Researchers at UC Santa Barbara were able to create three-dimensional images of the objects behind a brick wall in a series of experiments with the drones.
The two flying machines work in tandem. In the demonstration, they fly around a four-sided brick building. One drone transmits a continuous Wi-Fi signal, while the other, on the opposite side of the house, measures its power after it passes through.
By circumnavigating the house several times, the drones can generate high-resolution, accurate 3D images of the objects inside.
"Our proposed approach has enabled unmanned aerial vehicles to image details through walls in 3D with only Wi-Fi signals," said Yasamin Mostofi, a professor of electrical and computer engineering at UCSB. "This approach utilises only Wi-Fi RSSI [received signal strength indicator] measurements, does not require any prior measurements in the area of interest and does not need objects to move to be imaged."
The technique could have applications for search-and-rescue, archaeology or monitoring unstable buildings.
It builds on earlier work from the same lab, which pioneered using everyday radio signals such as Wi-Fi for imaging purposes, and had previously used ground-based robots for 3D imaging. The advantage of drones is that they can approach the area from several angles, and create optimised flight paths to collect the relevant data as quickly as possible.
In the past, Wi-Fi signals had been used to create 3D images, including in one recent paper from the Technical University of Munich. “Of course, this raises privacy questions,” said Friedemann Reinhard, the leader of that project. “After all, to a certain degree even encrypted signals transmit an image of their surroundings to the outside world. However, it is rather unlikely that this process will be used for the view into foreign bedrooms in the near future. For that, you would need to go around the building with a large antenna, which would hardly go unnoticed.”
Drones could change that, with covert surveillance another clear application of the technology. Researchers have previously developed methods of using Wi-Fi signals to detect the movement of people inside buildings. A technology called FreeSense, developed by Chinese researchers, can even identify individuals based on their body shape and the way they walk. Combined with the drones, this could be a powerful tool.
However, it could be a while before these devices are peeking into your homes. The real world is a much noisier place, with millions of signals bouncing around. “Enabling 3D through-wall imaging of real areas is considerably more challenging due to the considerable increase in the number of unknowns," said Mostofi.