Radio waves for crowd protection

The death toll in Jos, Nigeria after the most recent sui­cide bomb has climbed to 19. In our jaded world, that doesn’t seem so high. But nearly 13,000 indi­vid­uals died from sui­cide attacks between 2003 and 2010, and clearly that number con­tinues to rise.

Pro­fes­sors Carey Rap­pa­port and Jose Mar­tinez are using their skills in elec­tro­mag­netic radi­a­tion to put an end to that.

In a recent paper, the duo pre­sented a first-​​of-​​its-​​kind standoff detec­tion system for iden­ti­fying sui­cide bombers in a crowd. The imaging tech­nique uses mil­limeter (mm) wave radi­a­tion (the shortest wave­length in the radio wave fre­quency range) to dis­tin­guish between dif­ferent mate­rial types. The elec­tro­mag­netic wave, Mar­tinez explained, prop­a­gates at dif­ferent speeds when it travels through air, the human body, or an explo­sive material.

They can assign a char­ac­ter­istic speed to these dif­ferent mate­rials, thereby detecting not only the fact that an anom­alous object is present under a person’s clothing, but also what it’s made out of.

Rap­pa­port reminded me of the harsh reality that a sui­cide bomber would not be sat­is­fied merely with a bag of explo­sives taped to his/​her body. Nails and other metal objects, once shot through a crowd, could do much more damage at fur­ther dis­tances. So, being able to dis­tin­guish between mate­rial types could be quite valuable.

In the image above, the black line rep­re­sents the front of a human body and the gray cir­cles rep­re­sent images of metal rods taped to the stomach. The col­ored cir­cles are SAR (Syn­thetic Aper­ture Radar) images of wax bars, through which the mm wave prop­a­gates at a sim­ilar speed as through TNT. At first their loca­tion seem­ingly inside the body con­fused the team, until they real­ized what was hap­pening. It was their first clue about the speed differences.

Waves that must first pen­e­trate wax (or TNT) before reaching the body and ulti­mately the detector, slow down suf­fi­ciently so that com­pared to waves that did not pass through wax, they look far­ther removed from the surface.

In order to use this tech­nique in a crowd set­ting, the team devel­oped an array of mm wave radar devices and detec­tors that can be sta­tioned throughout an area one wishes to secure. As people pass by the radars, the array col­lects a series of data that, when taken together, can iden­tify a metal or explo­sive anomaly.