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Earth's deterrence weapon — a network of buried antennae that, if fired, broadcast Trisolaris's coordinates (and unavoidably, the Sun's) to the dark forest.
The gravitational-wave broadcaster is humanity's instrument of mutually assured destruction. Fired, it announces Trisolaris's coordinates to the universe — and by simple geometry, also Earth's. Three antennae are built on Earth, one is integrated into the warship Gravity. The system is controlled exclusively by the Swordholder. The Trisolaran droplets destroy all three Earth antennae minutes after Cheng Xin's investiture; the Gravity antenna survives long enough for Chu Yan to use it once, beginning the Broadcast Era.
Each antenna is a buried cyclical structure roughly a kilometer across, sunk under steppe and tundra and Pacific seabed. From the surface there is nothing to see but a small access bunker and a perimeter fence. The output is a slow modulated gravitational-wave pulse that propagates outward at lightspeed, perfectly directional, and undetectable by Trisolaran sensors after the fact.