University College London researchers have developed a system that can sense people moving behind masonry walls 25 centimeters thick using only passive radiation. The system detects the Doppler shifts of ubiquitous Wi-Fi and mobile telephone signals, so there is nothing to betray the surveillance. The researchers say the method could be useful in situations ranging from hostage-takings to traffic control.
The system calculates the position of hidden targets by comparing the reference and surveillance signals and interpreting the very small frequency shifts, which reveals the location and motion of the subject. At present the device only produces a radar-style scatter plot, a flare of color along with a variety of signal data, instead of a photograph resembling the subject.
The team built the high Doppler resolution passive Wi-Fi radar on two multi-frequency, software-defined field programmable gate array-based transceivers. The system compares the reference and surveillance signals, interprets the very small frequency shifts, and exposes the hidden subject's location and motion.
From IEEE Spectrum
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