The University of Utah has used a network of ZigBee transceivers to track people using radio tomography.
In a demonstration, 28 nodes were placed a round a 4m square at torso level.
"Radio signal strengths between all nodes were measured as a person walked in each area," said the University.
The experimental system has operated through walls, and from these early experiments researchers foresee police scattering nodes around buildings where hostages are held.
Fixed burglar alarms are another possibility, and as people cannot be identified, it could also monitor people flow in supermarkets.
"RF signals can travel through obstructions such as walls, trees and smoke, while optical and infrared imaging systems cannot," said the University. Even "where video cameras could work, privacy concerns may prevent their deployment. A system provides current images of the location of people and their movements, but cannot be used to identify a person."
Videos demonstrating use of radio tomographic imaging may be viewed online.
Radio tomography. People creates shadows in the radio links between transceiver nodes. University of Utah engineers also showed this method can see through walls.