How ‘MONICA’ was tricked into failing to protect RAF bomber crews


ARI 5664 ‘MONICA’ was an active radar device designed to protect the rear of heavy RAF bombers. Mounted below the rear turret it sent out a radio signal in a cone shape for 1,000 yards behind the aircraft. When this signal was interrupted by another aircraft, it also picked up the resulting ‘echo that bounced back.  

This ‘echo’ was translated into a ‘beeping’ tone that was heard in the crew’s earphones, warning them of the approach of an attacking Luftwaffe night fighter. Many of the attackers would approach from the behind and below as that was judged to be the most vulnerable area on the bombers, as it was undefended and a blind spot for all the crew.

So far so good. Except MONICA had fatal flaws.

It registered ALL aircraft with the same sound – it could not distinguish between enemy attackers and fellow bombers – and the constant beeping of surrounding friendly aircraft in a bomber stream of many hundreds often drove the crews to distraction,

Some turned the radar off, relying on their wits and eyes as they did before MONICA was introduced.

This meant that it was possible for the night fighter to ‘hide’ amongst the traffic of the bomber stream. And no approaching aircraft warning increased the chances of a collision between bombers in the often moonless and blacked-out skies of Europe.

Image: The shattered rear turret and amaged tail of Lancaster LM535, No. 207 Squadron RAF. The aircraft was in a collison over the target on 19/20 May 1944. Flying Officer Trevor Smart managed to bring the aircraft and his crew safely back to Britain and was awarded an immediate Distingiushed Flying Cross for doing so. He and his crew were lost on a raid to Wesseling near Cologne just over a month later on 20/21 June 1944. Image and more information at The MONICA antenna (looking like a bent coat hangar) can be seen hanging down near the bottom of the frame.

And worse was to come.

As always happens, examples of MONICA fell into German hands as RAF aircraft were shot down and the wreckage scrutinised. German scientists worked out what the equipment was for and then developed Flensburg, a passive radar detector – one that simply received pulses and does not emit signals.

Then the pendulum swung the other way.

Seventy-three years ago, at 04:25 on 13 July 1944 a Junkers Ju 88 G-1 night fighter made a navigational error and landed at RAF Woodbridge in Suffolk.

This was a complete surprise for all concerned.

The German crew (who were very low on fuel) thought they had landed in Germany and the airfield believed an RAF Mosquito had landed – the truth only becoming apparent to all when transport for the ‘Mosquito’ crew arrived at the aircraft and the quick thinking RAF driver drew a flare pistol and arrested the Germans!

According to an RAF Air Intelligence report (1), the Junkers carried a variety of radars and detectors not seen by the British before and they quickly set about analysing the equipment’s capabilities.

What they found was unpleasant answers to a number of questions.

The Lichtenstein SN-2 radar was an improvement of the original Lichtenstein radar encountered by the RAF. SN-2 was immune to jamming by standard WINDOW foil. It could be jammed by the improved SPECIAL WINDOW foil strips that reflected a different part of the signal spectrum.

However SN-2 used part of the frequency spectrum where Freya ground-based early warning radars also operated, so even if the airborne radar’s pulses where picked up by RAF aircrew they would appear to be Freya signals.


Image: The Woodbridge Junkers Ju 88 G-1 (werk nummer: 712273) now in RAF markings on a test flight over Britain. The  different sets of Flensburg aerials can be seen on the leading edge of the port wing and above and below the starboard wing. Image

Also on the aircraft was Flensburg and another passive device called Naxos. And these provided perhaps the greatest shock of all.

Wing Commander Derek Jackson (a former scientific colleague of the Director of Scientific Intelligence at the Air Ministry, R V Jones) tested the captured aircraft against a Lancaster bomber using MONICA over the UK.

Using Flensburg, he tracked it for a distance of 130 miles. Next he flew against a loose formation of five MONICA equipped Lancasters and was able to identify each as an individual aircraft.

His final test was against a group of seventy-one Lancasters, each with MONICA working and orbiting between Gloucester and Cambridge. Again Flensburg was able to pick out individual bombers.

Naxos was first used operationally in September 1943 and was designed to home in on the signals of H2S – an air-to-ground radar that was being used to identify targets through cloud and on moonless nights.

Naxos could find H2S from about 35 miles. Because of this relatively close range – giving about 5 minutes warning in the approaching German aircraft – and the fact that it could not identify an individual aircraft (only a general signal), the crew described it of little practical use.

Initially fitted to ‘Pathfinder’ aircraft, H2S was first used operationally in early 1943 and an example of this device was recovered from a Short Stirling bomber shot down over Holland on the night 2/3 February 1943 – the second raid it was used on.

Codenamed Rotterdam Gerät – ‘Rotterdam Device’ or ‘Rotterdam Apparatus’ it provided a model to develop Naxos from.

Previously H2S was switched on as soon as the aircraft climbed to operational height over the UK.

Later it was discovered that Naxos was coupled with Wurzburg radars dishes to produce Naxburg – a radar that could track the bomber formations over the UK and thus give advance warning of a raid setting off.

On learning of the results of trials Arthur Harris, head of Bomber Command ordered MONICA not to be used and the sets removed from all aircraft carrying it and H2S to be used for short intervals only.

Writing about Harris’ decision regarding MONICA, in his book Night Fighters: A development and Combat History the late aviation historian Bill Gunston wrote:

This supposed guardian of the bombers was probably responsible for more bomber losses than any other device, Allied or enemy, until in August 1944 (nearly two years later) crews were told not to use it’


(1) Air Intelligence 2(g) report No. 242 dated 16th July 1944 retrieved 25 April 2017


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