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Radar Decoys

The Kriegsmarine deployed two types of “radar deceiver” (Funkmeßtäuscher) to protect U-boats from aircraft. Aphrodite (FuMT 1) was manufactured by AEG while Thetis IIc (FuMT 2) came from Telefunken. Both were directed against 120–180 cm waveband (166–250 mHz) and could be deployed by the submarines themselves or by the surface vessels supporting them.


This was a tethered balloon carrying metal foil radar reflector strips and was inflated and deployed from the surfaced boat’s deck. It was cleared for operational use on 15 June 1943, three days after crews had been assured that the hydrogen bottles used to inflate it would leak rather than explode if hit by gunfire. A comprehensive set of instructions for the system’s use was appended to the U-boat Command diary for June 1943. These included:


Aphrodite is intended to deceive enemy units fitted with radar and so provide tactical advantages for the U-boat.


The device consists of a balloon of 70–90 cm diameter filled with nitrogen [other sources say hydrogen], connected to a plate anchor by a wire rope 50 metres long. Three strips of aluminium foil 4 metres long are attached to the wire rope, 8 metres apart.


The radar pulses transmitted by the enemy are reflected by the aluminium foil and the blips made in the enemy radar set are almost the same as those made by pulses reflected from a U-boat. At first the enemy radar operator will not be able to distinguish them unless he is within visual range, and even later, when he knows of the existence of the device and has gained a certain amount of experience from its frequent use, he will easily be misled.


The device lasts from about 2–8 hours, according to weather, and makes effective reflections during this time.


The anchor prevents the balloon rising or sinking rapidly and thereby giving away the fact that it is a decoy.


According to tests made so far … the device cannot be used in wind force above 5–6, sea above 4–5, as in a stronger wind the balloon is swept into the sea or the mooring rope and/or strips of foil are torn away.


So far only the results of trials in the Baltic are available and only guiding principles can be given for operational use, based on theory. It is left to the boats to collect practical experiences for effective operational use. Information obtained should be reported by radio at next opportunity for the benefit of other boats.


As the balloon is comparatively large and easily visible especially by surface vessels, for the present [it] is only to be used at night or in very bad visibility (fog), so as not to give the enemy information before necessary. It is also not to be used near the coast if there is any danger that it will drift ashore.

Although advice was given on how the decoys might be deployed in a variety of situations, the paper ended by saying: "Boats must try out suitable methods for themselves on the basis of these guiding principles and at their own discretion".

That August, another directive was issued to submariners:

The use of … Aphrodite is also most strongly recommended. It gives one boat the opportunity of simulating a whole group—for instance by letting several Aphrodite drift past the convoy at night and so make a gap in the escort before she attacks. Or if the boat wishes to attack from a certain quarter and the escort there is too strong, she can precede her attack by a group of scattered decoys so that the escort vessels will occupy themselves with them and thus make things easier for the boat.

What was thought to be Aphrodite’s first successful use was off the North African coast on 5 September when the U-boat observed the release point being illuminated 30 minutes later by shore searchlights. Three days later there came a report from the Atlantic:

By night flown over twice in quick succession by a locating flying boat. After each [pass] an Aphrodite released. After about three hours, dropping of parachute flares observed beyond horizon in direction in which Bolden released. Thereafter no further flight over U-boat.

NOTE: Bolden had been the term for chemical canisters released to deceive sonar by generating clouds of bubbles (known as SBT or Submarine Bubble Targets to the Royal Navy). The present context suggests the word could be used for decoys generally although Allied interrogators thought that Bold might be the inventor’s name.

Not two weeks after that incident boats were being told how the balloons had been approached by aircraft and chased by destroyers and that “skillful use is likely to result in great success, particularly in [the] case of a convoy”. On 21 November, U-969 used the device to escape detection by an aircraft. Nine days later, U-68 sank Fort de Vaux (5186 GRT) off Monrovia “after a successful Aphrodite deployment” drew off the freighter’s escorts. A directive to the »Borkum« wolf pack on 11 January 1944 melodramatically urged them to, “disseminate numerous Aphrodites over the area. When attacking the convoy, break up its escort by means of [them] … The eyes of the Fatherland are upon you.” On 11 February, U-413 was able to report “Aphrodite used successfully on several occasions against Naxos”. (German seamen used the name Naxos for both the warning receiver and the signals it was designed to detect). Crews were advised on 19 June 1944 that boats operating in coastal waters had successfully hidden in a “forest” of 20–60 Aphrodite while on the surface charging flat batteries. They remained unmolested despite cruising for 1–2 hours with a continuous strength-5 indication on Naxos.


A deciphered signal of 11 January 1943 described »Thetis« in some detail:

In order to interfere with the enemy location activity, type Thetis IIC decoy buoys (Täuschungs-Bojen) will be laid out in Biscay as far as approximately 12º West with effect from 11 January.

Description: a thin, upright wooden pole 5 metres long on a float, weighted at the bottom by a 5-metre long steel tube. On the wooden pole, thin metal dipoles visible only at very close range.

Method of operation: the buoy produces the same radar response as a U-boat.

On the morning of 30 January, two patrol boats, V401 and V402 were to sail from Royan “for Thetis experiments”. The intention of BdU earlier in the month had been for his boats to lay a line of 100–150 Thetis at 25-mile intervals across the Bay of Biscay from the 200 metre line out to 10º W: “The efficiency and life of these buoys will then be tested by aircraft”. On 18 March 1944 the Naval Staff was briefed that:

Radio-monitoring established repeated ASV locations on our own U-boats which were subsequently rescinded. Observation therefore permits the conclusion that the deployment of Thetis has proved itself.

An evaluation nine days later concluded:

Clearly … the enemy has had numerous false radar locations from Thetis laid in the Bay and this fact will have contributed to his change of tactics … [the] many reports of radar locations followed by a cancelled position are transmitted from areas where there are no U-boats but possibly Thetis buoys [provide] confirmation of [their] effectiveness …

As late as 23 June 1944, U-boat Command was noting that U-154 (sailed 20 June, lost 3 July) and U-802 (sailed 22 June, surrendered 11 May 1945) had dropped 10 Thetis apiece during their passage across Biscay from Lorient.


“Chimney Sweep” was a project to develop radar-absorbent coatings for U-boat conning towers and possibly for exposed hull areas also. Initial work was concerned with the 1.5 m waveband of ASV Mk. II and progressed only slowly. The adoption of the Schnorchel early in 1944 made the concealment of conning towers less of an issue and it was some time before it was appreciated that the Schnorchel itself was detectable by Allied centimetric radars. Work then switched to materials effective in the 3–9 cm band and these were fitted to a number of operational boats from late in 1944.