear a series of short tones followed by long pauses, meaning the aircraft was on the "dot" side of the antenna. Hearing the "dots", they would know that they had to veer to the right to fly down the centreline. If the crew was on the right side of the centerline, they would hear a series of long tones followed by short pauses, meaning the aircraft was on the "dash" side of the antenna. Hearing the "dashes", they would know that they had to veer to the left to fly down the centreline. In the centre, the radio would receive both signals, where the dots filled in the gaps in the dashes and produced a continual signal, the so-called "equisignal". Flying in the known direction of the runway and keeping the equisignal on the radio, Lorenz-equipped crews could guide an aircraft down a straight line with a relatively high degree of accuracy, so much so that pilots could then find the runway visually except in the worst conditions. During the early 1930s, the concept of a night bombing strategic campaign began to become paramount in military aviation circles. This was due to the ever-increasing performance of bombers, which were beginning to have the capability to strike across Europe with useful bomb loads. These aircraft were slow and lumbering, easy prey for interceptors, but this threat could be essentially eliminated by flying at night. A bomber, painted black, could be spotted only at very short ranges. As the bomber's altitude and speed increased, the threat from ground-based defences was greatly reduced. Planners believed that "the bomber will always get through". The problem with night bombing is that the same limitations in visibility meant the bomb crew would have a difficult time finding their targets, especially a blacked-out target at night. Only the largest targets, cities, could be attacked with any probability of success. To support this mission, the RAF invested heavily in navigation training, equipping their aircraft with various devices, including an astrodome for taking a star fix and giving the navigator room to do his calculations in an illuminated workspace. This system was put to use as soon as the war began and was initially regarded as successful. In reality, the early bombing effort was a complete failure, with the majority of bombs landing miles from their intended targets.[3] The Luftwaffe continued to research accurate night bombing against smaller targets. Not depending on celestial navigation, they invested their efforts in radio navigation systems. The Luftwaffe concentrated on developing a bombing direction system based on the Lorenz concept through the 1930s, as it made night navigation relatively easy by simply listening for signals on a radio set, and the necessary radios were already being installed on many aircraft. Map of Knickebein transmitters For bombing use, the modifications to Lorenz were fairly minor. Much larger antennas were needed to provide the required accuracy. This was achieved by using aerials with many more elements, but it retained the simple switching of two of the reflector elements to alter the beam directions very marginally. The beam angles were so dramatically reduced that it was only a few tens of yards wide over the target. It was the shape of the aerials that gave the system its code name, Knickebein, which means "crooked leg".[4][a] The word is also the name of a magical raven in Germanic mythology.[5] For the required range, transmitted power was increased considerably. The Knickebein receivers were disguised as a standard blind landing receiver system, consisting apparently of the EBL-1 and the EBL-2 receivers. The beam from a single transmitter would guide the bombers towards the target, but could not tell them when they were over it. To add this ranging feature, a second transmitter similar to the first was set up so its beam crossed the guidance beam at the point where the bombs should be dropped. The aerials could be rotated to make the beams from two transmitters cross over the target. The bombers would fly into the beam of one and ride it until they started hearing the tones from the other (on a second receiver). When the steady "on course" sound was heard from the second beam, they dropped their bombs.[6] Later, smaller Knickebein antenna The first of these new Knickebein transmitters was set up in 1939 on Stollberg hill in Nordfriesland near the border with Denmark; at Kleve (Cleves) near the Dutch border, almost the most westerly point in Germany, and at Lörrach near the border with France and Switzerland in south-western Germany.[7][8] Following the fall of France in June 1940, additional transmitters were installed on the French coast. Stations were also constructed in Norway and the Netherlands.[9][10] Knickebein were used in the early stages of the German night-bombing offensive and proved to be fairly effective, but the tactics for using the system in a widespread bombing effort were not yet developed, so much of the early German night bombing offensive was limited to area bombing. The search for the beams[edit] Efforts in Britain to block the Knickebein system required time to implement. British intelligence at the Air Ministry, led by R. V. Jones, became aware of the system when the Royal Aircraft Establishment analysed a downed German bomber's Lorenz system and observed it was far more sensitive than required for a mere landing aid. Secretly recorded transcripts from German prisoner of war pilots indicated this may have been a bomb aiming aid.[11] Winston Churchill had also been given Ultra intelligence from decrypted Enigma messages that mentioned 'bombing beams'.[10] When Jones mentioned the possibility of bombing beams to Churchill, he ordered further investigation. The British codenamed the system Headache.[12] Many in the Air Ministry did not believe that the system was in use. 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.[13]