Y-Gerät (Y-gadget), 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.
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 from the ground station pointed over the target, broadcasting a series of "pulses". A new piece of equipment in the bomber was required for the system to work; it 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. Fortuitously, the system operated on the same 45 MHz frequency as the BBC's dormant Alexandra Palace television station. Using it to broadcast random "return" pulses, 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.
Y-geräte were also used to locate and guide German fighter planes intercepting allied bombers. The system could establish direction and distance, thus location, of a fighter and use the same radio signal for two-way voice communications. Over a dozen of these systems were located near Fliegerhorst Deelen.