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, developed by Johannes Plendl, 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 distortion of the radio waves. The period ended when the Germans moved their bomber forces to the East in May 1941, in preparation for the attack on the Soviet Union.

Background


Prior to the war, Lufthansa and the German aircraft industry invested heavily in the development of commercial aviation and various systems and methodologies that would improve its safety and reliability. Among these was a considerable amount of research and development of blind landing aids which allowed aircraft to approach an airport at night or in bad weather. The primary system developed for this role was the Lorenz system, which was in the process of being widely deployed on large civilian and military aircraft.

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 found they were on the left side of the centreline they would hear a series of short tones followed by longer pauses - the pauses being the time the signal was being sent out the other side of the antenna. Hearing the "dots", they would know they had to turn to the right in order to be flying down the centreline. If they started on the right side, they would instead hear a series of longer tones followed by short pauses, while the signal was on the "dot" side of the antenna. Hearing the "dashes", they would turn to the left to capture the centreline. In the centre, the radio would receive both signals, which sounded like a continual signal, the so-called "equisignal". Flying in the known direction of the runway and keeping the equisignal on the radio, the Lorenz could guide a plane down a straight line with a relatively high degree of accuracy, so much so that the aircraft could then find the runway visually in all but the worst conditions.

Night bombing
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. And as the bomber's altitude and speed increased, the threat from ground based defences was greatly reduced. Simply put, planners believed that "the bomber will always get through".

The problem with night bombing is that the same limitations in visibility meant the bomb aimers 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. The concept of huge fleets of bombers flying unhindered over their enemy was so reprehensible that the Royal Air Force (RAF) built up its night bomber force largely as a deterrent. It was hoped that the fear of unrestrained attack against German cities would prevent the Luftwaffe from considering attacks against England.

To support this mission, the RAF invested very heavily in navigation training, equipping their aircraft with various equipment, 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 into 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 away from their intended targets.

The Luftwaffe did not take such a fatalistic view of air warfare, and continued to research accurate night bombing against smaller targets. Neglecting any training in celestial navigation, they invested their efforts in radio navigation systems to solve the same problem. 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.

Lorenz had a range of about 30 mi, enough for blind-landing but not good enough for bombing raids over the UK. This could be addressed by using more powerful transmitters and highly-sensitive receivers. In addition the Lorenz beams were deliberately set wide enough that they could be easily picked up at some distance from the runway centreline, but this meant their accuracy at long ranges was fairly limited. This was not a problem for blind landing, where the distance covered by the fan-shaped beams decreased as the aircraft approached the transmitters, but for use in the bombing role this would be reversed, and the system would have maximum inaccuracy over the target.

Knickebein
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 "knock-knee" or "crooked leg". For the required range, broadcast 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.

A single broadcaster would guide the bombers towards the target, but could not tell them when they were over it. To add this ranging feature, a second broadcaster similar to the first was set up so it 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.

The first of these new Knickebein transmitters were 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. 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.



Knickebein was used in the early stages of the German night-bombing offensive, and proved to be fairly effective. However, 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
Efforts in Britain to block the Knickebein system took some time to get started. British intelligence at the Air Ministry, led by R V Jones, was aware of the system initially because a downed German bomber's Lorenz system was analysed by the Royal Aircraft Establishment and seen to be far too sensitive to be a mere landing aid. Also, secretly recorded transcripts from German POW pilots indicated this may have been a bomb aiming aid. Winston Churchill had also been given Ultra intelligence from decrypted Enigma messages that mentioned 'bombing beams'.

When Jones mentioned the possibility of bombing beams to Churchill, he ordered more investigation. The British codenamed the system "Headache". However, many in the Air Ministry did not believe that the system was actually in use, and Frederick Lindemann, leading scientific adviser to the government, claimed 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.

Eckersley's claim was eventually demonstrated after Churchill ordered a flight to try to detect the beams. An Avro Anson was equipped with an American Hallicrafters S-27 amateur radio receiver, the only known receiver capable of receiving the 40 MHz signal at the time, requisitioned from a shop in Lisle Street, London. The receiver was operated by a member of the Y Service. The flight was nearly cancelled when Eckersley withdrew his assertion that the beams would bend round the earth enough to be received. Only R V Jones could save the flight by pointing out that Churchill himself had ordered it and he would make sure that the Prime Minister would get to know who cancelled it.

The crew were not told any specifics, and were simply ordered to search for radio signals around 40 MHz having Lorenz characteristics and, if they found any, to determine their bearing. The flight took off and eventually flew into the beam from Kleve. It subsequently located the cross beam from Stollberg (its origin was unknown prior to this flight). The radio operator and navigator were able to plot the path of the beams and discovered that they crossed right over the Rolls-Royce engine factory at Derby, at that time the only factory producing the Merlin engine. It was subsequently realised that the argument over whether the beams would bend round the earth or not was entirely academic as the transmitters were, more or less, in the line of sight to a bomber flying at high altitude.

British sceptics started regarding the system as proof that the German pilots were not as good as their own, who they believed could do without such systems. It was Lindemann himself who proved this wrong, when aerial reconnaissance systems started returning photographs of the RAF bombing raids, showing that they were rarely, if ever, anywhere near their targets.

Countermeasure
Efforts to block the Knickebein headache were brilliant in their simplicity, and aptly codenamed "Aspirin". Initially, modified medical diathermy sets transmitted interference, but later, on nights where raids were expected, local radio transmitters broadcast an extra "dot signal" at low power. 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 in an attempt to capture the beams' location, a successful capture would then be reported to nearby broadcasters.

The low-power "dot signal" was initially broadcast essentially at random, so German navigators would hear two dots. This meant there were many equi-signal areas, and no easy way to distinguish them except by comparing them with a known location. The British broadcasters were later modified to send their dots at the same time as the German transmitters, making it impossible to tell which signal was which. In this case the navigators would receive the equi-signal over a wide area, and navigation along the bombline became impossible, with the aircraft drifting into the "dash area" and no way to correct for it.

Thus the beam was seemingly "bent" away from the target. 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. A side effect was that as the German crews had been trained to navigate solely by the beams, many crews failed to find either the true equi-signal or Germany again. Some Luftwaffe bombers even landed at RAF bases, believing they were back in the Reich.

X-Gerät
As good as Plendl's Knickebein was, it was never invented to be used in the long-range role. 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"), also known as Wotan. Jones had already concluded the Germans used code names which were too descriptive. He asked a specialist in the German language and literature at Bletchley Park about the word Wotan. The specialist realised Wotan referred to Wōden a one-eyed Germanic god, and might therefore be a single beam navigation system. Jones agreed and knew it would have to include a distance-measurement system. He also concluded it might well work on the system described by an anti-Nazi German mathematician and physicist called Hans Mayer, who while visiting Norway, had passed a large amount of information in what is now known as the Oslo Report.



In fact X-Gerät used a series of beams to locate the target, each beam named after a river. The main beam, Weser, was similar in concept to the one used in Knickebein, but operated at a much higher frequency. Due to the nature of radio propagation, this allowed its two beams to be pointed much more accurately than Knickebein from a similarly sized antenna; the equi-signal area was only about 100 yd wide at a distance of 200 mi from the antenna. The beams were so narrow bombers could not find them on their own, so a low-power wide-beam version of Knickebein was set up at the same station to act as a guide. The main 'Weser' broadcast antenna was set up just to the west of Cherbourg in France.

The "cross" signal in X-Gerät used a series of three very narrow single beams, 'Rhine', 'Oder' and 'Elbe'. At about 30 km from the target, the radio operator would hear a brief signal from 'Rhine', and set up his equipment. This consisted of a special stopclock with two hands. When the 'Oder' signal was received the clock automatically started and the two hands started to sweep up from zero. When the signal from 'Elbe' was received the clock reversed, at which point one hand would stop and the other would start moving back towards zero. 'Oder' and 'Elbe' were aimed to be roughly 5 to 10 km from the bomb release point along the line of 'Weser' (the exact distance depending on the distance from the transmitter), meaning that the clock accurately measured the time to travel between the first two beams along the flight path. Since the time taken to travel that distance should be the same as the time needed to travel the last 5 km from 'Elbe' to the target, the moving hand reached zero and the bombs were automatically released. To be exact, the 'Elbe' signal was adjusted to correct for the distance the bombs would travel between release and impact.

Since X-Gerät operated on a much higher frequency than Knickebein (around 60 MHz) it required the use of new radio equipment. There were not nearly enough sets to go around, so instead the experimental unit Kampf Gruppe 100 (KGr 100) was given the task of using their sets in order to guide other aircraft to the target. To do this, KGr 100 planes would attack as a small group first, dropping flares which other planes would then see and bomb visually. This is the first use of the pathfinder concept that the RAF would perfect to great effect against the Germans only a few years later.

X-Gerät was used effectively in a series of raids known to the Germans as "Moonlight Sonata", against Coventry, Wolverhampton and Birmingham. In the raid on Birmingham only KGr 100 was used, and British post-raid analysis showed that the vast majority of the bombs dropped were placed within 100 yd of the midline of the 'Weser' beam, spread along it for a few hundred yards. This was the sort of accuracy that even daytime bombing could rarely achieve. A similar raid on Coventry with full support from other units dropping on their flares almost destroyed the city centre.

Countermeasure
X-Gerät proved more difficult to stop than Knickebein. 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. Although Jones had correctly guessed the beam layout (and acknowledges it was only a guess), the modulation frequency had been measured incorrectly as 1500 Hz, but was in fact 2000 Hz. At the time it was believed that this would not make any difference, as the tones were close enough that an operator would have a hard time distinguishing them in a noisy aircraft.

The mystery was eventually revealed after an X-Gerät-equipped Heinkel He 111 crashed on 6 November 1940 on the English coast at Chesil Beach. Although the plane sank during the recovery operation, the waterlogged X-Gerät equipment was recovered. On examination, it was learned that a new instrument was being used that automatically decoded the dots and dashes and displayed a pointer in the cockpit in front of the pilot. This device was fitted with a very sharp filter which was sensitive only at 2000 Hz, and not the early British 1500 Hz counter-signals. While the jammers were modified accordingly; this came too late for the raid on Coventry on 14 November, but the modified jammers were able to successfully disrupt a raid on Birmingham on 19 November.

X-Gerät was eventually defeated in another manner, by way of a "false 'Elbe' " which was set up to cross the 'Weser' guide beam at a mere 1 km after the preceding 'Oder' beam — much earlier than the expected 5 km. Since the final stages of the release were automatic, the clock would reverse prematurely and drop the bombs kilometres short of the target. Setting up this false beam proved very problematic as the Germans, learning from their mistakes with Knickebein, did not switch the X-Gerät beams on until as late as possible, making it much more difficult to arrange the "false 'Elbe' " in time.

Y-Gerät
As the British slowly gained the upper hand in the Battle of the Beams, they started considering what the next German system would entail. Since Germany's current approaches had been rendered useless, an entirely new system would have to be developed.

British monitors soon started receiving intelligence from Enigma decrypts referring to a new device known as Y-Gerät, which was also sometimes referred to as Wotan II, and was also developed by Dr. Plendl. Y-Gerät used a single narrow beam pointed over the target, broadcasting a modulated radio signal. The system used a new piece of equipment that received the signal from the beam and immediately re-broadcast it back to the ground station. The ground station listened for the return signal and compared its phase to the transmitted signal. This is an accurate way of measuring the transit time of the signal, and hence the distance to the aircraft. 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.

Countermeasure
The British were ready for this system even before it was used. By chance, the Germans had chosen the operating frequency of the Wotan II system very badly; it operated on 45 MHz, which just happened to be in the frequency range of the powerful-but-dormant BBC television transmitter at Alexandra Palace. All Jones had to do was arrange for the return signal to be received from the aircraft and then sent to Alexandra Palace for re-transmission. The combination of the two signals modified the phase shift &mdash; and thus the apparent transit delay. Initially, the signal was re-transmitted at low power, not powerful enough for the Germans to realise what was happening, but enough to spoil the accuracy of the system. Over subsequent nights, the transmitter power was gradually increased.

As Wotan II's use went on, the aircrew accused the ground station of sending bad signals and the ground station alleged the aircraft had loose connections. The whole scheme appealed to Jones as he was a natural practical joker, and remarked that he was able to play one of the largest practical jokes with virtually any national resource that he required. The gradually increasing power conditioned the Germans such they did not realise that anyone was interfering with the system, but believed that it suffered several inherent defects. Eventually, as the power was increased enough, the whole Wotan II system started to ring with all the feedback.

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, although by this time Hitler's attention was turning toward Eastern Europe.