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After Newtown: a new use for a weapons-detecting radar?

02 April 2013

Polarimetric radar is being tested by researchers at the University of Michigan as a possible means of detecting concealed weapons among groups of people.

Professor Kamal Sarabandi shows the difference in the visual response from the radar of a subject with a gun and without a gun (photo: Marcin Szczepanski/Multimedia Producer, University of Michigan, College of Engineering)

Kamal Sarabandi, an electrical engineering professor at the University of Michigan, specialises in remote sensing techniques. Following the appalling tragedy of the Newtown Connecticut shootings, he thought he might be able to use his US Department of Defence funded work on millimetre-wave radar to find weapons and bombs concealed on a person's body.
The new technology isn't in the radar system itself, which, like all radar, senses objects by sending out radio waves and listening for the signals that bounce back. Sarabandi's particular millimeter wavelength is used today in collision avoidance systems in cars, in satellite communications and in military targeting and tracking, for example.
Sarabandi paired it with Doppler radar signal processing to pick out the signature of a person walking in the noisy radar scene. Then he uses a technique called radar polarimetry to concentrate on the signal coming from the pedestrian's torso and identify the telltale glare that a metal object hidden there would cause.
He hasn't tested the technology on people, or people with real guns. But he has done computer simulations. He has also conducted experiments in an anechoic chamber with a mannequin hiding a nail-studded block of wax under a leather jacket - the nail block representing a makeshift explosive.
Sarabandi uses the Doppler effect to identify which parts of the reflected signal come from, say, the limbs and which part from the torso of a walking human.
To do this, he first attached motion-capture electrodes on the limbs and torsos of several human subjects moving about the University of Michigan's 3D lab, using the recordings of their motions to make true-to-life animations.

Then by simulating their Doppler radar reflection over time, he identified a general pattern that a suitably programmed computer could recognise. In Sarabandi's words, it represents "the DNA of walking". To confirm that it was specific to humans, he compared it with the signal from a walking dog.
The research subject - a mannequin - was coated with a material that reflects radar in a similar way to human skin, and set up on a turntable in the anechoic chamber. Metal objects concealed within the torso of the mannequin, such as a gun, will change the polarisation of a scanning radar beam. When the polarisation of the returning signal is different to that sent (a metal object will change the polarisation of the radar signal), then the assumption is that the figure is concealing a metal object, possibly a weapon.
Sarabandi's idea would be to scan a large group of people from a distance. Then security guards could watch flagged individuals more closely or pull them aside for scans with more sensitive devices.
Today Sarabandi and colleagues are working on a smaller system for a robot, and they're funded by the US Army. Developing and deploying a system for the homefront would take a lot more research, and, likely, public discussion. But it's one idea as the nation searches for solutions.

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