Chaff (radar countermeasure)

Chaff, originally called Window by the British, and "Düppel" by the World War II era German Luftwaffe, is a radar countermeasure in which aircraft or other targets spread a cloud of small, thin pieces of aluminium, metallised glass fibre 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 (in naval applications, for instance, using short-range 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.

World War II

The idea of using chaff was independently developed in the UK and Germany.

As far back as 1937, R. V. Jones had suggested that a piece of metal foil falling through the air might create radar echoes. [Jones. p.290] In early 1942, a 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. [Goebel. section 8.3 "The British Begin Countermeasures"] From a practical point of view it was found the most effective version were strips of black paper backed with aluminium foil cut to strips exactly 27 centimetres by 2 centimetres and packed into one pound (weight) bundles.

The Head of the TRE, A.P. Rowe , code-named the device as Window.

Meanwhile in Germany, similar research had led to the development of Düppel. [The German code name was that of the estate on which the first German tests with chaff had been made, circa 1942. Jones. p.299] Once the idea had been passed to the US, Fred Whipple developed a system (according to Harvard Gazette Archives) for dispensing strips for the USAAF, but it is not known if this was ever used.

The systems were all essentially identical in concept, small aluminium strips (or wires) cut to one-half of the target radar's wavelength. When hit by the radar, such lengths of metal resonate and re-radiate the signal. [Jones. p.291] Opposing defenses would find it almost impossible to pick out the "real" aircraft from the echoes from the chaff. Other radar-confusing techniques included Mandrel, [Mandrel was an airborne jammer targeted at the German Freya radars. Jones. p.295] Piperack, and Jostle.

However, 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 concern in Fighter Command and Anti-Aircraft Command, who managed to suppress the use of Window until July 1943. [Jones. pp.291-299] At this time it was felt the new generation of centimetric radars available to Fighter Command would deal with any Luftwaffe response to RAF Bomber Command use.

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 fire raids against Hamburg.

The first aircrew trained to use it were 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 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, their radar displays swamped with false echoes, utterly failed to find the bomber stream. A vast area of Hamburg was devastated with the loss of only 12 bombers. Squadrons 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 unable to track their targets in the night sky and left radar-guided guns and spotlights useless. In response to this, a new tactic, called 'Wild Boar', was developed to cope with the lack of accurate ground guidance. 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), and allow the fighters to visually acquire their targets, often against the fires and searchlights below.

A lesser known fact is that Luftwaffe used this technology just six weeks after the above mentioned Hamburg raid. The German strips were cut into 80 centimetre by 1.9 centimetre lengths and first dropped during a raid on 7-8 October 1943. [The Blitz-Then and Now (Volume 3) page 309.] In a series of raids in 1943, and the 'mini-blitz' of Operation Steinbock between February and May 1944, Düppel allowed German bombers to once attempt to operate over 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.

Falklands War

Chaff was heavily used by British warships in the Falklands War. The absence of chaff launchers on the "Atlantic Conveyor", while used by all other Royal Navy ships in the group, may have led to the ship's sinking by an Exocet missile — although given the vessel's large radar cross section, it is unlikely that chaff would have been effective.Fact|date=March 2008

During the war British Harrier aircraft lacked their conventional Chaff dispensing mechanism.cite book |author=Sharkey Ward |title=Sea Harrier Over the Falklands (Cassell Military Paperbacks) |publisher=Sterling*+ Publishing Company |location= |year= |pages=pg 245 |isbn=0-304-35542-9 |oclc= |doi= |accessdate=2008-03-20] Therefore Royal Navy engineers designed an impromptu 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. It was often referred to as the "Heath Robinson Chaff modification", due to its complexity.cite book |author=Morgan, David L. |title=Hostile Skies: My Falklands Air War |publisher=Orion Publishing |location=London |year= |pages=pg 59, 73 and photo section |isbn=0-297-84645-0 |oclc= |doi= |accessdate=2008-03-20]

ee also

*Countermeasure
*Infrared countermeasures
*Flare (countermeasure)
*Electronic countermeasures
*Anti-aircraft
*Anti-ballistic missile

External links

* [http://www.bbc.co.uk/dna/h2g2/brunel/A591545 BBC: The History of Radar]
* Obituary of Joan Curran in [http://findarticles.com/p/articles/mi_qn4158/is_19990219/ai_n14214115 The Independent, Feb 19, 1999 by Tam Dalyell]

References

*Goebel, Greg. [http://www.vectorsite.net/ttwiz.html "The Wizard War: WW2 & The Origins Of Radar"] v.2.0.2, retrieved 2008-03-18
* Jones, R. V. (1978). "Most Secret War: British Scientific Intelligence 1939–1945." Hamish Hamilton, London. ISBN 0 241 89746 7

Notes