Duga radar

The Soviets had been working on early-warning radar for their anti-ballistic missile systems through the 1960s, but most of these had been line-of-sight systems that were useful for rapid analysis and interception only. None of these systems had the capability to provide early warning of a launch, within seconds or minutes of a launch, which would give the defences time to study the attack and plan a response. At the time, the Soviet early warning satellite network was not well developed. An over-the-horizon radar sited in the USSR would help solve this problem, and work on such a system for this associated role started in the late 1960s.

The first experimental system, Duga, was built outside Mykolaiv in Ukraine, successfully detecting rocket launches from Baikonur Cosmodrome at 2,500 kilometres (1,600 mi). This was followed by the prototype Duga, built on the same site, which was able to track launches from the far east and submarines in the Pacific Ocean as the missiles flew towards Novaya Zemlya. Both of these radar systems were aimed east and were fairly low power, but with the concept proven, work began on an operational system. The new Duga-1 systems, built in 1972, used a transmitter and receiver separated by about 60 kilometres (37 mi).[4][5]

Duga-1 receiver

Duga-1 transmitter

Duga transmitter

Duga receiver

Locations of Duga and Duga-1 systems in Ukraine

At some point in 1976, a new and powerful radio signal was detected simultaneously worldwide, and quickly dubbed 'the Woodpecker' by amateur radio operators. Transmission power on some Woodpecker transmitters was estimated to be as high as 10 MW equivalent isotropically radiated power.[6][7] Even prior to 1976, a similar 'woodpecker' interference is remembered by radio amateurs occurring in the high frequencies. As early as 1963, or before, radio amateurs were calling this "the Russian Woodpecker".[8] Little is known about the power levels or Russian designation but it was probably a forerunner of the Duga radar systems. It was also speculated at that time, at least among radio amateurs, that this was an over-the-horizon radar.

These signals even caused interference on 27 MHz CB radios in the late 60s and early 70s sometimes completely blocking even local communications in Portugal for example, leading to the supposition of several Megawatts of RF power transmission.

Triangulation by amateur radio hobbyists and NATO showed the signals came from a location in present-day Ukraine, at the time called the Ukrainian Soviet Socialist Republic (part of the USSR). Confusion due to small differences in the reports being made from various sources led to the site being variously located near Kyiv, Minsk, Chernobyl, Gomel or Chernihiv. All of these reports were describing the same deployment, with the transmitter only a few kilometers southwest of Chernobyl (south of Minsk, northwest of Kyiv) and the receiver about 50 kilometres (31 mi) northeast of Chernobyl (just west of Chernihiv, south of Gomel). At one time there was speculation that several transmitters were in use.[6]

The radar system was given the code 5Н32-West by the Soviets, and was set up in two closed towns, Liubech-1 held the two transmitters and Chernobyl-2 the receivers.[7] Unknown to civilian observers at the time, NATO was aware of the new installation. A second installation was built near Komsomolsk-on-Amur, in Bolshya Kartel and Lian, but did not become active for some time.

The NATO reporting name for the Duga-1 is often quoted as STEEL YARD. Many online and several print references use this name. However some sources also use the term STEEL WORK (or STEEL WORKS). As any "official" sources using NATO Reporting Names are likely to be classified, establishing the true name will be difficult. The earliest found open source mention of a NATO Reporting Name for this system, a reference publication in print while the system was still active, unambiguously uses the term STEEL WORK.[9] Jane's Information Group is an often quoted open source reference for information across several military fields and subjects. Their "Jane's Radar and Electronic Warfare Systems", several editions over a number of years, uses the term "Steel Works".[10][11][12]

Even from the earliest reports it was suspected that the signals were tests of an over-the-horizon radar,[6] and this remained the most popular hypothesis during the Cold War. Several other theories were floated as well, including everything from jamming western broadcasts to submarine communications. The broadcast jamming theory was discarded early on when a monitoring survey showed that Radio Moscow and other pro-Soviet stations were just as badly affected by woodpecker interference as Western stations.

As more information about the signal became available, its purpose as a radar signal became increasingly obvious. In particular, its signal contained a clearly recognizable structure in each BPSK modulated pulse, which was eventually identified as a 31-bit pseudo-random binary sequence with properties much like Barker codes, with a bit-width of 100 μs resulting in a 3.1 ms pulse.[13] The auto-correlation of this pulse/sequence results in a single 100 μs pulse 31 times the amplitude of the received sequence, giving a resolution of 15 kilometres (9.3 mi) (the distance light travels in 50 μs) and process gain of almost 30 dB. This system took advantage of pulse compression to increase the power of the received echoes thereby increasing the sensitivity and effective range. When a second Woodpecker appeared, located in eastern Russia, but also pointed toward the US and covering blank spots in the first system's pattern, this conclusion became inescapable. These further installations allowed for more precise pin-pointing of potential targets or missiles in multiple dimensions thanks in part to the properties of the Barker codes that allow for multiple radars operating on the same frequencies without significant interference.

In 1988, the U.S. Federal Communications Commission (FCC) conducted a study on the Woodpecker signal. Data analysis showed a pulse repetition interval (PRI) of about 90 ms, a frequency range of 7 to 19 MHz, a bandwidth of 0.02 to 0.8 MHz, and typical transmission time of 7 minutes.

The pulses transmitted by the Woodpecker had a wide bandwidth, typically 40 kHz. Their repetition frequencies were 10 Hz, 16 Hz and 20 Hz with the most common frequency of 10 Hz, while the 16 Hz and 20 Hz modes were rarely used.

Steel structure of Duga-1 from below

To combat this interference, amateur radio operators attempted to jam the signal by transmitting synchronized unmodulated continuous wave signals at the same pulse rate as the offending signal. They formed a club called The Russian Woodpecker Hunting Club.[14]

Starting in the late 1980s, even as the FCC was publishing studies, the signals became less frequent, and in 1989, they disappeared. Although the reasons for the eventual shutdown of the Duga systems have not been made public, the changing strategic balance with the end of the Cold War in the late 1980s maybe had a part to play. Another factor perhaps was the success of the US-KS early-warning satellites, which began entering service in the early 1980s, and by this time had grown into a complete network. The satellite system provides immediate, direct and highly secure warnings, whereas any radar-based system is subject to jamming, and the effectiveness of OTH systems is also subject to atmospheric conditions.

According to some reports, the Komsomolsk-on-Amur installation in the Russian Far East was taken off combat alert duty in November 1989, and some of its equipment was subsequently scrapped. The original Duga-1 site lies within the 30 kilometres (19 mi) Zone of Alienation around the Chernobyl power plant. As of today, the radar appears permanently deactivated and will not likely receive future maintenance because such arrangements were not implemented within Russian and Ukrainian talks; with regards to the Dnepr early warning radar systems at Mukachevo and Sevastopol. Most of the antenna still stands and is often used by radio amateurs who visit the area using their own portable radio equipment.