D. of S. [Director of Signals] said that “Moonshine” was now required as soon as possible by Fighter Command.

R.C.M. Projects meeting (7 May 1942)

The experimental stage has been passed and “MOONSHINE” now seems to have become a practical proposition which we intend to use …

A/C Hugh Saunders, No. 11 Group (29 July 1942)

The first RAF airborne jamming operations were flown by No. 11 Group Fighter Command’s Northolt Defiant Flight. This unit’s machines were fitted with ARI TR1427 “Moonshine” which amplified and returned the pulses of the Freya early-warning radar, giving the appearance of a large formation and (it was hoped) drawing the defences’ attention away from a real attack. The first of these devices were built by the Telecommunications Research Establishment (TRE) at Swanage.

Decisions over what aircraft to use (should an existing night fighter squadron be disbanded, for example?), where the servicing personnel might be found and where to base the new unit generated a lot of correspondence. The intention was that the new formation should employ both Moonshine and Mandrel (a barrage jammer against German early-warning sets). Defiants were giving way to Beaufighters and Mosquitoes as night fighters and thus becoming surplus to front line requirements. The decision to establish a flight at Northolt under the aegis of the Station Flight came on 6 June 1942, initially with nine Defiant Mk. II (long range type), expansion to full squadron strength coming once “Mandrel” entered service. Each aircraft as to have the racks and wiring normally provided for A.I., plus “the old H.F. retractable aerial”. A week later AOC Fighter Command minuted that:

His fear was that “Mandrel” might, once revealed, be replicated by the enemy and turned against British defences, proving “more disadvantageous to ourselves”. As he saw it, only a really major operation would justify the risk inherent in using “Mandrel” for the first time.

The first research flights with a Defiant had begun from Tangmere on 28 May, the crew consisting of F/O Wilkins of No. 264 Squadron and Mr. D.W.F. Mayer of the TRE’s Countermeasures Group. Flights were also made from Boscombe Down (Aircraft & Armament Experimental Establishment). On 20 June, 264’s F/L Thomas was detached to Northolt to command the new Defiant Flight. Tests continued, with results ranging from “not very successful” to “poor”, even when the aerial was moved to the top of the aircraft. More pilots joined and test results progressed from “fairly good” on 22 June to “good” three days later. It seems that the problem had been cracked because this was the verdict on subsequent occasions too. On 8 July P/Os Leonard (pilot) and Simmonds (gunner) had their engine cut out during a formation-flying exercise. In attempting a forced landing at Aldenham (RAF Elstree), Leonard undershot, hit a tree and was killed. Simmonds, “badly shaken”, was taken to hospital. On 20 July, eight aircraft left for Drem, east of Edinburgh, “conducting special experimental tests” over the ensuing five days. This aerodrome was chosen to be well away from German eavesdroppers and in a mobile Ground Controlled Interception (GCI) set, modified by TRE, had been set up there.

Rather than say what the unit had actually been doing, the Operations Record Book simply advises: “For details of operational flights … please refer to No. 11 Group Most Secret file, Ref. 11G/S/600/2”. That file also notes that that first three operational flights each involved not only nine Defiants but two “spotters” (type unstated) whose role presumably was to monitor German reactions.

A trial operation was attempted by nine Defiants on the afternoon of 6 August but aircraft AA435 (Sgts. Ecclestone and McLaren) developed engine trouble, putting down at Middle Wallop, so the whole flight was recalled. Each transmitter could spoof only one frequency while the Freya operator had a choice of several, all of which had to be covered for the deception to succeed.

At 18.55 (GMT+2) nine aircraft again took off on “Moonshine Flight Serial 1A”, returning two hours later from a sortie which the Flight judged “completely successful”. The Defiants had spent an hour within German radar coverage; some 26 fighters were scrambled from Cherbourg-Maupertus, this being described by the British as a maximum effort by the two Staffeln (of IIII./JG 2) based there.

On the 11th, Acting S/L Thomas was appointed to command the unit, with Acting F/L Wilkins as his deputy. The next day’s operation was a carbon copy of “1A” but without Moonshine, to provide a comparison. Enemy reaction was “nil” although the Defiant of P/Os Wingfield and Simmons experienced a problem over the Channel and returned to Tangmere. On 13 August aircraft AA401 and AA382 were again detached to Drem while on the following day, 10 Defiants were operating, returning to Northolt at 16.50. The weather was clear and bright on 17 August and nine Defiants were in the air for an operation lasting from 16.20–17.55 and according to the ORB: “The flight was very successful, S/L Wells of No. 11 Group telephoned congratulations”. Acting as a diversion for Circus 204, the Flight made a rendezvous over Walton-on-the-Naze with three B-17s and nine Spitfire squadrons before making to a point 40 km east the North Foreland. The formation was swept by five Freya frequencies and no fewer than 144 German fighters were directed to the Dunkirk area to intercept what was evaluated as bombers out to raid Ostend. The diversion was within German radar coverage for 29 minutes and meanwhile 12 B-17s bombed marshalling yards at Sotteville-lès-Rouen, the US 8th Air Force’s first heavy bomber raid.

Two days after this, nine aircraft operated from 11.00–12.30 in support of a Circus flown against Abbeville while Operation Jubilee, the ill-fated Dieppe Raid was taking place. This seems to have had little effect: British RDF stations were being heavily jammed and the Germans simply directed all their fighters to Dieppe, with little attempt at further ground control. Not surprisingly, the German Naval Staff received several reports on the operation but nothing appears to have been said specifically about Allied jamming. These extracts may however be relevant:

On all three escorts the W/T immediately failed at the commencement of the action [with the approaching landing force].

Prior to the arrival of the report of the sea battle off Dieppe, the radar service picked up no indications of the enemy forces.

The build-up to the operation remained imperceptible since radio silence was maintained until the start of the landing at 0628 hours [GMT+2].

More operations followed on the 20th (160 German fighters reacting during 60 minutes in their early warning cover) and 21st, then all aircraft were grounded for two days of engine overhauls before ops were resumed on the 27, 28 and 29 August. The Flight would become No. 515 Squadron in October.