There's another prototype meant to enhance security, but will it too eventually turn into an assault on privacy?

Researchers in London have devised a stealthy system that gives off no radio waves so it can't be detected, but by sniffing Wi-Fi signals, it can pinpoint a person's movement inside a building. University College London scientists Karl Woodbridge and Kevin Chetty developed this suitcase-sized prototype that has successfully been tested through a one-foot-thick brick wall to determine "a person's location, speed and direction." PhysOrg added, "See Through The Wall (STTW) technologies are of great interest to law enforcement and military agencies; this particular device has the UK Military of Defense exploring whether it might be used in 'urban warfare,' for scanning buildings. Other more benign applications might range from monitoring children to monitoring the elderly."

"Fundamentally, this is a radar system - you're just using radio waves that have been emitted by an external WiFi router, rather than creating your own," explained ExtremeTech. "Compare this with MIT's through-the-wall (TTW) radar, which is 8 feet (2.4m) across and requires a large power source to generate lots and lots of microwaves."

Popular Science reported to look at this image and follow the steps below for how it would work:

1. MOVING SUBJECT: When Wi-Fi radio waves bounce off a moving object, their frequency changes. If, for example, a person is moving toward the Wi-Fi source, the reflected waves' frequency increases. If a person is moving away from the source, the frequency decreases. 2. REGULAR OL' ROUTER: A Wi-Fi Internet router already in the room fills the area with radio waves of a specific frequency, usually 2.4 or 5 gigahertz. 3. BASELINE SIGNAL: One antenna of the radar system tracks the baseline radio signal in the room. 4. SHIFTED SIGNAL: A second antenna detects radio waves that have reflected off of moving objects, which changes their frequency. 5. PERP, SPOTTED: By comparing the two antennas' signals, the computer calculates the object's location to within a few feet as well as its speed and direction.

If you think the answer would be to hold perfectly still in order to avoid detection, to trick it into thinking you are nothing more than a piece of furniture, think again. As Engadget previously pointed out, engineers at the University of Utah developed a wireless network capable of seeing through walls to detect and monitor breathing patterns. In this case, it's not meant to be a surveillance system, but an inexpensive way to monitor patients' breathing.

As we move forward and more covert surveillance tech that was previously science fiction becomes real-life technology, it will continue to clash with civil liberties. In the military where our soldiers' lives are on the line, then seeing through the walls could be a good thing. In the case of a bank robbery or another such crime where regular surveillance cameras have been disabled, then this tech could again be used for good. However, as we've seen historically, technology that starts off for military or law enforcement use often bleeds out and onto the public for covert surveillance. One example of this is Z Backscatter, full-body scanners in the form of mobile X-ray scanning units covertly driving around on streets that can scan you without you ever knowing it happened. Another example is Homeland Security's portable molecular-level scanning devices that can see through clothing at 164 feet away. It could scan everyone at airports without anyone knowing it happened. DHS said "this scanning technology will be ready within one to two years."

Here's the abstract for the research paper about seeing through walls by using Wi-Fi signals:

In this paper, we investigate the feasibility of uncooperatively and covertly detecting people moving behind walls using passive bistatic WiFi radar at standoff distances. A series of experiments was conducted which involved personnel targets moving inside a building within the coverage area of a WiFi access point. These targets were monitored from outside the building using a 2.4-GHz passive multistatic receiver, and the data were processed offline to yield range and Doppler information. The results presented show the first through-the-wall (TTW) detections of moving personnel using passive WiFi radar. The measured Doppler shifts agree with those predicted by bistatic theory. Further analysis of the data revealed that the system is limited by the signal-to-interference ratio (SIR), and not the signal-to-noise ratio. We have also shown that a new interference suppression technique based on the CLEAN algorithm can improve the SIR by approximately 19 dB. These encouraging initial findings demonstrate the potential for using passive WiFi radar as a low-cost TTW detection sensor with widespread applicability.

The paper, "Through-the-Wall Sensing of Personnel Using Passive Bistatic WiFi Radar at Standoff Distances" is behind a paywall for $31, so I didn't read it.