Lorenz beam
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The Lorenz beam blind landing system was a Ground-Controlled Approach device in use from around the 1930s.
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[edit] Operation
The Lorenz beam obtained a sharper beam than could be created by an aerial array by having two lobes of signal, one transmitting dashes and the other dots, with a narrow overlap. The transmitter was switched between one aerial and the other to ensure that an aircraft flying along the 'equisignal' beam would hear a constant tone. If the aircraft deviated from the 'equisignal' it would hear one signal, say the dashes, stronger than the other. Later derivatives of the system had signals of equal length in the pattern left-right-silence, to operate a visual indicator in the cabin.
[edit] Use for blind bombing
In the Second World War the Lorenz beam principle was used by the German Luftwaffe as the basis of a number of blind bombing aids, notably 'Knickebein' ('crooked leg') and the 'X-Gerät' ('X-Apparatus'), in their bombing offensive against English cities during the winter of 1940/41. These systems involved cross-beams of the same characteristics but on a different frequency, which would both enable the pilot to calculate his speed (from the time between crossing the Fore Cross Signal and crossing the Main Cross Signal), and indicate when he should drop his payload. The calculation was performed by a mechanical computer.
[edit] Allied jamming effort
When the Allies discovered the existence of the 'Knickebein' system, they rapidly jammed it, however the 'X-Gerät' was not successfully jammed for quite some time. A later innovation by the Germans was the 'Baedeker' or 'Taub' modification, which used supersonic modulation. This was so quickly jammed that the Germans practically gave up on the use of beam-bombing systems, with the exception of the 'FuGe 25A', which operated for a short time towards the end of the Baby Blitz.
[edit] Technical considerations
The reason why the Lorenz beam principle was necessary, with its overlapping beams, was because the sharpness of a beam increases approximately logarithmically with the length of the aerial array with which it is generated; a law of diminishing returns operates, such that to attain the sharpness achieved by the Lorenz system with a single beam (approximately 1 mile wide over a range of two hundred miles), an array of prohibitive size would be required.
