Duga (Russian: Дуга́, literally "arc" or "curve") was a Soviet over-the-horizon radar (OTH) system used as part of the Soviet missile defense early-warning radar network. The system operated from July 1976 to December 1989. Two operational Duga radars were deployed, one near Chernobyl and Chernihiv in the Ukrainian SSR (present-day Ukraine), the other in eastern Siberia. The Duga systems were extremely powerful, over 10 MW in some cases, and broadcast in the shortwave radio bands. They appeared without warning, sounding like a sharp, repetitive tapping noise at 10 Hz repetition rate, which led to it being nicknamed by shortwave listeners the Russian Woodpecker.
The random frequency hops disrupted legitimate broadcasts, amateur radio operations, oceanic commercial aviation communications, and utility transmissions, resulting in thousands of complaints by many countries worldwide. The signal became such a nuisance that some receivers used on amateur radio and television, such as the Datong Model SRB2 Auto Woodpecker Blanker, began including 'Woodpecker Blankers' in their circuit designs in an effort to filter out the interference.
The unclaimed signal was a source for much speculation, giving rise to theories such as Soviet mind control and weather control experiments. However, because of its distinctive transmission pattern, many experts and amateur radio hobbyists quickly realized it to be an over-the-horizon radar system. NATO military intelligence had already given it the reporting name STEEL WORK or STEEL YARD likely based on the massive size of the transmitter which spanned 700 m in length and 150 m (492 ft) in height. While the amateur radio community was well aware of the system, this theory was not publicly confirmed until after the fall of the Soviet Union.
The original Duga was supplanted by a pair of installations: western, Duga-1, and eastern, Duga-2. Duga-1 was built in northern Ukraine, between Liubech and Chernobyl. The receiver is located a few kilometers (miles) west-north-west of Chernobyl; the transmitter is located about 50 km (30 miles) northeast of Chernobyl. The site is open for pre-arranged visits, for which a permit must be obtained in advance; it has been open since October 2013.