If these walls could talk (and see)

Before mounting an assault, information that an assault team might want to know about an occupied room on the other side of a wall could include: the number of combatants in the room, who's the leader, what weapons they have, what are they doing, are there hostages and non-combatants present, and where's the exit point.

In 2017, the Iraqi Counter Terrorism Service took heavy casualties as they fought their way house-to-house through west Mosul. The terrorist and insurgent group Islamic State had tunnels and holes connecting buildings and made use of IEDs and boobytraps to inflict maximum casualties on the advancing Iraqi assault force.

In the past, a common surveillance methodology in terrorist siege-hostage situations was to drill a small hole through the wall and insert a viewing probe and listening device. That technology has been around for 50 years, but it can take several hours to covertly develop an observation hole without the drilling being obvious to people on the other side of the wall.

During the Iranian embassy siege in London in 1980, COBRA (the cabinet office command group) instructed British Gas to commence road drilling in an adjacent road to mask the wall-drilling operation. The road drilling had to be stopped after it agitated the terrorists inside the embassy. Instead, the British Airports Authority, owner of London's Heathrow Airport, was told to instruct approaching aircraft to fly over the embassy at low altitude.

Wall-drilling is clearly not a satisfactory option in a time-urgent or covert surveillance situation.

What other options are there?

In some circumstances, micro-UAVs (Unmanned Aerial Vehicles) disguised as insects can be used to penetrate a building to provide live imagery coverage, but airborne access is often not available.

In the excellent SBS TV series The Bureau, the DGSE (the French external intelligence service) gives Iraqi Special Forces an iPad-sized device that when held up against a wall provides an X-ray image through mud and concrete walls. In reality, such a device does not exist. (One of the problems with an X-ray-based system would be exposing one's own operatives to radiation.)

There are however devices with a limited capability to penetrate walls, depending on the construction of the wall. A wall made of lightweight materials is less of a penetration challenge than a solid brick or thick mud wall. An assault team needs to be very quiet to avoid being shot through a wall that's not substantial. (A 7.62mm AK-47 round can penetrate up to one layer of brick.)

The cheapest surveillance option is to use the Israeli 3D imaging sensor company Vayyar's Walabot app with an Android phone. It was designed to be an aid to plumbers and electricians to locate pipes and wiring inside walls, but it can also detect a person in close proximity on the other side of the wall.

Unfortunately, thermal or infrared cameras cannot see through walls. Walls are generally thick enough - and insulated enough - to block infrared radiation from the other side. If you point a thermal camera at a wall, it will detect temperature variations from the wall, but not what's behind it.

US company L-3Harris markets the Range-R which is a hand-held radar device capable of detecting humans through walls inside closed spaces, using radio waves. Its advertising states: "L3Harris' patented Stepped-Frequency Continuous-Wave (SFCW) radar technology and proprietary target detection algorithms enable Range-R to operate as a highly sensitive Doppler motion detector." It has been in use with the FBI and US Marshall Service and other US law enforcement agencies since 2013. It can identify known individuals 83 per cent of the time through their unique features and style of movement.

Israeli company Camero uses similar technology. It claims to be a world leader in the design and production of Ultra-WideBand (UWB) radio imaging systems, primarily for Sense Through The Wall (STTW) and body scanning applications. Its Xaver family of products provide "real-time information on multiple stationary and moving objects concealed behind walls - providing operators with unprecedented situational awareness and operational advantage, utilizing unique pulsed based UWB micro-power radar technology."

There are several other manufacturers of this technology. A Russian manufacturer demonstrated a similar system at the Milipol exhibition in Paris in 2019.

MIT's Computer Science and Artificial Intelligence Laboratory has created a more sophisticated system that can monitor a human body through walls and portray movement. It uses Radio Frequency (RF) waves to sense where the person is and then recreates the person as a simple stick figure - it's called RF-Pose. A future option would be to use Artificial Intelligence to analyse what people are doing on the other side of a wall. The system could operate on low-power RF signals, which reflect off living tissue and metal but pass through wooden or plaster interior walls.

In January 2020, the US Department of Defense advertised a contract opportunity to develop a US-based STTW system. Defense wants a man-portable system that will support soldiers and give them the ability to "track, locate, isolate, range, and count personnel and animals in a building or structure."

The system would also be required to ascertain whether people are sitting or standing and confirm whether they are friend or foe by analysing their biometric data. Defense also wants the SSTW system to be able to determine if there are any booby traps, explosives or weapons within a structure, and collect data that could be used to create 3D maps of buildings of interest, including identifying any tunnels. The main customer would be the US Army's Special Operations Forces.

Seeing or sensing through walls technology - when refined further - will have many surveillance and search and rescue applications. An enhanced capability could be available by the end of 2021.