Sunday, February 14, 2021

The appearance of the "Chinese Woodpecker" on the HF bands

Listening about on 40 meters this morning I heard a familiar sound - the "putt-putt-putt" of what sounded very much like the infamous "Russian Woodpecker" Over The Horizon Radar (OTHR) of the 1970s and 1980s - even having the same 10 Hz repetition rate that was common for the woodpecker.

Just by looking at the waterfall display, I could see that this signal was quite different:  Rather than taking 100s of kHz of bandwidth, this signal seemed to be fairly well contained within a bandwidth of a few 10s of kHz implying techniques unlike those of the signal from the 70s.

In other words:  It sounded like the bad old Russian Woodpecker, but it clearly was not.

Pulse versus chirp:

In the "old" days, a lot of RADAR systems simply blasted out a pulse of RF energy and then listened for the echo.  "Because physics", there is a 4th power distance relationship of reflected RF (e.g. doubling the distance causes a signal to decrease by a factor of 16) so tremendous radiated power levels were required to receive enough energy from the return pulse - which had to be intercepted over a wide bandwidth to get precise timing - from the object off which it had bounced.  Distance may be ascertained by timing the delay between the transmit and return pulse, often integrating this information over many pulses.

Audio clip of possible Chinese OTHR as heard on the KFS WebSDR system on
40 meters.  Significant backscatter is apparent in this recording, but the signal was
very much stronger and "cleaner" on remote receivers in Asia.
(Another type of signal - the "buzz-buzz" - was heard mid-recording and is not related.)

Many modern RADAR systems transmit a CW (continuous) signal that, instead of being pulsed, is swept in frequency.  Rather than relying solely on the time between the transmit and return pulse, one can measure the difference in frequency between the transmit signal and received (reflected) signal because the transmitter will have shifted frequency by the time the reflected signal arrives, and the greater the round-trip distance, the greater the frequency difference.  In other words, instead of timing the pulse directly - because there isn't one - the frequency difference, using a receiver that has a local oscillator that effectively tracks the transmitter's frequency, is what indicates distance.

Because the latter case uses a CW signal and a tracking receiver, one may use narrowband techniques (anything from a simple, narrow filter or an FFT with multiple "narrow" bins) on the receive end, potentially obtaining 10s of dB of processing gain.  In other words, to obtain the same Direction, Range and Distance information, far lower power may be required than with the old-fashioned pulse-type RADAR for comparable results.

It is likely because this "new" OTHR radar is chirp - that is, a swept-frequency transmitter - that its spectrum is far-better contained than the "bad old" Russian Woodpecker, and it's likely that the effective radiated power - while still quite high - is far lower.

A bit of sleuthing:

In the past, amateur radio operators could ascertain the location of the Russian Woodpecker only by obtaining antenna headings from multiple stations around the world and then compiling the data to determine a likely location of the transmitter.  These days, we have other methods at our disposal - and as readers of this blog will be aware, one of these is the KiwiSDR "TDOA" network.

In short, the TDOA network consists of a number of participating KiwiSDR receivers around the world that, when commanded to do so via the software extension's control panel, will record a GPS time-stamped audio file from the selected receivers and send them to a server that will analyze this data and determine the apparent location of the received signal.

This, I have done several times over the past several weeks, and today I had the opportunity to do it several times more - the results appearing in the maps below:

KiwiSDR TDOA results from the "Woodpecker" signal heard on 40 meters from remote receivers.

The above maps show the results of several TDOA sessions using several receivers scattered across the Pacific and Australasia - the lower image being derived from higher-quality data as propagation improved - and, possibly, as the transmitted beam was better-oriented in the direction of the receivers, reducing the appearance of backscatter.  Because these transmissions are (apparently) rapid frequency sweeps, they are nearly ideally suited for the type of analysis needed to determine the Time-Difference Of Arrival (TDOA) techniques employed - in other words, a RADAR in reverse.

Due to the vagaries of ionospheric propagation - and as should be apparent from the roughness of the numbers given - the absolute location of the transmitter is likely to be accurate within only about 100km at best for this sort of exercise - but something is clear:  It is probably not likely a "Russian Woodpecker", but more likely something akin to the " 啄木鸟   中国人 " - which Google tells me means something roughly akin to "Woodpecker from China".


There is a known Chinese OTHR transmit site near Nanjing (approx. 32.05°N, 118.78E) but that location does not correlate with the results above.  It's very possible that the TDOA error is quite large, but the coordinates on the maps shown above were similar for several runs:  More TODA runs, over time, may help to resolve this uncertainty.

"Will my radio's noise blanker help?"

Probably not!

Unlike the bad old Russian Woodpecker that consisted of narrow (and very broadband) pulses, this appears to be a swept carrier, meaning that unlike the old, Russian variant, a pulse-type noise blanker is unlikely to work well at all:  Rather than the pulse being "everywhere" within a few 10s (or 100s) of kHz of the desired receive signal simultaneously in the case of the Russian Woodpecker - and other impulse noises like vehicle ignition, electric fences and lightning - this signal is only on one frequency at any given instant and the wideband amplitude detector comprising a standard impulse-type noise blanker will likely be ineffective.  This "new" signal sounds like a pulse only because it spends a small amount of time in an SSB receiver's passband during each sweep.

In theory, it should be possible to design a software-based filter that will remove this signal as it is very repeatable, but it's likely that no software-defined receivers in common use at this time (e.g. as of the original posting of this article) will have anything at all that will touch it!

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This is not the first time that a Chinese OTHR has appeared on the ham bands:  For years, now, one can hear what sounds like a loud "buzz" that comes and goes as described on some of the links below.  The appearance of the "10 pps" version - possibly a refinement for longer-distance observation - seems to be comparatively new.

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Related links about the Chinese OTHR:

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This page stolen from


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