Long before radar and satellites, we tracked enemy movements with wireless direction finding. This early form of Signals Intelligence got its start in World War One.
As sea, land and air forces made increasing use of wireless during the Great War, wireless direction finding also grew. Most radio signals at that time were on lower frequencies – long and medium waves. Since signals at these frequencies traveled mainly as ground waves, it was fairly easy to find their source. If two or three different receiving stations plotted the direction, the origin would be triangulated.
RDF first started as a navigational aid for ships at sea. Radio beacons provided directional information. Later, RDF was also adopted by aircraft. Most of the modern navigational and homing methods used by air and sea transportation were developed as part of wireless direction finding efforts during the war.
Direction finding loops work quite well for getting a fix on a LW or MW signal. These devices have two challenges, though. First, due to their small size, signals picked up are quite weak. This problem was overcome with the shift to tube receivers with good amplifiers. Second, various electrical effects with loop and feed line balancing and capacitive coupling to ground can distort the reading. These problems were overcome by careful construction, as well as innovative DF antennas. The Bellini-Tossi system, used two fixed loops joined together by a radiogoniometer (mechanically rotating field coils). It had the advantage of ease of mounting on ships and aircraft.
Wireless Direction Finding – WWI “Radar”
The interception and decoding side of SIGINT has received much attention from historians. We all know about the code breakers of the Second World War, but the DF side has been largely ignored. If you are interested in the 1914-1918 equivalent of Bletchley Park, read about the British Admiralty’s Room 40.
Specialized wireless direction finding stations sprang up along Britain’s coastline. Motorized direction finding vehicles probed near the front lines. By 1915, military intelligence was producing weekly maps of enemy wireless locations. This included locations on land, sea and air. RDF was being used to track troop movements in Europe and U Boat tracks in the Atlantic.