X-Gerät (navigation)
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X-Gerät (X-gadget) was a radio navigation system used by the Luftwaffe early in World War II to aid bomber navigation. It was preceded by the Knickebein system and was replaced with the Y-Gerät (navigation) system.
The previous system used by the Luftwaffe called Knickebein 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.
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 Knickebein. The system operated at unknown frequencies 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.
