- Dissemination to U-boats at sea.
A constant effort was critically made to inform U-boats at sea of any intelligence which might assist them in their task. Thus, in addition to information on convoys and independents, both general and particular in its application for the offensive war, hundreds of messages concerned Allied anti-submarine activities. Intelligence for the U-boats defensive war included not only the number and disposition of anti-submarine units, whether surface or air, but also tactics, armament, and especially anti-submarine location devices. From time to time general estimates of Allied defenses for the various U-boat operational areas were added to the voluminous files of instructions which U-boats were obliged to carry and which were kept up to date by radio transmissions. The nature and tempo of the U-boat war required, in German eyes at least, a reliance on radio communications not only for the dissemination of current intelligence for offensive operations but also every scrap of information that could be gotten together on Allied defenses. Hence the reader of U-boat traffic was supplied with a surprisingly large background for judging German anxieties, suspicions, fears, and misconceptions, together with plans and hopes, or expedients, for counter action. As the U-boat task changed or as the conditions surrounding its execution altered, the intelligence sent to the U-boats was modified.
- The course of the war as reflected in intelligence sent to U-boats at sea.
- During the winter offensive, 1942-1943.
Intelligence disseminated to U-boats during the winter offensive of 1942-1943 was almost altogether on convoys, with emphasis on UK-US lanes. Other intelligence was issued only in so far as it bore upon or could be worked in with the convoy offensive. Intercepts of Allied contact and attack reports were rarely repeated on U-boat circuits, and then merely to request
a clarification from the U-boat concerned.1 Reports of merchant sinkings in distant areas were occasionally relayed on appropriate circuits with requests for the identity of the U-boats responsible for the sinkings.
- During the summer of 1943.
As the U-boat went on the defensive and sought out distant areas of operations, a distinct type of U-boat message gradually became a commonplace, and was to remain such: namely, the repetition of Allied contact and attack reports. The Allied reports became a kind of substitute for U-boat unit transmissions in view of the increasing need for radio silence on the part of the U-boat. The general defense situation reports for the Atlantic became remarkable for length and for new editions. Instead of convoy intelligence on the old scale, traffic situation reports of distant coastal areas and the Caribbean were on the air.
- Resumption of convoy offensive. Winter 1943-1944.
The renewal of the North Atlantic convoy offensive brought back the convoy intelligence messages. Indicative of German difficulties in finding the convoys of an enemy who was reading almost everything the German navy put on the air and reading it currently was the appearance of new types of intelligence messages: the relay of D/F fixes on Allied unit transmissions and special reports from intercept parties onboard U-boats. Allied knowledge of the U-boats whereabouts was reflected in the constant flow of messages which endeavored to analyze the success of Allied location devices.
The repetition of contact and attack reports continued, increasing noticeably in the spring of 1944, particularly for the Biscay area, and gradually working around to include the Indian Ocean as well. The disposition and habits of U.S. Navy CVE groups were pressing concerns which necessitated
revisions of current orders of the defense situation in an effort to determine where and when U-boats might safely surface. Attempts were made to evaluate all underwater sounds reported by U-boats in terms of new kinds of Asdic, search buoys, counter-devices for the acoustic torpedo, bluff, or marine biology.
- After the summer of 1944.
The German attempt to fight with an outmoded U-boat which could not escape detection by a superior enemy gradually filled U-boat traffic with messages concerning the problem of U-boat defense. A time was reached when U-boat traffic seemed to reflect more Allied activity than German activity. With the introduction of the schnorchel U-boat German interest in underwater sound was intensified and concern with Allied radar remained as acute as ever. Operational intelligence messages became very detailed accounts of Allied shipping in coastal areas. In April 1945 the "Harke" gesture towards a revival of convoy warfare was accompanied by convoy intelligence indicating that the convoy plot had been kept up to date even though not used.
- Sources acknowledged in U-boat traffic.
A summary of the various sources of intelligence which were acknowledged in U-boat traffic will not contain any startling revelations, for these sources are the ones which the enemy is expected to have. They should be borne in mind, however, for it was against this background that one had to judge possible sources when no acknowledgment was given.
- Aerial reconnaissance.
It was not always possible to know when convoy intelligence could be accounted for by GAF sightings, even when the convoys were in the areas of GAF range, for acknowledgments were not consistently made. Such a source could usually be presumed with a degree of safety in the Mediterranean, along the English-Gibraltar convoy lane, along the Arctic route to Russia, and to some extent over the western approaches to Great Britain. Attempts were made in the spring
of 1944 to home U-boats on UK-US convoys by means of special long range aircraft in the area of 20°W.
- Submarine reconnaissance and observations of enemy conduct.
Submarines were themselves used in the effort to accumulate detailed observations of shipping and defense in distant coastal areas. The cumulative results were customarily repeated as "Situation and Traffic Reports" for the benefit of U-boats about to enter the area concerned. Some of this information could be traced to a particular U-boats own transmitted reports, but here again there was no certainty on many points. U-boat war logs were sometimes acknowledged as the source.
When schnorchel U-boats undertook a close-in blockade of British ports during the winter of 1944-1945 their situation reports on British coastal waters became especially detailed and systematic. The "Halm" ("Blade of grass") series of Offiziers sent to U-boats on the 13-14 February 1945 offered a correlation of information on shipping which undoubtedly used non-U-boat sources. By such means U-boats were given a clear and accurate summary against which to judge the significance of their own observations.
In addition to reconnaissance, U-boats were required to make special reports on Allied location devices, briefs of which were transmitted by radio. In this way it was possible to follow the struggles of the U-boat with Allied radar, from reports of radar transmissions intercepted on the early U-boat search receivers through all the subsequent attempts to isolate the mysterious source of Allied superiority. A numbered series of "Experience Messages" kept U-boats informed of Allied antisubmarine behavior and German interpretations.
- Gibraltar area.
Information from agents, as seen through U-boat traffic, was confined largely to the Gibraltar area: Ceuta, Cape Tres Forcas, Gibraltar, Alboran, Cape Spartel. The Germans followed all ship movements in and out of the Straits. Cape Spartel would report size and composition of an inbound convoy and
its escort, giving exact time of sighting, line of bearing, and speed. Gibraltar would follow up with what ships had put in to or out of Gibraltar. German aerial reconnaissance would pick up the convoy after it had passed into the Mediterranean. All of this information was relayed on Mediterranean U-boat circuits, or on Atlantic circuits in the case of an outbound convoy. Clandestine traffic from the agents themselves was available to the Atlantic Section.2
- Agents elsewhere.
Particularly active in 1943 were the agents at Lourenco Marques and in the Cape Town area. Their traffic was also available to the Atlantic Section. It was possible to identify information passed to U-boats with specific reports which had gone in from these agents - both Italian and German. Occasionally information, presumably from agents, was disseminated on independent ships out of Takoradi, Lagos, Egypt, Persian Gulf, etc. In 1945 agents furnished information of a minefield off Fastnet, Ireland.
In addition to details on shipping in the Gibraltar area, Japanese Military Attaché traffic from Lisbon to Berlin carried much information on trans-Atlantic convoys including dates of departure from the U.S. A "reliable" Italian agent claimed the U.S. Naval Attachés office in Lisbon as the source of his report on the disposition of the U.S. fleet. (PPB 33, 3 November 1944) The reports of agents in England were seen in clandestine traffic via Spain. There is at least one case in which the sailing date of a US convoy (UGS 27) was attributed to an agent's report. (2013/16 December 1943 to Alsterufer and Osorno) As a rule, however, German U-boat traffic reflected only a small part of an organization which was apparently extensive and active but whose outline could not be discerned.
- Prisoners of war and survivors of encounters at sea.
Statements from survivors were occasionally passed immediately to Control by U-boat commanders. On one such occasion information on an England bound convoy (SC 118) was forwarded while the operation was still in progress. U-266 (Jessen) sank a straggler and captured the ship's captain and engineer. Within a few hours Jessen transmitted the following:
Prisoner's statement: Rudloff's convoy approximately 45 ships of which 15 are tankers. Broad formation, 10 columns. Destination North Channel. Inner and outer defense. Steamer frequency at present 50 meters.
Convoy formation: 10 columns, each with 4 to 5 ships. Distance between columns 900 meters. Distance between ships 550 meters. Speed 7-8. (2031/6, 0047, 0120/7 February 1943)
Some information on the general routing of convoys and independents in the South Atlantic and on the Caribbean-New York run was gained in this manner. With the increasing effectiveness of Allied anti-submarine measures U-boats were urged to take prisoners, especially from aircraft shot down, and interrogate them on tactics and devices for U-boat location. In December 1943, a prisoner from a Wellington helped materially in dispelling German fears of submarine location by amazingly effective search receivers.
- Radio interception.
- Direction finding.
Prior to the fall of 1943 little or no attempt had been made to supply U-boats with current D/F's. Beginning with the resumption of the North Atlantic battle, however, U-boat circuits relayed an increasing number of fixes on Allied unit transmissions. During January 1944, for example, no less than 51 D/F fixes were sent to U-boats in the North Atlantic. The area covered was usually north of 40°N and east of 30°W, but a few fixes were made as far west as 56°W. It does not appear that effective use was or could be made of this information by
U-boats at sea, although a certain amount of correlation with the current convoy chart was attempted from shore for their immediate benefit.
- Traffic analysis.
Acknowledgments of traffic analysis as a source of information were sometimes seen in Mediterranean traffic, and more frequently in Arctic U-boat traffic. In 1944-1945 U-boats in the Far East were furnished with the results of Japanese traffic analysis on the movements of major fleet units in the Indian Ocean. Although acknowledgment of traffic analysis as a major source of information on Atlantic convoys was extremely rare, it was assumed in the Atlantic Section that German knowledge of the convoy cycles came principally from this source, particularly in view of the stereotyped nature of convoy traffic. GC&CS recognized that valuable information on Atlantic convoys was gained through the recovery of delivery groups and the study of call signs, on which it was known that German communication intelligence placed considerable emphasis. (see ZIP/ZG/252, p. 4). Captured German documents have confirmed the extensive use of traffic analysis not only in reconstructing convoy cycles but also in the correct identification of convoys by designator and number.3 RAF Coastal Command traffic was also exploited in connection with convoy movements. The GAF communications intelligence organization work in close collaboration with the navy in such matters.
- Allied transmissions in plain language or in self-evident code.
The repetitions of merchant vessel distress signals, BAMS submarine contact and attack reports, and aircraft reports on U-boats were frequently acknowledged by the phrase "according to B-Service." It became quite evident that German intercept service guarded the BAMS circuits with care and that U-boat Command correlated these reports with his submarine tracks, issuing orders and reprimands on the basis of them. The Atlantic Section watched the repetitions of these reports in
German traffic and invited COMINCH's attention to the advantages derived from them by the enemy.
- Interception by U-boats.
Although there are a few cases of U-boat monitoring on the international distress frequency in connection with attacks on merchant shipping, U-boat traffic does not show this to have been of any importance. It was certainly never stressed by Command. The only serious attempt, by U-boats at sea, to exploit Allied radio transmissions was that made on convoy voice traffic, for which trained operators were provided in 1943. (See "B-Dienst Aboard U-boats" in Chapter V) The most persistent attempt at interception of Allied transmissions was that directed against radar. In addition to warning for the individual U-boat, radar interception was intended to build up a knowledge of Allied radar characteristics and tactics. Early in 1944 certain U-boats were equipped with special search gear and trained men to carry out "Feldwache" tests in an effort to determine what frequencies the Allies might be using which the standard U-boat receiver could not pick up.
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1. For example - see 1034/2 January 1943: B-Service report of attack on U-Boat in 08°N - 55°W.; Mohr (U-124) replied. Also 1324/5 February 1943 to Group Nordsturm: "We have two English reports of attack." Gretschel (U-707) replied.
2. The majority of the clandestine traffic received in the Atlantic Section came from the U.S. Coast Guard and other government agencies. These documents are now on file at the National Archives, Washington, D.C. Records of the National Security Agency, "Messages of German Intelligence/Clandestine Agents, 1942-1945," SRIA 01-1550, SRIB 017361, SRIC 01-4164, and SRID 01-73. Record Group 457, National Archives, Washington, D.C.
3. In addition to the naval radio intelligence bulletin of 23 June 1944 already referred to, see ZIP/SAC/P. 7, a GAF radio intelligence bulletin.