page_title: Chaff (countermeasure)
section_title: Second World War
hierarchical_section_title: Chaff (countermeasure) / Second World War
caption_attribution_description: English: The effect of Window on the display of a German Giant Würzburg (Würzburg-Riese) radar. Imperial War Museum? - picture scanned by me Ian Dunster 12:56, 27 March 2006 (UTC) from the The Radar War article in the November 1978 issue of Aeroplane Monthly and uncredited.
context_page_description: Chaff, originally called Window by the British and Düppel by the Second World War era German Luftwaffe, is a radar countermeasure in which aircraft or other targets spread a cloud of small, thin pieces of aluminium, metallized glass fibre or plastic, which either appears as a cluster of primary targets on radar screens or swamps the screen with multiple returns.
  Modern armed forces use chaff 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 as well as penetration aids such as decoy balloons and chaff.
  Modern radar systems can distinguish chaff from target objects by measuring the Doppler shift; chaff quickly loses speed compared to an aircraft and thus shows a characteristic change in frequency that allows it to be filtered out. This has led to new techniques where the chaff is further illuminated by an additional signal from the target vehicle with the proper Doppler frequency. This is known as JAFF or CHILL.
context_section_description: The idea of using chaff developed independently in the United Kingdom, Germany, the United States and Japan. In 1937, British researcher Gerald Touch, while working with Robert Watson-Watt on radar, suggested that lengths of wire suspended from balloons or parachutes might overwhelm a radar system with false echoes and R. V. Jones had suggested that pieces of metal foil falling through the air might do the same. In early 1942, a Telecommunications Research Establishment (TRE) researcher named Joan Curran investigated the idea and came up with a scheme for dumping packets of aluminium strips from aircraft to generate a cloud of false echoes. An early idea was to use sheets the size of a notebook page; these would be printed so they would also serve as propaganda leaflets. It was found that the most effective version was strips of black paper backed with aluminium 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 of Düppel. The German code name came from the estate where the first German tests with chaff took place, circa 1942. 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.
  The systems used the same concept of small aluminium 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. 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 devices codenamed Mandrel, Piperack and Jostle. 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. This caused concern in RAF Fighter Command and Anti-Aircraft Command, who managed to suppress the use of Window until July 1943. It was felt that the new generation of centimetric radars available to Fighter Command would cope with Luftwaffe retaliation.
  Examination of the Würzburg radar equipment brought back to the UK during Operation 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 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 fire raids against Hamburg. The first aircrew trained to use Window were in 76 Squadron. Twenty-four crews were briefed on how to drop the bundles of aluminised-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. 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. Over a week of attacks, 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 we
caption: The effect of chaff on the display of a Würzburg Riese radar. The effect of jamming appears in the left "jagged" half of the circular ring, contrasting with the normal "smooth" (unjammed) display on the right half of the circle, with a real target at the 3 o'clock position – on the jammed left side the real target "blip" would have been indistinguishable from the jamming.
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