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AN/FLR-9

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 refrac= ts the signals so that they return to earth rather than disappear out into spa= ce. 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, us= ing 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.

* DD Kavanagh writes: "I was the ESD Field Officer, stationed at Fuchu, Japan, responsible f= or construction of the FLR-9s at Clark and at Misawa in 1964. My NCO, statione= d at Misawa, was C/MSgt. Joseph E. Rabig. The work those days was very hush, hus= h. As the structure went up there was much speculation at Misawa about what the purpose was. Joe came up with the explanation that it was an 'Elephant Cage= .' He maintained that was their purpose and justified the explanation by point= ing out: 'They work pretty well, don't they. You don't see any elephants running around loose do you?' I guess the name stuck."

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 monitor= ing 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 f= rom 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 t= he center of the array.

DD Kavanagh writes: I happened to come across your FLR-9 page while surfing the net the other day= . I regret to inform you that some aspects of your description of its operation were very incorrect.

The outer ring of 48 monopole antenna elements covered the portion of the spectrum from about 3 = Mhz. to just below 6 Mhz. (These band frequency numbers are only very approximat= e. My memory isn't that good, 40 years later.) Just behind them was an inner r= ing of 96 monopole elements that covered from about 6 Mhz to about 12 Mhz. Behi= nd the second ring was the vertical reflecting screen, 105" in height who= se purpose was to assure the calculated directivity of the 6 vertical elements that were combined by the goniometer in the central building to provide the requisite directivity. Just outside the central building was a circular, vertical structure that held an assembly of 48 bow tie type antennae that w= ere similarly combined in appropriate receiving groups to provide the requisite = directivity on the bands from about 18 Mhz. to 30 Mhz.<= /p>

The surrounding grou= nd screen, combined with the circular, vertical "elephant cage" assu= red the proper combination of earth conductivity and reflective backdrop to ass= ure the proper radiation pattern of the vertical elements. It was practically transparent to the generally horizontally polarized pattern of the high ban= d, bow tie antenna elements. --DDK

CINCARDF writes:

I'm with DD. Plus, I= differ with your statement, "The station's . . . operators work in an operati= ons building in the center of the array." That would be a pretty small operation, given that building's size. Through the miracle of cable, they worked outside the Elephant Cage in much larger facilities befitting the AN/FLR-9's cost and capabilities.

[And, since you incl= ude anecdotal evidence, you might like to know that someone in Japan inappropriately said the FLR-9s were "Missile Catchers," which ca= used quite a stir in the Misawa area for a time. Who wants to be an intentional Ground Zero?]

Back to the purely f= actual side, a cross reference to the Soviet Krug system might be appropriate (plus maybe Fix-Four and Fix-Eight). Plus, I am glad you said "nominal range" given troposcatter, thermal inversions, and just plain bad radio weather: I'd put it at 20-5500+ km.

In 1964 the San Vito base and the base at Chicksands w= ere 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.

Bob Ney writes (05/2= 006):

I read with great in= terest the information about the FaiLuRe 9 as it was sometimes derogatorily known.= I was one of the two senior engineers who oversaw the installation of the equipment in the Central Building back in 1962/1963. Actually, the system w= as up and running by the middle of 1963. I personally phased and cut in the fi= rst six operational beams on the "A" band at San Vito so that we could demonstrate contractual compliance before a deadline for operation in either late February or early March of 1963. DD Kavannaugh was the lead senior tec= h at Brindisi and I still have a photograph of the group from PSO/ESD that was t= here for the installation.

You do not mention t= hat the FLR-9 design was based on the Wollen-Weber antenna used by the German Navy = to allow Admiral Donnitz to communicate with his submarines during WW II. I did see the reference at the bottom of the page. I believe that antenna was not= a full 360 degrees as the FLR-9s were.

I was also involved = in the installation in England at Chicksands.

There is an interest= ing little anecdote having to do with the vertical antennas. From a distance, t= hey resembled the old Redstone missiles and someone in the British Communist Pa= rty started the rumour that this was a "Missile Base" that was being built on British soil. A protest march was being organized and the American military was just waiting to embarass the organizers when somebody, probably KGB, informed them of the true use and the rally was called off.=

Also, in Italy, some= one goofed in the design of the facilities around the central building and buri= ed the two septic tanks on top of a quiescent artesian spring. Everything was = good until the rainy season arrived. I remember looking out and seeing the concr= ete covers floating on top of the upwelling water. The Air Force then contacted= the local "Honey Wagon" to come out and pump the tanks and he got to = them all right. However, when he tried to leave, he was much heavier and with the mud, he bogged down and after about 30 metres of travel, he couldn't move a= ny farther. Well, the Corps of Engineers went to get their D-8 Cat, but it wouldn't start. Then they got a tank recovery vehicle, but it was too tall = to get through the gate in the HF array. The FordAll tractor just spun his whe= els, so, the honey wagon guy sort of shrugged his shoulders in a typically Itali= an way and opened the valves on the truck. Since this was just a few metres fr= om the front door and upwind at that, getting to the central building was a 'hold-your-breath-and-run' event for about a week. I wanted to plant waterm= elon seeds but they wouldn't let me.

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Maintained by Steven Aftergood
Created by John Pike
Updated October 16, 2006