In the 1970s, during the Cold War, if you wanted to see if someone was launching an
intercontinental ballistic missile at your country, you had two options: put a satellite
in space -- which at that point was expensive, impractical, and likely to fail -- or build
a radar system.
But radar can only see to the horizon. It'll show you a missile in flight, but by the time
you've seen it your chain of command might not have time to react. You'd need to know
it's launched, when it's launched. You need radar powerful enough to bounce off the ionosphere,
spread out for thousands of miles, and still have enough signal to be detectable after
it bounces back. You need megawatts of power and one of the biggest radar arrays ever constructed.
You need something like this.
Welcome to the Duga-3 Array, in Chernobyl.
This was known in the west as the Russian Woodpecker. For more than a decade, it randomly
hopped shortwave frequencies, trying to find the best one to get a return signal, sending
out a repetitive rat-t-t-t-t-t-t that sounded like a woodpecker. It was so powerful that
countries around the world filed official complaints with the Soviet Union, and there
was a small industry of "Woodpecker filters" or "Moscow mufflers" that would notch it out
on your radio.
There's another one about 60km away, they act as a pair: one transmits, one receives.
And it needs to be this big to get any sort of resolution, so you can tell roughly -- very
roughly -- where the missile's headed. But the really clever thing is how any radar system
like this can tell the difference between a missile and the ground. Because this isn't
like pointing a radar into the air: if you're bouncing the signal, then the ground is going
to reflect back just as much as the missile that's flying above it. How do you tell the
difference?
Doppler effect. The same way that the siren on a police car moving towards you sounds
higher, the radar reflections from a missile will be at a higher frequency. And missiles
move at speeds measured in miles per second. So Duga didn't listen for the same frequency
that its friend transmitted. It listened for a slightly higher one. If it found that return
frequency, then -- well. It didn't happen. Fortunately, all that transmission power,
all that disruption, was for nothing. Wonderfully. Because no-one ever launched, and somehow,
despite everything, humanity got through the Cold War.
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