New Microwave Imaging Technology Can “See” Through Walls


We’ve had radar for almost a century. A subtle piece of mankind’s technological toolbox, radar is elegant and simple in concept: They send out radio waves, record their reflections with a receiver, and examine the waves to see where and how far something is.

Today designers are able to create sophisticated radar systems with multiple receivers. When a single device, such as your cell phone, pings multiple receivers, it can be triangulated with incredible precision.

Now researchers at the US National Institute of Standards and Technology (NIST) have developed a new radar scanning system that uses the opposite configuration, with one receiver and many transmitters. This new radio can take pictures and videos in real time, even when objects are hidden behind walls or moving at hypersonic speeds.

The technology could potentially enable first responders to more easily find people in smoke-filled burning buildings or track accelerating debris in space. The explorers reported their results in the Nature communication.

NIST’s system is based on microwaves – the waves themselves, not the device – although microwaves jump around to heat your food. Microwaves are the smallest inhabitants of the radio wave family. Their wavelengths can range from the size of a school ruler to the thickness of a dime. They are already used in the radar; the devices that track the speed of a car on the road rely on microwaves, for example.

If you shorten the microwave wavelengths much further – ten thousand times from the shortest microwavesYou get visible light. All of these are in the electromagnetic spectrum, and apart from the wavelengths, there is basically no difference between them.

Although invisible to the human eye, microwaves can be used to create real-time images. CDC

But microwaves have certain advantages over light when it comes to creating images. “Obviously, I can’t see through these walls because the wavelengths to which the human eye is sensitive don’t penetrate such objects very well,” says Fabio da Silva, one of the researchers who led the effort at NIST. “However, if you go to longer wavelengths, like the microwave range … you should be able to ‘see’ through the walls.”

That’s why your router can be reached anywhere in your house or your cell phone intercepts a signal underground – they’re connected to the internet via microwaves that can penetrate walls and floors. This also means that the researchers’ new system can penetrate even relatively dense materials such as drywall and concrete. It is also not hindered by bad weather like thick clouds and rain.

[Related: China’s version of GPS is almost complete. Here’s what that means.]

However, traditional radar is not very good at quickly producing detailed images. “To get a picture, [radar systems] usually have to scan their lighting patterns, which usually slows down the imaging process, ”says Mohammadreza Imani, a researcher who studied microwave imaging, and a professor at Arizona State University who was not involved in NIST research.

The NIST researchers wanted to develop a system that could overcome this limitation. For inspiration, the NIST team looked for new equipment so-called “single pixel cameras” these are essentially cameras without lenses. They emit light and use ultra-fast and highly sensitive detectors to measure how long it takes for the light to return. If you can get a system that processes the data quickly, you can create images quickly and efficiently.

Another important part of the researchers’ system is based on what is known as “light in flight”, which works with the way waves bounce off things in order to reach their recipients. “They use the idea that light is reflected by other objects and these objects then illuminate other objects through multiple reflections,” says da Silva.

Aside from imaging, light in flight is often used by 3D rendering software to create more realistic images. The researchers at NIST have adapted it to their microwaves.

It was a major hurdle for da Silva and his colleagues to incorporate all of these concepts into a usable scanning system. So they turned to GPS, which locates you by finding your location relative to multiple satellites. Da Silva and his colleagues used multiple microwave sources and a single receiver and relied on computer algorithms to link everything together.

The result is a system that can scan an area the size of an above-average apartment and then find objects up to 1 millimeter – all within microseconds. The researchers also hope the system can be used for applications similar to the original purpose of radar: tracking objects that are supersonic and move at more than five times the speed of sound. Examples are explosions or space debris, the latter becomes a bigger threat as the earth’s orbit becomes more and more cluttered with man-made garbage.

[Related: The sky is full of potential UFOs—here’s why]

“The system discussed in this article not only provides rapid imaging over a large scene, but can also distinguish between first and second scatter events,” says Imani. In other words, it can detect not only an impact, but the impact after an impact. “You can use the second-scattering events to depict obstacles through and around them.”

The team’s results are based on laboratory demonstrations at NIST. Since then, da Silva has left NIST for a startup. Wavsenswhere he hopes to keep working on the system and eventually make it something that can be brought to market and used more widely.

“We assume that something like a prototype test will be carried out in the next few months,” he says. “Then we can probably have something in the next year that can either be used or tested in real applications.”

Aside from this system, other researchers want to develop microwave security scanners that are faster, less intrusive, and much less bulky than the giant pneumatic mailboxes you see at airports. Microwave imagers are also the backbone of self-driving cars. And small microwave cameras, Imani says, could even find their way into your phone.

“It can also be used just for fun, for example to inspect [the] Adjusting a shoe without harmful effects, ”says Imani – something like that old x-ray shoe fittersbut without unnecessary radiation.

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