My home is not my castle anymore: Technologies that allow to see through the walls

In the past, technologies that allow to see through walls were available only for some governmental services. Today, the situation changes owing to technological advancement that is followed by the price decrease.

In the beginning of 2015, the USA Today instigated a wave of publications on handheld radar called RANGE-R that had been used by the U.S. police and other government services. The radar system is able to ‘see through the walls’. To be more accurate, it registers motion inside closed spaces. High sensitivity of this radar system is able to detect breathing of a man hiding somewhere deep inside a building behind several walls.

The existence of such device turned out to be surprising for many journalists who were listing the features of RANGE-R. While at the same time, these radars have been long in mass production for military and intelligence-service needs. The radar systems have been used in FBI hostage-rescue missions, in firefighting collapse search-and-rescue operations, in the U.S. Marshals Service operations to catch fugitives, and so on.

Prior to that, the technology was available only for some governmental services. Today, the situation changes owing to technological advancement that is followed by the price decrease. The RANGE-R radar system costs around $6,000, while prototypes for new radar systems are being built from accessible low-cost Wi-Fi modules.

Therefore, it will not be surprising if the same equipment will be used by criminals tomorrow, and it is just the right time to scrutinize the technology and its features.

Is there anyone alive out there?

The sensor technology of RANGE-R was named Through-the-Wall Sensors (TTWS). Just like in all of the other radars, the sensors ‘light up’ the field of view with radio waves and register the reflected radiation afterwards.

It is only easy to do it in theory. In practice, the creators of TTWS have to combine several technologies and advanced data processing methods in one device. Radar operators have to undergo a long process of learning how to interpret the radar data.

Most TTWS radars operate in the frequency range of 1-10 GHz. The radiation in this range is good at penetrating concrete, wood, plastic, glass, and other walls. To demonstrate the validity of the statement, scan your home or office, which usually is congested with Wi-Fi networks.

The higher the frequency is, the worse the radiation penetrates through the walls. At the same time, higher frequencies increase the accuracy of object size and distance measurements. Besides, some materials absorb radio waves selectively in a narrow range. That is why advanced scanners can switch frequencies on the go or use a wide part of the radio spectrum.

Working with short impulses allows to evaluate distances to objects by the time it takes for a wave to travel back and forth. Motion detection is based on the Doppler shift: a wave reflected from a moving object slightly changes its frequency, allowing, for example, to detect a near-static motion of the chest of a breathing man.

There is no doubt that TTWS devices have a lot of limitations. The main restriction is that the radio waves cannot penetrate metal. Thus, detecting a man in the closed body of a car or in a building encased in aluminum siding is impossible. Water has properties similar to metal: a wet porous concrete is quite an effective defense.

Generally, a thick layer of concrete or brick weakens signals well. It is usually impossible to detect anything if the total thickness of walls that separate the radar and the object is over 12 inches (about 30 centimeters).

The detection range of most devices is 50-65 feet (about 15-20 meters), while devices with bigger antennae and more powerful power supplies can ‘knock through’ approximately 230 feet (about 70 meters). Not only a man, but a dog, or a curtain caught in a draft can move inside of a house. The interpretation of an object is not always definite, especially when the measurement process is very limited in time. A standard measurement takes approximately a minute.

Most radars are made handheld. A radar operator has to press radar against the wall of the inspected building to eliminate tremor. Sometimes, there are situations when it is impossible to approach the wall. That is why some radar models are equipped with tripods or are mounted on robots or drones.

The least complicated TTWS radars just show whether someone is alive or moving in the building. More complex devices detect the distance to the object and its direction and give out an approximate layout of the building and objects in two or three dimensions.

Experimental solutions already look promising (at least under laboratory conditions). For instance, a mobile radar Wi-Fi system mounted on a couple of robots allows to generate a map of a house that is completely unknown with a 0.7-inch (2-centimeter) accuracy. It is still a fiction for mass production.

How to counter it. The best defense against TTWS-spying is a shielded building. If your house has reinforced thick concrete constructions everywhere, then you do not need to do anything. Otherwise, aluminum siding or metallized wallpaper is good. You can keep three dogs in your house as well, as mass-production radars cannot detect more than three targets at once yet.

That’s a (not so) scary terahertz

If you follow news on popular science, then you surely have heard something about terahertz detectors that can see through walls and sense bombs from afar. Publications of similar kind appear periodically on the Internet after a brisk press release from some science laboratory that announces for the nth time about some considerable success towards something.

As a matter of fact, terahertz radars are only used for passenger inspections at airports. These radars became famous because of their ability to ‘strip people’, which means that they reveal a very detailed picture of a human body that is hidden under clothes.

The majority of other applications of ‘the terahertz’ (which covers the range of 300GHz-10THz) remain science fiction. In reality, there are a lot of unsolved problems ranging from the signal fading while coming through various barriers to the construction of high-power compact radiation emitters.

There is one more urban myth: infrared cameras that look through the walls. Contrary to the popular belief, thermal detectors cannot do anything like that. Even a layer of frosted glass or plywood is not transparent for an infrared detector.

How to counter it. Take off your foil cap or put it on. It’s your call.

What kind of voices do you hear?

Everyone who watched spy films, even just once, knows that any conversation can be eavesdropped on from afar, even through the windows. The glass vibrates under the impact of sound waves, and the motions can be read with a laser. As a countermeasure, cheap and effective ‘jammers’ were invented that can be attached to the glass to generate random noise.

Contemporary spies lead easier lives. They may recover a conversation by analyzing a silent video with only a part of the room visible on camera. The general principle is the same: any object subjected to vibrations can act as a membrane. This can be a bag of chips, the surface of a glass of water, or the leaves of a ficus plant.

The standard window jammers will not prevent devices from picking up the sound. Even though to decode the conversation, the video must be shot with a special high-speed camera that can record several thousand frames per second (the value must be higher than the voice frequency).

Nevertheless, high-speed cameras get into our lives fast. For example, a lot of contemporary smartphones can make videos with a high frame-per-second frequency that helps to extract valuable data. For example, it can help to identify participating individuals in a conversation.

Spying through windows is not a problem these days as drones become cheaper and more advanced.

How to counter it. A curtain or blinds will prevent the possibility of ‘video-eavesdropping’. It is important that the curtain should not act as a sound membrane; so, opt for something heavier for your curtains or blinds or attach the aforementioned jammer to them.

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