The German Army’s field signal intelligence effort was organized into nine SIGINT regiments (known as KONAs, Kommandeur der Nachrichten Aufklaerung) stationed on every front.
Signals intelligence, the intercept and analysis of electronic signals, had its birth just before World War I as telecommunications became increasingly important in diplomacy and military operations. In many European countries, the monitoring of this new communications system came under the same bureaus (called “black chambers”) that previously had intercepted and read foreign mail. Their contributions to military operations in that war and the importance of intercepting and analyzing a likely opponents communications (called signals at that time) were widely recognized during the interwar period.
By 1939, virtually every power in Europe had a signals intelligence agency within its foreign ministries, and more often than not, within its military departments as well. Signals intelligence (called SIGINT) was expanded during World War II to include the intercept and analysis of electronic signals not related to communications. This reflected the growing importance of radar and electronic systems in warfare. Winning the electronic war determined the outcome in the Battle of the Atlantic and contributed to the success of the Allied bombing campaign. Signals intelligence played a key role in every military campaign in World War II.
The more spectacular code breaking or cryptographic aspects of signals intelligence are well known if not well understood. Successful code breaking can have an immediate strategic impact on a country’s activities, both military and diplomatic. With it, one can read an enemy’s actual thoughts and plans and prepare one’s own countermeasures or reactions accordingly. For that reason, every major SIGINT agency had a code breaking section, and those of the respective foreign ministries generally had the largest and best funded.
The U.S. State Department was the only major foreign ministry that did not maintain a signals intercept service during the interwar period. Even minor European countries, such as Romania and Hungary, had such services. Fortunately for the United States, its military services retained their SIGINT agencies after World War I and had begun to expand their capabilities as World War II approached. At first, the U.S. military’s focus on protecting America’s territories in the Pacific limited its code breaking successes to the major countries in that theater, but with British assistance, the Americans were able to make a significant contribution to the Allied SIGINT effort in Europe after 1943.
Although spectacular in its impact, code breaking is neither the only element of signals intelligence nor the most important. Much can be gained from analyzing the nature of the signal itself, as well as from whom and to whom the signal is directed. Individual Morse code operators tap out their messages in a unique way that can be identified. This “fist” could be used to track the operator’s movements. Since many military and political leaders of the period used their own personal communicators, the leaders or their units could be located by finding the communicators. Also, unit call signs or the code names that units and commanders used to identify themselves in electronic communications provided a means by which to track the movements and activities of those units. For that reason, many successful deception plans of World War II revolved around the placement and activities of communications personnel and the exchanging of call signs.
Determining with whom units and agencies are communicating can do much to identify the intentions, capabilities, and likely missions of those units or their leaders. For example, communications between a major enemy ground component commander’s headquarters and most of the enemy’s armored or mobile divisions in the area might indicate an impending offensive in that sector. The addition of a major air force HQ communicating with that commander’s HQ would be an additional indicator. Indeed, those were exactly the SIGINT indicators available to the Allied ground commanders in France on 8-9 May 1940, some twenty-four hours prior to the German invasion of Belgium.
Denying this sort of information to an opponent was as important as gaining it from them. Hence, SIGINT was very much a chess game between those collecting the signals and their opponents’ efforts to protect their own communications from interception or deny the collectors the details required to analyze those communications. This was called communications security. The communications security effort had to be balanced against the likely impact on one’s own forces. Using a permanent call sign, for example, simplified friendly force identification on the net, but also made it easier for hostile SIGINT services to do the same. Thus, most nations rotated their call signs periodically, except where speed of identification or communications outweighed the need for security—such as for aircraft, individual minor combatants, or tactical communications among units in combat.
Another aspect of SIGINT was direction finding, or DF. This technique used directional antennas to determine the azimuth or direction from which a signal emanated. Two or more intercept stations operating in concert against a single transmitting station (or emitter) could triangulate its position. In other words, they could fix the emitter’s location by plotting the respective azimuths on a map or chart. The emitter’s location, or fix, was where the azimuths intersected. The closer the angle of intersection was to 90 degrees (a “cross bearing”), the more accurate the fix. The fix’s accuracy also improved with the number of azimuths, since the primitive directional antennas of the period required the operator to judge the azimuth himself by deciding from which direction the signal was strongest. The more experienced the intercept operator, the more accurate his judgment. A larger number of azimuths also enabled the plotter to discard the most inconsistent azimuths, or to determine an area of probability if no clear fix emerged.
The transmitters frequency also affected the accuracy of a DF fix. Higher frequency systems had narrower beams and were easier to judge. Lower frequencies could be detected at longer ranges since their signals often traveled along the earth’s surface beyond the horizon (ground wave), or reflected off the stratosphere back onto the earth’s surface (sky wave). Radars, airborne navigation systems, and the higher frequency communications systems—such as very high frequency (VHF) air and tactical ground communications— could only be intercepted if the receiver was located within a direct line of sight of the transmitter. When detected by multiple intercept stations, however, an emitter’s position could be fixed very accurately and within a very short time.
Proximity to the transmitter was another consideration. The closer the intercept station was to the transmitter, the more accurate the azimuth was likely to be. That was why Allied naval authorities wanted high frequency DF (HF/DF) systems installed on destroyers escorting convoys. It enabled the escorts to locate the U-boat reporting the convoy’s location more accurately, significantly improving the chances of destroying the U-boat.
High frequency communications were so important to naval operations, because of the vast distances involved, that all nations employed huge naval SIGINT infrastructures with numerous HF/DF sites to track hostile and neutral naval forces. The Western Allies had the most extensive and effective networks, with SIGINT stations at virtually every British and American overseas base. The Italians and Soviets had networks that enabled them to track naval units operating in the waters near their shores. The Germans had only a limited capability to track naval units in the Atlantic accurately until they established HF/DF sites in France and Norway. Even then, the lack of “cross bearings” inhibited the accuracy of their fixes against units in the South Atlantic and Indian Oceans.
Britain had the world’s largest and most capable SIGINT organization at the start of World War II. All military and diplomatic SIGINT activities theoretically came under its umbrella, but in practice, the tactical and operational SIGINT collection of the military services came under the service chiefs. Still, Britain’s SIGINT effort was better coordinated than that of any of the war’s participants. Britain entered the war at a slight disadvantage since most of its SIGINT efforts had been directed against the Soviet Union during the interwar period—Germany had been identified as a threat only in the late 1930s. Britain did benefit from its good relations and cooperation with France, and from 1939 on, with Poland.
The French had a good picture of the German military communications networks, much of it gained from the Czech intelligence services that transferred many of their files to the French just before the German occupation in 1938.
The activities of Bletchley Park and its decoding successes are generally well known, but the British military SIGINT services also made a significant contribution to the Allied war effort. The Anglo-American HF/DF sites greatly facilitated the Allies’ antisubmarine efforts, particularly after HF/DF equipment was installed aboard Allied escort ships. The British Army assigned signals intelligence companies, called special wireless sections (SWS), to each British field army HQ. Equipped primarily with HF intercept equipment and lacking DF capability at war’s start, these units became more mobile as the war progressed. The doctrine changed as well. In 1940, British SWS units in France detected the movement of the Panzer divisions up to the border and into the Ardennes. British commanders, however, did not accept their reports, waiting instead for confirmation by aerial reconnaissance. The resulting forty eight-hour delay was an important element in the Allied defeat in France. Afterward, such reports of “immediate significance” went directly to the British field army commander.