WEBVTT
Kind: captions
Language: en

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In the 1970s, during the Cold War, if you
wanted to see if someone was launching an

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intercontinental ballistic missile at your
country, you had two options: put a satellite

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in space -- which at that point was expensive,
impractical, and likely to fail -- or build

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a radar system.

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But radar can only see to the horizon. It'll
show you a missile in flight, but by the time

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you've seen it your chain of command might
not have time to react. You'd need to know

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it's launched, when it's launched. You need
radar powerful enough to bounce off the ionosphere,

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spread out for thousands of miles, and still
have enough signal to be detectable after

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it bounces back. You need megawatts of power
and one of the biggest radar arrays ever constructed.

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You need something like this.

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Welcome to the Duga-3 Array, in Chernobyl.

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This was known in the west as the Russian
Woodpecker. For more than a decade, it randomly

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hopped shortwave frequencies, trying to find
the best one to get a return signal, sending

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out a repetitive rat-t-t-t-t-t-t that sounded
like a woodpecker. It was so powerful that

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countries around the world filed official
complaints with the Soviet Union, and there

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was a small industry of "Woodpecker filters"
or "Moscow mufflers" that would notch it out

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on your radio.

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There's another one about 60km away, they
act as a pair: one transmits, one receives.

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And it needs to be this big to get any sort
of resolution, so you can tell roughly -- very

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roughly -- where the missile's headed. But
the really clever thing is how any radar system

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like this can tell the difference between
a missile and the ground. Because this isn't

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like pointing a radar into the air: if you're
bouncing the signal, then the ground is going

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to reflect back just as much as the missile
that's flying above it. How do you tell the

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difference?

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Doppler effect. The same way that the siren
on a police car moving towards you sounds

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higher, the radar reflections from a missile
will be at a higher frequency. And missiles

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move at speeds measured in miles per second.
So Duga didn't listen for the same frequency

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that its friend transmitted. It listened for
a slightly higher one. If it found that return

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frequency, then -- well. It didn't happen.
Fortunately, all that transmission power,

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all that disruption, was for nothing. Wonderfully.
Because no-one ever launched, and somehow,

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despite everything, humanity got through the
Cold War.

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