They Ate the "Window" http://www.clintoncountyfiles.com/usafprs/chaff.htm by Phil Rowe -------------------------------------------------------------------------------- Aluminum or aluminized mylar (plastic) strips, thrown out of high altitude airplanes, was one form of countermeasure against radar. Later called chaff, it was originally called "window" because flight crews tossed it out of bomber and transport plane windows and hatches. Chaff was quite effective as a means of creating false targets on the screens of enemy radars. Small clouds of the material lingered in the air long enough to clutter radar screens and confuse real targets from false ones. Strip chaff was bundled in light cardboard packets about the size of a paperback novel. Hundreds of small individual pieces of foil, barely bigger than lengths of wire, would scatter as soon as the slipstream tore open the surrounding cardboard. From altitudes of 20,000 feet it took several minutes for the chaff to reach the ground. Ribbon chaff, also packed in light cardboard, was one long piece of aluminized material. It looked like a length of shiny magnetic tape from today's cassette music players. One end of the ribbon was attached to a rectangular piece of cardboard which acted like a parachute, with the ribbon dangling down below as it fell to ground. The strips of chaff were cut to particular lengths, according to the radio frequency band of the radars meant to be countered. Lengths of four to ten inches were used, though individual packets typically contained but one length. Shorter lengths were intended to counter high frequency radars and longer strips were used for lower frequencies. Each individual strip of chaff acted as a reflecting dipole (antenna) to send echo signals back to the radar receiver. Ribbon chaff acted similarly, but one twisting and turning ribbon could fool radars of many frequencies. As the reflective ribbon twisted and untwisted in descent, the length between twisted segments varied, causing the reflective lengths to effect higher and then lower frequency radars. Ribbon chaff did not create the large false images on radar screens that strip chaff clouds did, but the versatility of varied frequency reflectivity made it useful in some applications. One problem with aluminized mylar chaff, both strip and ribbon, was its effect on hitting the ground. If the material landed in pastures of grazing cattle, the stuff would be eaten by the animals just as if it were grass. Accumulations of the metalized strips eventually caused stomach problems for the livestock. Farmers were not happy when their cows ate the "window". When ribbon chaff got down near the ground, it often caused quite different problems. Long twisting ribbons of the metalized foil, draped over electric power lines, occasionally played havoc with public utilities. Electrical power blackouts, sometimes effecting wide areas, were caused by the ribbon chaff. Military training flights were soon barred from dispensing ribbon chaff over the continental U.S.. Another difficulty with chaff bundles arose from moisture and the cold of high altitudes. Military aircraft, with bundles of chaff loaded into their dispenser hoppers, often sat out in the rain. Cardboard wrappers around the chaff bundles absorbed water, sometimes enough to soak the chaff within. When the airplanes climbed to altitude, where temperatures are well below freezing, the chaff packets froze solid. Chaff bundles dispensed in a frozen state tend to remain as solid bricks and the slipstream does not scatter the material as intended. Those chaff "bricks" posed a hazard to people and property on the ground too. The effectiveness of chaff depends upon a variety of factors. It is much less useful today, with the advent of very high speed airplanes, than it was in the 1940's and 50's. Once a bundle of chaff is tossed or dispensed from an airplane, its relative velocity stops almost instantly. The real radar target, the airplane, quickly moves away from the slowly descending chaff. The cloud of chaff continues to appear on the radar scope, unless special echo processing techniques are employed, and the airplane's echo can be seen moving away from its protective chaff. Discerning which echo is the airplane and which is the chaff is not difficult. The obvious motion differences, between the high speed airplane and the nearly motionless chaff become a problem. The advent of radars which discern targets of different velocities ( high, medium and low ), by echo signal processing, has greatly diminished the utility of chaff. These radars feature signal processing circuits, known as moving-target-indicators (MTI), which quickly ignore stationary targets and portray only moving ones to the radar operator. In WWII days, however, chaff was widely used and proven very effective, when properly employed. For example, One airplane dropping a stream of chaff bundles could form a long cloud echo pattern on a radar scope, masking airplanes flying through it. Several airplanes dropping multiple trails of chaff could obscure many airplanes. An airplane dropping random clusters of chaff creates multiple echo patterns on the radar screen. Several airplanes, flying along in parallel, or better yet crossing tracks, could create a screen-full of false targets. Sorting out the real from the false echoes took time and a lot of work for radar operators, in the early days. Some techniques were developed to extend the usefulness of chaff in protecting high speed airplanes. Chaff rockets, fired ahead of the airplane, dispensed reflective material in the path of the craft. The radar echo of the chaff would then include the same airspace as the plane flying through the chaff. This technique was not very effective, and certainly much more costly than dropped chaff bundles. It also failed to solve the immediate velocity difference between the fast moving airplane and the low velocity chaff. Today's modern equivalent of chaff is the flare. Flares are pyrotechnic devices which are used to decoy infrared (IR) homing missiles. The guidance systems of some ground-to-air anti-aircraft missiles employ electronic sensors which track the heat signals of airplanes. The heat of jet engines is easy for a missile to separate from the cold IR characteristics of blue sky. The missile tracker systems simply home in on the heat source. The purpose of the flare, for the target aircraft, is to confuse the incoming missile and induce tracking errors causing it to miss. Where chaff used to be effective in the low frequency (radio wave) frequency spectrum of radars, the flare is still moderately effective in the very high frequencies of IR. A great deal of sophistication has evolved in the design of IR trackers and IR decoys meant to counter them. Effective use of chaff, in earlier times, and IR flares today, depends upon the timing of their use. Knowing when to dispense such decoys is critical. The problem for the flight crew is determining when, how and if to dispense the life-saving decoys. In fact, that's the crux of the survival problem, and the subject of an entire treatise outside the context of this presentation.
I've also heard from vets that this was sometimes used to decorate christmas trees in the winter of 44-45
Actually throwing the stuff out of the aircraft was a chore much detested by aircrew. Several first-hand accounts I have read describe packets of 'window' coming unbundled inside the aircraft - the result being chaos as the foil strips flew around inside the bomber.
Martin and guys, it maybe interesting here but.......I interviewed RK bordfünker Anton Heinemann who was crewed with RK winner pilot Gerhard Raht of I./NJG 2. I asked Anton about window in 1945 when the FuG 220d sets were getting screwed by window and his comments were "we tried to adjust the frequency and when all of this failed I would home in on the strongest blips on my screen(cathode tubes of the radar set), as these got brighter and brighter we knew that the window was the stongest in this area and were assured to have bombers close by." So the worse case scenario for the RAF is that sometimes it worked in the reverse and acted like a magnet for experienced German night fighter crews..... E
No, I wasn't the buyer... http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=155038&item=3204310352
http://www.vectorsite.net/ttwiz8.html R.V. Jones knew that disrupting German radars would blind the Kammhuber Line, and he knew just how to do it. As far back as 1937, he 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, the only woman among the boffins, 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. Although R.V. Jones wanted to use Window right away, he was overruled. Lord Cherwell, Watson-Watt, and RAF Fighter Command opposed the use of Window, since they believed that once Window was used, the Germans would immediately learn the trick and use it on raids over the British Isles in turn. In reality, the trick was much too obvious. Like so many other inventions in the Wizard War, the British hadn't been the only ones to think of it. The Germans also performed tests in 1942 on using foil strips to jam radar, calling the scheme "Dueppel". German leadership proved every bit as nervous about the idea as their British counterparts. When Hermann Goering heard about the tests, he ordered them to be stopped immediately and reports on the experiment suppressed. By this time, Allied offensive bombing capabilities clearly outstripped the German ability to retaliate in kind, and in fact the imbalance would only get worse. Dueppel clearly would hurt the Reich far more than it would hurt the British.
Chaff is still quite effective today. It requires that the defending radar switch modes or, be capable of operation that will 'see' through chaff. Also, Raytheon invented 'smart' chaff called CHEAP (Chaff electronically augmented payload) that includes an active noise jammer in each piece. In any case, chaff remains a very effective ECM measure. In WW II the Germans reacted to chaff only after the British started dropping in in July of 1943 (the Hamburg raid). A main reason for this was drüppel as it was known had been invented in Germany in 1940. Göring, in his infinite wisdom, upon reviewing an experimental use of this by technicians was appalled at its effectiveness. He immediately ordered all research stopped, all documents, materials, etc. associated with the experiments destroyed. His fear was that the British might discover the German's research and use this device against their radar system. It apparently never occured to him that the British might discover chaff on their own. Anyway, after Hamburg the Germans did try to develop effective anti-chaff countermeasures. The main three they produced were Würzlaus, a pulse-doppler range gated circuit for use in Würzburg radar, Taunus, a video differentiation circuit and, Nürnburg an audio demodulator circuit that worked off of the audio frequency of bomber aircraft propellers. Late war developments K-Laus (an improved doppler discriminator) and Wismar (wide bandwidth tuning) were actually found to mitigate all but the worst of chaff clutter. However, neither reached production by war's end. Windmar was another ECCM that the Germans tried late war. It used a time / phase lock generator to assist K-laus in eliminating low doppler rate signals as would be caused by drifting chaff. These developements were overtaken by the introduction of the Egerland (FuMG 74) and Marbach (FuMG 76) centimetric radars based on developments from captured British H2S radars and cavity magnetrons.
T.A. good materials again. minor correction....Deutsche Düppel not druppel. Wismar was used under a different name in the Ju 88G-6 nachtjägers in December 44 throughout 1945 till war's end. Also the angle of the late War SN-2 sets aerials offered a wider band width. FuG 218 aerials were set in the vertical but were also set at an 45 degree angle for minor disturbance. The infra red devices were also brought back into some useage but I will cover this in one of my two books. a little point of interest. NJG 7 with JU 88C's and A's also dropped chaff over the RAF bomber formations in prelude of an attack to throw off RAF guidance systems, or so the Germans thought.....
Sorry for the typo. A few notes. I listed all the basic counter-measures the Germans took against chaff. Most applied to ground based search and fire control sets for the most part. With AI sets, the switch to Lichtenstein SN-2 (FuG 220....FuG 218 is the Neptun family of radars developed by Siemens (FuG 217 is Neptun by FFO))cured most of the chaff problems simply by switching frequencies. Lichtenstein SN-2 appeared in four major variants: SN-2 Developed from Lichtenstein S a variant of the original BC series for use as an ASV radar in the same category as Hohentweil (FuG 200). Frequency was 73, 82 or, 91 Mhz. Later this was enlarged to 37.5 to 118 Mhz. Range was 300 - 500 meters to 4000 meters. Beam width was 100 - 120 degrees depending on installation. SN-2b This set added a fifth antenna in the middle for Lichtenstein C-1 Weitwinkel in an attempt to reduce the large minimum detection range. SN-2c Removed the C-1 installation. SN-2d The antenna array was rotated 45 degrees to minimize interference with the tail waring antenna added to this model. None of the Spanner series infra-red search devices developed by AEG proved highly useful. Spanner 1 - 4 saw some limited service but were unpopular due to their insensitivity, lack of range and small sight picture. The FuG 280 Keil Z passive detector developed by Zeiss, using a lead-sulphide selenium cell feeding to a CRT display, late war saw no actual service although a few trials units were made. It improved the on the Spanner series having a maximum range under ideal conditions of about 4000 meters.
The Crash of Junkers 9/KG30 at Cavendish The resumption by the Luftwaffe of their mini-blitz in the spring 1944 was code named “Steinbock”. This saw large numbers of bombers over the British Isles; more than had been seen for many months. The night of 21/22 of March saw around 95 aircraft sent out to attack London. Four of the raiders were brought down on land. 4D+AT fell to the guns of Squadron Leader Bunting and his radar operator Flt/Reed in a Mosquito of 488 Squadron flying out of Bradwell Bay. They found the Germans with the help of searchlights and despite evasive manoeuvres and the release of “Duppel” foil strips to hamper the Mosquito's radar , the 9/KG30 was hit by two bursts of 20mm cannon fire. The bomber went down in an inverted dive and exploded on impact in a field at Blacklands Hall, just to the north of Cavendish at 00:45. http://www.foxearth.org.uk/ww2Crashes.html ---------- Was it used elsewhere by the Germans? Anyone know?
Thanks for the link to a fascinating site, Kai ! That area of the Suffolk/Essex border is very well-known to me and I had never come across this site before......
You´re most welcome, Martin! I was quite interested in finding that the Germans actually used the duppel during the war.
they used it on many occassions during the Ardenne battles and on the Unternehmen Gisela the German nf's dropped all along the coast line to "bang" British radar
Dumb question... I know about this stuff but would like to know if it was used to any great degree by bombers feigning the thought of pre-emptive the amphibious strike on Calais leading up to D-Day???
It certainly was - 'Operation Taxable', one of 617 Squadron's less glamorous adventures ! Don't have time to post more just now - go to go to work....
( Later ) Back to Operation 'Taxable' - 16 Lancasters of 617 Squadron in two waves, each carrying 14 crew, dropped huge amounts of 'Window' at sea in precisely-guided routes to simulate the approach of a large shipping convoy approaching the Pas-de-Calais immediately before dawn on D-Day. The Operation was led by Les Munro with Leonard Cheshire flying as 2nd pilot. So secret was the operation that there was no de-briefing and no mention of the nature of the work appears in 617s Operational Record Book.
I remember reading about this operation years ago. The aircraft had to dump the chaff out to make it look like ships moving across the water. I think I remember afterward it was all done for nothing because anouther air unit knocked out the German ground radars that would have picked up this special activity. It was interesting that someone would even think of making bombers look like ships on radar but you have to have everyone on the same game plan to make things work right.
