The 'x-ray' technique is based on the fact that the strength of a wireless network's signal is dependent on what lies between the transmitter and receiver. By monitoring variations in strength, a receiver can be made to keep track of what is moving between it and the transmitter.

The researchers used a wireless network of 34 nodes to create a matrix of signals and monitor their strength. The pair said that, when set up outside a standard living room, they were able to locate moving objects inside to within a metre.

The pair envisage a scenario where emergency responders could blanket the outside of the building with such wireless signal sensors, which would then 'self localize', understanding where they are, perhaps using built-in GPS systems. They could then beam back wireless signal strength information to a base station which could estimate the positions of moving people and objects inside the building.

"They deploy radio sensors around (and potentially on top off) the building area, either by throwing or launching them, or dropping them while moving around the building", the researchers explained.

Reports in MIT's Technology Review suggest that the researchers have tested the idea using the same wireless radio protocol employed by home automation sensor technologies like Zigbee.