Peabody and Charvat came up with the idea of amplifying the faint signal that does make it through, and subtracting out the signal produced by a concrete wall (above).
The first problem wasn't that hard, as signal amplifiers are easy to make. Teasing out an image from the reflected radiation was the challenge. Longer wavelengths pass through solids more easily (this is one reason your radio works inside your house). But the longer the wavelength, the bigger your antenna has to be and the slower the data is transmitted.
The researchers chose the wavelength they did because it is small enough that the radar apparatus can be about eight feet long. Then they looked at how to filter the signal and see what is behind the wall. When a radio signal passes through a wall, hits something and bounces back, it's wavelength changes. Using an analog crystal filter, they were able to build a receiver that will only pick up signals of the altered wavelength. That effectively deletes the wall from the image.
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Right now their device shows people as blobs beyond the wall, but they hope to improve it. As it is, they're able to take video at about 11 frames per second.
There are some limitations: the way the device takes images is by subtracting certain signals and comparing it to a previous image, so anything that isn't moving will not show up. That means a person would look like an inanimate object, provided they stay still.
But even in its raw state, it can still help first responders look through walls to see if anyone is in a building -– or aid soldiers in urban combat zones. It was the latter that Peabody and Charvat had in mind when they designed it. To that end they are working on improving the image quality to make the system more user-friendly.
Via MIT News
Images: David Waldorf/Getty Images and MIT Lincoln Lab