Battle of the beams
Encyclopedia
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. British "scientific intelligence" at the Air Ministry
fought back with a variety of 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.
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 opened the mid-point 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 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 center, the radio would receive both signals, which sounded like a continual signal, the so-called "equisignal". Flying the known direction of the runway and keeping the equisignal on the radio, Lorenz could fly a plane down a straight line with relatively high accuracy, enough so that the aircraft could then find the runway visually in all but the worst conditions.
s from fighter interception and ground-based anti-aircraft systems were greatly reduced. However, the disadvantage of this strategy was the difficulty of finding a blacked-out
target at night.
The Royal Air Force
(RAF
) thus 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 calculations in a lit workspace. They put this system into use as soon as the war began and were initially happy with its success. In reality, the early bombing effort was a complete failure, with the majority of bombs landing miles away from their intended targets.
The Luftwaffe instead invested heavily in radio navigation
systems to solve the same problem, notably neglecting any training in celestial navigation
. 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 miles (48.3 km), 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 beams of Lorenz 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 airplane approached the transmitters, but for use in the bombing role this would be reversed, and the system would have maximum inaccuracy over the target.
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 ("crooked leg") transmitters were set up in 1939 on the 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, further 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 anyway.
, were 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 Enigma
messages) mentioning 'bombing beams'.
When Jones mentioned the possibility of bombing beams to Churchill, Churchill 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, though T S Eckersley of the Marconi company had claimed 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
, operated by a member of the Y Service. The flight was nearly cancelled when Eckersley withdrew his claim 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 Churchill 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.
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 his "photoflash" systems started returning photographs of the RAF bombing raids, showing that they were rarely, if ever, anywhere near their targets.
sets transmitted interference, but later, on nights where raids were expected, local radio transmitters broadcast a surplus "dot signal" at low power. The German predilection for turning on the beams long before the bombers reached the target area aided the British efforts. Ansons
fitted with receivers would be flown around the country in an attempt to capture the beams' location, and 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 with a known location. The British broadcasters were later modified to broadcast their dots at the same time the German transmitters would, 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 "bent" away from the target. Eventually, the beams could be bent 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 bombers even landed at RAF bases, believing they were back in Germany.
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 yards (91.4 m) wide at a distance of 200 miles (321.9 km) 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
.
The "cross" signal in X-Gerät used a series of three very narrow single beams, Rhine, Oder and Elbe. About 30 kilometres (18.6 mi) 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 (3.1 to 6.2 mi) 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 kilometres (3.1 mi) from Elbe to the target, when the moving hand reached zero 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 new radio equipment to be used. There were not nearly enough sets to go around, so instead the experimental unit KGr 100 (Kampfgruppe 100) was given the task of using their sets in order to guide other planes 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 to great effect 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 yards (91.4 m) of the midline of the Weser beam, spread along it 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 nearly destroyed the city centre.
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 kilometre (0.621372736649807 mi) after the preceding Oder beam — much earlier than the expected 5 kilometres (3.1 mi). 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, didn't switch the X-Gerät beams on until as late as possible, making it much more difficult to arrange the "false Elbe" in time.
British monitors soon started receiving intelligence intercepts referring to a new device known as Y-Gerät, which was also sometimes referred to as Wotan. R V Jones had already concluded the Germans used code names which were too descriptive. He asked a specialist in German language and literature at Bletchley Park
about the word Wotan
. The specialist realised Wotan, the name of a one-eyed god, might be a single beam navigation system. Jones agreed and knew it would have to be based on a distance-measurement system. He also concluded it might well work on the system described by an anti-Nazi German mathematician and physicist Hans Mayer
, who while visiting Norway had passed a large amount of information in what is now known as the Oslo Report
.
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 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 gave radio instructions to the pilot to correct the flight path.
The British were ready for this system even before it was used. By chance, the Germans had chosen the operating frequency of the Wotan system very badly; it operated on 45 MHz, which just happened to be the frequency 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 a 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's use went on, the aircrew accused the ground station of sending bad signals and the ground station accused the aircraft of having 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 system started to ring with all the feedback.
Luftwaffe
Luftwaffe is a generic German term for an air force. It is also the official name for two of the four historic German air forces, the Wehrmacht air arm founded in 1935 and disbanded in 1946; and the current Bundeswehr air arm founded in 1956....
) used a number of increasingly accurate systems of radio navigation
Radio navigation
Radio navigation or radionavigation is the application of radio frequencies to determine a position on the Earth. Like radiolocation, it is a type of radiodetermination.The basic principles are measurements from/to electric beacons, especially...
for night bombing. British "scientific intelligence" at the Air Ministry
Air Ministry
The Air Ministry was a department of the British Government with the responsibility of managing the affairs of the Royal Air Force, that existed from 1918 to 1964...
fought back with a variety of 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, LufthansaLufthansa
Deutsche Lufthansa AG is the flag carrier of Germany and the largest airline in Europe in terms of overall passengers carried. The name of the company is derived from Luft , and Hansa .The airline is the world's fourth-largest airline in terms of overall passengers carried, operating...
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
Lorenz beam
The Lorenz beam blind landing system was an air radio navigation system in use from the late 1930s. The name refers to the company that produced the system; Lorenz referred to it simply as the Ultrakurzwellen-Landefunkfeuer, German for "ultra-short-wave landing radio beacon", or LFF...
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
Dipole
In physics, there are several kinds of dipoles:*An electric dipole is a separation of positive and negative charges. The simplest example of this is a pair of electric charges of equal magnitude but opposite sign, separated by some distance. A permanent electric dipole is called an electret.*A...
, which had a slightly longer reflector element
Reflector (antenna)
An antenna reflector is a device that reflects electromagnetic waves.It is often a part of an antenna assembly.The most common reflector types are...
on either side set slightly back. A switch rapidly alternated opened the mid-point 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 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 center, the radio would receive both signals, which sounded like a continual signal, the so-called "equisignal". Flying the known direction of the runway and keeping the equisignal on the radio, Lorenz could fly a plane down a straight line with relatively high accuracy, enough so that the aircraft could then find the runway visually in all but the worst conditions.
Night bombing
Both the British and Germans based much of their pre-war bombing strategy on night bombing, in which the threats to the bomberBomber
A bomber is a military aircraft designed to attack ground and sea targets, by dropping bombs on them, or – in recent years – by launching cruise missiles at them.-Classifications of bombers:...
s from fighter interception and ground-based anti-aircraft systems were greatly reduced. However, the disadvantage of this strategy was the difficulty of finding a blacked-out
Blackout (wartime)
A blackout during war, or apprehended war, is the practice of collectively minimizing outdoor light, including upwardly directed light. This was done in the 20th century to prevent crews of enemy aircraft from being able to navigate to their targets simply by sight, for example during the London...
target at night.
The Royal Air Force
Royal Air Force
The Royal Air Force is the aerial warfare service branch of the British Armed Forces. Formed on 1 April 1918, it is the oldest independent air force in the world...
(RAF
Royal Air Force
The Royal Air Force is the aerial warfare service branch of the British Armed Forces. Formed on 1 April 1918, it is the oldest independent air force in the world...
) thus invested very heavily in navigation training, equipping their aircraft with various equipment, including an astrodome
Astrodome (aviation)
An astrodome is a hemispherical transparent dome fitted in the cabin roof of an aircraft to allow the use of a sextant during astro-navigation....
, for taking a star fix
Celestial navigation
Celestial navigation, also known as astronavigation, is a position fixing technique that has evolved over several thousand years to help sailors cross oceans without having to rely on estimated calculations, or dead reckoning, to know their position...
and giving the navigator room to do calculations in a lit workspace. They put this system into use as soon as the war began and were initially happy with its success. In reality, the early bombing effort was a complete failure, with the majority of bombs landing miles away from their intended targets.
The Luftwaffe instead invested heavily in radio navigation
Radio navigation
Radio navigation or radionavigation is the application of radio frequencies to determine a position on the Earth. Like radiolocation, it is a type of radiodetermination.The basic principles are measurements from/to electric beacons, especially...
systems to solve the same problem, notably neglecting any training in celestial navigation
Celestial navigation
Celestial navigation, also known as astronavigation, is a position fixing technique that has evolved over several thousand years to help sailors cross oceans without having to rely on estimated calculations, or dead reckoning, to know their position...
. 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 miles (48.3 km), 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 beams of Lorenz 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 airplane 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. It was the shape of the aerials that gave the system its code name. 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. For the needed 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 blind landing 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 ("crooked leg") transmitters were set up in 1939 on the Stollberg hill
Stollberg (North Frisia)
At 43.4 metres above sea level, the Stollberg is the fourth highest hill in the district of North Frisia in northern Germany, after the Sandesberg near Ostenfeld , the Uwe Düne in the municipality of Kampen auf Sylt and the Rantzauhöhe within the Langenberg south of the village of Leck...
in Nordfriesland
Nordfriesland
Nordfriesland, English "Northern Friesland" or "North Frisia", is a district in Schleswig-Holstein, Germany. It includes almost all of traditional North Frisia along with adjacent areas to the east and south and is bounded by the districts of Schleswig-Flensburg and Dithmarschen, the North Sea and...
near the border with Denmark
Denmark
Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...
, at Kleve (Cleves)
Kleve
Kleve , is a town in the Lower Rhine region of northwestern Germany near the Dutch border and the River Rhine. From the 11th century onwards, Kleve was capital of a county and later a duchy...
near the Dutch border, almost the most westerly point in Germany and at Lörrach
Lörrach
Lörrach is a city in southwest Germany, in the valley of the Wiese, close to the French and the Swiss border. It is the capital of the district of Lörrach in Baden-Württemberg. The biggest industry is the chocolate factory Milka...
near the border with France and Switzerland in south-western Germany. Following the fall of France in June 1940, further 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 anyway.
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 JonesReginald Victor Jones
Reginald Victor Jones, CH CB CBE FRS, was a British physicist and scientific military intelligence expert who played an important role in the defence of Britain in -Education:...
, were aware of the system initially because a downed German bomber's Lorenz system was analysed by the Royal Aircraft Establishment
Royal Aircraft Establishment
The Royal Aircraft Establishment , was a British research establishment, known by several different names during its history, that eventually came under the aegis of the UK Ministry of Defence , before finally losing its identity in mergers with other institutions.The first site was at Farnborough...
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
Winston Churchill
Sir Winston Leonard Spencer-Churchill, was a predominantly Conservative British politician and statesman known for his leadership of the United Kingdom during the Second World War. He is widely regarded as one of the greatest wartime leaders of the century and served as Prime Minister twice...
had also been given Ultra (intelligence from Enigma
Enigma machine
An Enigma machine is any of a family of related electro-mechanical rotor cipher machines used for the encryption and decryption of secret messages. Enigma was invented by German engineer Arthur Scherbius at the end of World War I...
messages) mentioning 'bombing beams'.
When Jones mentioned the possibility of bombing beams to Churchill, Churchill 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
Frederick Lindemann, 1st Viscount Cherwell
Frederick Alexander Lindemann, 1st Viscount Cherwell FRS PC CH was an English physicist who was an influential scientific adviser to the British government, particularly Winston Churchill...
, leading scientific adviser to the government, claimed that any such system would not be able to follow the curvature of the Earth, though T S Eckersley of the Marconi company had claimed it could.
Eckersley's claim was eventually demonstrated after Churchill ordered a flight to try to detect the beams. An Avro Anson
Avro Anson
The Avro Anson is a British twin-engine, multi-role aircraft that served with the Royal Air Force, Fleet Air Arm and numerous other air forces prior to, during, and after the Second World War. Named for British Admiral George Anson, it was originally designed for maritime reconnaissance, but was...
was equipped with an American Hallicrafters
Hallicrafters
The Hallicrafters Company manufactured, marketed, and sold radio equipment beginning in 1932. The company was based in Chicago, Illinois, USA.-History:William J. Halligan founded his own radio manufacturing company in Chicago in late 1932...
S-27 amateur radio
Amateur radio
Amateur radio is the use of designated radio frequency spectrum for purposes of private recreation, non-commercial exchange of messages, wireless experimentation, self-training, and emergency communication...
receiver, the only known receiver capable of receiving the 40 MHz signal at the time, requisitioned from a shop in Lisle Street, London
St. James's
St James's is an area of central London in the City of Westminster. It is bounded to the north by Piccadilly, to the west by Green Park, to the south by The Mall and St. James's Park and to the east by The Haymarket.-History:...
, operated by a member of the Y Service. The flight was nearly cancelled when Eckersley withdrew his claim 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 Churchill 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
Derby
Derby , is a city and unitary authority in the East Midlands region of England. It lies upon the banks of the River Derwent and is located in the south of the ceremonial county of Derbyshire. In the 2001 census, the population of the city was 233,700, whilst that of the Derby Urban Area was 229,407...
, at that time the only factory producing the Merlin
Rolls-Royce Merlin
The Rolls-Royce Merlin is a British liquid-cooled, V-12, piston aero engine, of 27-litre capacity. Rolls-Royce Limited designed and built the engine which was initially known as the PV-12: the PV-12 became known as the Merlin following the company convention of naming its piston aero engines after...
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.
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 his "photoflash" 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 were brilliant in their simplicity, and aptly codenamed "Aspirin". Initially, modified medical diathermyDielectric heating
Dielectric heating, also known as electronic heating, RF heating, high-frequency heating and diathermy, is the process in which a high-frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. At higher frequencies, this heating is...
sets transmitted interference, but later, on nights where raids were expected, local radio transmitters broadcast a surplus "dot signal" at low power. The German predilection for turning on the beams long before the bombers reached the target area aided the British efforts. Ansons
Avro Anson
The Avro Anson is a British twin-engine, multi-role aircraft that served with the Royal Air Force, Fleet Air Arm and numerous other air forces prior to, during, and after the Second World War. Named for British Admiral George Anson, it was originally designed for maritime reconnaissance, but was...
fitted with receivers would be flown around the country in an attempt to capture the beams' location, and 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 with a known location. The British broadcasters were later modified to broadcast their dots at the same time the German transmitters would, 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 "bent" away from the target. Eventually, the beams could be bent 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 bombers even landed at RAF bases, believing they were back in Germany.
X-Gerät
As good as Knickebein was, it was never invented to be used in the long-range role. Efforts had been underway for some time to produce a much more accurate version of the same basic concept, which was eventually delivered as X-Gerät (translated "X-Apparatus").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 yards (91.4 m) wide at a distance of 200 miles (321.9 km) 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
Cherbourg-Octeville
-Main sights:* La Glacerie has a race track.* The Cité de la Mer is a large museum devoted to scientific and historical aspects of maritime subjects.* Cherbourg Basilica* Jardin botanique de la Roche Fauconnière, a private botanical garden.* Le Trident theatre...
.
The "cross" signal in X-Gerät used a series of three very narrow single beams, Rhine, Oder and Elbe. About 30 kilometres (18.6 mi) 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 (3.1 to 6.2 mi) 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 kilometres (3.1 mi) from Elbe to the target, when the moving hand reached zero 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 new radio equipment to be used. There were not nearly enough sets to go around, so instead the experimental unit KGr 100 (Kampfgruppe 100) was given the task of using their sets in order to guide other planes 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
Pathfinder (RAF)
The Pathfinders were elite squadrons in RAF Bomber Command during World War II. They located and marked targets with flares, which a main bomber force could aim at, increasing the accuracy of their bombing...
concept that the RAF would perfect to great effect against the Germans only a few years later.
X-Gerät was used to great effect in a series of raids known to the Germans as Moonlight Sonata, against Coventry, Wolverhampton and Birmingham. In the raid on Birmingham
Birmingham
Birmingham is a city and metropolitan borough in the West Midlands of England. It is the most populous British city outside the capital London, with a population of 1,036,900 , and lies at the heart of the West Midlands conurbation, the second most populous urban area in the United Kingdom with a...
only KGr 100 was used, and British post-raid analysis showed that the vast majority of the bombs dropped were placed within 100 yards (91.4 m) of the midline of the Weser beam, spread along it a few hundred yards. This was the sort of accuracy that even daytime bombing could rarely achieve. A similar raid on Coventry
Coventry Blitz
The Coventry blitz was a series of bombing raids that took place in the English city of Coventry. The city was bombed many times during the Second World War by the German Air Force...
with full support from other units dropping on their flares nearly 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 total 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
Heinkel He 111
The Heinkel He 111 was a German aircraft designed by Siegfried and Walter Günter in the early 1930s in violation of the Treaty of Versailles. Often described as a "Wolf in sheep's clothing", it masqueraded as a transport aircraft, but its purpose was to provide the Luftwaffe with a fast medium...
crashed on 6 November 1940 on the English coast at Chesil Beach
Chesil Beach
Chesil Beach, sometimes called Chesil Bank, in Dorset, southern England is one of three major shingle structures in Britain. Its toponym is derived from the Old English ceosel or cisel, meaning "gravel" or "shingle"....
. 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
Coventry Blitz
The Coventry blitz was a series of bombing raids that took place in the English city of Coventry. The city was bombed many times during the Second World War by the German Air Force...
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 kilometre (0.621372736649807 mi) after the preceding Oder beam — much earlier than the expected 5 kilometres (3.1 mi). 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, didn't 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. It was thought that if the British could defeat this new system very quickly, the Germans would abandon their attempts entirely.British monitors soon started receiving intelligence intercepts referring to a new device known as Y-Gerät, which was also sometimes referred to as Wotan. R V Jones had already concluded the Germans used code names which were too descriptive. He asked a specialist in German language and literature at Bletchley Park
Bletchley Park
Bletchley Park is an estate located in the town of Bletchley, in Buckinghamshire, England, which currently houses the National Museum of Computing...
about the word Wotan
Woden
Woden or Wodan is a major deity of Anglo-Saxon and Continental Germanic polytheism. Together with his Norse counterpart Odin, Woden represents a development of the Proto-Germanic god *Wōdanaz....
. The specialist realised Wotan, the name of a one-eyed god, might be a single beam navigation system. Jones agreed and knew it would have to be based on a distance-measurement system. He also concluded it might well work on the system described by an anti-Nazi German mathematician and physicist Hans Mayer
Hans Ferdinand Mayer
Hans Ferdinand Mayer was a German mathematician and physicist and perhaps most notable for the Oslo Report which revealed German technological secrets to the British Government shortly after the start of World War II.-Biography:Hans Ferdinand Mayer studied mathematics, physics and astronomy at...
, who while visiting Norway had passed a large amount of information in what is now known as the Oslo Report
Oslo report
The Oslo Report was one of the most spectacular leaks in the history of military intelligence. Written by German mathematician and physicist Hans Ferdinand Mayer on November 1 and 2, 1939 during a business trip to Oslo, Norway, it described several German weapons systems, current and future.Mayer...
.
Y-Gerät
Y-Gerät (navigation)
Y-Gerät , also known as Wotan, was a radio navigation system used by the Luftwaffe in World War II to aid bomber navigation. It was preceded by the X-Gerät system....
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 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 gave radio instructions to the pilot to correct the flight path.
The British were ready for this system even before it was used. By chance, the Germans had chosen the operating frequency of the Wotan system very badly; it operated on 45 MHz, which just happened to be the frequency of the powerful-but-dormant BBC
BBC
The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...
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 a 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's use went on, the aircrew accused the ground station of sending bad signals and the ground station accused the aircraft of having 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 system started to ring with all the feedback.
See also
- Chain HomeChain HomeChain Home was the codename for the ring of coastal Early Warning radar stations built by the British before and during the Second World War. The system otherwise known as AMES Type 1 consisted of radar fixed on top of a radio tower mast, called a 'station' to provide long-range detection of...
- GEEGEE (navigation)Gee was the code name given to a radio navigation system used by the Royal Air Force during World War II.Different sources record the name as GEE or Gee. The naming supposedly comes from "Grid", so the lower case form is more correct, and is the form used in Drippy's publications. See Drippy 1946....
, the early war RAF navigation system for night bombing - Kammhuber LineKammhuber LineThe Kammhuber Line was the name given to the German night air defense system established in July 1940 by Colonel Josef Kammhuber.- Description :...
- List of World War II electronic warfare equipment
- OboeOboe (navigation)Oboe was a British aerial blind bombing targeting system in World War II, based on radio transponder technology. Oboe accurately measured the distance to an aircraft, and gave the pilot guidance on whether or not they were flying along a pre-selected circular route. The route was only 35 yards...
, the December 1941-launched successor to GEE for RAF night bomber raids.