Airborne countermeasures against Allied ASV radars in the Mediterranean theatre seem to have ceased in November 1942 as ground was lost in Africa and Axis supply convoys were no longer routed to Libyan ports. Efforts appear to have been revived by the dramatic reverses suffered by the U-boat fleet the following Spring—advice went out to all U-boats during the evening of 22 May 1943 that “the danger of being taken by surprise by locating [i.e. radar-equipped] aircraft is very great, especially at night”.
During the summer of 1943 a new unit appeared under Fliegerführer Atlantik. For some time, Allied intelligence analysts, reliant on sporadic and limited information, were as unsure of its name as they were of its purpose. Its activities generally entailed an He 111 taking off from Bordeaux-Mérignac (or latterly Nantes) during the hours of darkness and spending about eight hours over Biscay before returning to base next morning. Deciphered messages and Y-Service monitoring occasionally revealed intended operations, largely through routine safety warnings to the Kriegsmarine and Flak that friendly aircraft would be crossing the coast at particular times. They also logged contacts with ground stations regarding intended landings but nothing seems to have been transmitted about any results achieved. That August, prisoners from KG 40 disclosed that a Ju 88 and an He 111 had been operating from Bordeaux/Mérignac since July, under the direct control of the German Air Ministry and charged with the observation of Allied Radio Traffic.
The situation over the Bay of Biscay had been worrying the Kriegsmarine for more than a year. Noting that existing shore-based transmitters had a range of only 65 km, in August 1942 the Chief of Signals Division had concluded that:
Jamming transmitters on board submarines are both unsuitable and dangerous. They prevent only range-finding but not direction-finding by aircraft. On the other hand, the use of aircraft of Luftflotte 3 equipped with jamming transmitters over the approach routes is considered very valuable.
The German Naval Staff was understandably alarmed by 38 submarine losses during May 1943 (as far as they knew: in fact it was 42) in the operational area or on passage through the Bay. To guard against air attack, U-boats were fitted with radar warning receivers such as “Metox” and "Wanze". Nevertheless their signals traffic is full of reports of these devices’ failings and indeed a breakdown was sufficient reason for the boat to return to port. What was more, the Germans had been aware since early March that, “the submarines' radar interception gear can apparently no longer detect locations by enemy planes because they have gone over to new wavelengths” (the first Coastal Command operations with 9 cm ASV Mk. III had taken place that month). On 4 June, after three more losses attributed to Allied aircraft, the Chief of Staff reported to Hitler that “the Navy’s own measures are exhausted” and that “immediate, concentrated measures on a large scale” were required from the Luftwaffe. A U-boat specially fitted out to detect Allied airborne radars had itself been bombed soon after leaving port and compelled to turn back, no replacement was yet ready but some airborne assistance was in prospect:
Four days later:
The excellent location devices developed by the enemy, which have special effect when used by enemy planes, have no countermeasure on our part and are greatly jeopardizing our submarines … OKL … informs Naval Staff for information, of a request to Quartermaster General to re-equip the next three Ju 188s which are delivered, for use against enemy search planes and to assign them to … Fliegerführer Atlantik for operations in the Bay of Biscay.
Here then, were the beginnings of Kommando Rastedter, named for its leader, Obltn. Siegfried Rastedter.
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