210: Flying Blind

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This episode delves into the fascinating, high-stakes cat-and-mouse game of radar and radio countermeasures during the Battle of Britain. Discover how British intelligence meticulously deciphered the German “Knickebein” and “X-Gerät” bombing guidance systems—starting with insights from captured soldiers and culminating in signal detection—and how they scrambled to develop countermeasures like the surprisingly simple “Aspirin” system. From strategically placed radio transmitters to deliberately set countryside fires, this episode reveals the ingenuity—and occasional rushed decisions—on both sides as they raced to disrupt enemy bombing raids, highlighting the crucial role of signal intelligence in the skies over Britain. Hello everyone and welcome to History of the Second World War Episode 210 - The Battle Before Britain Pt. 2 - Flying Blind. This week a big thank you goes out to Sybran, Autumn, and Tom for choosing to support the podcast by becoming members, you can find out more about becoming a member and other ways to support the podcast over at historyofthesecondworldwar.com/members. The Second World War would be a technology accelerator, with tremendous amounts of money poured into a variety of technological innovations with the purpose of winning the war. The innovations from the war years would have eventually happened either way but the requirements of the war brought them into the world, and into operational use much sooner. But as we look back from the modern day how we interpret and understand the war is impacted by technologies that we have easy access to that did not exist in 1940. There are some obvious examples of this, the internet, smart phones, modern medicine, of course podcasting, but the one we are going to discuss today is something you may not think about very often. If you are listening to this, it is very likely that within arms reach you have a device that can tell you at any moment precisely where you are located on the earth to within a few meters of accuracy. This is just one of those facts that many people never even really think about, but is really pretty amazing particularly in comparison to how difficult or impossible it would be to get that information 80 years ago. Often in warfare during the Second World War knowing your exact position was only marginally valuable, and relative position is often more important. It does not matter exactly where I am, just that there is a tank 100 meters over there, that kind of stuff. But one area where knowing the exact location was important was around strategic bombing. The goal of strategic bombing was, even in an area bombing campaign, to hit a specific target. That target could be as small as a factory or as large as a city, but there was always a target. Finding that target could be complicated by any number of factors, but the most impactful was darkness. Bombing a target at night was a known challenge at the start of the war, but if an air force could master it then there were certain key benefits that they could take advantage of. A lack of visibility for the defenders made it more likely that the bombers would arrive over the target without being detected, even if they were detected aerial interception was almost impossible, and the defenders were less likely to be ready to act against a bombing raid. All of these benefits would slowly be eroded throughout the war, primarily with the proliferation of radar both on the ground and eventually in the air, but it would still be advantageous to bomb targets at night. But, to effectively bomb targets at night you had to find the target, and without the aid of GPS like we have today, entire systems and technologies had to be developed to help bombers know where they were going and when they needed to release their payloads. In the skies over Britain in 1940 this would take the form of radio navigational aids that were used by the Germans to give their bombers the ability to follow certain signals along a line to the target, and then another signal would intersect that line to tell the bombers when they were over the target. The first systems were based on off the shelf commercial radio navigation systems that had been in use before the war, but after these were counteracted by the British further systems would be developed that used different frequencies to achieve the same result. Radio waves could not be hidden, but it would take the British months, and they had to get lucky with a downed He-111 before they found the frequency that this new system was operating at, which would finally allow them to begin to counteract its usefulness. This episode is going to cover the development of those German systems, and then how the British found out about them and developed ways to impact their effectiveness. It is an interesting way to discuss the kind of technological back and forth that would occur in so many different areas of the war, just in this case in the radio waves above Britain during the last 6 months of 1940.

One of the best ways that military could understand a system used by the enemy was to develop their own similar system, that helped to explore what was possible and give some insight into how an enemies system might also work. This was done for technologies like radar, with every nation developing their own and that resulted in their being teams in each nation that understood the problem space and the general parameters for how systems might be built. When it came to navigational aids for bombers, while the Germans were working on radio navigational aids the British were not really putting an official RAF effort into developing the technology and the skills to use it. Instead, they believed two competing, and in retrospect incompatible facts. The first was that the Germans would not be able to bomb at night, they would instead be forced into strictly daylight operations, and operations in good weather, which would limit their ability to launch a campaign over Britain and would make them very vulnerable to the efforts of Fighter Command. The second was that the plucky pilots and navigators of Bomber Command would not have any problem finding their targets at night through such novel technologies as celestial navigation and dead reckoning. I think that this is a good example of a very common British viewpoint before the Second World War, which was I guess you could call it British Exceptionalism, the idea that the British were just better than everybody else and that they would find a way to do things successfully where other nations couldn’t. In some instances this was influenced by a racism that often was added into the mix, which came into play with the completely incorrect evaluations of the Japanese which is of course a story for another day. But even with the Germans, there were at times a complete under estimation of what Germany as a collective entity was capable of. Before we talk about the downsides of these views, it is worth pointing out that there were also benefits to this kind of confidence, particularly at the higher levels of national leadership when faced with a national challenge like having an enemy preparing to invade. But there were also downsides, like with radio navigation. Working from the assumption that it was impossible for the Germans to launch a night time bombing campaign, the RAF leadership downplayed the needs of night time defense. This would be most impactful in the area of aerial interception, which the British were, for lack of a better term hopeless at for most of 1940. If they had seen night bombing as a serious threat maybe things would have changed, and more effective countermeasures would have been designed.

On the other side were the Germans, and they recognized quite early on that it would be impossible for their bombers to find targets without some help. This belief was formed after the German bombing experiences during the Spanish Civil War when not only did they find it almost impossible to find targets at night, but even during the day bad weather had a disastrous effect on bombing accuracy. The result of these experiences was an effort made into creating a radio navigation system, which was an expansion of what many nations already used on their airfields, both military and civilian. To help aircraft find their airfields to land on during peacetime, a radio signal would be broadcast which the aircraft could hone in on to land. The Germans used something similar during the war, because while it was hard to find targets to bomb it was equally difficult to find out where your home base was after a combat sortie. This system would then be extended through the use of radio beacons and towers setup throughout the newly occupied French territories. On a technical level the system was built around a series of radio beams that were broadcast in a line across Britain, intersecting with the target. But it is interesting how this was used by the pilots actually flying the bombers, and for that we turn to the description of a German airman that bailed out over Britain during the fighting in 1940. He would describe the system, not to the British but to another prisoner like this: “It cuts every human weakness, every human error, because it is purely mechanical. […] When I’m on this beam, and have tuned this apparatus in, then I hear, “Da, da, da, da, da”. And when I hear that then I know for certain that I’m on the left of the beam.’ So he responds by steering to the right. ‘If I hear, “Dit, dit, dit, dit”, then I know I’m on the right of the beam. [If I hear a continuous note] then I know for certain that I’m on the navigation beam. That is the wonderful part of it, I need not do anything except fly my aircraft quite mechanically along this continuous note.” This helped to remove the possibility of human error, and when the plane arrived at the target it would encounter another radio note which would begin the drop sequence. One radio signal could tell them that they were flying in the correct direction, but the precise area to drop their bombs was a different story. To accomplish that there were a series of three beams the first was a warning that the bombers were approaching their drop zones. Then there would be a second beam that was 50 kilometers closer to the target, which served a critical purpose when it came to navigation. After crossing the second beam the aircraft would fly at a constant speed, with a stopwatch running, now the exact speed did not matter because airspeed was often not really known. The airspeed values given in an airplane were at best rough approximations of air speed, which could not really account for things like wind speed, and so they bore little resemblance to what an aircraft’s ground speed was. But the beam system fixed that because there were precisely 15 kilometers between the 2nd and third beams of the system, and then precisely 15 kilometers between the third beam and the target. This meant that all the plane needed to do was pick a speed on the air speed indicator, start a stop watch at the second beam, then fly that exact same amount after the third beam and they would be over the target. There was still some margin of error in a system like this, if wind changed between the beams, or if the beams were slightly off, but it was still far more effective than just trying to determine location based on speed and bearing.

With the Germans using this radio navigation system, and the results of their bombing raids making it clear that they were able to find targets even at night, how did the British learn about the system, and eventually begin to work against it? This is an area where our knowledge today, even what I have shared with you so far in this episode, is actually harmful to our understanding of how the information was discovered by the British at the time. One of the great challenges of understanding history is that we know much more about the overall situation than the people did at the time, and all that the British knew at the beginning was that the Germans were finding their targets, and they did not really have much more information than that. Their primary source of additional information was delivered to them though, in the form of German airmen that survived their aircraft being either shot down or having mechanical problems over Britain. There were many survivors that were captured after landing in Britain, either gently or not so gently, and as soon as they were on the ground a process would begin. When any aircraft crashed the site was quickly brought under the control of the local police, both to keep any downed airmen near the aircraft, but also to protect the site until the military and then also intelligence officers arrived. The intelligence officers would then investigate the site for information, of particular importance were things like codebooks, maps, or any other documents. They would also speak to the airmen and make a judgement call on whether or not each of the airmen might have additional information. Officers, navigators, pilots, or others that were thought to be possible sources of information would not be sent directly to Prisoner of War camps but were instead taken to sites operated for the purpose of gaining additional intelligence. The German airmen knew that these type of actions would be taken, and so efforts would be made to destroy evidence and information before the British arrived. For example in one case a German radio operator was found in a British field ripping up every single page of his notebook and burying it in the dirt. Now in that case many of the pieces were then dug up and reassembled, but the German airman did try to prevent their use. It was also known that downed airmen would be questioned by the British, because of course they would be, although it is worth pointing out that the British were not using forceful interrogation methods, and it was more just questioning. One of the best ways to improve the effectiveness of this questioning was for the British intelligence officers to bring together a lot of information gained from all kinds of different sources and to make it seem like they knew a lot more about the German situation than they actually did. Or to quote Group Captain Samual Denys Felkin, the chief interrogator of the RAF: ‘The most common method of inducing prisoners to talk was a display on the part of the interrogation of profound knowledge of all aspects of the German air force, even to the names, nicknames and idiosyncrasies of officers and their crew in a prisoner’s own unit.’ In the 2024 series Masters of the Air, there is a pretty good scene that depicts this type of interaction, only from the other side with a downed American airman meeting with a German intelligence officer, but his approach to interrogation is the same. Basically the idea is you gather up all of these little tidbits from POW letters, radio intercepts, microphones in POW housing, other interrogations, and you make it seem like you already know everything and so what would be the harm of the airman saying what they know, nothing can possibly be new information, right? One of the phrases that Felkin and his team picked up on was “X-Gerät” which just translates to X-equipment. This started popping up as early as February 1940, so well before the focus of German bombers shifted to Britain, but there was no clear indication of what it was or what it meant. It would take some number of months before the British were able to begin piecing it together.

The primary method that additional information was gathered was not though interrogations, but instead through microphones that were positioned in the POW housing. This allowed the British to use a system whereby they bluffed their way through the initial interrogation, again making it seem like they knew a whole bunch of stuff that they did not know, then of course the prisoners would lie to them because of course the prisoners would lie to them. Then when the prisoners were taken back to their accommodations their conversations with other prisoners would be recorded, and those recordings were a very lucrative source of information. They were particularly good for things like this “X-Equipment” because it very quickly became apparent that the equipment was almost ubiquitous in the German aircraft, and that information about it was well known. This made the prisoners more chatty about it, and its impact. For example one prisoner would be recorded saying that the X-equipment was so good that ‘blind bombing’ was the future of the German bombing efforts. These were the kinds of comments that would help the British intelligence officers piece together what things were used for and then use that information in future interrogations. It was during these prisoner conversations that another word began to pop up in the same context ‘Knickebein’ which translates to crooked leg. This was another piece of the puzzle, because this was the more official name for the system at the time, and the phrase that was used in other German communications. For example in mid-June one of the Enigma decrypts contained the word ‘Knickebein’ because it was a message which contained information about the exact position of one of the radio towers used for the system. Unfortunately for the British efforts, there were limits to what they could gain from these methods, most German airmen, while they knew that a system was being used to guide them to their targets did not know anything about how it worked. The real breakthrough would come when a prisoner, who would be designated A231 would make it clear that he planned to tell the British everything he knew, because he was a German that held anti-war beliefs. He would describe Knickebein as a radio system that used intersecting radio beams to guide the German bombers to the target, through the use of a radio receiver on the bombers themselves. This was a key breakthrough, because the British, while they knew some of the terminology did not know that there was a specific piece of equipment on the planes that was being used for this purpose. And when they found it, it became clear why it was overlooked. Of course every single German aircraft that was brought down over Britain was examined, and anything of value was noted and studied, however, the Knickebein system was not some fancy or interesting looking device, ti was instead just a common radio receiver. It, in essence, hid in plain sight but just being a boring unexceptional radio receiver.

Once there was an understanding of the basics of how the system worked, that it was based on radio waves, and that there was not some kind of special receiver device, it was time to try and intercept the signal. To do this an Avro Anson was used, which was one of those aircraft types that nobody talks about, but which would end up being quite important to the overall British war effort. It was originally designed as a two engine maritime patrol aircraft, but its range and speed meant it was not very good at that job and was replaced at the very start of the war. However, it would take on another life as a training aircraft and then as what I would call a utility aircraft, used for things like light transport and people moving. Eventually over 10,000 Ansons would be built, and it would go on to another life as a light civilian transport after the war, being used by small charter aircraft companies for decades. So yeah, Avro Anson, very underappreciated aircraft. In this case it was put in the air over the Midlands with the purpose of finding the German radio signals and verifying the information that had been gained from other sources. The pilots were told that they would be able to find transmissions that were similar to the Lorenz radio signals that had been used before the war for airfield position broadcasts. On their second path over the predicted area of the beam, they found exactly what they were searching for, while flying south they ran into an area where they received dots, then around 400 meters of continuous signals, and then dashes. Now they knew exactly what the Germans were doing, and they moved on to the next logical development, countermeasures.

The system that was developed would be code named Aspirin and it was so simple that it did not even require any kind of special device. Instead a group of 28 old army radio transmitters were placed around the country and then they just tuned to the same frequency as the German signals and then they blasted out the dash signal in all direction. This helped to confuse the German bombers, by simply making it more difficult to follow the correct path with so much additional noise in the way, but it was not a perfect system. In some ways rushing out the Aspirin system was a mistake, because as soon as it started broadcasting the Germans knew what was happening, and as soon as pilots began to return with news of all of these dash signals the Germans would shift their tactics. The first of these steps would be taken in late September when, in the middle of the bombing raid, the Germans swapped their guidance signals to a different radio frequency. The bombers then shifted their radio frequency at exactly the same time, so they kept the signal, but the British kept broadcasting on the old frequency, making for another crystal clear and uninterrupted signal for the German bombers. It can be easy to criticize the British for rushing out Aspirin, and producing a system that was easily detected. The better system would have been to develop their own radio waves that pushed the Germans off target by matching the exact structure of the system, but pushing the German bombers off target, so that they were dropping bombs on fields. But that system would have taken time, and almost every night the German bombers were overhead, causing the solution to the rushed, even though it was known that it was not perfect.

While the Germans began to adjust to British countermeasure efforts by doing things like swapping frequencies, the longer term solution was simply to build out a more complex and capable system. This would eventually be referred to primarily as ‘X-Gerät’ or X-device, which was also what Knickebein had been known as to the British before they knew the Knickebein name, which just adds some confusion. But this new system was really just a more advanced version in every way. Instead of just a regular radio signal that a person listened to and interpreted, the new system involved a visual display which made it easier to interpret and easier for the airmen to ensure the radio signal was legitimate and that it was followed correctly. The new signal also used much shorter wavelengths which made it a generally tighter and more accurate beam, which was only possible due to the better ways of following it. These new devices however, were not fitted to as many aircraft, and they were instead primarily used by the specialized K.Gr 100 pathfinders unit. This squadron would fly ahead of the primary bomber and drop incendiary bombs on the target, all the following bombing raid had to do was go roughly the correct direction and then drop their own bombs on the fires. The arrangement was not perfect, but the Germans also chose expediency over perfection, because they did not want to wait for enough of the new visual devices to arrive to equip all of their aircraft. This created the interesting countermeasure that was attempted, and was successful a few times, where the British would race around the countryside setting fires near the expected targets of the German bombers. The raid would be detected by radar, and there were only so many targets that specific raid paths would target, and so some fields would be set alight with the hope that it would cause the bombers to drop their payloads in the wrong place. Sometimes countermeasures are technologically advanced, required specific technical information and then radio transmitters, sometimes all it takes is a match and a bit of grass, war is weird like that. In all of the cases we have discussed in this episode this work was being done to try and impact the efficacy of the German bombing efforts. Next episode will focus on the state of the British strategic bombing efforts during the summer of 1940, the best summary I can give is that they were certainly trying, but they were struggling against the fact that some of the core assumptions that had driven British strategic bombing theory during the 1930s were proving to be inaccurate, incomplete, or actively harmful to a successful bombing campaign.