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 , 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 . German systems . " 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 wide at a distance of 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 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 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 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 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 after the preceding ' Oder ' beam — much earlier than the expected . 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 — 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 .