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Through-the-wall surveillance technologies


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Invited paper, pp. 30-33 IEEE X-plore
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Ultrawideband and Ultrashort Impulse Signals, 17-21 September, 2012, Sevastopol, Ukraine pp. 30-33
©2012 IEEE
Yanovsky F. J., Ivashchuk V. E., and Prokhorenko V. P.
National Aviation University, Kiev, Ukraine
This paper will present a brief overview of methods for solving the problem of
surveillance behind the opaque wall. A low cost method for detecting moving objects
behind the wall, based on the analysis of reflected pulse UWB signal is considered in
more details. Demonstration of real work of the method is carried out with use of the
operating breadboard model of the device implementing this method.
Keywords: Through-the-wall radar, UWB radar, GPR.
The possibility to see through obstructions that are
opaque to visible light always was very attractive for
human. However until recently such vision seemed
impracticable dream. Now several approaches to solve
different problems of through-the-wall surveillance
are known.
Considerable progress in this area is caused by co-
incidence in time of two circumstances: a) the in-
creased needs of the modern society caused by
activization of terrorist activity, and also increase of
requirements to the efficiency of rescue operations
after natural cataclysms like earthquakes and volcanic
activity that became more frequent; b) development of
new methods for generating and radiation of signals in
a wide frequency range, including ultra wideband
(UWB) signals, and advent of fast and effective de-
vices of digital signal processing. In other words, de-
mand increase has coincided with occurrence of new
engineering possibilities.
That is why through-the-wall imaging has recently
become a topic of intense research as it concerns de-
tection and localization of people behind impenetrable
walls [1].
Possible applications of such devices are ranging
from rescue operations in rubble up to terrorists'
movements tracking inside a building as well as other
law enforcement operations.
There were different techniques proposed using
millimeter electromagnetic radiation [18, 21], UWB
sounding signals [19], acoustic signals [20], etc.
This paper will present a brief overview of methods
for solving the problem of surveillance behind the
opaque wall.
One of methods for detecting moving objects be-
hind the wall, based on the analysis of reflected pulse
UWB signal is considered in more details. Demon-
stration of real work of the method is carried out with
use of the operating breadboard model of the device
implementing this method.
In this overview we restrict ourselves to the con-
sideration of radar techniques only.
First of all it is logical to consider continuous wave
(CW) and pulse radar systems. Both kinds of sound-
ing waveforms were considered to be used for
Through-the-Wall Surveillance (TWS). Pulse sound-
ing waveforms can be: short pulse (without intrapulse
modulation) and long pulse sounding waveforms with
intrapulse modulation [2]. Application of short pulses
is more frequent for TWS because of technological
reasons and absence of range sidelobes worsening
signal selection after processing.
In work [3], short pulse sounding waveforms with
carrier and without carrier (videopulses) were com-
pared, and pulses with carrier were recommended as
preferable because (according to [3]) matched filtra-
tion or correlation processing with transmitted signals
as templates are possible for them in contrast to pulses
without carrier. Nevertheless, the disadvantage of
pulses with carrier is the growth of signal attenuation
in construction materials with increasing frequency,
and this disadvantage may have a crucial influence in
some cases, particularly for thick and concrete walls.
Another classification distinguishes Doppler radars
[4] and interferometric radars [5, 6]. Doppler radar
usually uses CW sounding waveform but it can be
also a pulse (pulse-coherent) system.
While the Doppler and interferometric radars are
normally narrow band systems, the pulse TWS radars
[7, 8] are normally UWB systems.
2 Ultrawideband and Ultrashort Impulse Signals, 17-21 September, 2012, Sevastopol, Ukraine
There are some works on TWS radar based on
noise sounding waveforms [9, 10]
Various signal processing algorithms can be used
including even SAR principle [11] and compressed
sensing technology [22].
Development of the microwave ultra-wide band
step-frequency radar and application of the radar for
through-obstacles object detection and imaging sys-
tems is presented in [12]. The radar quickly sweeps
through frequency range sequentially generating a set
of equally distributed frequencies and collects re-
ceived signal on each frequency. Sensitivity of the
radar for the behind wall objects is improved by
build-in hardware first reflection suppression sub-
system. Tests of the system have shown its ability to:
track position of the target even behind three brick
walls; detect breathing and heartbeat of a human
throw several meters of soil; obtain image of human
behind wall by applying tomography processing to
data collected by the radar.
A noncoherent, stepped-frequency TWS system
approach, based on a trilateration technique, is pre-
sented in [13]. This approach involves multiple inde-
pendent radar units and, as such, provides flexibility
in positioning the units with various stand-off distanc-
es and inter-element spacing.
The interplay between coherent and non-coherent
data fusion in a widely distributed MIMO sensor net-
work is considered in [14] and seems to be a very
interesting topic.
Another attempt to use a MIMO configuration and
UWB signals in order to detect scatterers behind a
wall is discussed in [15], where method of Decompo-
sition of the Time Reversal Operator (DORT) is pro-
posed for detection and localization of a moving
target behind a wall. According to [15] one of the
DORT method major strengths is that detection re-
mains possible through a distorting medium.
Spectral variations of the reflections using
videopulses are used in UWB radar for human being
detection [16]. Novelty of the proposed [16] system
radar lies in its large operational bandwidth combined
with excellent time stability. It has been shown that
due to breathing the range to a person varies within
0.6 cm.
As a rule, operating frequencies in all methods are
rather high. They lie in the frequency band of 1 to 4
GHz. Due to strong attenuation of electromagnetic
radiation in the material of main walls a thick wall
still is a problem.
The proposed UWB system is based on the princi-
ple used previously in the prototype of ground pene-
trating radar (GPR) [17]. However in contrast to GPR,
which survey normally a stable situation under the
ground, in the system, a motion is the feature to detect
Actually a living person as any breather is always
characterized by a kind of motion including its wig-
gling and even a movement of the chest when breath-
ing. So, the system should detect any movement. Let
us distinguish between two kinds of movement: a)
indubitable movements like walking, and 2) small
movements, say, caused by breathing.
Actually, presence of a moving target causes
change of the situation behind the wall. Let us sup-
pose that we are able to compare the reflected signal
over a period of the order of
s. In this case we can
definitely detect the change of the situation, if a target
or any part of the target is changing its position during
s. Detection of a movement means presence of the
target in the resolution volume.
Two principle problems of this approach are re-
quired high resolution of the system and good pene-
trability of the radiation that is used.
In order to detect small movements a system with
high range resolution is necessary. Wideband and
UWB technology is suitable to solve the problem of
high resolution.
Another problem is to provide a detectable level of
reflection from the object behind the wall in spite of
twofold passage through the wall. All construction
materials and soils cause attenuation of a propagating
radiation (sounding signal). Especially important is
the fact that the attenuation on average increases con-
siderably when frequency increases. Therefore, for
penetration of the sounding signal through the thick-
ness of obstruction, the operating frequency of the
system should be as low as possible; and this is one of
the features of the accepted approach to design the
system. Thus, the system for searching people under
the rubble and behind thick walls should use an UWB
signal that maximizes the receive power at compara-
tively low operating frequency.
The operating principle of the system for searching
for people behind the wall and under the rubble is
illustrated in Fig. 1
Fig. 1. Operating principle of the system for
searching for people under the rubble or wall
The transmitter sends a short UWB pulse into the
direction of a wall or rubble. The receiver takes scat-
tering signals from different objects in rubble or be-
hind the wall. The processing system that can be
based on a laptop compares the signals over the given
Short Paper Title
Ultrawideband and Ultrashort Impulse Signals, 17-21 September, 2012, Sevastopol, Ukraine 3
If the shape of the signals was changed, there was a
movement. If there are no changes, that means no
The estimated parameters of the system prototype
are following:
Frequency range 100 ... 1000 MHz
The pulse duration 1 .. 10 ns
Maximum depth of detection (thickness of a wall)
up to 5 m
Average radiated power< 250 mW
The test of the system was done in the building us-
ing the plastered brick wall of about one meter thick-
ness. A man was sitting on the chair, making different
kind of movements, namely:
test 1: waving his hand from side to side;
test 2: bending the body from side to side;
test 3: just breathing, keeping the body in the stable
The result of the experiment is presented in Fig. 2,
where the screen of the laptop is indicated. One can
see very strong changes in the picture that correspond
to test 1 and test 2, as well as comparatively weak but
still detectable change of the picture corresponding to
test 3.
Fig. 2. Imaging of three types of movements (Test 1
is movement of a hand; Test 2 is movement of a body;
Test 3 is movement of a chest when breathing)
Another experiment was on detection a person who
walked up and down behind the combined (brick and
concrete) wall of 1 m thickness. The result is present-
ed in Fig. 3.
Fig. 3. The image of sequential (in time) situations,
when a person went past behind a thick (1 m) wall
seven times
There are a wide variety of UWB radar techniques
for TWS with different configurations, sounding sig-
nals and signal processing.
A possibility to detect moving objects under the
rubble and behind the thick wall with a simple low
cost UWB pulse radar has been demonstrated. Range
resolution is achieved using UWB signal. Relatively
big depth of penetration through the thick walls is
provided by “low” frequency.
High sensitivity of motion detection is provided by
special signal processing software. All parameters of
the implemented physical demonstrating model can be
significantly improved during the research work and
modernized prototype development.
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