According to her crew, U-128 had been the first boat to take the Metox warning receiver to sea, on her fourth patrol, from 2–10 September 1942.
She had been assigned to escort U-108 which was returning to Lorient having claimed three ships off the coasts of Venezuela, British and French Guiana, and Suriname. Fitted with the receiver, U-128 went to sea with a party of five technicians aboard, headed by a doctor of engineering; none of them wore a uniform. The voyage was confined the Bay of Biscay and several Allied aircraft were successfully detected, the boat diving each time. Both U-128 and U-108 returned safely to Lorient on 10 September and prisoners said that in future a similar escort would be provided for damaged submarines or those without their own detector.
Although the presence of a party of technicians points to something special, U-108 was not the first boat to be fitted with a warning receiver, nor even the first in her own 2. Flotille. As early as 5 June the Führer der Schnellboote was asking for expedited deployment of radar receiving devices on his boats in the Western Theatre. By the 17th he was able to report favourable experiences with “radar observation sets” both in the Mediterranean and the West and asked for a feasibility study of putting the equipment into mass production so that it could be fitted to every S-boat.
The Seekriegsleitung considered on 27 July how British airborne radar activity over the Bay of Biscay might best be neutralised. U-boat Command had proposed a network of coastal jamming stations but it turned out that existing transmitters had too little range for the job. This left the deployment of warning receivers aboard the U-boats themselves and 60 sets were now ready for installation. There was however another means of defence for two weeks later, the MND (Marine Nachrichten Dienst = Naval Signals Service) reported on the use of radar decoys by U- and S-boats.
The Italians wanted radar Funkmeßontrollgeräte (radar control/inspection devices) for their boats too, but on 13 August SKL concluded that German needs must be met first and only in September could a start be made on supplying the Regia Marina. Meanwhile “the planned seven-day test of the devices by a U-boat in Biscay” would go ahead. Operating in mid-Atlantic, U-214 had sailed from Brest on the 9th, carrying radar detection gear. BdU noted on 19 August that when it gave a warning she had dived despite seeing no aircraft and that shortly afterward bombs had fallen close aboard, inflicting “tolerable damage”. U-107 had sailed from Lorient on the 15th with “the newly-installed control device” and five days later BdU recorded how: “According to boat, it was located four times by day and twice by night … Dived for every location transmission and was not attacked the whole way. A very satisfactory result”.
SKL’s verdict was that, “the usefulness of the Kontrollgerät is herewith clearly demonstrated by a practical example”, adding on 21 August that, “As a defensive measure … the fitting of radar interception gear seems to be successful, as far as can be seen at present”. U-69 was said to have had similarly good experiences, avoiding all attacks by diving whenever the set gave a warning. Three days later however, a further report on the “Fu.M.B.” (Funkmeßbeobachter = radar observer) urged that it should only be viewed as a stopgap. What was wanted was a ground network along the Biscay coasts of France and Spain, capable of locating the enemy with precision. On 26 August, BdU set out a requirement to fit all U-boats with a “combined radar and radar interception set”.
Early deployments of radar warning receivers aboard U-boats.