The Duga-3 was a Soviet-era long-range over-the-horizon anti-ballistic missile early warning radar. With a roughly 10 Hz. knocking tone (when AM demodulated), it was nicknamed the "Russian Woodpecker." The spectrum of its signal looked something like these three samples:
From left to right, the two pronounced peaks are (probably) ham SSB signals. The flat-topped broadband signal near the middle (green bar in the waterfall plots) is the one of interest. Finally, the strong peak on the right is (probably) a shortwave broadcast station.
For what it's worth, the Fourier transform of this boxcar frequency spectrum is a nice sharp pulse:
Nice, easy to handle. Gives good localization.
Wonder who's feeling the need to switch on this sort of early warning radar. We shall see.