World War II SEAD [Suppression of Enemy Air Defence] Part I


An RAF Avro Lancaster drops “Window” chaff. These clouds of aluminum strips showed as false echoes on German radar screens, making it difficult for the Luftwaffe to respond appropriately to bombing raids. “Window” was first used in the raids on Hamburg in July-Aug 1943. The Germans copied the technique in raids on England, calling their chaff “Düppel”, but so late in the war, their lack of bombers and the abundance of Allied night interceptor fighters made these raids ineffective.


AS-69/APT Crossed Element Phased Antenna
This antenna was one of the first “fishook”, or turnstyle antennas designed to provide circular polarization to counter Wurzburg radars. This antenna was among a long series of such antennas in an attempt to be more effective against a wide variety of target antennas, both horizontally and vertically polarized.

It is hard to say when SEAD began. Undoubtedly during World War I early combat aircraft probably fired bullets at those taking pot shots at them, but it seems that the first dedicated application of a SEAD effort occurred during the early stages of the Battle of Britain between the Royal Air Force (RAF) and the Luftwaffe in 1940.

The early development and advances in Radio Direction Finding and Ranging, which became better known as radar, were not lost on the RAF. The efforts of Sir Robert Watson Watt, considered to be the inventor of radar, yielded a network of EW stations positioned around the United Kingdom and given the code-name Chain Home. Chain Home was intended to provide long-range ‘eyes’ for the RAF to warn them of approaching Luftwaffe aircraft. The system was a breakthrough. It gave the RAF as much as 30 minute’s advanced notice of an incoming formation of German bombers, enough time for fighters to be scrambled to meet the aircraft as they neared the British Isles. The Germans of course, put two-and-two together and realised that Chain Home was a de facto force multiplier which could exact a heavy toll on their aircraft. If future missions over the UK were to succeed then Chain Home would need to be destroyed.

The Luftwaffe began their attacks on the radar stations on the 12 August when destruction arrived from a fine and misty sky onto radar stations at Dover in Kent, Dunkirk in northern France, and Pevensey and Rye in East Sussex. The attack was enough to put all but Dunkirk out of action for 6 hours and would herald the Luftwaffe’s battle for air superiority, known as Adler Tag (Eagle Day); the prelude to Operation Sea Lion, the invasion of the British Isles. Yet, despite these successful attacks Luftwaffe commander Hermann Göring called off further strikes believing them to be somewhat lacking in effectiveness. Göring’s lack of tactical understanding was perhaps not surprising, he did after all, famously switch the Luftwaffe’s priorities from hitting RAF airfields to bombing London, raining hell on the civilians below but leaving the airfields relatively untouched and the RAF free to destroy his bombers. This unwittingly ignited a debate over the virtues of strategic bombing which has raged ever since. Our Prussian friend would have been appalled, the Chain Homes were a perfect example of a CoG as they allowed fighters to be directed towards the bombers, and their comprehensive destruction could have made the outcome of the Battle of Britain, and the Second World War, very different.

That said, the Allies were also at risk from German air defences. Before the Battle of Britain had begun, the RAF had started to take the war to the Third Reich, performing a bombing raid on Berlin in August 1940. The dangers of AAA increased throughout the war. As such, formations of Allied bombers had to become larger to ensure that, to paraphrase British Prime Minister Stanley Baldwin, at least some of the bombers always got through to their target. The AAA was exacting a heavy loss on aircrew and aircraft, and subsequently degrading the effectiveness of the Allied bombing campaign against Nazi Germany. One report by the United States Army Air Force (USAAF) as it was known until 1947, notes that while the damage from enemy fighters wrought on its bombers operating over Europe had been reduced, the devastation caused by AAA or ‘flak’ (an acronym for Flugabwehrkanone; ‘anti-aircraft cannon’) had increased damage ten-fold for bombers compared to damage inflicted by enemy fighters. The puffs of black smoke that exploded around the B-17 Flying Fortresses and Avro Lancasters which threaded their dangerous paths to their targets, were much more harmful than their pint-sized explosions would suggest. Well-placed rounds could damage an aircraft, preventing it from dropping its bombs and, at worse shoot the aircraft out of the sky. Realising the effectiveness of flak, the Luftwaffe saturated its AAA coverage, clustering the weapons in citadel-like structures known as Flaktürme or Flak Towers, their forbidding, slab-like appearance leaving little doubt as to their destructive purpose.

The Germans had also developed the Würzberg gun-laying radar during World War II which was deployed from 1941 and used to connect flak batteries for guiding AAA. The Würzburg had a range of around 18 miles (29 km) and could pick up targets after they had been detected at long range by the Freya system. The fire control radar allegedly got its name from Wilhelm Runge, its inventor, blindly sticking a pin into a map and hitting the city of Würzburg. Production of Würzburg began in 1940 and around 3,000 would be built in three versions; ‘A’, ‘B’ and ‘C’ with the accuracy of the system steadily improving as time went by. The accuracy of the Würzburg was particularly good up to a range of around ten miles (16 km) and could give the radar operator an accurate picture as to the location of an incoming formation of aircraft.

Würzburg was joined by the Freya long-range EW radar system which had a range of 100 miles (160 km), and over 1,000 were constructed during the war. The radar was linked to Germany’s network of ground observers to provide an embryonic Integrated Air Defence System (IADS). The first engagement that the RAF had with a Freya radar was on 18 December 1939, when eighteen RAF Wellington bombers were performing a daylight bombing attack on Germany. The radar was able to detect the bombers and direct fighter aircraft to intercept them.

The long-range radar could detect the incoming formation of aircraft and alert the Würzburg operators as to their expected avenue of attack. Würzburg would then watch the skies and once it had detected the formation, hand the targets off to flak batteries which could bring accurate fire to bear on the formation. The Germans invested heavily in radar technology, and it would not be long before networks of radar were positioned throughout occupied Europe covering many directions of possible advance.

The British took strenuous efforts to seek and destroy German radar and a high priority was placed on the Würzburg sets. During a reconnaissance sortie on 22 November 1941, an RAF photo-reconnaissance unit (PRU) Spitfire had taken a picture of a German radar installation at Bruneval near Le Havre on the French coast. Reginald Victor Jones, the British Assistant Director of Intelligence (Science) and radar expert, had stressed the need for an intensive effort to hunt down German radar systems after the acquisition of the famous Oslo Report by the British in 1939 which, amongst other things, detailed the characteristics of German EW radar technology.

The installation at Bruneval prompted further interest and another PRU sortie was performed on 5 December, this time with Flight Lieutenant Tony Hill at the Spitfire’s controls. Flt. Lt. Hill’s pictures revealed a 10 ft (3 m) diameter radar dish which resembled the Würzburg design. The British government decided that the Bruneval site would be attacked (codenamed Operation Biting) by a commando force of 120 troops from C Company, 2nd Battalion of the Parachute Regiment led by Major John Frost and including Flight Sergeant Charles Cox, a technician, who would assist with dismantling and carrying any parts of the Würzburg that the force could carry. The troops would parachute from twelve Armstrong-Whitley-V bombers commanded by Wing Commander PC Pickard from RAF 51 Squadron.

Once on the ground the force met some heavy resistance and suffered two killed and four taken prisoner. That said, Major Frost’s force was able to photograph the radar and take some components back to the United Kingdom for examination. One of the most important things revealed by the analysis was that the radar was susceptible to countermeasures. However, one unwanted side-effect of the raid was that the Germans fortified their radar sites, determined that they would not be attacked in this way a second time. However, the positive side of this was that the fortified structure made enemy radar installations easier to locate. Operation Biting was notable in its use of commandos to assist the defence suppression task and was arguably the first example of the joint approach to SEAD, something that would later be utilised by the Israelis with great effect during the 1960s.

While the British performed commando raids to assist the defence suppression fight, aircraft were also brought to the fray, particularly hitting the factories which produced the Würzburgs. On 20 June 1943, RAF bombers were sent to attack the Zeppelin factory at Friedrichshafen which was thought to house a Würzburg production line. This was following photographs that had been taken in June 1943 which appeared to show a collection of radar reflectors outside the factory. Two days after Winston Churchill had personally viewed the photographs at RAF Medmenham, home of the RAF PRU, orders were sent to the RAF’s No.5 Group to attack the Zeppelin works with their Lancaster bombers during the next full moon. The raid, which occurred on 20 June 1943, was a success and the Würzburg assembly line was smashed.

Thoughts also turned to defensive methods. This was still the early days of defence suppression, before the principles of surprise, weight, mass, balance and persistence were defined for the mission, but the germ of these ideas were present in how the Allies chose to confront the flak menace. True, the Germans had robust air defences around their cities and their key targets, but they were just as susceptible as any other defender to the rule that you cannot protect everywhere at once. Those prosecuting the bombing campaign therefore realised that one solution could be to fly around the flak emplacements on their way to and from the target. Another recommendation was for the crews to fly their bombers as high as possible. However, the use of altitude to frustrate flak also had to be squared with the need to see the target being attacked, so hiding behind cloud cover could not be an option. A major railway marshalling yard may be quite visible and characteristic from a considerable height, but a ball-bearings factory may not and what is more, the latter may look rather like surrounding buildings which do not have a military function. The other idea was to space bombers at certain intervals or within designated sections of airspace that would enable them to take full advantage of the emerging radar countermeasures that the Allies were starting to field.

A snapshot of the losses suffered by the Eighth Air Force over one month, August 1944, illustrates how devastating radar-guided flak could be. The USAAF sent almost 45,600 aircraft over their targets. Of those aircraft, almost 11,700 suffered hits from flak, around 26 per cent of the total. In 250 cases, the flak was sufficient to destroy the aircraft. Of all Eighth Air Force aircraft lost during August 1944 in the European theatre to all causes, flak claimed 70 per cent. In terms of damage suffered to Eighth Air Force aircraft in theatre, flak was responsible for 84 per cent of that damage. The fighter menace may have begun to dissipate as the Allies smashed up the Luftwaffe, but undoubtedly ground-based air defence was still a considerable force to be reckoned with.

However help was at hand from a surprisingly simple invention. Chaff, or Window as it was initially called by the British, was used from 1943; initially as part of Operation Gomorrah, the bombing of Hamburg. It was dispensed by aircraft and would provide a confetti of thousands of tiny metallic strips of aluminium foil which would lazily drift around the air and eventually down to earth. As they did this, they would reflect radar beams back on themselves. The result was that these millions of metal strips would show up as a series of returns on the radar operator’s scope. Situated well-away from the action, the operator could not be sure if the strips were a deception or a formation of bombers heading for a target. Window was famously used on the eve of D-Day on the 4 June 1944 to create a phantom bomber force attacking in the direction of Calais rather than Normandy to fool the Germans as to the locations of the Allied landings by convincing the operators of the Würzburg and Freya radar that a huge force of bombers was heading in that direction.

Window gave bomber crews protection from radar provided that they flew within 2,000 ft (609 m) of the cloud of metallic foil. Guidance to the bomber crews of the USAAF Eighth Air Force directed that they should not attack on a heading of more than five degrees difference from the Window dispensing aircraft to ensure that they did not appear as a separate radar return and thus a lucrative target. However, the system worked for a while, but eventually German radar operators got wise to Window and learned to differentiate between the cloud of foil and the bomber formations. Initially, The British Government’s Chief Scientific Advisor, Professor Alexander Lindemann had counselled against employing Window on raids over Germany fearing that once the Germans realised that the foil strips were accompanying bombing raids where their air defence radar was being disrupted, they would see the ruse and use similar techniques during their raids on the UK. Ironically, the Germans had developed their own version of Window called Düppel, and held off using it over the UK for very similar reasons.

Window was not the only defence against radar, it was joined by Carpet. Unlike its cousin, this was not a physical countermeasure, but was instead one of the first ECMs to be used by attacking aircraft. In essence, Carpet transmitted radio waves to jam the Würzburgs thus depriving the flak batteries of their eyes which told them where the attacking bomber formation was located. However, like Window, the efficacy of Carpet was degraded the farther a bomber was operating from the aircraft carrying the equipment. Therefore, the spacing of the bomber formation vis-à-vis the Carpet-equipped aircraft became a matter for dextrous handling.

Carpet was the product of the rigorous research that both British and American scientists were performing to defeat the radar menace. The effort was two-fold: not only were countermeasures designed, but for these countermeasures to be effective, the scientists had to develop a comprehensive understanding of how German air-search radar actually worked as part of getting inside the German air-defence network’s mindset. In modern parlance, this is known as building an understanding of the enemy’s Electronic Order of Battle.

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