The AN/FLR-9, operated by the Air Intelligence Agency, is part of the DOD World Wide High Frequency Direction Finding System. High-frequency radio communication signals travel to receivers over the horizon by bouncing off the ionosphere. The shell of ionised particles which surrounds the earth refracts the signals so that they return to earth rather than disappear out into space. The clarity of the signal received depends on atmospheric and topographical conditions. HF-DF stations detect radio signals from aircraft or ships, and calculate the direction, or line of bearing, of the radio transmitter from the direction finding antenna. When the same signal is received by two or more antennae, the intersection of the lines of bearing mark the transmitter's location, using either precision single station location (SSL) capability, or in a network of DF stations using both multi-station azimuth triangulation and SSL. High Frequency Acquisition (AQ) and Direction Finding (DF) operations are performed with the Narrowband System (NBS) and Wideband Direction Finding (WBDF) Subsystem in support of normal and degraded communications modes, using both adaptive reception and super-resolution direction finding techniques.
The AN/FLR-9 circularly disposed antenna array (CDAA), popularly known as elephant cages, have a nominal range between 150 to 5000 kilometers. Consisting of two rings of HF antennae, the inner ring, for monitoring longer longer wavelength signals, is typically some 230 meters in diameter with some containing 40 folded dipoles. The outer ring for monitoring shorter HF wavelengths is is about 260 meters in diameter and contains some 120 sleeve monopoles. Inside each ring is a large wire screen, supported by 80 towers, to shield antennae on the other side of the array from HF signals from crossing the array, which would interfering with geolocation operations. A horizontal ground screen about 400 meters in diameter surrounds the entire site. The station's intercept operators work in an operations building in the center of the array.
In 1964, the San Vito base and the base at Chicksands were the first to be equipped with the FLR-9 radio interception system and were part of a new intelligence network codenamed `Iron Horse'. Three other FLR-9 antennae are known to have been built at Misawa Air Base in Japan, Clark Air Base in the Philippines, and Elmendorf Air Base in Alaska.
- Augsburg, Germany
- RAF Chicksands, UK
- Clark AFB, Phillipines
- Elmendorf AFB, AK
- Menwith Hill, UK
- Misawa AFB, Japan
- San Vito dei Normanni AS, Italy
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