Researchers at the National Institute of Standards and Technology (NIST), together with Wavsens LLC, have developed a method for using radio signals to create real-time images and videos of hidden and moving objects. One application could be to help firefighters find escape routes for victims inside buildings filled with fire and smoke. The technique could also help track hypersonic objects such as missiles and space debris.
The new method could provide critical information to help reduce deaths and injuries. “Our system allows real-time imaging around corners and through walls, and tracking of fast-moving objects such as millimetre-sized space debris flying at 10 kilometres per second, all from standoff distances,” said physicist Fabio da Silva, who led the development of the system while working at NIST. “Because we use radio signals, they go through almost everything, like concrete, drywall, wood and glass,” da Silva added. “It’s pretty cool, because not only can we look behind walls, but it takes only a few microseconds of data to make an image frame. The sampling happens at the speed of light; as fast as physically possible.”
The NIST imaging method is a variation on radar, which sends an electromagnetic pulse, waits for the reflections, and measures the round-trip time to determine distance to a target. Multisite radar usually has one transmitter and several receivers that receive echoes and triangulate them to locate an object.
“We exploited the multisite radar concept but in our case use lots of transmitters and one receiver,” da Silva said. “That way, we are able to locate and image anything that reflects anywhere in space.”
The NIST team demonstrated the technique in an anechoic (non-echoing) chamber, making images of a 3D scene involving a person moving behind drywall. The transmitter power was equivalent to 12 cell phones sending signals simultaneously to create images of the target. Da Silva said the current system has a potential range of up to several kilometres. With some improvements, the range could be much further, limited only by transmitter power and receiver sensitivity, he said.
The transmitting antennas operated at frequencies from 200 MHz to 10 GHz, roughly the upper half of the radio spectrum, which includes microwaves. The receiver consisted of two antennas connected to a signal digitiser. The digitised data were transferred to a laptop computer and uploaded to the graphics processing unit to reconstruct the images.
The NIST team used the method to reconstruct a scene with 1,5 billion samples per second, a corresponding image frame rate of 366 fps. With 12 antennas, the NIST system generated 4096-pixel images, with a resolution of about 10 centimetres across a 10-metre scene. This image resolution can be useful when sensitivity or privacy is a concern. However, the resolution could be improved by upgrading the system using existing technology.
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