Researchers at the University of Utah SPAN lab have found a way to see through walls to detect movement inside a building using wireless sensor networks!
The surveillance technique is called variance-based radio tomographic imaging and works by visualizing variations in radio waves as they travel to nodes in a wireless network. A person moving inside a building will cause the waves to vary in that location, the researchers found, allowing an observer to map their position.
The researchers, electrical engineering graduate student Joey Wilson and his faculty advisor Neil Patwari, have tested the technique with a 34-node wireless network using the IEEE 802.15.4 wireless protocol and TelosB hardware. By “interrogating” the space with signals and multiple receivers, the researchers found they were able to read the waves to detect the location of a moving object within a meter of accuracy.
In a mission-critical application, the researchers envisioned a building imaging scenario similar to the following: Emergency responders, military forces, or police arrive at a scene where entry into a building is potentially dangerous. They deploy radio sensors around (and potentially on top of) the building area, either by throwing or launching them, or dropping them while moving around the building. The nodes immediately form a network and self-localize, perhaps using information about the size and shape of the building from a database (e.g., Google maps) and some known-location coordinates (e.g., using GPS). Then, nodes begin to transmit, making signal strength measurements on links which cross the building or area of interest. The RSS measurements of each link are transmitted back to a base station and used to estimate the positions of moving people and objects within the building.
Locating interior movement from outside of a building is extremely valuable in emergency situations, enabling police, military forces, and rescue teams to safely locate people prior to entering. Variance-based radio tomography is a powerful new method for through-wall imaging that can be used to track the coordinates of moving objects. The cost of VRTI hardware is very low in comparison to existing through-wall imaging systems, and a single network is capable of tracking large areas. These features may enable many new applications that are otherwise impractical.
