‘X-ray vision’ technology could soon see through rubble and help rescue disaster victims


Seconds after reaching a city, earthquakes can cause immense destruction: houses collapse, skyscrapers turn to rubble, people and animals are buried in debris.

Immediately after such carnage, emergency personnel desperately search for any signs of life in what was once a home or office. Often, however, they find they were digging in the wrong pile of rubble, and precious time has passed.

Imagine if rescuers could see through the debris to spot survivors under the rubble, measure their vital signs and even generate images of the victims. This is quickly becoming possible with sight-through-the-wall radar technology. Early versions of the technology that indicates whether a person is in a room have been in use for several years, and some can measure vital signs, albeit under better conditions than through rubble.

I am a Electrical Engineer who studies electromagnetic communication and imaging systems. Me and others use fast computers, new algorithms, and radar transceivers that collect large amounts of data for allow something much closer to x-ray vision science fiction and comic books. This emerging technology will help determine how many occupants are behind a wall or barrier, where they are, what items they might be carrying and, in law enforcement or military applications, even what type of body armor they might be wearing.

These transparent radars will also be able to track the movements of individuals, and heart and respiratory rate. The technology could also be used to remotely determine the entire layout of a building, down to the location of pipes and wires in walls, and detect hidden weapons and traps.

Wall transparency technology has been in development since the Cold War to replace drill holes through walls for espionage. There are some commercial products in the market today, like Tidy radar, which are used by law enforcement officers to track movements behind the walls.

How radar works

Radar stands for radio detection and telemetry. Using radio waves, a radar sends a signal that travels at the speed of light. If the signal hits an object such as an airplane, for example, it is reflected back to a receiver and an echo is seen on the radar screen after a certain delay. This echo can then be used to estimate the location of the object.

In 1842, Christian doppler, an Austrian physicist, described a phenomenon now known as Doppler effect or Doppler shift, where the change in frequency of a signal is related to the speed and direction of the signal source. In the original case of Doppler, it was the light of a binary star system. This is similar to changing the height of a siren when an emergency vehicle rushes towards you, overtakes you and then drives away. Doppler radar uses this effect to compare the frequencies of transmitted and reflected signals to determine the direction and speed of moving objects, such as thunderstorms and high-speed cars.

The Doppler effect can be used to detect tiny movements, including heartbeats and chest movement associated with breathing. In these examples, the Doppler radar sends a signal to a human body and the reflected signal differs depending on whether the person is breathing in or out, or even depending on the person’s heart rate. This allows the technology to accurately measure these vital signs.

How radar can pass through walls

Like cell phones, radars use electromagnetic waves. When a wave hits solid walls like drywall or wooden walls, a fraction of it is reflected off the surface. But the rest go through the wall, especially at relatively low radio frequencies. The transmitted wave can be fully reflected if it hits a metallic object or even a human, because the high water content of the human body makes it highly reflective.

If the radar receiver is sensitive enough – much more sensitive than regular radar receivers – it can pick up signals that are reflected through the wall. Using well-established signal processing techniques, reflections from static objects like walls and furniture can be filtered out, allowing the signal of interest to be isolated – like a person’s location.

The key to using radar to track objects across a wall is to have a very sensitive antenna that can pick up dramatically diminished reflected radio waves. Image via Abdel-Kareem Moadi, CC BY-ND

Turn data into images

Historically, radar technology has been limited in its ability to aid disaster management and law enforcement because it did not have enough computational power or speed to filter out background noise from complex environments. like foliage or rubble and produce live images.

Today, however, radar sensors can often collect and process large amounts of data – even in harsh environments – and generate high-resolution images of targets. Using sophisticated algorithms, they can display the data in near real time. This requires fast computer processors to quickly process these large amounts of data, and broadband circuits which can quickly transmit data to improve the resolution of images.

Recent developments in millimeter wave Wireless technology, from 5G to 5G + and beyond, will likely help improve this technology even further, delivering higher resolution images through an order of magnitude wider bandwidth. Wireless technology will also speed up data processing times as it dramatically reduces latency, the time between transmitting and receiving data.

My lab is developing rapid methods to remotely characterize the electrical characteristics of walls, which help calibrate radar waves and optimize antennas to pass waves more easily through the wall and essentially make the wall transparent to waves. We are also developing the software and hardware system to perform big data analysis of radar systems in near real time.

On the left, a laboratory installation showing a cinder block wall and a cardboard silhouette covered with foil of a person, and, on the right, a radar image showing a corresponding silhouette in three-dimensional space
This wall penetrating laboratory radar provides more detail than current commercial systems. Image via Aly Fathy

Better electronics promise portable radars

The low frequency radar systems usually required to see through walls are cumbersome due to the large size of the antenna. The wavelength of electromagnetic signals corresponds to the size of the antenna. Scientists have pushed transparent radar technology to higher frequencies in order to build smaller, more portable systems.

In addition to providing a tool for emergency services, law enforcement and the military, the technology could also be used to monitor the elderly and read the vital signs of patients with infectious diseases like COVID- 19 from outside a hospital room.

The interest of the US military is an indication of the potential of transparent radar. They are technology research which makes it possible to create three-dimensional maps of buildings and their occupants almost in real time. They’re even looking for a transparent radar that can create images of people’s faces that are precise enough for facial recognition systems to identify people behind the wall.

Whether or not researchers can develop transparent radar sensitive enough to distinguish people by their faces, the technology is likely to go far beyond smudges on a screen to give first responders something like superhuman powers.

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This article from Ally fathy, Professor of Electrical Engineering, University of Tennessee, is reposted from The conversation under a Creative Commons license. Read it original article.




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