Thursday, June 02, 2005

The FRD-10: An endangered species

FRD-10 array

In the early 1960s the U.S. Naval Security Group began deploying a network of large high-frequency direction-finding (HF-DF) circularly disposed antenna arrays, the AN/FRD-10s, to detect, monitor, and plot the location of Soviet submarines and other radio emitters in the Atlantic and Pacific oceans.

Sometimes dubbed Elephant Cages or Dinosaur Cages, the FRD-10 arrays were enormous structures. In the centre of each array was a two-storey operations building, about 40 metres square, where the station personnel worked. Surrounding this building were two concentric rings of HF antennas, one for shorter HF wavelengths, containing 120 sleeve monopoles, and one for longer HF wavelengths, containing 40 folded dipoles. The shorter wavelength ring was about 260 metres in diameter and the longer wavelength ring was about 230 metres in diameter. Inside each ring there was also a large wire screen, supported by 80 towers, which was designed to prevent HF signals from crossing the array and interfering with its operations. The inner screen, corresponding to the longer HF wavelengths, was roughly 36 metres high. A horizontal ground screen about 390 metres in diameter surrounded the entire site. (Aerial views of an FRD-10 array here.)

Fourteen of the huge arrays were eventually deployed by the NSG (not counting two built at Sugar Grove, WV, for communications rather than intelligence-gathering):
  • Adak, Alaska
  • Edzell, Scotland
  • Galeta Island, Panama
  • Guam
  • Hanza, Okinawa
  • Homestead, Florida
  • Imperial Beach, California
  • Marietta, Washington
  • Northwest, Virginia
  • Rota, Spain
  • Sebana Seca, Puerto Rico
  • Skaggs Island, California
  • Wahiawa, Hawaii
  • Winter Harbor, Maine
Another two were built by the Canadian Forces Supplementary Radio System: one at Gander, Newfoundland and one at Masset, British Columbia (both built in 1970-71).

The FRD-10 arrays became the backbone of the BULLSEYE net, the Atlantic and Pacific HF-DF nets. They were supplemented by a number of smaller, simpler CDAAs known as Pushers, including a Canadian Pusher in Bermuda. (Canada also has Pushers deployed at Leitrim and Alert.)

The FRD-10s offered four major improvements over their predecessors, the GRD-6 in NSG service and the GRD-501 in Canadian service:
  1. transmissions could be recorded for immediate or subsequent DF-ing;
  2. bearings were four times as accurate;
  3. antenna gain was about four times higher than previous systems; and
  4. the system had the ability to select wanted signals and reject interfering signals or noise.
As noted in the Supplementary Radio Activities Consolidation Plan (30 May 1966), the improvement expected as a result of deploying the FRD-10s was "a combination of more accurate and reliable fixes, producing reduced search areas in ocean areas of prime responsibility so fresh in time as to enable maritime commanders to deploy their forces more economically and with much greater prospect of making contact with the target than is now the case."

The following maps show the locations of FRD-10 (black circles with pink dots) and Pusher arrays (empty black circles) during the system's heyday.

Click to see full-sized image

Click to see full-sized image

In the mid-1990s, however, the NSG began to close down its FRD-10 arrays. The demise of their Soviet targets, a desire to refocus collection efforts and cut costs, and, presumably, a decision to rely on alternative ocean surveillance technologies has led to the near-extinction of the FRD-10. Canada's two arrays are the only ones left in service. The others have since been dismantled. (Al Grobmeier has written more on the fate of the NSG FRD-10s.)

The Canadian FRD-10s themselves were converted in 1997 so they could be remotely operated from CFS Leitrim. Presumably they are primarily used for HF intercept operations now, although DF remains as a secondary capability. The Masset and Gander arrays no longer have other FRD-10s to work with, but they can still work with the Canadian Pusher arrays at Leitrim and Alert, and presumably with other HF-DF sites still operated by UKUSA allies and other partners. (This leaked NSA document (see p. 3) confirms that the agency continues to operate the "world-wide CROSSHAIR HFDF geolocation service", and that 2nd Parties and other countries participate.)

The first step in this direction may have been Project Polo (G1777), which was established in the late 1980s to "modernise the CFSRS High Frequency/Direction Finding (HF/DF) system at CFS Masset and Gander, and to equip CFS Alert for netted DF Operations."

Did Canada see the demise of the FRD-10 network coming? It seems unlikely that the authors of Challenge and Commitment, the 1987 Defence White Paper (précis: "The Soviet Threat will go on forever"), expected the UKUSA allies' main ocean surveillance networks to be shut down within a decade, so assuming the two sites remain useful and we're not just stuck with a couple of White Elephant Cages, maybe we got lucky.

Update 25 August 2005: Corrected to include the FRD-10 built at Marietta, Washington, and update info on the fate of the NSG FRD-10s. The FRD-10 at Marietta was dismantled in 1972, possibly as a result of the Masset array's entry into service. [A comment dated 12 September 2015 (see below) states that the Marietta array had continuing problems with inaccurate bearings due to the presence of nearby aluminum smelters, so it was probably pointless to keep the site in service once Masset was complete.]

Update 14 December 2007: As noted in the comments, the NSG detachments at US Army/Air Force FLR-9 CDAA sites also participated in the BULLSEYE net, as did some older NSG sites that continued operating the GRD-6 system (and older Canadian sites operating the GRD-501 system) for a number of years.

Update 24 June 2009: Information on the roughly equivalent Soviet Krug HF-DF network here.

Update 9 March 2015: Updated Pacific HF-DF map to include the Panama FRD-10, as suggested in comments.

Update 13 September 2015: Added some information on problems at Marietta, as suggested in comments, and updated a few other dated sections of the text.


Anonymous Anonymous said...

Check out cage at Elmendorf (Anchorage)

Not on your list

December 13, 2005 2:38 am  
Blogger Bill Robinson said...

Thanks for the feedback!

The Elmendorf "elephant cage" is an FLR-9, part of an Army/Air Force network that was separate from the FRD-10s of the BULLSEYE net. You can see the full list of FLR-9 sites here.

December 13, 2005 11:34 pm  
Anonymous Anonymous said...

Thanks for all the info.
My name is Peter Lewis. I was stationed at some of the sites. My father, Edward J. Lewis Jr. was involved in the location and construction of these sites. He was a CTCM and served 31 years in the Navy. He also taught code in Imperial Beach. It is gratifying to see what a challenge he had to face in the work he did.
Thanks again.
My email is:

April 04, 2006 9:33 am  
Anonymous Anonymous said...

You did not include some of the USAF sites. One of which was San Vito, IT. Also, Clark AFB, PI.

May 02, 2007 6:38 pm  
Anonymous Anonymous said...

In 1975 to 77 I was ststioned in my home town of Homestead, Florida. At that time I was amused that we had Canadian Forces stationed there also and they were on Foriegn Duty. Never thought the US was foriegn duty till then.

May 02, 2007 7:14 pm  
Anonymous Anonymous said...

I was an HDFD instructor and "taught" many a Canadian (or in some cases they taught me). Also served a stint at Gander (I miss the Screech!!). You guys were/are a bunch of pro's and I value my time served with you.

May 02, 2007 7:59 pm  
Blogger Bill Robinson said...

Hi, all. Thanks for the recent comments. This is not a high traffic blog, so the 400 or so visits in the last 24 hours equal about a month's worth of normal traffic around here. I'm assuming that somebody sent a notice about the FRD-10 posting to some massive NSG retirees list. I deliberately left the FLR-9 sites off my list, but I guess the NSG dets at those sites did participate in the BULLSEYE net so I really should mention them. Did all the FLR-9s participate?

May 03, 2007 5:10 pm  
Anonymous Anonymous said...

Check out Sidi Yahia, Morocco for an FRD-10 site.

May 04, 2007 11:05 am  
Anonymous Anonymous said...

I was stationed at the Hanza Okinawa Japan CDAA for about 3 years in the mid to late 60's. The physical facilities are still there, including the antennas. This CDAA facility is clearly visible using Google Earth images. I'm not sure what the date of the image is,but within the last few months the image resolution in the area was increased significantly. So if it has since been removed, it has been only in the last few months.

I'm ptetty sure that Naval Security Group is no longer using the site. for its originally intended purpose.

May 06, 2007 1:23 am  
Anonymous Anonymous said...

Interesting info on your web site. I worked in the FRD10 in the swamp at NSGA Homestead from 1981 to 1984. What a great mission and a great time to be in Florida (no hurricanes). It would be great to find some of the Canadians I worked with in Florida. Very sharp troops.

May 22, 2007 10:00 pm  
Blogger Bill Robinson said...

Hi, Steve.

Thanks for the comment. Check out Online Oldtimers and 291 Happenings for lots of contact information for current and former 291ers.

May 22, 2007 10:38 pm  
Anonymous Anonymous said...


The new HBO series, John from Cincinnati has the Imperial Beach station in the show. I had no idea what the character was looking at until an IMDB post directed me to your site. I'm now much more enlightened.


Kleinzeit & Archie

July 06, 2007 11:58 am  
Blogger bombadil said...

A few months ago I went to Google earth and looked up the old array at Clark AB in the Philippines and it appeared that someone had turned the thing into something that looks like a pavilion.

September 10, 2007 1:09 pm  
Anonymous Anonymous said...


You asked if all the FLR-9 sites participated in the network. I can tell you that Elmendorf did. There were also still some sites participating using the old GRD-6 arrays also (Iceland).

November 11, 2007 8:11 pm  
Anonymous Anonymous said...

Also, in the early 70's the Navy was deploying a flr-15, haven't heard much of what happened to them

March 30, 2008 4:43 pm  
Anonymous Anonymous said...

Just 'stumbled' on your blog...
'Velly Intellesting...'
I did about 60-75 TDY's to Gander from 1975 to 1999, plus visits to all the other CFSRS sites(less Alert!).
I retired from the 'business' in 2000.
Overall info in your Blog + comments about 90-95 % accurate, Well Done!

"Long may your big jib draw"

Dennis Skiffington

August 17, 2008 3:27 pm  
Anonymous Anonymous said...

My father helped invent and deploy the BULLSEYE network in the 1960s. As a former submariner turned writer, I'm writing a book about submarine programs that includes this technology. I'm interested in talking with anyone involved with these systems during the cold war. Please contact me at

November 07, 2008 1:33 pm  
Blogger Unknown said...

Thanks for web page, took me back to when I was in SecGru 61 - 69. Worked on FRD-10 on Guam, Edzell and communicated with Chicksands when working on Audley St. London(66-69). Hope maybe one day to see full storey about FRD's. Thanks again and Well Done.

Bob H.

September 30, 2009 11:12 am  
Anonymous Anonymous said...

As a ham radio operator I found your site about the HFDF antennas interesting. I also am interested in what is now being called COLD WAR relics!


January 23, 2010 3:46 pm  
Anonymous Anonymous said...

Has anyone noticed lately that there are a lot of vehicles parked around the ops building at the Imperial Beach FRD-10A? I'd swear the thing was in operation.

117° 7'48.16"W

March 10, 2011 10:24 pm  
Anonymous Mike P said...

I worked with the Pongo's at both CFS Massett and CFS Gander back in the 70s and early 80s. Good duty, boy, could the guys at Massett put away the Labatt's!

December 31, 2011 4:57 pm  
Anonymous said...

'Anonymous' mentioned Iceland; assume
he was referring to the Keflavik NSG
sites at Hafnir and 'Rockville'. The
Hafnir and mainbase (NSG comms relay)
sites were closed in 1978 and conso-
lidated into a new building at Rock-
ville with a 'Pusher'. That site has
been closed now for a number of years
unless USAF (or civilians) are using
it. 'Bob H' mentioned his time at
7 N Audlry in 66-68 time frame. Were you on 7th deck or at 2 PC? I
was there 1/65-12/67 at both loca-
tions (watched 'em build 2PC. Jim

August 22, 2012 3:19 am  
Anonymous Anonymous said...

Was stationed at NSGA Adak (was a plank owner) we always joked is that the FRD-10 facility was where they kept Godzilla caged.

September 23, 2014 12:39 pm  
Anonymous Anonymous said...

I notice that you only show the Panama FRD-10 as being in the Atlantic HFDF Net. I remember talking to a USN maintenance tech who was posted there, he said that the OPS was divided in half - one half was in the Atlantic HFDF net and the other half was in the Pacific HFDF net.

March 08, 2015 5:50 pm  
Blogger Bill Robinson said...

Thanks for the tip, anonymous of 8 March 2015! I've updated the map.

- Bill

March 09, 2015 2:43 pm  
Anonymous Anonymous said...

FYI. The Marietta, WA FRD-10 was shut down because of inaccurate bearings. This was caused by nearby aluminum smelt plants that affected the ground conductivity and pulled bearings off.

September 12, 2015 10:06 pm  
Blogger Bill Robinson said...

Thanks for the update about Marietta!

The decision to build the Canadian FRD-10s was made around 1966. Canada probably wanted to have a modern Pacific coast station to ensure its continued participation in the Pacific HFDF net, but the problems with the bearings at Marietta must have been apparent by that time as well, so it sounds like the Masset array was also intended to be a replacement for Marietta. Marietta's closure within about a year of Masset's completion is certainly consistent with that explanation.

- Bill

September 13, 2015 3:43 pm  
Anonymous Anonymous said...

I was navy(68-76) and stationed at Misawa and Clark AFB. We did indeed participate in the Bullseye program. So did San Vito Italy, Elmendorf and Karamisal Turkey. Kamisaya Japan was also a station but was moved to Misawa in late 70 or early 71, I believe. I was in Misawa before the move. Was transferred from Kamisaya to Misawa late 69 and left in Aug 70. Was also stationed at Imperial Beach CA and Edzell Scotland. Shared duty at Imperial Beach with some Canadians. They loved it there, no snow.

September 27, 2015 6:22 pm  
Blogger Unknown said...

I worked in the elephant cage in 70 and 71. We had the poor Marine guards convinced that there was a tunnel coming through the island from the Marianas trench and submarine would surface in our building in order to receive highly secret repairs and orders.

November 09, 2016 12:43 am  
Blogger Bill Robinson said...

@ Joseph Lawlor - That's hilarious!

- Bill

November 13, 2016 4:01 pm  
Blogger Unknown said...

Kamiseya sent their RS work to Guam in late 70, early 71. I was sent from Guam to Kamiseya TAD to be trained on RS intercept. I returned to Guam to find a blazing hot room in the bottom floor of the CDAA building (only one air conditioning vent installed) full of AN/R-390's and ready to go. tuned in Vladivostok and off we went.

April 07, 2017 7:59 pm  
Blogger tracker said...

I would like to know the source of "anonymous" claim that Marietta system was shut down due to inaccurate bearings.


May 18, 2019 8:03 pm  
Anonymous Anonymous said...

Quick question, any idea why Gander and Masset remain when all the other AN/FRD-10 have been disbanded.

November 05, 2019 4:18 pm  
Blogger Bill Robinson said...

I don't know for certain why Canada chose to continue operating the arrays at Gander and Masset. The two sites were (and are) important intercept locations for CSE, as well as DF sites, and we wouldn't have wanted to lose that intercept capability. My best guess is that it was as cheap or cheaper to keep them in service, especially once they were remoted to Leitrim, than to build entirely new facilities in the same or similar locations.

November 05, 2019 10:43 pm  
Anonymous Anonymous said...

I served USN from 1948 to 1967. Most of this time was in HFDf (Operator type). Served in both Pacicic and Atlantic Net Controls, Kami Seya, Sabana Seca, Imperial Beach, Northwest, Edzell
and Guam. Operated equipment from the DAB to the FRD-10. Was instrumental in converting Edzell from GRD-6 to FRD-10. It was a wonderful trip and I enjoyed it all. Been retired since 1967

January 23, 2020 3:51 pm  
Anonymous Anonymous said...

Did anyone find out if there were any active duty personnel who are now suffering from any diseases after working in one of these antennas radio frequency FRD-10? My dad was stationed in Sabina Seca and in Rota - 4 years at each duty station. He now has dementia and Parkinson's disease. Could this FRD be responsible? Any guidance would be most helpful. thank you

February 18, 2023 4:16 pm  

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