Chapter VIII
U-Boat Communications and U.S. Navy Communications Intelligence
(Intercept and High Frequency Direction Finding)

  1. Summary

  2. Operating procedure

  3. U-boat circuits

  4. Use of medium frequencies

  5. Use of very low frequencies

  6. Norddeich off-frequency operation. (Standing War Orders 217 and 218)


  8. U.S. intercept operations

  9. HF/DF operations ashore

  10. HF/DF operations afloat

  11. TINA and RFP


U-Boat Associated Transmitters


1. Summary

Communications between U-boats and their bases was carried out by H/F radio, with repetition on very low frequency to insure reception. Either the "Broadcast" or "Intercept" method was used by Control to deliver traffic to U-boats. The large number of circuits, designed to serve all areas in which U-boats were operating, consisted of regular U-boat circuits, convoy circuits, and special circuits. In addition to this system for communication between control and its boats, medium frequencies were used for cooperation of U-boats with each other, with surface vessels, and with aircraft, mainly for homing purposes.

The chief concern of the German communications service was to maintain contact between Command and the U-boats without offering too good an opportunity for D/F-ing and traffic interception by the Allies. Proper regard for radio discipline was in evidence in traffic at all times, but became more pronounced as unfavorable conditions for the U-boat, particularly at rendezvous and in group operations, became more acute; and the summer and fall of 1943 brought the campaign of COMSUBs against D/F to a new height.

During the period, restriction and supervision of the use of radio became stricter than ever. Restrictions were placed on the use of certain receiving sets, and COMSUBs demanded employment of off-frequencies of the Norddeich service even more insistently than before. Also the off-frequency procedure already in effect was completely revised with a view to increasing both efficiency of operation and difficulty of interception. This stage of development of the off-frequency procedure offered little difficulty to the Allied D/F organization, but a later refinement introduced in January 1945 was quite effective in combating interception.

The summer of 1944 brought the most important development in German communications in the form of KURIER experiments. KURIER was a system of high-speed automatic "flash" transmissions involving the use of special sending and receiving equipment. During several periods of experimentation, no KURIER signal was ever D/F'd, and if the


system could have been brought into operational use, the Allied D/F organization would have been rendered helpless until counter-equipment could have been devised.

American efforts to intercept German naval circuits were made as early as 1938, and by the time the United States entered the war almost 100 percent coverage was obtainable. HF/DF-ing of U-boat transmissions was begun in January 1941 by shore stations and was extended to shipboard stations the next year. The D/F organization was much improved during the course of the war through liaison with the British, an increase in the number of stations, and better operating technique. The "tip-off system was used to advantage for alerting all D/F stations simultaneously. OP-20-G cooperated fully in the establishment of shipboard HF/DF stations, in training personnel to man them, and in keeping the forces afloat abreast of all the latest trends in U-boat communication with HF/DF for the identification of transmitters.

2. Operating Procedure

German U-boats maintained contact with and received orders from their home bases by high frequency radio communications, but to insure that U-boats received all traffic, and elaborate very low frequency repetition service was in operation over which practically all U-boat traffic was repeated.

The control stations on the U-boat circuits delivered traffic to U-boats by either the "Broadcast" or "Intercept" method. The same H/F circuits were used for both shore-ship and ship-shore traffic. The control stations observed fixed silent periods on all circuits for the reception of U-boat transmissions. In addition to these fixed silent periods, the shore stations normally observed a five-second pause at the end of each fortieth group in long transmission in order to permit U-boats with traffic of higher precedence to break in. Thus a U-boat could transmit during silent periods, during the five-second pause, or at any time when control signified that the circuit was clear by use of "VA." Neither external address nor call sign were used by U-boats. Control acknowledged a U-boat transmission by repeating the message and inserting a serial number in the heading.


3. U-Boat Circuits

Communications were carried out on a large number of frequencies which were divided into circuits and selected so as to be suitable for any sea area in which the U-boats were operating. Several of these circuits were known as "Convoy circuits," and were used by U-boats while making group attacks on convoys. The following is a list of the circuits used. It is to be noted that the name of the circuit in most cases indicates the general area in which the circuit was used.

  1. Regular U-boat Circuits
    • Coastal
    • Ireland
    • America 1, A & B
    • America 2, C & D
    • America 3 E & F
    • Africa 1, A&B
    • Africa 2, C & D
    • Africa 3, E & F
    • Arctic
    • Mediterranean
    • Aegean
    • Penang
  2. Convoy Circuits
    • Diana
    • Hubertus
    • Wotan
  3. Other circuits used by U-boats
    • Bruno 3 (Norddeich)
    • Anton (Kootwijk

All of the circuits in list (a) were used at one time or another during the war. Certain circuits were dropped or put into effect depending upon the disposition of the U-boats at sea. The Coastal circuit was normally used by boats operation in the Channel and Biscay areas. Ireland was used for the eastern North Atlantic. The America and Africa circuits consisted of three separate circuits each in which two channels


or frequencies were generally keyed simultaneously. Each of these two could be disconnected and separated as a separate circuit if necessary. America 1 was generally used in the same area as Ireland circuit. America 2 served the western Atlantic north of a line from about the Azores to Key West. America 3 served the Middle Atlantic, Caribbean and South American coastal areas. The Africa circuits, of which never more that two were employed at one time, served the southern East Atlantic and the Indian Ocean.

All of the above circuits were controlled form Lorient, Paris, Bernau (Berlin), or Wilhelmshaven, depending on the progress of the war.

The Arctic circuit, which was controlled from Northern Norway, served U-boats operating off Northern Norway and on the Murmansk routes. The Mediterranean circuit, controlled from Toulon, served U-boats in the Western Mediterranean, while the Aegean circuit, controlled form Salamis, served the Eastern Mediterranean. Penang acted as control for the circuit of the name and served the Indian Ocean area, supplementing the Africa services.

Of the three convoy circuits in list (b), either Diana or Hubertus was always in effect, at times both were employed simultaneously. Wotan was in force only once and for only a short period during the latter part of the war. The purpose of these special circuits was to separate communications with U-boat operating groups from the traffic with other U-boats in the area which were not members of the group. Normally U-boats guarded the area circuits until contact with a convoy was contemplated or made, then shifted to guard the designated convoy circuit. When operations were broken off, the U-boats returned to the area circuit.

In the event that communications with home stations were broken off because of ionospheric disturbances or for other reasons, provisions were made for U-boats operating as a group to maintain contact with each other by means of what was known as a "Group Circuit". The procedure was for all boats concerned to shift to a predetermined frequency, appoint one boat as control, and continue operations until conditions permitted a return to the assigned circuit.

Other circuits available for use of U-boats were Bruno 3, designated Series Norddeich by the Allies, and Anton, otherwise known as Series Kootwijk. Bruno 3 consisted of four frequencies keyed simultaneously. The frequencies were


chosen from the various wave band to permit world-wide coverage both day and night. While this circuit was used mostly by U-boats, it could also be used by surface craft. In fact, it was on one of the Bruno 3 frequencies that the ill-fated Bismarck transmitted her last message. Far East surface blockade runners were known to guard this circuit while outside of Japanese-controlled territory. Off-frequency operation was also carried out by U-boats in connection with this series.

Circuit Anton normally consisted of four frequencies keyed together and was intended for the use of boats in the Eastern North Atlantic and Northern Norway. This circuit was available to both U-boats and surface craft. A system provoking for off-frequency operation in connection with Anton was in effect but apparently seldom, if ever, used.

4. Use of Medium Frequencies

Medium frequencies were used by U-boats for homing purposes and sometimes for general communication in home waters. Three types of homing on M/F were noted: homing for convoy attack, sometimes in cooperation with aircraft; homing for rendezvous; and homing to port. In homing for convoy attack, the U-boat which first made contact on a convoy was ordered by control to send beacon signals to lead other U-boats to the convoy. Homing for rendezvous, usually for the purpose of supplying provisions and fuel, was accomplished by having the supply boat transmit beacon signals on M/F. Such signals were ordinarily sent only during bad weather when precise navigation was not possible. In homing to port, the beacon signal was sent by the escort vessel or by regular shore beacon transmitters.

5. Use of very low frequency

High-powered very low frequency transmitters were used extensively by the Germans to augment the regular U-boat transmissions on H/F. All traffic carried on the regular U-boat series was repeated on very low frequency to enable reception while submerged or when ionospheric conditions prevented reception on H/F. Practically every known high-powered very low frequency transmitter in occupied Europe was employed for this purpose at one time or another. What


was estimated to be one of the most powerful very low frequency transmitters in the world, called "Goliath" by the Germans, appeared on the air late in 1943. This transmitter, having a power believed to be nearly 1000 kilowatts, was capable of being shifted in frequency and had a range form 15 to 25 kcs. One U-boat reported a signal strength of 5 from the Caribbean while submerged at 60 feet.

6. Norddeich off-frequency operation. (Standing War Orders 217 and 218)

A method of transmitting by U-boats to defeat the Allied HF/DF organization was brought into operation in October 1943. This system was used in conjunction with the Norddeich (DAN) transmitters. Briefly the system was as follows: Norddeich sent on four frequencies in four different wave band simultaneously. Each wave band had ten different "off-frequencies" above and below the basic frequency available for use. Norddeich selected one off-frequency for each wave band in use and informed the U-boat of the chosen frequencies by inserting several indicating letters in the call sign idling strip. The U-boat desiring to use this system listened to determine the valid frequencies and then transmitted on the off-frequency in the wave band best heard.

The above system which offered little hindrance to HF/DF operations, was continued until January 1945. At that time a more complicated system was inaugurated and continued in use until a few days before the end of the war. The latter system provided for using any one of 336 kcs above or below each basic frequency. Further, the system of indicating valid off-frequencies was changed to compose three four-letter groups which were encoded on a special table. Otherwise it was essentially the same as the former system. This new system proved difficult for the HF/DF organization because a very exact frequency calibration was required to detect each transmission. Only mediocre success was experienced in D/F-ing such transmissions.

A set of four "check" transmitters were operated in conjunction with the Norddeich series for assisting U-boats to select the proper wave band in which to transmit. The transmitters were of the same power as the regular U-boat transmitters. Therefore a U-boat, after listening to all four,


could select the wave band best heard, and could transmit with a good chance of being heard in Germany.


A system of high-speed automatic "flash" transmission known by the covername KURIER was first introduced on an experimental basis in August 1944. This equipment consisted of a separate unit designed to be attached to a radio transmitter. The unit could be sit up by a series of levers to reproduce dots, dashes, and spaces, and was capable of sending about ten letters of encoded text at a speed of approximately 600 words per minute. Nothing is known of the receiving equipment, but presumably and automatic recorder of high sensitivity was required. Obviously this system was designed as a counter-measure against D/F. Further security was obtained in the use of frequencies, in that deviation from a given basic frequency of as much as 200 kcs plus of minus was provided for. In no case were KURIER transmissions ever D/F'd, and it is significant that if this system had been brought into effective operational use, the Allied D/F organization would gave been rendered helpless until counter-equipment could have been devised.

8. U.S. Intercept Operations

First efforts to intercept German naval circuits were made by OP-20-G early in 1938 from the East Coast of the United States. The results obtained were very poor, mainly because German operations at that time were confined to the Baltic and North Sea areas, with resulting poor signal strength. In October 1938, an intercept team consisting of four men was established in the flagship of Squadron 40-T, then operating in European water, results were excellent and this team remained in operation until the squadron returned to the U.S. in September 1940. At the outbreak of World War II, all German U-boat circuits were known and were being intercepted by OP-20-G.

Interception was undertaken on a full scale by shore intercept stations on the U.S. East Coast thereafter. This work was done at Cheltenham, Maryland until January 1943 and was


then transferred to Chatham, Massachusetts where a major intercept station was placed in operation. Practically 100 percent intercept coverage was obtained until all but the final week of the war. Information relating to German communications procedure obtained by radio intelligence methods was compiled and published by OP-20-G in a document known as RIP 42 early in 1941. The information contained in this publication was greatly enlarged upon when subsequent liaison with the British was effected.

9. HF/DF operations ashore

First attempts to take bearings on U-boat transmissions were begun in January 1941. At this time there were only seven HF/DF stations in operation in the Atlantic area and bearings were taken independently. However, through liaison with the British, added D/F stations, and improved operating technique, the D/F organization was fairly efficient by the time of U.S. entry into the war. Improvements were rapid thereafter, and at the war's end, fixes were being made with bearings furnished by more than forty U.S., British, and Canadian stations. Three U.S. nets, and East Coast, Caribbean, and South American, were in operation, each with plotting facilities to serve the Sea Frontier Commands required. Centralized and interlocked control by both land-line and radio was maintained, making it possible to alert or "tip-off all D/F stations.

The use of the "tip-off system was gradually expanded to a point at which, in addition to our own stations, it was utilized by all Canadian stations, outlying British stations and all U.S. vessels engaged in convoy escort or antisubmarine warfare operations. The "tip-off frequencies were controlled by San Juan, sending simultaneously on four frequencies which were selected to cover the entire Atlantic. "Tip-offs" from U.S east coast stations on landlines were relayed by radio to San Juan by Jupiter. The "tip-off system helped greatly in increasing the efficiency of the HF/DF nets and proved particularly helpful to HF/DF equipped ships at sea.

10. HF/DF operations afloat

Every possible assistance was rendered by OP-20-G in furthering the shipboard HF/DF program as an additional antisubmarine


warfare weapon. Two technicians from OP-20-G were sent to England in 1942 to gather information relating to the operation and maintenance of this equipment. These two men assisted in establishing an HF/DF school for the training of shipboard operators in December 1943. The school was subsequently administered by Bureau of Personnel. Constant liaison with the school and forces afloat was maintained by OP-20-G personnel to insure that all concerned were in possession of the latest trends in U-boat communication procedures. Such liaison was in the nature of exchange visits by radio intelligence an D/F officers both ashore and afloat, letters, and dispatches. It is considered that this assistance was of great value to the forces afloat. A special publication dealing with German U-boat communication procedure, designed primarily for Atlantic Fleet vessels engaged in convoy escort and ante-submarine warfare, was published on 1 July 1943. This publication, CSP 1774 Series, was particularly valuable to HF/DF equipped vessels in detecting U-boat transmissions at sea.

11. TINA and RFP

Both TINA and RFP were extensively utilized in conjunction with HF/DF in attempting to identify individual U-boats. TINA, the purpose of which was to identify a radio operator by his sending characteristics, consisted of making a tape recording of each U-boat transmission and taking mathematical measurements of each dot, dash, and space. RFP was a method of transmitter identification in which high-speed photographs were taken of a transmission, making possible an analysis of the transmitter's power supply.

12. Chronological account of the German struggle against D/F and radio interception

An early illustration of precautions against HF/DF during refueling operations occurred in December 1942, when several U-boats were to be refueled by Schnoor (U-460). COMSUBs warned the boats against too frequent use of radio when approaching the provisioning point and directed that no beacon signals should be requested until several hours had passed without finding the provisioner (2158/3 December 1942) In an earlier message of the same day (1056/3 December 1942),


the boats were ordered to report their fuel stocks after provisioning, but only after having moved 30 miles away form the provisioning point. Other traffic of the same month indicated that Atlantic U-boats were making use of the Norddeich off-frequency procedure in transmitting short signals, with acknowledgment by control on the basic frequencies.

On 4 January 1943, a new Standing War Order was issued containing an outline of points to be considered before sending any radio message. (0516/4 January 1943) The purpose of the order was to eliminate the transmission of unnecessary reports and to provide for transmission of necessary ones in the shortest possible form. This was merely the first step in a campaign aimed at the tightening of radio discipline, for during the first months of 1943 there were constant reminders on the subject, with emphasis on the necessity for radio silence and the danger of tuning transmitters with radiation.

Repeated and emphatic instructions underlined the danger which could result from the use of radio during rendezvous operations and whenever groups of U-boats were in the same area, and there were many explicit reprimands for infractions. On 5 May an emphatic reiteration of the ban on tuning with radiation addressed to Arctic U-boats required that main and spare transmitter aerials be plugged in only when the situation might require quick use of the transmitter and that all-wave, Broadcast, and "Radione" receivers be used by boats in patrol lines only for reception of tactically important reports, owing to the danger of D/F.

The increased difficulty for U-boats at refueling rendezvous was reflected in COMSUBs' message 2306/5 of June 1943, in which he directed Group Trutz and Bartke (U-488) to carry out provisioning operations under radio silence and without any beacon signal unless the rendezvous had not taken place after searching for two days. Comparison of this order with the December 1942 order to boats refueling form Schnoor, cited above, illustrates the trend toward extreme caution which had developed in the intervening period.

A more direct indication of German appreciation of the effectiveness of the allied radio intelligence organization and a further intensification of efforts to escape D/F and radio interception became apparent from traffic of 10 August 1943. On that date COMSUBs1 Current Order 38 summed up the situation as follows:


By employing long-range reconnaissance aircraft and aircraft carriers the enemy is today able to operate aircraft on offensive patrol on the basis of D/F bearings not only near the coast but also in all danger of bearings being taken on U-boats when using their radio has thereby become more serious. The inaccuracy of the bearings of between 50 and 60 miles is compensated for by radar location by the aircraft."

Because of this situation U-boats were ordered to take advantage of discovery by the enemy, occasioned by sightings and attacks, to send their reports; to be especially careful in the use of radio when its use was necessary; after using the radio to remain submerged for several hours when there was danger of being taken by surprise; to supervise radio operation and to tune without radiation; and to use alternative frequencies of the Norddeich service (0827 and 0845/10 August 1943 ff), was devoted to a complete revision of the off-frequency procedure already in effect, to go into force on 1 September (later postponed to 1 October). Under the new procedure, deviations from the basic frequencies could be changed frequently by the control station and would never be known in advance of the hour of transmission even by the transmitting U-boat. Several messages of September and October urged increased use of the alternative frequencies when giving passage reports in order to make it more difficult for the enemy to take bearings and establish the number of inward and outward bound boats.

From the middle of August to the end of November 1943, there was a renewal of preoccupation with the possibility that the Allied D/F services was profiting by receiver radiations. A message of 19 August ordered that certain types of receivers were not to be used until investigation of that possibility had been completed, and directed that very low frequency transmissions were to be received with the D/F receiver only, while the all-wave receiver was to be restricted to essential services (1922/19 August 1943). On 24 October however, Hartmann (U-441) reported a suspicion that bearings could be taken on the D/F receiver because he had been flown at shortly after switching it on (2237/24 October 1943); consequently on 5 November a new order directed outward and inward bound boats east of 18°W, as well as boats outbound from Germany as far as Naval Grid Square AE (Iceland Area), to receive very low


frequency only when submerged. H/F reception on the surface was permissible only if a boat was equipped with a "Main" receiver. (2049/5 November 1943) The ban on some of the other receivers was lifted on 8/12, however, presumably after satisfactory results in radiation tests (1837/8 December 1943).

The emphasis on radio discipline and restriction of transmissions continued in 1944, with few significant developments in the first seven months of the year. An attempt was made to speed up the transmissions of short signals and short weather reports in January, and provisional Standing War Order 254, providing for visual signals between cooperating aircraft and U-boats as a means of avoiding D/F, was issued on the 30th of that month. In the spring, weather reports became so important to the German High Command that the possibility of D/F fixes had to be accepted in order to get the weather information through (1416/4 May 1944). However, boats were ordered to proceed submerged for several hours and to change position constantly after sending their reports.

The summer of 1944 brought a new and potentially important development in German communications in the form of experiments with KURIER procedure. The experiments started on 4 August and continued, with some interruptions for correction of faulty equipment, until 28 August, when the tests were canceled temporarily because transmitting installations were still not reliable enough. During this period there had been a few successful transmissions, but a large number failed to be received or could not be deciphered owing to failure to attain the required fine adjustment of the transmitting installation.

Another period of trial started in November 1944, was discontinued in December, and resumed in January 1945. During December tests were made in transmission of KURIER signals over the Schnorchel round dipole. There was no indication of the results actually obtained, but COMSUBs held little hope of success because the radiation obtained through use of the Schnorchel round dipole was insufficient. At 2325 on 27 January the first operational KURIER signal was sent by Schumann (U-245), operating under the cover-name "Brutus", but on 2 February the use of KURIER procedure was discontinued and never resumed, probably because of the evacuation of Bernau, where the receiving installation was located.


One final development of importance during the last months of the war was the introduction on 25 January 1945 of a new and complicated cipher for determining valid Norddeich off-frequencies, directions for which had been given in a long series of messages between 20 and 26 November 1944. The new system was effective to a considerable extent in evading D/F, but was never used as extensively as the former KONRAD system, probably because of the many failures to get through to Control which were occasioned by the lack of necessary accurate tuning to proper frequencies.


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