AN/FLR-9

The AN/FLR-9, also known as "Iron Horse," was a massive, circular "elephant cage" HF (High Frequency) direction finding antenna array used by the United States during the Cold War and for some time afterward. Its primary purpose was to locate and monitor high-frequency radio transmissions originating from the Soviet Union and other nations. These transmissions often included signals related to military communications, diplomatic activity, and intelligence operations.

Located at several sites around the world, including Misawa Air Base in Japan, Chicksands in England, and San Vito dei Normanni Air Station in Italy, each AN/FLR-9 installation consisted of a circular array of antenna elements surrounding a central receiving building. The diameter of these arrays was substantial, typically exceeding 1,300 feet.

The AN/FLR-9 worked by analyzing the time difference of arrival (TDOA) of radio signals at different points around the circular array. This information was then used to calculate the bearing, or direction, from which the signal originated. Multiple AN/FLR-9 sites could then be used in a process of triangulation to pinpoint the location of the radio transmitter.

The system was considered a crucial component of US signals intelligence (SIGINT) capabilities during the Cold War, providing valuable information on the activities of potential adversaries. As technology advanced and satellite-based communication became more prevalent, the AN/FLR-9 became less critical. Many of the sites have since been decommissioned and dismantled.