No 100 Group – Confound and Destroy

Handley-Page Halifax, No.462 Sq

At the end of 1943 Royal Air Force Bomber Command gained a new capability to assist its night raiders: No 100 Group, a force with the task of reducing the effectiveness of the German air defences. The Group’s commander, Air Vice-Marshal Edward Addison, planned a two pronged attack on the defences. First, by jamming and spoofing the German radar systems and blocking the radio channels, he would make it more difficult for night fighters and anti-aircraft gunners to find and engage the bombers. Second, by sending long-range night fighters to seek out their German counterparts and attack their airfields, the Group would impose constraints on the operations of the defending night fighter force. In recognition of the dual-nature of its role, the Group’s official motto was ‘Confound and Destroy’.

During the autumn of 1944 No 100 Group began to make its presence felt during the night attacks on targets in Germany. The role of the formation was termed ‘bomber support’ (what is now called ‘defence suppression’). No 100 Group’s jamming force comprised four squadrons of converted heavy bombers: No 171 with Halifaxes, No 199 with Stirlings (later it would convert to Halifaxes), No 214 with Flying Fortresses and No 233 with Liberators (later it would convert to Flying Fortresses). These aircraft were modified to carry a menagerie of specialized electronic jamming equipment: ‘Mandrel’ and ‘Carpet’ to jam the Germans’ ground radars, ‘Piperack’ to jam their night fighters’ airborne interception radars and ‘Jostle’ to jam their communications radio channels. In addition, several of the aircraft were modified to carry large quantities of ‘Window’ metal foil to create thousands of false targets on the enemy radar screens to distract and confuse the defenders.

The Group’s destroying element comprised seven squadrons of Mosquito night fighters. In addition to their normal airborne interception radar, some of these aircraft carried the ‘Serrate’ or ‘Perfectos’ homing devices. ‘Serrate’ picked up the transmissions from the German night-fighter radars and gave a bearing that enabled the Mosquito crews to home on their source. ‘Perfectos’ was even cleverer: it radiated signals to trigger the IFF identification equipment of any German aircraft within a range of about fifteen miles, and the latter’s coded reply signal betrayed its exact whereabouts. ‘Perfectos’ provided the three pieces of information necessary for a successful interception: it gave relative bearing and distance, as well as providing a positive hostile identification of the aircraft under observation. The last of these was particularly valuable, since the Mosquitos were too deep in enemy territory where there might be a few German night fighters in an area of sky filled with several hundred friendly bombers. No longer could Luftwaffe night fighters cruise over their homeland concerned only with finding and shooting down bombers: now these hunters were liable at any time to become the hunted.

As well as seeking out German night fighters in the air, two of the Mosquito units, Nos 23 and 515 Squadrons, specialized in flying night intruder missions against the enemy night-fighter bases. These aircraft would orbit over the enemy airfields for hours on end, and bomb or strafe any movement seen on the ground.

To provide ELINT support for these operations the Group had its own ‘Ferret’ squadron, No 192, with Halifaxes, Wellingtons and Mosquitos fitted with special equipment to collect signals from the various German radar systems.

No 100 Group’s jamming element flew four general types of mission in support of the night bombers: the ‘Mandrel’ screen, the ‘Window Spoof’, the Jamming Escort and the Target Support operation. The ‘Mandrel’ screen usually involved between ten and sixteen aircraft orbiting in pairs along a line just clear of enemy territory, with an interval of fifteen miles between pairs. Each aircraft carried several ‘Mandrel’ transmitters, and the purpose of the operation was to produce a wall of jamming about 100 miles long, to prevent the German early-warning radar operators from seeing aircraft movements behind the screen. Usually the ‘Mandrel’ screen was employed to conceal the approach of a raiding force, but when no raid was planned it was erected to cause the enemy controllers to think that a raid was in the offing and so force them to scramble night fighters to waste their dwindling supplies of aviation fuel.

The ‘Window Spoof’ comprised up to twenty-four aircraft in two formations of twelve aircraft flying in line abreast, with 2¼ miles between the aircraft and the second line some 30 miles behind the first. Each aircraft released ‘Window’ at a rate of thirty bundles a minute, one every two seconds. In this way the formation could produce on enemy radar the illusion of a bomber stream of some 500 aircraft. The aim of the tactic was to lure night fighters away from the real raiding forces (each real bomber stream also dropped ‘Window’, though at a lower rate, so that it was impossible to tell the real attacks from the feints).

The Jamming Escort role involved Fortresses and Liberators of No 100 Group flying above the main bomber stream and jamming with ‘Jostle’ and ‘Piperack’ transmitters to blot out, respectively, the German night fighters’ radio channels and their airborne interception radars. When the Jamming Escort aircraft arrived at the target they often assumed the Target Support role, in which they orbited in the area and operated their ‘Carpet’ transmitters to jam the frequencies used by the German flak control radars.

Sergeant Kenneth Stone, an air gunner of No 233 Squadron flying Liberators, described his impressions of the types of operations his unit flew:

‘Window Spoof’ raids were carried out by a few aircraft dropping the metal foil to simulate a large force of aircraft on the enemy radar. The operation was very precisely worked out; there would be a rendezvous point in a safe area and timing was critical to within two minutes. If a crew arrived later than this it had to abort the mission because one aircraft late was a sure give-away on radar. All aircraft had to go in together and dispense ‘Window’ at a regular pace. The aircraft flew a ‘corkscrew’ course to disperse the ‘Window’ more effectively and give the illusion of a larger force than was actually present. Generally six spoofers would show up on radar as 300-plus plots. A spoof target was generally selected which either committed the defences in that area and left the genuine target free, or at worst split the defences and rendered a proportion of them useless. There were two methods of ending this type of spoof. Either we continued to the spoof target and dropped target markers there; or we stopped ‘Windowing’ short of the target and dived away at a great rate of knots. There is no doubt that the ‘Window Spoofs’ were highly successful in achieving their purpose, by fooling the defences and diverting their effort. The casualty rate for spoofing aircraft was not so high as might have been expected, because the enemy night fighters would be tied up with the paper-chase and could not winkle out the real aircraft.

In Stone’s view the Target Support operation was the most dangerous of those flown by his unit, and in recognition of the hazards these missions were shared out amongst the crews in strict rotation:

The general principle was to cover the target from five minutes before the initial marking began until five minutes after the bombing stopped. The big hazard was having to hang around while the bomber boys ran in, bombed, and got the hell out of it! Fifteen minutes seemed a long, long time suspended over the inferno below. The support aircraft generally flew some 2,000 to 4,000 feet above the bomber stream and jammed the flak and searchlight radars, the night-fighter H/T frequencies, the night fighter radars, etc. — in other words diverting the defensive forces away from the bombers during the most critical period.

The reader may gain an impression of the way in which No 100 Group’s tactics dovetailed with those of the rest of Bomber Command from a more or less typical operation of the late war period, that on the night of 20/21 March 1945. The targets were the oil refineries at Bohlen near Leipzig and at Hemmingstedt near Hamburg; the former was to be attacked by 235 heavy bombers, the latter by 166.

The first action by Bomber Command that night was a large-scale nuisance raid on Berlin by 35 Mosquitos, beginning at 9.14 p.m. The Mosquitos, flying fast and high, required no support from No 100 Group’s jamming force. As the raiding force moved in, night-fighter Mosquitos of Nos 23 and 515 Squadrons fanned out over Germany making for the enemy night-fighter bases likely to become active that night. When the intruders reached their objectives they orbited, waiting to pounce on any aircraft seen taking off or landing.

Just after 1 a.m. the main raiding force bound for Bohlen crossed the French coast and headed south-east towards southern Germany. Also heading across France, on a track almost parallel to that of the Bohlen attack force but a little further south, was a feint attack force of 64 Lancasters and Halifaxes. These aircraft belonged to operational conversion units and were flown by crews in the final stages of their training.

No 100 Group’s electronic trickery began at 2.05 a.m. on the morning of the 21st. Established in a line 80 miles long over France and just inside Allied-held territory, seven pairs of Halifaxes of Nos 171 and 199 Squadrons turned on their ‘Mandrel’ equipment to provide a wall of jamming to conceal the approach of the Bohlen attack force, which by then had split into two separate parts.

Running across France outside the cover of the ‘Mandrel’ screen, the feint attack flown by trainee crews continued heading east in full view of the enemy radars. German night fighters moved into position to block the threatened incursion but, at 2.55 a.m., when the bombers were just short of the German border, the feint attackers turned round and went home. A few minutes later, well to the north, the two Bohlen attack forces burst through the ‘Mandrel’ jamming screen and crossed the Rhine into German-held territory. Twenty miles ahead of the bombers flew four Halifaxes of No 171 Squadron and seven Liberators of No 223 Squadron dropping ‘Window’ to conceal the strength of the attacking forces. Flying ahead and on the flanks of each of the bomber streams, 33 Mosquito night fighters of No 100 Group began their deadly game of hide-and-seek with their enemy counterparts.

Shortly before 3 a.m. a Mosquito of No 85 Squadron, with pilot Flight Lieutenant Chapman and radar operator Flight Sergeant Stockley, picked up IFF identification signals on ‘Perfectos’ from an enemy aircraft at a range of 12 miles. Chapman afterwards reported:

At 0255 hours just after passing Hamm on the way in to escort the bomber stream we got a ‘Perfectos’ contact at 12 miles’ range — height 12,000 feet. Range was closed to 1 mile but no AI [radar] contact was obtained and the range started to increase again, so deciding that the contact must be below we did a hard diving turn to port down to 9,000 feet and finally D/F’d [took a bearing] on to the target’s course at 7 miles’ range. We closed to 6 miles’ range on a course of 120° and an AI contact was obtained…The target was still climbing straight ahead and was identified with the night glasses as an Me 110. I closed in to 600 feet and pulled up to dead astern when the Hun started to turn to port. I gave it ½ ring deflection with a three-second burst whereupon the E/A [enemy aircraft] exploded in the port engine in a most satisfying manner with debris flying back. It exploded on the ground at 0305 hours, position 25-30 miles NW of Kassel.

From start to finish the engagement lasted ten minutes and, as can be seen, this type of operation tended to involve the Mosquito crew in a lengthy chase before they reached a firing position.

The spoof tactics that night were successful. The German fighter controller seriously underestimated the strength of the two raiding forces heading for Bohlen; he estimated their strengths as about 30 aircraft each and thought they might even be ‘Window’ feints. Only after the raiders had crossed the Rhine and reports had started to come in from German ground observers did it become clear that the southerly force was far larger than had been thought: no amount of electronic jamming could conceal the roar of 800 aircraft engines.

By now 89 German night fighters were airborne and orbiting over their holding beacons, waiting for their controller to clarify the air situation and direct them against the bombers.

That night the intention of the No 100 Group operation was to create the impression that the raiders’ main objective was Kassel, and soon after the main raiding force had crossed into German-held territory it turned north-east towards the city. The German fighter controller swallowed the bait and ordered almost all of his night-fighters to head for radio beacons Silberfuchs, Werner and Kormoran in the vicinity of Kassel. He ordered the rest of his force, a single Gruppe of Ju 88s, to move to radio beacon Otto near Frankfurt to cover a possible threat to that city. Soon afterwards there came reports from Kassel that the city was under imminent threat of attack, as Pathfinder flares blossomed overhead and a few bombs detonated.

German night fighters were ordered to move on the city, but this was no full-scale onslaught, merely a feint by Mosquito bombers backed by No 100 Group Liberators and Halifaxes dropping ‘Window’. During the course of this spoof a German night fighter shot down a Liberator of No 233 Squadron; only one member of the crew survived.

Meanwhile, some 25 miles south of Kassel, the main raiding force had turned away from that city and was heading for Bohlen. The Liberator crewmen had not sacrificed their lives in vain, however, for the feint against Kassel kept most of the German night fighters uselessly in that area for nearly half an hour. Not until 3 a.m. did the German fighter controller realize that he had been tricked and order his force to head east in pursuit of the raiders. Six minutes after that he gave the probable target as Leipzig, the city nearest Bohlen, but by then the vanguard of the raiding force was within thirty miles — eight minutes’ flying time — of the target.

Still No 100 Group had not exhausted its repertoire of tricks. Just short of Bohlen six Flying Fortresses and Halifaxes broke away from the main raiding force and ran a ‘Window’ trail to the important oil refinery complex at Leuna which lay twenty miles to the north-west. Twelve Lancaster bombers accompanied the jamming aircraft to give substance to the spoof. When the feint attackers arrived over the complex they dropped further target markers and the Lancasters put down their loads of bombs. Leuna lay directly in the path of the German night fighters streaming to the east, and the spoof attack delayed their arrival at the real target still further. One Lancaster crew paid the supreme price for the precious minutes of additional delay inflicted on the defending night fighters in reaching the main target.

The 211 Lancasters assigned to the Bohlen raid reached their objective and carried out a concentrated eleven-minute attack. The five Flying Fortresses and the Liberator that had provided Jamming Escort Support along the route to the target now orbited over the refinery in the Target Support role throughout the period of the attack. Not until 4.10 a.m., as the last raiders were leaving Bohlen, did the first of the German night fighters arrive in the area. Their radar operators encountered severe jamming and they had great difficulty picking out their prey amongst the large numbers of ‘Window’ returns. To add to the defenders’ confusion, as the Bohlen attack force withdrew to the west, the No 100 Group Halifaxes that had operated in the ‘Mandrel’ screen role had a further part to play. They now ran a further ‘Window Spoof ‘attack’ on Frankfurt, and dropped target markers to simulate the opening of a large-scale raid on that city.

As the Bohlen attack force crossed the Rhine to safety, Bomber Command’s operations for the night were only half complete. While the defenders’ attention had been concentrated over central Germany, the raiding force of 166 Lancasters bound for Hemmingstedt ran in at low altitude, maintaining strict radio silence. Shortly before reaching their target the bombers rose above the radar horizon and began climbing to their attack altitude of 15,000ft. Each aircraft released large amounts of ‘Window’ to give the impression on radar that this was yet another feint attack. At 4.23 a.m. the attack on the refinery began, supported by jamming from a Fortress and a Liberator of No 100 Group. Because of the low-altitude approach and the clever use of ‘Window’, the German raid tracking organization failed to appreciate the strength of this force and the bombers were well on their way home before the first radar plots on ‘weak formations’ were reported in the target area. Night fighters were scrambled to engage the force but there were few interceptions and only one of the bombers was shot down.

During the two raids the oil refineries at Bohlen and Hemmingstedt were both hit hard and neither would resume production before the war ended. The night’s action cost Bomber Command thirteen aircraft, including a Liberator and a Fortress of No 100 Group. Eight of the losses were attributed to attacks from night fighters and one to flak, two bombers were lost in a mid-air collision and the cause of the remaining two losses could not be established.

That night No 100 Group’s Mosquitos had several skirmishes with German night fighters but only two of the latter were shot down — and both fell to Chapman and Stockley. The bombers’ gunners claimed the destruction of two more enemy night fighters. German records indicate that the Luftwaffe lost seven night fighters that night, however. The fates of the other three aircraft will probably never be known but it is not difficult to speculate: a tired pilot, trying to land quickly on a dimly lit airfield patrolled by Mosquitos, might misjudge his approach and crash; a crew flying at low altitude to avoid being intercepted by a Mosquito might run into a hillside; a night fighter crew would switch off their IFF equipment to avoid betraying their position on ‘Perfectos’ and be shot down by ‘friendly’ anti-aircraft guns. Such losses, which were frequent, were the result of No 100 Group’s efforts as surely as were those brought about by its night fighters. By this stage of the war the wide-ranging Mosquitos had become the bane of the German night fighter crews’ existence.

When the war in Europe ended in May 1945, No 100 Group had honed its tactical stills to fine edge. Many of the electronic warfare techniques that it had pioneered for supporting bomber attacks would prove useful nearly more than forty-five years later, in the skies over Iraq.