The AN/FLR-9 is a type of very large antenna array, built at many locations during the cold war to gather signals for western signals intelligence (SIGINT). The world-wide network, known collectively as "Iron Horse", could eavesdrop on HF communications from almost anywhere on the planet.
Each antenna consisted of an inner ring 230 metres in diameter, consisting of 40 individual folded dipoles, and an outer ring about 260 metres in diameter consisting of 120 monopoles. The whole array is surrounded by a ground shield some 400 metres in diameter. The arrangement permits accurate direction finding of signals from up to 5000 km away.
FLR-9s were constructed at the following places:
Advances in technology have made Iron Horse obsolete. It is not known how many antennae remain in existence - some have already been demolished such as the one at Chicksands. In late 2001, the US Air Force sought bids for the demolition of the site in Italy.
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