Sources of Frontline Intelligence: Radio Intercept & Radiogoniometry (Direction Finding)

U.S. Army Listening Post

By Terrence J. Finnegan

Radio intercept and radiogoniometry (direction finding) had a tremendous impact on key battles during the first weeks of mobile warfare. Both had evolved as intelligence disciplines before the war started. Intercepting open enemy radio transmissions played critical roles at Tannenberg in eastern Poland and the First Marne northeast of Paris. Tannenberg was the first battle in history where interception of enemy radio traffic played the decisive role.

Success came to the Germans when they intercepted Russian radio transmissions containing exact force disposition and locations. Aerial reconnaissance had reinforced German command decisions but was not as decisive as the radio intercept. Ironically for the Germans, Allied radio intercepts of transmissions describing German maneuvers led to successful analysis of German army intentions a few days later at the First Marne, due partly to the German army's "lack of discipline in radio operation." By September 4 French intelligence confirmed that "The German First Army, neglecting Paris and our Sixth Army, before which nothing has shown itself, continues its march toward the Marne." German fatigue and logistical shortfalls were also determined. Finally, the monitoring of German General von Kluck's order to withdraw was evidence to the French radio intercept analysts that the German retreat had commenced. 

The successes at Tannenberg and the Marne clearly illustrated radio intercept and radiogoniometric value to the combatants. The ensuing demand for intelligence from wireless intercepts for each headquarters required better organization. By 1917, the Germans established Army Wireless Detachments [Armee-Funkerabteilung] to conduct wireless intercepts for centralized units at Army and Corps Headquarters. Traffic analysis studied enemy radio procedures and call signs. (By the war's end, a large part of the interceptions were of signals in Morse code.) Radiogoniometric analysis was a highly favored method for confirming enemy order-of-battle and determining the depth of echelons in a given sector. It allowed Allied forces to position their own forces to provide effective counter against an enemy attack.

U.S. Army Mobile Directional Detection Station

By 1915, any telephone wires strung out along the trench connecting back to the communications hub at the front had been destroyed by artillery, requiring buried telegraphy to provide communications within the trenches. Electro-magnetic currents of comparatively low frequency could be detected directly by the telephone receiver. Wire-tapping sections intercepted ground telegraph lines (French term for this telegraphy through the ground operation was télégraphie par le sol [T.P.S.]). The normal range for transmissions was three kilometers, enough to support the average frontline unit sector.

Intercept stations working from the most forward trenches used the earth lines to listen in to the enemy telephone conversations in the opposite trenches. They were able to report when the enemy was relieved and to warn of imminent attacks. Radio discipline in the front lines was called for and enforced. Soldiers were warned that the enemy overheard all telephone conversations. Radio intercept operators not only monitored for enemy conversations but also kept track of communication violations by friendly forces at the front. One T.P.S. operator recalled one such violation. "It is the French Artillery observer, but he is not using code. It is against the rules. He should not spot gun fire from the front lines without code. He will be reported." 

One of the world's most beloved monuments, the Eiffel Tower, served as a collection site for enemy radio transmissions in the Great War. Recognition of its service in the war was introduced by General Clergerie, chief of staff to the French military commander of the Paris sector during the threatening times of the First Marne. Clergerie cited the German cavalry commander of the First Army, General von der Marwitz, violating radio discipline on September 9 by transmitting "Tell me exactly where you are and what you are doing. Hurry up, I am going to bolt." The next day, French intelligence discovered at the transmission site—as determined by triangulation—abandoned stacks of munitions, vehicles and a field kitchen with a great store of flour and dough half-kneaded. 

Source: Over the Top, February 2009