Antiaircraft defenses now in use are cued by radars that detect and track incoming aircraft. But the radars are vulnerable: their signals can be jammed or missiles can be launched to ride back down the radar beams and destroy the transmitters. Instead of transmitting its own signals, China’s new Passive Coherent Location system tracks the signals of civilian radio and TV broadcasts and picks up aircraft by analyzing the minute turbulence their flight causes in the commercial wavelengths. Because PCL doesn’t transmit, its receivers can’t be detected and jammed or destroyed. In the United States, Lockheed Martin has developed a similar Silent Sentry system, which it is trying to sell for low-cost air defense or air-traffic control.
