Chaff (countermeasure)
Chaff, originally called Window[1] or Düppel, is a radar countermeasure involving the dispersal of thin strips of aluminium, metallized glass fiber, or plastic.
[9] Once the British had passed the idea to the US via the Tizard Mission, Fred Whipple developed a system for dispensing strips for the USAAF, but it is not known if this was ever used.
Ignorance about the extent of knowledge of the principle in the opposing air force led planners to judge that it was too dangerous to use, since the opponent could duplicate it.
The British government's leading scientific adviser, Professor Lindemann, pointed out that if the Royal Air Force (RAF) used it against the Germans, the Luftwaffe would quickly copy it and could launch a new Blitz.
The anti-aircraft guns fired randomly or not at all and the night fighters, their radar displays swamped with false echoes, utterly failed to find the bomber stream.
For over a week, Allied attacks devastated a vast area of Hamburg, resulting in more than 40,000 civilian deaths, with the loss of only 12 out of the 791 bombers on the first night.
Window rendered the ground-controlled Himmelbett (canopy bed) fighters of the Kammhuber Line unable to track their targets in the night sky and rendered the early UHF-band B/C and C-1 versions of the airborne intercept Lichtenstein radar (following the capture of a Junkers Ju 88R-1 night fighter by the British in May 1943 equipped with it) useless, blinding radar-guided guns and spotlights dependent on the ground-based radar.
Although theoretically effective, the small number of bombers, notably in relation to the large RAF night-fighter force, doomed the effort from the start.
The Germans obtained better results during the air raid on Bari in Italy, on 2 December 1943, when Allied radars were deceived by the use of Düppel.
[13] Following the British discovery of it in 1942 by Joan Curran, chaff in the United States was co-invented by astronomer Fred Whipple and Navy engineer Merwyn Bly.
In the Pacific Theatre, Navy Lieutenant Commander Sudo Hajime invented a Japanese version called Giman-shi, or "deceiving paper".
[18] Therefore, Royal Navy engineers designed an improvised delivery system of welding rods, split pins and string, which allowed six packets of chaff to be stored in the airbrake well and be deployed in flight.
[19] Although chaff produces large amounts of scattered reflections potentially clogging a radar display it is easily filtered by virtue of it moving relatively slowly through the sky.
To overcome this in use large amounts of chaff are deployed and then the aircraft will turn so that it moves predominantly perpendicular to the radar source.
As a result, after release it quickly loses any forward speed it had from the aircraft or rocket launcher, and then begins to fall slowly to the ground.
Modern radars use the Doppler effect to measure the line-of-sight velocity of objects, and can thus distinguish chaff from an aircraft, which continues to move at high speed.
A U.S. Department of Defense-sponsored 1998 research review stated that the "widespread environmental, human and agricultural impacts of chaff as currently used in training are negligible and far less than those from other man-made emissions.
