Seeing through walls, a capability available to law enforcement and military authorities for several years, could become a bit more predictable in the future thanks to a technique developed by researchers at Duke University.
In a paper published on Thursday in the journal Optica, Duke professors Daniel Marks and David R. Smith, and postdoctoral researcher Okan Yurduseven describe a method for through-wall imaging (TWI) that compensates for the varied distortion produced by different wall materials, to allow details to be captured more accurately.
"[I]n many practical situations, both the object to be imaged and the structure of the intervening medium are unknown," the paper explains. "Unfortunately, the medium then adds confounding variables so that the scattered field sampled by the sensor may no longer be sufficient to determine both the medium and object structures."
The paper describes a variety of ways to capture images through walls using microwave radiation. These techniques have already made their way into commercial products like L-3 CyTerra's RANGE-R and Camero-Tech's XAVER line of handheld through-wall radar units. The devices start at about $6,000.
TWI tech has been receiving government funding for more than a decade. DARPA debuted a handheld through-wall device called Radar Scope in 2006, to help US troops in Iraq. In 2012, according to USA Today, at least 50 law enforcement agencies in the US had such technology. CyTerra did not immediately respond to a request to provide more current figures.
A 2001 Supreme Court case, Kyllo vs. The United States, found that thermal imaging through walls without a warrant violated the Fourth Amendment. It's likely that radar-based imaging would be similarly constrained.
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More recently, radio-frequency imaging has become so commoditized that it's being contemplated for IoT and smart home applications by a startup called Vayyar.
In developing their wall-penetration technique, Marks, Smith, and Yurduseven found wall materials, while varied, all tend to have a high degree of symmetry and to produce consistent distortions in penetrating microwaves, unlike the objects in the scanned room.
So they were able to create an algorithm using blind deconvolution – a deblurring technique – that separates data showing consistency from more irregular data, with this latter data set representing objects beyond the scanned wall.
The boffins' approach also differs from most TWI radars in that it doesn't use temporal ranging to differentiate between the wave scattering produced by an object and a wall. A typical TWI system, to separate objects 10 mm apart, the paper explains, would need a bandwidth of 10 GHz, so might operate across a range of radio spectrum frequencies from 15 to 25 GHz.
Radar transceivers capable of handling those frequencies turn out to be fairly costly and to be difficult to get approval for from communications regulatory agencies, according to the researchers.
The Duke boffins propose using a narrow bandwidth – between 24 and 24.5 GHz – that has already been approved for industrial, scientific, and medical devices. They conclude their technique holds promise for developing imaging systems that would be useful in a variety of disciplines, such as construction and seismology.
The work was funded by the Air Force Office of Scientific Research. ®