The AN/FLR-9 is a type of very large circularly disposed antenna array, built at eight locations during the cold war for HF/DF direction finding of high priority targets. The worldwide network, known collectively as "Iron Horse", could locate HF communications almost anywhere on Earth. Because of the exceptionally large size of its outer reflecting screen, the FLR-9 was commonly referred to by the nickname "Elephant Cage." Constructed in the early to mid 1960s, in May 2016 the last operational FLR-9 at Joint Base Elmendorf-Richardson in Alaska was decommissioned. It can be confused with the US Navy's AN/FRD-10, which also used a circularly disposed antenna array.
AN/FLR-9 antenna array at Misawa Air Base c. 1980
Circularly disposed antenna array
A circularly disposed antenna array (CDAA), sometimes referred to as a circularly disposed dipole array (CDDA) or a wullenweber, is a large circular antenna array used for radio direction finding. They are used by military and government agencies to triangulate radio signals for radio navigation, intelligence gathering, search and rescue, and enforcement of broadcasting laws. Because their huge circular reflecting screens look like circular fences, some antennas have been colloquially referred to as "elephant cages". The term "wullenwever" was the World War II German cover term used to identify their secret CDAA research and development program; its name is unrelated to any person involved in the program.
Direction-finding system Galeta Island, Panama
CDAA at US Navy Sobe Communications base, Yomitan, Okinawa, Japan
An overhead view of Elmendorf AFB, Alaska, in late 2002. The antenna array is the large circle in the forested area, in the upper left, comparable in size to the adjacent airstrip.