Difficulties of
navigation at night

The Pathfinders


Electronic warfare






Electronic warfare

The first 1,000 bomber raid

From mid-1942 RAF bombers began to fly very large-scale night raids. Strategists believed that if hundreds of aircraft flew in a concentrated bomber ‘stream’ the sheer number of attackers would overwhelm the German defences, making raids more effective and reducing RAF losses. Each bomber had to navigate independently because they were unable to fly in formation at night. But it was vital they kept to planned routes and timings in order to create a heavy concentration of aircraft.

On the night of 30th May 1942, the first of the famous ‘1,000 bomber raids’ was launched against Cologne. It was conceived by Bomber Command’s new chief, Sir Arthur Harris, and was both a military and propaganda success – proving to both Britain’s allies and the enemy that the RAF was at last capable of launching devastating attacks on Germany and forcing the Nazis, who had invaded so much of Europe, to defend their own homeland.

But the RAF’s new effectiveness was not only due to greater numbers of aircraft. Electronics were increasingly playing a vital role, allowing bomber aircrews to navigate more accurately to the target. But there was a catch: The enemy was also using electronics - to detect the approaching bombers.



Ground crews load huge Sterling bombers
Ground crews load huge Sterling bombers

Eyes and ears of the Reich

From the start of the war both Britain and Germany used ground-based radar to detect approaching aircraft. British radar stations had been vital in detecting Luftwaffe bombers and had helped the RAF win the Battle of Britain.

The Germans had developed FREYA, a long-range radar able to detect approaching aircraft at a distance of 100 miles. They had also installed a chain of WURZBURG radars stretching down the whole of western Germany, These controlled an integrated defensive system of searchlights, flak and night-fighters.

The British countered by fitting some bombers with a device called MANDREL which was able to jam FREYA radar, rendering it useless. British scientists also came up with TINSEL, whereby a microphone placed in a bomber’s engine bay recorded the deafening engine roar and the wireless operator transmitted it on German ground-to-air radio frequencies, making it impossible for nightfighters to speak to their controllers. RAF personnel who spoke German would tune in to Luftwaffe night-fighters’ radio frequencies and give false and misleading instructions.


  A German FREYA early-warning radar dishGerman radar operators could see the British bombers coming
A German FREYA early-warning radar dishGerman radar operators could see the British bombers coming

An RAF wireless operator over enemy territoryGerman speaking WAAF radio operators
An RAF wireless operator over enemy territoryGerman speaking WAAF radio operators

A matter of life and death

The British scientists were constantly trying to find ways to ‘jam’ German radar and disrupt radio communications as the Nazis developed new electronic devices. But as each side struggled to gain the upper hand, the ebb and flow of technical advances became a matter of life and death for aircrews.

The aircraft loss rates fluctuated, depending on the interplay of initiatives from both sides: In mid-1943 losses averaged around 5%. In human terms this meant that on a typical operation involving 750 bombers, the RAF would lose about 37 aircraft – and 260 airmen.

However, British scientists had come up with an idea that would provide a spectacular, if temporary, decrease in the RAF’s loss-rate – a simple device that would completely confuse the German radar operators.


  A squadron photo
A squadron photo

Top secret technology

WINDOW was the code name for small metallised strips, like tin foil, designed to be dropped in bundles from RAF bombers. The result was a gently drifting cloud of metallic strips that created confusing signals on German radar screens and concealed the position of the actual bombers.

For some time, the British government had delayed the use of the top secret WINDOW. The fear was that as soon as the RAF used it, the Germans would find the metallised strips on the ground and take up the idea - using it on raids against Britain and disabling Britain’s own vital early warning radar that had been crucial in the winning of the Battle of Britain in 1940. However, Bomber Command’s losses were becoming unbearable. ‘Bomber’ Harris insisted the RAF be allowed to use WINDOW to save aircrews’ lives. The British didn’t know that the Germans had already thought of the idea of WINDOW anyway - but had not used it for exactly the same reasons as the British.


  A cloud of WINDOW falls behind the bomberDifferent lengths of WINDOW were used to disrupt radio
A cloud of WINDOW falls behind the bomberDifferent lengths of WINDOW were used to disrupt radio

‘Tin foil’ proves a success

WINDOW was first used by the RAF in a series of attacks on Hamburg: the first was on the night of 24/25 July 1943, followed in quick succession on 27/28 and 29/30 July. A fourth raid on 2 August was a failure due to a massive thunderstorm over the target. The strips 30 cm long and 1.5 cm wide were being dropped at the rate of a bundle a minute. 2000 of the strips would have appeared to Wurzburg operators as the echo of a heavy bomber. The effect, particularly on the first night, was dramatic. The entire radar system was disrupted. Searchlights were seen to be waving aimlessly; Anti-aircraft fire was hesitant and inaccurate and gave way to barrage fire; German fighter pilots were losing their cool in the general mayhem and the intercepted radio traffic showed the enemy ground controllers were hopelessly confused. 700-800 RAF aircraft dropping WINDOW had created on German radar screens the suggestion of 11,000 bombers about to attack.

The results spoke for themselves. Only 12 aircraft were lost on the first Hamburg raid - just 1.5%. On the second and third raids losses crept up a little - to 17 and 27 aircraft - but still significantly below the previous loss rates. From then on, WINDOW became routine on every flight until the end of the war. It was manufactured in varying dimensions to match different frequencies.


  WINDOW proved astonishingly effectiveGerman night fighter controllers
WINDOW proved astonishingly effectiveGerman night fighter controllers

Bomber Command’s darkest hour

The low loss rates did not last. The Germans became able to differentiate real and spurious radar echoes by filtering out the relative speeds, and their fighters were given more freedom to operate independently. RAF loss rates crept up again: 40 aircraft out of 600 were lost over Peenemunde in August 1943, and later in that year, losses mounted significantly over Berlin. The ‘Battle of Berlin’, as the period November 1943 to March 1944 came to be known, cost Bomber Command about 600 aircraft and 4,000 aircrew out of some 10,000 sorties. It was at this stage that the Stirling, the first of the four-engined heavy bombers, was pulled out of Bomber Command’s main force due to its vulnerability: they were being lost at the rate of 13% in the initial stages of the Battle. Not many Stirling crews were surviving the required ‘tour’ of 30 operations.

Losses of Bomber Command aircraft and crews climaxed over Nuremburg on the night of 30/31 March 1944: 95 aircraft lost out of 795 dispatched (11.9%).

Electronic countermeasures (ECM) had in the past helped reduce losses. But new ideas and developments were still desperately needed.


  During 1943 these airmen had a less than 50% chance of completing 30 operations.
During 1943 these airmen had a less than 50% chance of completing 30 operations.

100 Group – Decoys, spoofs & ghost bombers

In November 1943, 100 Group was created as the RAF unit responsible for all electronic and radio counter-measures. The Group used about 100 aircraft for bomber support. B-17s (Flying Fortresses) and B-24s (Liberators) carried jamming equipment and flew with the bomber stream; Halifaxes and Stirlings flew separate missions creating false radar echoes of spoof and decoy raids by non-existent ‘ghost’ squadrons using WINDOW and other devices. The Group also flew Beaufighters and Mosquito night fighters, carrying airborne radar for tracking and destroying enemy night fighters.

The B-17 Fortresses were chosen because of their ability to fly particularly high. Their radio transmitters jammed German early warning radar and fighter control communications. A German speaking crew member would operate the jamming equipment and give false instructions to Luftwaffe night-fighter pilots. The B-17s were able to fly 5,000 ft above the bomber stream, throwing out a protective electronic ‘cloak’ to help conceal the attack. Similar work was carried out by B-24 Liberator bombers.



Loaded with electronics: B-17 Flying FortressAn RAF 100 Group Flying Fortres
Loaded with electronics: B-17 Flying FortressAn RAF 100 Group Flying Fortres

Electronic ‘raids’ on D-Day

On the night of 5/6 June 1944, a few hours before the D-Day assault on Normandy, five of 100 Group’s B-17 Fortresses, together with a similar force of Lancasters, were given an unusual task: To fly back and forth across the Channel, penetrating 80 miles into France then turning around. On each inward journey WINDOW bundles were tossed out as fast as possible. Just 10 aircraft created on German early warning radar a ‘ghost’ bomber stream of hundreds of non-existent raiders, distracting the Nazis’ attention from Normandy. The decoy bombers also jammed German radio using on-board transmitters. An electronic wall, blocking all German communications, was established for several hours over northern France, masking the presence of 1,000 Allied transport aircraft on their way to drop paratroops at the start of the greatest invasion in history.


  D-day: The skies above Normandy
D-day: The skies above Normandy

A deadly game of ‘spoof’

100 Group was continually changing tactics in order to remain convincing. They developed a technique called ‘conditioning’: A spoof raid on 17/18 Aug 1944, involving a small number of 100 Group aircraft dropping WINDOW gave an impression on German radar that hundreds of bombers were heading for Kiel. Luftwaffe night fighters went up in strength to meet this supposed threat - which came to nothing. The following night the real Main Force of hundreds of bombers was targeted on Bremen and flew over the same area: they were totally ignored in the belief that it was just another spoof.

Spoof raids often took place on the same night as a real raid – a handful of 100 Group aircraft would decoy German radar – using systematically placed WINDOW to create a ghost bomber stream and send night-fighters hunting for shadows. Meanwhile the real bomber force headed for a different target, dropping WINDOW to create a confusing radar signal and accompanied by 100 Group specialist ‘jamming’ aircraft that flew above them.

The RAF also used airborne radar to launch an offensive against the Luftwaffe night-fighters. Intruder missions into Europe were flown by RAF night-fighters equipped with SERRATE airborne radar. This device tracked German night-fighters who were themselves using electronic equipment to find RAF bombers. The hunters had become the hunted.

The innovative work and success of British scientists in the ‘electronic war’ saved as many as 1,000 bomber aircraft and up to 7,000 aircrew.


Electronic victory

The Allies’ combination of scientific and military skill was recognised by the Germans themselves. General Galland, the Luftwaffe’s leading fighter ace commented: “The combination of the Pathfinders’ operations, the activities of No. 100 Group, the British advantage in radar, jamming and WINDOW techniques combined with intelligent attacking tactics, as well as the discipline and bravery of the RAF crews, have been remarkable. We had our severe problems in trying to defend Germany in the air.


  A JU-88 German night-fighter with airborne radar to track RAF bombers
A JU-88 German night-fighter with airborne radar to track RAF bombers

Mosquito night fighter

ECM (Electronic Counter-Measures) code-names:


Window - foil strips dropped from RAF aircraft to swamp German radar with false signals.

Mandrel, Carpet, Shiver – airborne radio transmitters used to jam and swamp German ground radar such as Freya and Wurzburg.

Boozer – on-board equipment that warned bomber crews when they was being tracked by German radar.

Monica and Fishpond - fitted to bombers to give warning of approaching German fighters.

Tinsel and Jostle - radio transmissions from the bombers to drown out radio communications between German fighters and their controllers.

Cigar and Corona – German-speaking RAF operators assigned to transmit false and confusing directions to German fighters.

Serrate – airborne radar fitted to RAF night-fighters on intruder operations to enable them to track and attack German night-fighters.


- ground based long-range radar.

Wurzburg- ground based shorter range radar giving information on approaching bombers to searchlights, flak and fighters.

Naxos and Flensburg - fitted to German fighters to permit them to home onto radio transmissions from the bombers (e.g. onto the bombers’ H2S transmissions)

Leichenstein - airborne radar fitted to fighters to allow them to detect the bombers.