From Stealth Radar System Sees Through Trees, Walls — Undetected:
The radar scatters a very low-intensity signal across a wide range of frequencies, so a TV or radio tuned to any one frequency would interpret the radar signal as a very weak form of static.“It doesn’t interfere because it has a bandwidth that is thousands of times broader than the signals it might otherwise interfere with,” Walton said.
Like traditional radar, the “noise” radar detects objects by bouncing a radio signal off them and detecting the rebound. The hardware isn’t expensive, either; altogether, the components cost less than $100.
The difference is that the noise radar generates a signal that resembles random noise, and a computer calculates very small differences in the return signal. The calculations happen billions of times every second, and the pattern of the signal changes constantly. A receiver couldn’t detect the signal unless it knew exactly what random pattern to look for.
The radar can be tuned to penetrate solid walls — just like the waves that transmit radio and TV signals — so the military could spot enemy soldiers inside a building without the radar signal being detected, Walton said. Traffic police could measure vehicle speed without setting off drivers’ radar detectors. Autonomous vehicles could tell whether a bush conceals a more dangerous obstacle, like a tree stump or a gulley.