Battle of the Beams

The Battle of the Beams was a period in early World War II when bombers of the German Air Force ("Luftwaffe") started using radio navigation for night bombing. British "scientific intelligence" at the Air Ministry fought back with a variety of increasingly effective means, and the Germans eventually gave up trying to use any such system.

Background

Both the British and Germans based much of their pre-war bombing strategy on night bombing, in which the threats to the bombers 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 bombing effort was a complete failure, with the vast 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. They already had some experience with these sorts of systems due to their deployment of the Lorenz blind-landing aid at many airports, which also equipped most of their bombers in order to allow them to land at night or in bad weather.

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 element, which had a slightly longer reflector element on either side set slightly back. A special switch rapidly and alternately opened the mid point connection of each reflector in turn opening one for longer than the other. This sent a stream of dots to the left of the centreline and a stream of dashes to the right. Due to the directional characteristics of the this arrangement, aircraft to the right of the runway centerline would receive a much stronger long signal ("dash") while those to the left would receive a stronger short signal ("dot"). The two signals overlapped along a relatively narrow centre line, and since the received strengths of the dashes matched those of the dots such that a continuous "equisignal" was received. 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.

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 needed radios were already being installed on many aircraft. Lorenz directed an aircraft down a line, so two Lorenz-like systems with crossed beams could be used to fix a single spot. All that would be needed was a second Lorenz receiver.

Lorenz had a range of about 30 miles, 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 centerline, 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. To address this all that was needed was to make a much larger antenna array.

German systems

Knickebein

For bombing use the modifications to Lorenz were fairly minor. Much larger antennas with considerably smaller beam angles were set up, and broadcast power was increased considerably. The first two of these new Knickebein ("crooked leg") transmitters were set up at Stollberg in northern Germany near the border with Denmark, and the other at Kleve (Cleves), almost the most westerly point in Germany. The two aerials could be rotated to make the two beams 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 the second receiver). When the steady "on course" sound was heard from the second beam, they dropped their bombs.

It was the shape of the aerials that gave the system its code name. Unlike the wide-pattern Lorenz, Knickebein required far more 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 Knickebein receivers were disguised as a standard blind landing receiver system, consisting apparently of the EBL-1 and the EBL-2 blind landing receivers. The sensitivity of the receivers though had been considerably enhanced from the standard equipment in the hope that the British wouldn't appreciate their purpose. Information overheard from captured aircrew revealed that the aircrew believed that the British would never find it (indicating that the equipment was on board the aircraft). In the event, the Royal Aircraft Establishment at Farnborough realised that the equipment was far more sensitive than it needed to be for a standard blind landing receiver. Knickebein was codenamed "Headache".

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.

Efforts in Britain to stop the system took some time to get started. British intelligence at the Air Ministry, led by R V Jones, were aware of the system initially because a downed German bomber's Lorenz system was analysed 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 put two and two together and ordered more investigation. However, many in the Air Ministry didn't 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 (then the only known receiver capable of receiving the 40 MHz signal) requisitioned from a shop in Lisle Street, London, operated by a member of the Y Service The flight was nearly cancelled when Eckersly 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 weren't as good as their own, who 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.

Efforts to block the Knickebein 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 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. [http://www.vectorsite.net/ttwiz_07.html]

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 (Secret (i.e. unknown) Apparatus).

X-Gerät used a series of beams to locate the target, each beam named for 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 wide at a distance of 200 miles from the antenna. The beams were so narrow that bombers couldn't 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 km from the target the radio operator would hear a brief signal from Rhine, and set up his equipment. This consisted 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 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 10 and 5 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, 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 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 later 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 less than 100 yards of the midline of the Weser beam, spread along it a few hundred yards. This was the sort of accuracy that day bombing could rarely achieve. A similar raid on Coventry with full support from other units dropping on their flares nearly destroyed the city centre.

Stopping X-Gerät proved to be more difficult than with 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 R V Jones had correctly guessed the beam layout (and he acknowledges that that is all they were), the modulation frequency had been measured incorrectly at 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 the English coast at Chesil Beach. Although it crashed just prior to the Coventry raid, an inter-service dispute prevented recovery of the X-Gerät equipment until after the incoming tide had covered and damaged it. On later 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.

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 gap. Since the final stages of the release were automatic, the clock would reverse prematurely and drop the bombs kilometers 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 about the word Wotan. The specialist realised Wotan, the name of a one-eyed god, might be a single beam navigation system. [Jones (HH) p.120] Jones agreed and knew it would have to be based on a distance-measurement system. [It turned out their conclusion about the origin of the code name Wotan was incorrect. The Y-Gerät was actually code named Wotan-II, Wotan-I being the X-Gerät. The name Wotan did not in fact imply a single beam. Jones (HH) p.177] He also concluded it might well work on the system described by a German well-wisher in Norway, who had passed a large amount of information in what is now known as the Oslo Report. [The information in this Report was plentiful and seemingly far too useful to be true, and many considered it to be a German disinformation campaign. The Oslo Report's description of Wotan was accurate, however, and the Report was later realised to be "for real."]

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. A major disadvantage of the system was that it only allowed one aircraft to be guided at a time.

The British were ready for this system even before it was used. 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 sent it to Alexandra Palace for re-transmission. The combination of the two signals modified the phase shift — and 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.

The Germans eventually realised what was happening and the Luftwaffe's faith in electronic navigation aids was completely shattered. The bombing offensive of Britain waned, partly because of the abandonment of electronic aids, but mainly because Hitler's attention was turning to the Soviet Union.

ee also

* List of World War II electronic warfare equipment
* Kammhuber Line
* Chain Home

References

* Jones, R. V. (1978). "Most Secret War: British Scientific Intelligence 1939–1945." Coronet paperback edition 1979 ISBN 0-340-24169-1. First published 1978 Hamish Hamilton (HH), ISBN 0 241 89746 7.

*Goebel, Greg. [http://www.vectorsite.net/ttwiz_07.html#m1 Battle of the Beams: Knickebein]

*Goebel, Greg. [http://www.vectorsite.net/ttwiz_07.html#m2 Battle of the Beams: X-Geraet]