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Chaff, originally calledWindow[1] orDüppel, is aradar countermeasure involving the dispersal of thin strips ofaluminium,metallizedglass fiber, orplastic.[2] Dispersed chaff produces a largeradar cross section intended to blind or disrupt radar systems.[3]
Modern military forces use chaff to distractactive radar homing missiles from their targets.Military aircraft andwarships can be equipped with chaff dispensing systems for self-defense. During itsmidcourse phase, anintercontinental ballistic missile may release chaff along with its otherpenetration aids.
Contemporary radar systems can distinguish chaff from legitimate targets by measuring theDoppler effect;[4] chaff quickly loses speed after leaving an aircraft, and the resulting shift inwavelength of the radar return can be measured. To counter this, a chaff cloud can beilluminated by the defending vehicle with a doppler-corrected frequency. This is known as JAFF (jammer plus chaff) or CHILL (chaff-illuminated).[5]
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The idea of using chaff developed independently in theUnited Kingdom,Germany, theUnited States andJapan. In 1937, British researcher Gerald Touch, while working withRobert Watson-Watt on radar, suggested that lengths of wire suspended from balloons or parachutes might overwhelm a radar system with false echoes[6] andR. V. Jones had suggested that pieces of metal foil falling through the air might do the same.[7] In early 1942,Telecommunications Research Establishment (TRE) researcherJoan Curran investigated the idea and came up with a scheme for dumping packets ofaluminium strips from aircraft to generate a cloud of false echoes.[8] An early idea was to use sheets the size of a notebook page; these would be printed so they would also serve aspropaganda leaflets.[9] It was found that the most effective version was strips of black paper backed withaluminium foil, exactly 27 cm × 2 cm (10.63 in × 0.79 in) and packed into bundles each weighing 1 pound (0.45 kg). The head of the TRE,A. P. Rowe, code-named the device "Window". In Germany, similar research had led to the development ofDüppel. The German code name came from the estate where the first German tests with chaff took place, circa 1942.[9] Once the British had passed the idea to the US via theTizard Mission,Fred Whipple developed a system for dispensing strips for theUSAAF, but it is not known if this was ever used.
The systems used the same concept of smallaluminium strips (or wires) cut to a half of the target radar's wavelength. When hit by the radar, such lengths of metal resonate and re-radiate the signal.[9] Opposing defences would find it almost impossible to distinguish the aircraft from the echoes caused by the chaff. Other radar-confusing techniques included airborne jamming devicescodenamed Mandrel, Piperack, Jostle andCarpet. Mandrel was an airborne jammer targeted at the German Freya radars,[10] while Carpet targeted the gun-layingWürzburg radar. 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 theRoyal Air Force (RAF) used it against the Germans, theLuftwaffe would quickly copy it and could launch a newBlitz. This caused concern inRAF Fighter Command andAnti-Aircraft Command, who managed to suppress the use of Window until July 1943.[11] It was felt that the new generation of centimetric radars available to Fighter Command would cope withLuftwaffe retaliation.
Examination of theWürzburg radar equipment brought back to the UK duringOperation Biting (February 1942) and subsequent reconnaissance revealed to the British that all German radars were operating in no more than three frequency ranges, making them prone tojamming.Arthur Travers "Bomber" Harris, Commander-in-Chief (C-in-C) ofRAF Bomber Command, finally got approval to use Window as part ofOperation Gomorrah, the week long bombing campaign againstHamburg. The first aircrew trained to use Window were in76 Squadron. Twenty-four crews were briefed on how to drop the bundles ofaluminised-paper strips (treated-paper was used to minimise the weight and to 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 proved spectacular. The radar-guided master searchlights wandered aimlessly across the sky. Theanti-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. Squadrons quickly had special chutes fitted to their bombers to make chaff deployment even easier. Seeing this as a development that made it safer to go on operations, many crews got in as many trips as they could before the Germans found a counter-countermeasure.
Although the metal strips puzzled the German civilians at first, German scientists knew exactly what they were–Düppel—but had refrained from using it for the same reasons as Lindemann had pointed out to the British. For over a year the curious situation arose where both sides of the conflict knew how to use chaff to jam the other side's radar but had refrained from doing so for fear of their opponent replying in kind. Window rendered the ground-controlledHimmelbett (canopy bed) fighters of theKammhuber Line unable to track their targets in the night sky and rendered the early UHF-bandB/C andC-1 versions of the airborne interceptLichtenstein radar (following the capture of aJunkers Ju 88R-1 night fighter by the British in May 1943 equipped with it) useless, blinding radar-guidedguns andspotlights dependent on the ground-based radar.OberstHajo Herrmann developedWilde Sau (Wild Boar) to cope with the lack of accurate ground guidance and led to the formation of three new fighter wings to use the tactic, numberedJG 300, JG 301 and JG 302. Ground operators would radio-direct single-seat fighters and night fighters to areas where the concentrations of chaff were greatest (which would indicate the source of the chaff) for the fighter pilots to see targets, often against the illumination from fires and searchlights below. A few of the single-seat fighters had theFuG 350Naxos device to detectH2S radar (the first airborne, ground scanning radar system) emissions from the bombers.
Six weeks after the Hamburg raid, theLuftwaffe usedDüppel in 80 cm × 1.9 cm (31.50 in × 0.75 in) lengths during a raid on the night of 7/8 October 1943.[12] In raids in 1943 and the "mini-blitz" ofOperation Steinbock between February and May 1944,Düppel allowed German bombers again to attempt operations overLondon. Although theoretically effective, the small number of bombers, notably in relation to the large RAFnight-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 ofDüppel. The Germans obtained better results during theair raid on Bari in Italy, on 2 December 1943, when Allied radars were deceived by the use ofDüppel.[13]
Following the British discovery of it in 1942 by Joan Curran, chaff in the United States was co-invented by astronomerFred Whipple and Navy engineer Merwyn Bly. Whipple proposed the idea to the Air Force he was working with at the time.[14] Early tests failed as the foil strips stuck together and fell as clumps to little or no effect. Bly solved this by designing a cartridge that forced the strips to rub against it as they were expelled, gaining anelectrostatic charge. Since the strips all had a similar charge they repelled each other, enabling the full countermeasure effect. After the war, Bly received theNavy Distinguished Civilian Service Award for his work.
In thePacific Theatre, Navy Lieutenant Commander Sudo Hajime invented a Japanese version calledGiman-shi, or "deceiving paper". It was first used with some success in mid 1943, during night battles over theSolomon Islands.[15] Competing demands for the scarce aluminum necessary for its manufacture limited its use.[16] On February 21, 1945, during theBattle of Iwo Jima,Giman-shi was successfully used prior to aKamikaze attack on theUSS Saratoga.[17]
British warships in theFalklands War (1982) made heavy use of chaff.
During this war, BritishSea Harrier aircraft lacked their conventional chaff-dispensing mechanism.[18]Therefore,Royal Navy engineers designed an improvised delivery system ofwelding rods,split pins and string, which allowed six packets of chaff to be stored in theairbrake well and be deployed in flight. It was often referred to as the "Heath Robinson chaff modification", due to its complexity.[19]
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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.
Radar can make use of the doppler effect to distinguish between chaff and target aircraft which are fast moving.
The doppler effect only occurs for the component of velocity parallel to the radar beam.
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. It may also rotate to minimize its cross section exposed to the radar beam.
This leads to aircraft being more difficult to separate from the effectively stationary chaff and is known as "notching" as radar typically incorporate a notch of low sensitivity to frequencies associated with low velocity.
The effect is likely to be momentary against modern radar systems but can be prolonged by the use of Chill and Jaff as described below.
One of the important qualities of chaff is that it is lightweight, allowing large amounts to be carried. 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. From the viewpoint of an enemy radar, the chaff quickly decays to zero velocity. Modern radars use theDoppler 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. This allows the radar to filter out the chaff from its display.[20]
To counteract this filtering, the JAFF or CHILL technique has been developed. This uses an additional jammer broadcaster on the aircraft to reflect a signal off the chaff cloud that has the proper frequency to match that of the aircraft. This makes it impossible to use Doppler shift alone to filter out the chaff signal. In practice, the signal is deliberately noisy in order to present multiple false targets.[20]
In essence, the JAFF technique is a low-cost offboard decoy, moving the jammers from the launcher platform to the decoy, and using the chaff as a reflector to provide angular separation.[20]
While foil chaff is still used by certain aircraft, such as theBoeing B-52 Stratofortress bomber, this type is no longer manufactured. The chaff used by aircraft such as theFairchild-Republic A-10 Thunderbolt II,McDonnell Douglas F-15 Eagle,General Dynamics F-16 Fighting Falcon, andMcDonnell Douglas F/A-18 Hornet consists of aluminium-coated glass fibres. These fibre "dipoles" are designed to remain airborne for as long as possible, having a typical diameter of 1mil, or 0.025 mm, and a typical length of 0.3 inches (7.6 mm) to over 2 inches (51 mm). Newer "superfine" chaff has a typical diameter of 0.7 mils (0.018 mm). The chaff is carried in tubular cartridges, which remain attached to the aircraft, each typically containing around 3 to 5 million chaff fibres. The chaff is ejected from the cartridge by a plastic piston driven by a small pyrotechnic charge.[21]
Chaff countermeasures come in two main types: continuous wave (CW) chaff, used against radar-guided missiles that operate on a continuous frequency, and pulsed chaff, used against missiles that operate on a pulsed frequency.[22]
Little research has been conducted on the public health and environmental effects of chaff. 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."[2][23]
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