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FEATURE – Boulton Paul Defiants ‘spoof’ enemy radar

Photo: A flight of 515 Squadron Defiant Mk.IIIs. The two on the right are AA578/K and AA652/B. KEN ELLIS COLLECTION




They fooled German defences by ‘spoofing’ a sophisticated enemy radar system – but it was perilous work for those flying the Boulton Paul Defiants, as Sean Feast reveals.

As Arthur Harris took over at Bomber Command in February 1942, his arrival signalled a major change in the fortunes of the ‘Bomber Boys’.

Believing that aerial bombardment at an uncompromising level could win the war, Harris sought to prove his theory had merit with showpiece raids involving more than a thousand aircraft a time.

Apart from the massive organisational and logistical challenges, there was a problem with putting so many bombers in the air; they could be tracked easily by enemy radar. With the increased onslaught from Bomber Command, the Germans had established a defence infrastructure par excellence, with corridors of searchlights and heavy flak batteries to meet incoming waves. They had also fielded squadrons of night-fighters, guided to their quarry by sophisticated ground and air radar.

At the core of the German defences was Freya – a network of early warning radars capable of detecting an approaching aircraft from a range of up to 100 miles (160km). This system represented a threat to all RAF operations, including Fighter Command.

Fighter sweeps fell within the remit of 11 Group, then under the command of Trafford Leigh-Mallory. He wrote to his immediate superior, Air Marshal Sholto Douglas – the commander-in-chief of Fighter Command – in April 1942 about the increasing losses his forces were suffering. Leigh-Mallory made specific mention that the enemy’s radar defence: “…has improved so much in the last few months that it is very difficult to get a formation into France without it being detected and reported with great accuracy.”

Spoof and screen

There were several ways that the potentially devastating impact of Freya could be minimalised. One involved creating a ‘spoof’, deceiving the German radar into thinking that there were many more aircraft in the sky than were actually present. Another comprised a ‘screen’, jamming the radar to the extent that a large formation could pass effectively ‘unseen’.

The task was given to scientists at the Telecommunications Research Establishment (TRE) in Malvern, Worcestershire. For the spoof, codenamed ‘Moonshine’, they developed what was known as the Airborne Radio Installation Transmitter Receiver (ARI TR.1427). It worked by receiving pulses from the Freya installations, amplifying them and sending them back, creating the illusion of a much larger force flying in formation.

For Moonshine to work effectively, a small number of aircraft equipped with the device would fly in formation in daylight until the spoof prompted a response. At that point they could break formation and fly home independently.

For the screen, the scientists created ‘Mandrel’, a noise jammer that overwhelmed the signals from Freya. Echoing Moonshine, this was a radio transmitter that could be carried in the air.

The tactic for Mandrel required the jamming aircraft to fly out into the night until they reached a specific patrol area, approximately 50 miles off the enemy coast. Here they would orbit, sometimes for an hour or more, until the weather, shortage of fuel or the unwelcome sight of a German fighter obliged them to return home.

By using nine aircraft, each with their designated orbit, a 200-mile screen could be made in the German radar coverage – a gap big enough to allow a steady stream of bombers to progress to their target, without the enemy realising they were there.

The rest of this article is in the November issue of FlyPast. It’s available in UK shops now or can be purchased via the website – Alternatively, you can download a digital edition from – simply search ‘FlyPast’.

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