During the 1970s and 80s Russia deployed their Over-the-Horizon Radar (OTHR) which ultimately became known as the “Russian Woodpecker” to HAM Radio enthusiasts worldwide. Why that moniker? Well the signal emitted a sound pattern of its transmitted pulses at HF frequencies that was heard on worldwide shortwave radios to the annoyance of HAM operators. You can hear the sound of the Woodpecker signal on YouTube here.
Shown here is the Duga-1 RADAR array within the Chernobyl Exclusion Zone. On the right side can be seen an array of pairs of cylindrical/conical cages; these are the driven elements, fed at the facing points with a form of ladder line suspended from stand-off platforms at the top right of the image. A backplane axel reflector of small wires is seen left of center, most clearly at the bottom of the image. (By Necator – the English language Wikipedia (log), Public Domain)
This RADAR system was an early warning anti-ballistic missile radar system that was put into operation in late 1971—-based in Chernobyl. This RADAR’s pulses were hopping between four broadcast time windows at four distinct frequencies: 16450, 16490, 16570 and 16390 kHz—one for each window.
In the “static” mode (Static information does not require a high refresh rate; that is Name, Call sign, Tonnage, Destination, ETA do not change and are forwarded), each pulse had a bandwidth of 40 kHz, and a length of 3 to 6 ms. The signal was typically broadcast for 10, 16 and 20 times per second and then followed by a 72 ms silent period.
In the “dynamic” mode (Dynamic information, is position, course, speed, heading, ROT, etc. is changing constantly), four frequencies that could have also been used on other areas of the HF band (8070, 8230, 8310 and 8260 kHz, for example) were all transmitted sequentially in each and every time window in intervals of 6 ms.
In the highly used 10 Hz mode, where the four frequencies were transmitted every 100 ms, the duration of each distinct frequency’s transmission window was 7 ms, thus resulting in a 27 ms continuous broadcast followed by 72 ms of silence, all adding up to 100 ms. There were a number of transmission hopping and timing techniques that were used by the Soviets to avoid jamming of their transmit frequencies as well as to prevent spoofing of the received echoes. It was the power and range of this system that was a ‘thorn in the side’ to all shortwave listeners and amateur and commercial HF operators worldwide.
See my original article on this subject in the Spectrum Monitor.
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