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Wireless Network Modded To See Through Walls
KentuckyFC writes "The way radio signals vary in a wireless network can reveal the movement of people behind closed doors, say researchers who have developed a technique called variance-based radio tomographic imaging which processes wireless signals to peer through walls. They've tested the idea with a 34-node wireless network using the IEEE 802.15.4 wireless protocol (the personal area network protocol employed by home automation services such as ZigBee). The researchers say that such a network could be easily distributed by the police or military wanting to determine what's going on inside a building. But such a network, which uses cheap off-the-shelf components, might also be easily deployed by your neighbor or anybody else wanting to monitor movements in your home."
Aluminum oxide is a dielectric with breakdown around 16kV/mm and dielectric constant of 9. That puts the material in a class similar to glass. As such it would be among the most ineffective Faraday cages since the walls of such a cage must be conductive, and to be truly effective, VERY Conductive. In fact, at high RF ranges light weight cages and shielding have to be made of silver, gold, etc. to keep the skin effect thickness of the material down to manageable values. What is interesting about the aluminum oxide dielectric is its apparent very lossy nature to some RF frequencies, while being "transparent" to others. That is sort of similar to the behavior of pure water, which, if absolutely pure, is a dielectric, but as a polar dielectric it absorbs high frequencies in your microwave oven or in the atmosphere between me and my geosynchronous internet satellite.
The usefulness of aluminum oxide as a dielectric has been known a long time and electrolytic capacitors used as power supply filters, among other things, use its characteristics to make large capacitances in small volumes.
The image in the article isn't really good. If you want to see a demonstration of what they did in real time, it's here.
Check out their demonstration videos at http://span.ece.utah.edu/radio-tomographic-imaging.
I was fortunate enough to see the demo at Mobicom last year. It's a really neat application, even if the math is nothing new.
However, this isn't radar, it's tomography. Radio tomography. And the innovation isn't radio tomography, it's using stock WiFi hardware to do it, but I suspect you already knew that.
... wherein the Supreme Court (including Scalia, amazingly) held that peering into homes using equipment that was not available in common use by the layperson was within the bounds of the 4th amendment, and therefore requires a search warrant.