WWII Signals Intelligence II


Lydd HF Direction Finding Station 1945 Captain Louis Varney G5RV 2nd from left

The British “Y” Service, or voice intercept service, intercepted uncoded German voice communications—primarily the Luftwaffe’s, but those of the German Army as well. Field Marshall Sir Bernard L. Montgomery recognized and exploited the capabilities of the Y Service better perhaps than any other senior British commander.

Initially employed against German fighter and bomber communications to supplement data obtained from radar plots, the RAF’s Y Service took on a broader role as the British shifted to the offensive after 1942. Assigned to disrupt the German air defense system, Y Service operators actively intruded on the German night fighter and later on day fighter communications circuits. British operators would call the German pilots or HQ controller on the radio, issue false orders, request operational information, or question orders and otherwise disrupt communications. Beginning in 1942, every bomber raid included a specially modified bomber carrying Y Service operators who provided warning of German fighter activities and disrupted German communications as necessary. Since the German Army used tactical codes in their voice circuits, the British rarely used the Y Service operators to intrude on German ground communications.

To be effective, Y Service operators required an extensive knowledge of German communications procedures and call signs, as well as near native fluency in the language. Intrusion can be countered, however, by the use of regularly changing tactical codes and tight communications discipline. Since air operations occurred at a speed that precluded the extensive use of codes, air control nets were the most likely to suffer intrusion. The Germans countered the RAF’s Y Service by using female communicators, but the British introduced female operators of their own. Although the Y Service never completely disrupted the German air defense command and control system, it certainly reduced its effectiveness.

The Polish security service was much smaller than that of its allies and opponents, but it still managed to obtain copies of the German Enigma encryption machine and made significant inroads toward breaking Germany’s codes—particularly those of the Luftwaffe. The Poles also reportedly broke some of the lower tactical Soviet code systems. They shared their results with their Anglo-French allies, even smuggling the files and equipment to France after Warsaw’s fall in 1939. This collaboration was the foundation of the Allies’ successful and widespread penetration of the German Enigma encryption systems (ULTRA). Cooperation and collaboration between the United States and Great Britain only began after Dunkirk, and accelerated after the Battle of Britain.

Little is known about the Soviet SIGINT services, but given the lack of cooperation between the state security organs (see NKVD) and military intelligence before the war, it is reasonable to assume that each of those agencies operated its own separate SIGINT services. Although the extent of Soviet SIGINT capabilities in World War II probably will never be known, the Germans discovered during their early offensives that the Soviets had extensive knowledge of the German communications networks and command structure. Finland’s SIGINT services discovered in November 1939 that the Soviets had been monitoring their air defense and army communications for some time. The Finns also believed that they had discovered several instances of Soviet operators intruding on Finnish communications networks. The Germans reported similar experiences during the battle for Stalingrad and the later campaigns on the eastern front. At the very least, this suggests that the Soviets had a significant capability to operate against the communications of its two most important European foes.

Germany entered the war with three separate military SIGINT organizations: the Abwehr’s Chiffrierwesen, the Kriegsmarine’s Beobachdienst or B-Dienst, and the Luftwaffe’s Forschungsamt. The Gestapo had a SIGINT service as well, but used it primarily for counterintelligence. Each agency operated independently after 1938, and rarely shared information, much less cooperated with each other. The German military service chiefs, in fact, often used their SIGINT agency’s reports to curry favor with Adolf Hitler. The resulting dispersal of effort inhibited the overall effectiveness of Germany’s SIGINT effort. The Luftwaffe, for example, would not ask the Kriegsmarine to collect information about Allied land and air radars and electronic systems, nor would the Kriegsmarine ask the Luftwaffe for information about Allied sea-based systems. Conversely, neither service was enthusiastic about sharing what it knew about these systems. Both services had to collect against all possible Allied systems, and given the limited resources available, neither obtained all the information it needed to support its respective war efforts.

Nonetheless, all the German SIGINT services enjoyed some successes, particularly early in the war. The Germans concentrated on operational and tactical signals intelligence—that is, the rapid dissemination of SIGINT information directly and immediately to the operational-level commander. German Army SIGINT units were assigned to each army corps and contributed greatly to the successful ground offensives of 1939 and 1941. Detecting the movements of Allied ground units far ahead of the German advance, they provided timely warning of Allied countermoves that enabled German mobile units to outmaneuver their slower Allied opponents, who received their own SIGINT reports much more slowly. The German SIGINT equipment was also light and mobile and the units were totally motorized so they could keep up with the Panzer units. General Erwin Rommel’s signals intelligence company is probably the most famous of these units, and he credited much of his success to its efforts. That company was so good that Field Marshall Montgomery specifically targeted the unit for destruction before launching the El Alamein offensive.

The Luftwaffe used the same equipment and employed a structure similar to the army’s, providing each Luftflotte (Air Fleet) with a radio reconnaissance battalion. These battalions monitored enemy air force and ground service communications to provide warning of enemy air movements, locate enemy operating bases, and locate enemy air headquarters and command posts. The radio reconnaissance battalions in Germany and on the western front were consolidated in May 1944 to concentrate against the Allied bombing campaign. They provided warning of Allied bombing raids by intercepting the prelaunch testing of electronic systems aboard the bombers. They also helped to track night bomber streams by monitoring the bombers’ navigation systems and radar emissions.

The Kriegsmarine primarily employed permanent HF/DF sites located near its major bases in Germany and the occupied countries in the west. These sites enabled the B-Dienst to provide accurate locating data on enemy ship movements in the North Sea and northern Atlantic to German naval units involved in the Battle of the Atlantic. The data were disseminated immediately to naval and U-boat operational headquarters and disseminated regularly to units at sea. The accuracy was not very good on shipping in the Mediterranean, South Atlantic, and Indian Oceans, but it did give German naval units an idea of what they might encounter. Also, the B-Dienst was able to decrypt Allied convoy and Royal Navy operational communications during much of the war’s early years. That was critical to Germany’s success in Norway and contributed to the effectiveness of U-boat operations during early phases of the Battle of the Atlantic.

The Kriegsmarine also assigned B-Dienst detachments to Germany’s surface raiders and major surface combatants. Thus, every flagship and surface raider had the ability to detect Allied naval formations and convoys in their operating area. In some cases, the B-Deinst detachments also did local decoding of enemy communications, particularly merchant shipping and aerial reconnaissance reporting. Transmission security was provided by the use of “one-time” pads to protect against enemy decoding.

Italy’s state and military intelligence services had SIGINT units. Targeted mostly against the French and Yugoslavs, the Italians had a reasonably effective decoding service, which successfully broke the Yugoslavian Army and the French and British naval codes during the interwar period. The Italians, however, were similar to the French in that they relied on permanent signals-monitoring stations. The Italian services were totally dependent upon SIGINT sites in Italy and Albania. This not only inhibited the interception of Allied communications in North Africa and the more distant waters of the Mediterranean, but it delayed dissemination of the reporting. It also prevented the rapport and mutual understanding between the intelligence services and the supported commanders that are so essential to effective intelligence support.

The Italians’ most prominent success came during the invasion of Yugoslavia when Italian radio operators in Albania regularly intruded on the encrypted Yugoslav command net, countermanding attack orders and misdirecting Yugoslav units and logistics support.

In contrast to the cooperative Anglo-American SIGINT effort, the Axis countries did not trust each other enough to share information, much less coordinate their SIGINT efforts. Finland had the best SIGINT service of the minor countries, decrypting Soviet naval and other communications at various times during the Winter War. Monitoring of Soviet Air Force and army communications also provided the Finns with key information about Soviet intentions and preparations well in advance of actual attacks and bombing raids. Unfortunately, it was not enough to know when, where, and with what your opponent was attacking. One had to have the resources to defeat the attack, and such resources were what the Finns most acutely lacked. The Romanians and Hungarians also had fairly large and effective SIGINT services, but their efforts were directed more against each other, so they contributed very little to the overall Axis war effort.

SIGINT has become one of the most important elements of modern intelligence operations in a world of fast-moving, mobile military forces. Electronic signals are the means by which every modern commander sees and controls his forces, which are themselves dependent upon electronic systems to conduct their missions. SIGINT became an increasingly technological and complex affair as World War II progressed. The ability to read an enemy’s communications or know which of his units were in contact with each other provided insights into his operations and intentions. Knowing an enemy’s electronic sensor and navigation systems led to the development of effective countermeasures and the ability to blind him at a critical time, or just to avoid his forces when necessary. The Anglo-American cooperation in SIGINT enabled both countries to make the most of their SIGINT efforts and virtually guaranteed the Allied victory once the war-fighting forces had the resources to exploit the insights the SIGINT services developed.

Additional Reading

Andrew, Christopher, Codebreaking and Signals Intelligence (1986).

Clayton, Aileen, The Enemy Is Listening (1980).

Irving, David, Breach of Security (1968).

Kahn, David, The Codebreakers (1972).

Norman, Bruce, Secret Warfare: The Battle of Codes and Ciphers (1973).

West, Nigel, The SIGINT Secrets (1986).