An experimental system can take ghostly snapshots by measuring the reflections of the same Wi-Fi signals used for connecting to the internet. It has been suggested as a way of tracking packages in warehouses or mapping collapsed buildings to safely and quickly reach victims.
Writing in the journal Physical Review Letters, researchers at the Technical University of Munich reported a way to create a type of three-dimensional image or hologram using wireless signals. The holograms are somewhat similar to sound recordings, which store the vibrations of violins strings or vocal cords.
“Using this technology, we can generate a three-dimensional image of the space around the Wi-Fi transmitter, as if our eyes could see microwave radiation,” said Friedmann Reinhard, a physicist who authored the report with his colleague Philip Holl, in a statement.
Their proof-of-concept experiment used an antenna sliding along a pole to measure from different angles Wi-Fi signals emanating from a router. Using specialized software to make sense of the measurements, the researchers created a holographic image of a cross, which they made out of aluminum foil.
The system is still in the early stages of development. But the researchers said later systems could have multiple antennas that take tons of snapshots, stitching them into a stop-motion animation of equipment moving around a factory or people walking through a home. It would result in “video-like image frequency,” Holl said.
Other researchers are also seeking ways to turn stray cellular and wireless signals into coarse images of people through buildings and walls. Dana Katabi, an electrical engineer at the Massachusetts Institute of Technology, and her colleagues have been tinkering with such technology for years.
In 2013, the researchers built a system that detects wireless reflections off the human body, spitting out a silhouette of a person standing behind a wall. Their device monitors how these wireless ripples change over time, using software to reconstruct the silhouette in shade of blue, yellow, and red.
“The data you get back from these reflections are very minimal,” said Katabi, whose research team has also devised wireless transmitters that clock walking speed, in a statement. “However, we can extract meaningful signals through a series of algorithms we developed that minimize the random noise produced by the reflections.”
