Battle of the beams
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The Battle of the Beams was a period in early World War II when Luftwaffe bombers 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.
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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 was greatly reduced. The disavantage of this strategy was however, the difficulty of finding a target at night.
The RAF thus invested very heavily in navigation training, equipping their planes with various equipment 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 opened and were initially happy with its success. However in reality the bombing effort was a complete failure, with the vast majority of bombs landing miles away from their intended targets.
Lorenz
The Luftwaffe instead invested heavily in radio navigation systems to solve the same problem. Prior to the war they had deployed the Lorenz blind-landing aid at many airports and equipped most of their bombers with the radio equipment needed to use it.
The Lorenz system worked by placing two radio transmitters close to each other, near the end of the runway. The antennas were fairly directional, projecting their signals slightly to each side of the runway centerline, one slightly left, the other slightly right. One broadcast a series of widely separated dots, the other dashes, so the pilot could tell on which side of the runway centerline they were on by the sound of the signal. Key to the system was an area in the middle where the two beams overlapped, where the dots of the one signal "filled in" the dashes of the other, resulting in a steady tone known as the equi-signal.
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.
Knickebein
Lorenz directed an aircraft down a line, so two crossed beams could be used to fix a single spot. Several systems based on this idea were studied through the 1930s. Lorenz had a range of about 30 miles, enough for blind-landing but not good enough for bombing raids over England. 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.
For bomber 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 in northern Germany near the border with Denmark, and another to the south near France. The two beams could be pointed to cross over the target, and the bombers would fly into the beam of one and ride it until they started hearing the tones from the other (in another radio). When the steady "on course" sound was heard from both radios, they dropped their bombs. Using Knickebein required nothing more than the installation of a second Lorenz receiver.
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 England 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 due to a number of intercepts and captured documents from downed bombers. However many in the Air Ministry didn't believe that the system was actually in use, and Lindemann (the leading scientific adviser to the government) claimed that any such system would not be able to follow the curvature of the Earth anyway. Both claims were proven untrue when one enterprising British intelligence officer flew a light plane equipped with a ham radio into one of the beams and picked up the signals. At this point the same sceptics then started pooh-poohing the system as proof that the German pilots weren't as good as their own, who could do without such systems. However it was Lindemann himself who proved this wrong, when his "photoflash" systems started returning photographs of the RAF bombing raids, showing that they were rairly, if ever, anywhere near their targets.
Efforts to block the Knickebein were brilliant in their simplicity. On nights where raids were expected, local radio transmitters broadcast the "dot signal" at low power. This meant that as the bombers flew over England, almost any point at all would make the "on course" steady signal, and the planes would drift far to the left of their line. The crews knew something was wrong, but the nature of the problem was a mystery. Eventually a German radio engineer flew on one mission and, finding himself many miles from the target, concluded that the British had invented a system for bending the radio beams. They gave up on Knickebein soon after.
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 (X-gadget).
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, this allowed its two beams to be pointed much more accurately from a similarly sized antenna, the equi-signal area being only about 100 yards wide at 200 miles from the antenna. The beams were so narrow that a lower power set of beams with a much wider pattern was used to allow the bombers to find the main beam. The main Weser system was set up just to the west of Cherbourg.
The "cross" signal was different in X-Gerät. It 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 primarily of a stopwatch with two hands. When the signal from Oder was heard the operator started the clock and two hands started to sweep up from zero. When he heard the signal from Elbe he "started" the clock again, at which point one hand would stop and the other would start moving back towards zero. Oder and Elbe were aimed to be at exactly 10 and 5 km from the bomb release point at the line of Weser, meaning that the clock accurately measured the time to travel 5km, and thus calculated the ground speed of the plane. Since the time taken to travel that 5km should be almost identical to the time needed to travel the last 5km from Elbe to the target, when the moving hand reached zero the bombs were automatically released.
The accuracy of X-Gerät was considerably better than Knickebein, but as it operated on completely different frequencies it required new radio equipment to be used. There was not nearly enough 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 use 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 barely achieve. A similar raid on Coventry with full support from other units dropping on their flares nearly destroyed the city center.
Stopping X-Gerät proved to be more difficult than with Knickebein. The frequency at which the system operated was unknown until an X-Gerät equipped Heinkel He 111 ditched on the English coast. Defences against the system were soon developed in a similar fashion as Knickebein, but proved to be unreliable. It was later learned that a new pointing device 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 sensitive to the frequency of the beeps, and the early British signals didn't bother matching this accurately. X-Gerät was eventually also defeated in another manner, a "false Elbe" was set up to cross the Weser guide beam not at 5km, but at 1. Since the final stages of the release were automatic, the clock would count a very fast "5km" and then drop the bombs kilometers short of target.
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 be like. With the standard beam systems clearly no longer of any use, some sort of entirely new system would have to be developed. It was thought that if they could defeat this new system, whatever it was, very quickly, the Germans would give up on the whole idea in frustration.
They soon started receiving intercepts referring to a new device known as Y-Gerät, which was also sometimes referred to as Wotan. R.V. Jones had long realized that the Germans used code names that were far too literal. Asking around he learned that Wotan was the name of a one-eyed God. Based on nothing more than this, he assumed that Y-Gerät used a single beam. From this they thought up systems that could use a single beam, and concluded that Wotan would have to be based on a distance-measurement system. Perhaps shockingly, the guess was exactly correct.
Y-Gerät used a single narrow beam pointed over the target, broadcasting a series of "pulses". The system used a new piece of equipment that received the pulses from the beam and immediately re-broadcast them, where they were picked up at the original station. By listening for the return pulses, the distance to the plane could be calculated with fairly good accuracy. The planes did not have to "fly the beam", ground controllers would plot the position of the plane and give instructions over radio to correct their path. A downside was that only one plane could be guided at once.
The British were ready for this system even before it was used. As soon as they realized it was in use, they set their plan in motion: broadcasting random "return" pulses from BBC radio transmitters. The German ground stations received several pulses for every one sent, and had no idea which was the "real" one. They gave up on this system after only a few small raids, considering the British to be "way ahead" and any further radio navigation systems hopeless.