Soviet HF-DF

During the Cold War, the Soviet Union built an extensive network of large circularly disposed antenna arrays (CDAAs) for high-frequency radio direction-finding and monitoring. The largest of these CDAAs were known in the West as the Krugs. These huge arrays were laid out on a circular pad 200-300 metres in diameter and consisted of a large vertical screen about 105 metres in diameter surrounded by 40 large monopoles distributed around a circle about 120 metres in diameter. A building at the centre of the array housed the monitoring equipment and intercept operators (see diagram).

The United States also built an extensive network of large Wullenweber arrays: 14 huge FRD-10 CDAAs for the U.S. Navy and eight even larger FLR-9 CDAAs for the Army and Air Force. Canada also built two FRD-10s, one at Masset and one at Gander. With the end of the Cold War, however, most of these arrays were dismantled. Only two FLR-9s remain in service, and the two Canadian FRD-10s are now the only ones left in the world. (More on the history of the FRD-10 here.) The U.S., Canada, and the other UKUSA allies also built a number of smaller CDAAs known as Pushers, many of which are still in service.

Like its FRD-10 and FLR-9 counterparts, much of the Soviet (now Russian) Krug network has been dismantled. Desmond Ball (Soviet Signals Intelligence (SIGINT), Australian National University, Canberra, 1989) reported in 1989 that some 30 Krug systems were then operational in the Soviet Union. Declassified U.S. documents subsequently identified 31 current and former Krug sites, all of which have been located on Google Earth. About two thirds of the sites are clearly no longer operational, and it is quite possible that many of the sites that look like they may be intact (in imagery sometimes many years old) are also no longer in service.

Some sites, including the Gatchina Krug site (also known as Verolantsy) near Saint Petersburg (see photo), are apparently still in use, however. The current Russian Krug network could include 10 or more active sites.

The Soviets also built a network of smaller Fix-24 CDAAs similar to the Pusher arrays, many of which are also no longer in service.

The Soviet Union used its Krug network to monitor and determine the location of Western HF radio transmissions, especially aircraft and ship transmissions. CDAAs are capable of determining the direction of arrival of a radio signal with a high degree of precision, and by triangulating the bearings taken on the same transmission by several different Krug stations the Soviets could plot the location of the transmitter fairly accurately.

Due to their remote locations, however, many of the Krug stations had to report the bearings they took by radio, and Western SIGINT stations were apparently able to monitor and exploit these transmissions in turn. (Western SIGINT stations also monitored the reports made by Soviet air surveillance radar stations and thus were able to track Soviet aircraft flying in Soviet airspace. This was one of the reasons NORAD was routinely able to intercept Soviet aircraft on training missions before they entered North American radar coverage. But that's a story for another day.)

Jerry Proc's CFS Masset page recounts an example of Western monitoring of the Krug network that reportedly made the rounds in the 1970s:

A Soviet Air Force bomber traveled to the Abbotsford BC International Air Show and sent position reports in CW back to their home base as they flew across Canada. During this flight, the Russian KRUG network was using the aircraft transmissions for check bearings and was reporting the bearings in tenths of a degree. Our backplotting of these bearings is said to have confirmed their accuracy.

I'm not sure I would trust the veracity of a bearing supposedly taken on an aircraft that was reporting its actual position, but the story does appear to confirm Western monitoring of the Krug network.

[Update 25 May 2012: Updated the section on the number and location of Krug sites to reflect continuing research.]