Chaff (radar countermeasure)
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Chaff, also called Window, is a radar countermeasure in which aircraft or other targets spread a cloud of small, thin pieces of aluminum or plastic, which either appears as a cluster of secondary targets on radar screens or swamps the screen with multiple returns.
Modern armed forces use chaff (often deployed with short-ranged SRBOC rockets) to distract radar-guided missiles from their targets. Most military aircraft and warships have chaff dispensing systems for self-defense. An intercontinental ballistic missile may release in its midcourse phase several independent warheads, a large number of decoys, and chaff.
Chaff can also be used to signal distress by an aircraft when communications are not functional. This has the same effect as an SOS, and can be picked up on radar. It is done by dropping chaff every 2 minutes.
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[edit] World War II
The idea of using chaff was independently developed in the UK, Germany, and the United States.
As far back as 1937, R. V. Jones had suggested that a piece of metal foil falling through the air might create radar echoes. In early 1942, a TRE researcher named Joan Curran had investigated the idea and come up with a scheme for dumping packets of aluminum strips from aircraft to generate a cloud of false echoes. The British referred to the idea as Window. Meanwhile in Germany, similar research had led to the development of Düppel. In the US, Fred Whipple developed a similar system (according to Harvard Gazette Archives) for the USAAF.
The systems were all essentially identical in concept, small aluminum strips cut to one-half of the target radar's wavelength. When dropped, the strips would give a strong echo, appearing as a bomber on radar screens. Opposing defenses would find it almost impossible to pick out the "real" bombers from the false echos. Other radar-confusing techniques included Mandrel, Piperack, and Jostle.
Then something odd happened: no one used it. Unaware of the opposing air force's knowledge of the chaff concept, planners felt that using it was even more dangerous than not: as soon as it was used the enemy could easily duplicate it and use it against them. In particular the British government's leading scientific adviser, Professor Lindemann, balefully pointed out that if the RAF used it against the Germans, the Luftwaffe would quickly copy it and could launch a new Blitz. This caused panic in Fighter Command and Anti-Aircraft Command, who managed to suppress the use of Window until July 1943.
Examination of the Würzburg radar equipment brought back to the UK during Operation Biting and subsequent reconnaissance, revealed to the British that all German radars were operating in no more than three major frequency ranges, and thus were prone to jamming. "Bomber" Harris, Commander-in-Chief (C-in-C) of RAF Bomber Command, finally got approval to use Window as part of Operation Gomorrah, the raids against Hamburg.
The first to use it were 76 squadron. Twenty-four crews were briefed onto how to drop the bundles of aluminised-paper strips (treated-paper was used to minimise the weight and maximise the time that the strips would remain in the air, prolonging the effect), one every minute through the flare chute, using a stopwatch to time them. The results were spectacular. The radar guided master searchlights wandered aimlessly across the sky. The AA guns fired randomly or not at all and the night fighters utterly failed to find the bomber stream. A vast area of Hamburg was devastated with the loss of only 12 bombers. Squadron Commanders quickly had special chutes fitted to their bombers to make the deployment even easier. Seeing this as a development that made it safer to go on ops, many crews got in as many trips as they could before the Germans found a countermeasure.
Although the metal strips puzzled the German civilians at first (many thought they were radioactive or carrying anthrax, or some other disease), the German scientists knew exactly what they were because they had developed Düppel themselves and refrained from using it for exactly the same reasons as Lindemann had pointed out to the British.
The use of Window rendered the ground-controlled 'Himmelbett' fighters of the Kammhuber Line redundant overnight but the Germans responded quickly, using non-radar equipped free-ranging 'Wild Boar' day fighters to attack visually. Some argue that, by using Window, the British forced the Germans to devise a more effective night fighter defense and had they left well alone then Allied bomber losses may have been ultimately smaller, and not worth the momentary advantage Window gave.
A lesser known fact is that Luftwaffe used this technology just six weeks after the above mentioned Hamburg raid. In a series of raids in 1943, and a larger series known as Operation Steinbock between February and May 1944, Düppel allowed German bombers to once again return to London. Although theoretically effective, the small number of bombers, notably in relation to the RAF's now-large night fighter force, doomed the effort from the start. The British fighters were able to go aloft in large numbers and often found the German bombers in spite of their Düppel.
[edit] Falklands War
Chaff was heavily used by ships in the Falklands War. The absence of chaff launchers on the Atlantic Conveyor, whilst used by all other, Royal Naval, ships in the group, may have led to the ship's sinking by Exocet missile - although given the vessel's large radar cross section, it is perhaps unlikely that chaff would have been effective.
[edit] External links
[edit] Notes
http://www.vectorsite.net/ttwiz8.html#m3 THE BRITISH BEGIN COUNTERMEASURES from Greg Goebel's THE WIZARD WAR
