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In
the 1930s, U.S. Navy researchers stumbled upon the concept of radar when they
noticed that a plane flying past a radio tower reflected radio waves.
Scientists have now applied that same principle to make the first device that
tracks existing Wi-Fi signals to spy on people through walls.
Wi-Fi
radio signals are found in 61 percent of homes in the U.S. and 25 percent
worldwide, so Karl Woodbridge and Kevin Chetty, researchers at University
College London, designed their detector to use these ubiquitous signals. When a
radio wave reflects off a moving object, its frequency changes—a phenomenon
called the Doppler effect. Their radar prototype identifies frequency changes
to detect moving objects. It’s about the size of a suitcase and contains a
radio receiver composed of two antennas and a signal-processing unit. In
tests, they have used it to determine a person’s location, speed and
direction—even through a one-foot-thick brick wall. Because the device itself
doesn’t emit any radio waves, it can’t be detected.
Wi-Fi
radar could have domestic applications ranging from spotting intruders to
unobtrusively monitoring children or the elderly. It could also have military
uses: The U.K. Ministry of Defence has funded a study to determine whether it
could be used to scan buildings during urban warfare. With improvements,
Woodbridge says, the device could become sensitive enough to pick up on subtle
motions the ribcage makes during breathing, which would allow the radar to
detect people who are standing or sitting still.
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.
BREATHE
EASY
It’s
possible to detect a person’s breathing rate by surrounding him with radio
waves. Neal Patwari’s wireless engineering group at the University of Utah
designed a network of 20 inexpensive radio transmitters that are placed around
a patient’s bed. Then they created an algorithm that detects a stationary
person’s breaths better than current detectors do. Patwari plans to upgrade the
algorithm by the end of the year to filter out body movements too. The system
could someday be used in hospitals in place of tubes and masks.
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