The FRD-10: An endangered species
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
- 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
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:
- transmissions could be recorded for immediate or subsequent DF-ing;
- bearings were four times as accurate;
- antenna gain was about four times higher than previous systems; and
- the system had the ability to select wanted signals and reject interfering signals or noise.
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. Most of the others have already 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 would remain as a residual 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 possibly with other arrays still operated by UKUSA allies.
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.
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.