Chapter 2
Anti-Detection Devices

  1. Summary

  2. Introduction of APHRODITE

  3. 3. Radio intelligence action on the information at hand

  4. 4. APHRODITE successes

  5. 5. APHRODITE and the invasion

  6. 6. THETIS

  7. 7. BOLD


THETIS Radar Decoy


1. Summary

A clear-cut illustration of the value of communications intelligence is offered by the story of APHRODITE, a radar decoy balloon first used in the middle of 1943. The device was first mentioned on 17 April in Baltic naval traffic which was read by the British and reported by them to the Atlantic Section. With few details given in the original message and two other messages sent in June, it was possible for the Atlantic Section to inform COMINCH of the nature and description of the device before it was put into operational use. APHRODITE consisted of hydrogen balloon to which were attached metal strips tuned to known British and American radar frequencies. They were released to lead searchers off on a wild goose chase and thus allow a U-boat to escape detection. The device proved very successful at times, even leading, in connection with the T-V acoustic torpedo, to several sinkings of Allied craft. During the invasion, APHRODITEs were released in large numbers to deceive searching aircraft while U-boats not provided with Schnorchel surfaced for charging in the Bay of Biscay. Another device with the same object as APHRODITE was THETIS, a dummy buoy to which metal dipoles were attached, designed to give the same radar reflection as U-boat. All significant details of this device were given in the message which announced its impending use in January 1944. Traffic indicated that is was used to simulate patrol lines and for protection of non-schnorchel U-boats during the invasion. BOLD, a device which produced noises and bubbles when discharged into the water, was used against vessels searching with Asdic or hydrophones.

2. Introduction of APHRODITE.

The first mention of APHRODITE to come to the attention of the Atlantic Section was in a British routine report on communications matters, issued under date 1 May 1943. Speaking on a radio circuit guarded by the British, it was stated that at 0115/17 April, Communications Experiment Trial Command sent the following to the depot ship Wilhelm Bauer for Captain Giessler:


Communications Equipment Trial Command tests today without result. No APHRODITE signals established. Propose on 17/4 first experimental series from 1300 with 'A.S.V.' aircraft due to arrive at midday. For each series have ready 2 to 3 free and 2 to 3 captive. Characteristics of U-boat: swastika flag forward, morse lamp flashes on closing.

To this the British added the following notes:

  1. Giessler had long been connected with R.D.F.
  2. Nothing known of APHRODITE.

Nothing more was heard of APHRODITE until June, when two messages on all U-boat circuits mentioned the device. One of these, 1552/12, was addressed to "boats with APHRODITE II" and stated:

Hydrogen tanks not dangerous even if hit. At worst gases with blow off without exploding.

The other message, 1203/15, stated:

Beginning at once the use of APHRODITE according to instructions is authorized.

3. Radio intelligence action on the information at hand.

With the arrival of the June messages quoted in the preceding paragraph, a conclusion as to the nature of APHRODITE, based entirely upon the earlier British report and a knowledge of the place occupied by APHRODITE in Greek mythology, was formulated. Thus it was that on 23 June 1943 the Atlantic Section was able to send information in a memorandum to COMINCH giving an accurate theory as to the nature and use of the device. The memorandum stated:

It is believed that APHRODITE refers to deception equipment of submarines in combating aircraft radar. The assumption as to its working is as follows: When a surfaced submarine obtains via Metox any indication of radar, a hydrogen balloon is released by the submarine. To the balloon are attached metal strips of various lengths tuned to


known British and American radar frequencies. The submarine steams off into the wind while the balloon passes off the leeward, drawing the hunter off on a wild goose chase.

Later, incontrovertible evidence demonstrated that the above theory was in fact correct.

Of great importance was the fact that by means of communication intelligence COMINCH knew of the existence of APHRODITE before it was put into operational use.

4. APHRODITE successes.

In July the U-boat Command, apparently impatient because it had received no reports on the performance of APHRODITE, directed U-boat's to make experimental use of it, since there was little possibility of tactical use, and various reports were sent in answer to that order. A possible, but not confirmed, tactical use may have occurred on 21 July, when an Allied report cited a disappearing radar contact in 15°45'N -72°50'W; but the first tactical use acknowledged by the U-boat Command was in September and October, when several successful deceptions were claimed and an obvious campaign to instill confidence in the device was carried on by the U-boat Command. It must be remembered that for the previous several months the U-boat had existed under the heavy shadow of fear of radar and almost complete inability to detect or combat it. APHRODITE was a hope, and in the German struggle against surprise attack for the air, was worthy of every consideration and trial. Whatever comfort it could supply to the demoralized U-boat arm was sadly needed.

Actually, after the long dearth of U-boat successes, there was some occasion for rejoicing in the reports of APHRODITE successes. Aside from the general feeling of satisfaction resulting from any aid to avoidance of aircraft, a few reports of extremely advantageous use in offensive action must have been very encouraging. U-68 (Lauzemis), for instance sank the Free French escorted freighter Fort le Vaux on 30 November 1943 "after successful use of APHRODITE"; and, most striking of all, at the end of December, Petersen (U-541) used APHRODITE in conjunction with the very effective acoustic torpedo "Zaunkönig" to sink "three destroyers" in one attack. It is small wonder that the commander of U-541 thought that APHRODITE


was "first-rate," and that thereafter command urged upon all U-boats the advantage of "the skillful release of APHRODITE along with the use of 'Zaunkönig.'"

From the beginning of 1944 on, reports on APHRODITE showed no unanimity of opinion among U-boat captains as to its effectiveness in various instances and in differing situations. The high tide of its use seemed to have passed, and it assumed the position of a rather routine, not especially exciting, counter-measure, though it was apparent that the U-boat Command considered it a successful device on the whole. One significant reference during January directed Group Borkum to "infest the area with numerous APHRODITE to loosen up the defenses." Other references during the first months of 1944 showed that it was being used for the purpose of testing German radar sets, and that its range for deception covered not only meter length radar, but also frequencies around 10 centimeters.

5. APHRODITE and the Invasion.

One further episode of importance in the APHRODITE story occurred in connection with the difficult situation for U-boats in the Bay of Biscay during and after the Allied invasion of France. In the midst of the unprecedented danger from aircraft, U-boats caught in the danger area without Schnorchel were sometimes forced to surface in order to charge their batteries and ventilate the boat. In such cases a "forest of APHRODITEs" was successfully used to protect the boat from detection and attack during the period of emergency. In several messages read during June, emphasis was on the large number of APHRODITEs being used and the difficulty of supplying replacements through the French ports, a difficulty which persisted throughout the remainder of the time the ports were in use by U-boats. As time went on, reports on use of APHRODITE became more and more infrequent, and by the advent of 1945 they were almost non-existent. Boats leaving port continued to carry the device, however, so it was clear that it was still considered an effective counter-measure under certain conditions, even though the addition of Schnorchel had revolutionized the whole problem of U-boat attack and defense.



Message 1642/11 January 1944 on all U-boat circuits introduced a new German anti-radar device to Allied communications intelligence, and through the usual channel to COMINCH. The translation of the message read:

To interfere with enemy radar activity, dummy buoys of type 'THETIS 20' will be put out in Bay of Biscay as far as bout 12°W. beginning 11 January. Description: A thin wooden upright 5 meters long, on a float. Weighted underneath with a 5-meter steel tube. Then metal dipoles on the wooden mast, can be seen only from very short distances. Effect: Buoy gives same reflection as U-boat.

In this one short message the most significant details about THETIS and its purpose were given. Until July 1944 messages reporting the setting out of THETIS buoys occurred in considerable number, with positions in the Mediterranean and in the Atlantic as far as 31°W. Thereafter the number of references in traffic decreased, with final mention in message 1641 (Gray) of 15 August to French U-boat flotillas, in which it was ordered, concerning cargoes to be carried by U-boats escaping from French ports:

Relegate transfer of THETIS to last place. They are too unwieldy. Use space for important things such as APHRODITE, power cable, valves and BOLD.

Thus the relative unimportance of THETIS was finally announced.

The one significant detail not revealed in the original message about THETIS came to light in three almost simultaneously transmitted messages of 19 March, all directing the addressees to set out their remaining THETIS at 15-mile intervals, the object presumably being to simulate patrol lines. This type of mass use continued through July, including general use for protection of non-schnorchel U-boats during the invasion.



Individual BOLD Pellet

A counter-device less frequently mentioned in U-boat traffic than APHRODITE was the BOLD, a cartridge containing pellets of a chemical composition which produced noises and bubbles when discharged into the water. Their purpose was to confuse anti-submarine search by vessels using Asdic or hydrophones. The device was first mentioned in ENIGMA traffic when Bulow (U-404) requested a rendezvous with another U-boat so that he might take aboard some BOLD (1925/28 January 1943). That COMSUBs attached importance to the device was indicated by the fact that he granted the request as well as by a more direct statement in a message on 1531/24 April. Reminding U-boat commanders of the use of BOLD when taking evasive action, he said: "The BOLD technique has been used in recent times almost invariably with success." Only a limited amount of material about BOLD appeared in ENIGMA traffic, but information at hand from all sources was correlated and presented to COMINCH in a memorandum of 30 July 1943, and was sufficient to indicate the nature and tactical possibilities of BOLD. The instructions pamphlet taken from the U-505 and translated by the Atlantic Section proved helpful in elucidating several messages sent to U-boat's during the winter of 1944-1945 concerning the more effective use of BOLD in "Total Underwater Warfare."


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