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”.

U-353 was fitted with Metox shortly before she sailed on from Kiel on 22 September 1942. Sunk on 16 Octber, her survivors gave extensive information on the set and its operation. The crew had been told that the receiver was needed because so many boats crossing Biscay had been surprised recently by searchlight-carrying aircraft.

The set itself was a box of approximately 66 x 23 ix 18 cm with a central tuning dial calibrated from 0-100 and rotated by a knob beneath it. The figure on the dial could be compared with a graph to obtain the detected signal’s frequency and wavelength. According to prisoners from U-353, when a signal was detected, it was:

transmuted into a low-pitched sound signal heard by the operator in his earphones. The operator … reports to the Captain … and the U-Boat submerges immediately; since the set was fitted experimentally … the general procedure … was to come to periscope depth and observe the kind of craft the signal emanated from.

However, “all prisoners believed that different frequencies are used by different types of vessel or aircraft” and the aim was to correlate each threat with a reading on the dial so that in future crews would know right away what they were up against. A rating from U-203 (sunk on 25 April 1943) gave a slightly different account. He had been trained on Metox at the Westminster Hotel at Le Touquet, learning to get a bearing on signals accurate to within 10–15º. The large dial (graduated from 0–100) was turned until the signal was heard. The small dial (graduated from 0 to 30) was then turned until the maximum intensity was received. The antenna was rotated manually to find the bearing. On U-boats, however, the antenna was rarely rotated and the boat dived immediately after receiving the signal. He said that signals from aircraft fluctuated in tone, those from ships and sets on land were steadier but distinguishing between them was still difficult.

According to a Kriegsmarine signals conference on 7 March 1944, “There exist two jamming devices developed by the Navy, Olga II and Karl I, both of which work in the secondary band of 1.35–1.75 m”. When France was liberated an example of the former was captured by the Allies. It was categorised as “German ground jamming equipment … apparently designed to operate in the 160–222 [mHz] range”. It consisted of three plug-in units mounted in a cabinet plus a power pack and oscillator and modulator units. The transmitter’s output was estimated at 450 watts and there was a three-position control marked “pulse frequency” on the front panel which seemed to produce “railings” on enemy screens. A captured document mentioned the FuMS 1 jammer, codenamed Olga II, operating in the 167–222 Hz band and with a 450-Watt output and it was clear that this was the captured set. It is known from other sources that the device was manufactured by Blaupunkt. The set’s frequency range covered. a number of British systems including Chain Home Low (CHL), Ground Cntrolled Interception (GCI), A.I. Mk. IV and A.S.V. Mk. II.

Early deployments of radar warning receivers aboard U-boats and a captured jamming transmitter.