Tomographic Motion Detection

Here’s an interesting take on the security alarm problem. Among other things, passive infrared (PIR) sensors are horribly vulnerable to getting any kind of dirt or crud caked on the lens: industrial dirt and dust is often enough to render them useless. One solution? Use radio.

By throwing up a lot of cheap transmitters and measuring the signal loss between every pair of transmitters, you can precisely locate radio-absorbing and radio-reflection objects.

Like, say, burglars.

A burglar entering a particular space slightly absorbs the radio signal whenever he stands between any two transmitters. So do all the other objects in the room, but it’s easy enough to say “this is the room normally” and just look for changes.

As it happens, since these are radio transmitters, you can hide them in walls and the like, so it’s never clear where they are, either.

Granted, if they ever develop a properly functioning large-scale metamaterial RF invisibility cloak (or maybe they already have, and not told anyone), these things are toast. Same goes for anyone managing to attack or backdoor the one receiver in charge of monitoring the network.

But against ordinary criminals, incorporated into a well designed defense-in-depth system where each layer is on its own enough to provide adequate security… this looks pretty clever.

Cringe-worthy but still amusing is the “Interactive Tracking Graveyard,” where they show off the system’s ability to locate and characterize movement in the space. Being able to locate motion precisely means you can automatically point a CCTV camera at it for a security guard to check out.

(Thirty years ago, this idea probably would have seen applications in automating the “death strips” on various totalitarian borders… which just goes to show you how security is a double-edged sword.)

“Radio Tomographic Imaging (RTI) is an emerging technology that locates moving objects in areas surrounded by simple and inexpensive radios. RTI is useful in emergencies, rescue operations, and security breaches, since the objects being tracked need not carry an electronic device. Tracking humans moving through a building, for example, could help firefighters save lives by locating victims quickly.

RTI works by placing many small and inexpensive radios around an area of interest. Each radio is capable of transmitting and receiving wireless signals, creating a dense network of “links” that pass through the area. Objects that move within the area reflect and/or absorb the wireless signal, preventing some of the power from reaching its destination. An image of where the power is being absorbed can be formed using all the link power loss measurements, thus allowing one to know where objects within the area are located. This research uses theory found in imaging, estimation/detection theory, inverse-problems, regularization, signal processing, communications, electromagnetics, and wireless networking.”

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