Suicide Bomber Detection Using Millimeter-Wave Radar Richard Sullivan,

Suicide Bomber Detection Using Millimeter-Wave Radar Richard Sullivan,

Suicide Bomber Detection Using Millimeter-Wave Radar
Richard Sullivan, Morgan Galaznik, Jose Martinez, Carey Rappaport
Department of Electrical Engineering, Northeastern University, MA 02115
This work was supported in part by CenSSIS, the Center for Subsurface Sensing and Imaging Systems, under the
Engineering Research Centers Program of the National Science Foundation (Award Number EEC-9986821)

Problem Description

Explosive devices, most often
consisting of cylindrical, metal
objects filled with explosives,
require detection by radar.
Testing protocols must be
developed to model realistic
conditions.
Unique scattering patterns may
be obtained using a crosspolarized signal, but existing
radar can only facilitate one
polarization at a time.
Existing radar aperture does not
result in desired beam width.

Objective
Determine a reliable and detectable characteristic of non-standardized bodyworn IED assemblies.
Implement experimental test protocols to verify
delectability of objects with high conductivity for a variety of geometries.

L3

Bio-Med

EnviroCivil

S2S3

S1

S4

S5

Validating
L2 TestBEDs
Fundamental
Science

Compute average return
signal

Transmitter

R2

Max Value

16
14
12
10
8
6
4
2

Receiver

Repeat for various
cases and compare

As you can see in Figure 1, where the electric field is polarized so that the field
vector is parallel to the shorter dimension of the transmitter aperture, the radar
has the capability of producing electric field that is polarized vertically, parallel to
the length of the human body, and also field that is polarized horizontally,
perpendicular to the length of a human body.
The testing location provided is an anechoic chamber of size sufficient to perform
experiments that illuminate the width of one human torso and the height from
approximately knee to slightly above the head. We were also provided with a
canvas vest with multiple pockets for carrying primarily cylindrical metal pipes
though other metal objects could also be inserted.

0
Subjects

Declare Threat

Figure 1 77GHz Testing Radar

It should be noted that in Figure 2, each bin effectively reflects a progressive
distance. It is known through addition of objects in the chamber that the target
location occurs at the 20th bin. The distance from radar to target is known to be
10.1m; thus, range can be determined simply. Upon implementation of the data
processing outlined in Figure 3, a return similar to Figure 4 is obtained.

Testing Methods
Inasmuch as the goal of this testing is to determine the feasibility of detecting
body-worn IEDs on humans using high-frequency electromagnetic waves, the
goal may also be considered one of finding different geometries of metal worn by
a human, since we expect differing scattered fields only when our variable is one
of high conductivity. We also expect human flesh to exhibit highly conductive
characteristics at our frequency since human flesh has been shown in earlier
paper to exhibit the constitutive properties of dielectric constant 27 and
conductivity 30 S/m at 25 GHz [4].

Radar Software Operation
The provided radar came complete with data-processing software capable of
providing magnitude of electric field return from different distances. This is
achieved by taking the Fast Fourier Transform of each scattered voltage signal or
chirp. The transmitted chirp consists of a Gaussian pulse whose frequency is
linearly ramped over a range of approximately 300MHz to just below 77GHz.
When this is done, one effectively has both magnitude and range for radar input.
A sample of the data received is shown in Figure 2.

Subjects with
Empty Vest

5
Pipes

9
Pipes

Figure 6 Compiled Results For Human Subjects at Horizontal Polarization

Figure 3 Post FFT Data Processing

As you can see in Figures 5 and 6, the results vary dramatically between cases
and while some cases can provide desirable results independent of other cases, it
becomes difficult to set threshold levels for the general population that give any
acceptable false alarm rate.

Continuing Work
Data is in the process of being collected and there are twenty more human subject
cases that have already been collected though not yet processed and included
with the data shown in Figures 5 and 6. Other detectable characteristics of the
data are also being considered. First, it has been considered that the reflection
from cylindrical pipes may lend itself to more angular uniformity than that of a
human body without them. It is the same principal as a metal plate of finite
thickness versus that of a metal cylinder; where the reflected field off a plate will
vary dramatically when it is facing the radar and when it is facing at any other
angle, the return from a large metal cylinder would be the same regardless of
angle. Additionally, the effect of the target on the return from the background is
also being considered. Essentially, it has been hypothesized that the change in
the scattered field from ranges beyond the target could correlate to the objects
worn by a human subject.

Long Term Antenna Design

Figure 4 Processed Return Data
While many tests have been performed with a variety of objects including pipes
and metal plates not on humans the focus of the testing has been on collecting
data for a variety of human subjects wearing everyday clothes, the empty vest,
and the vest filled with increasing level of pipes. The most practical amount of
pipes that the vest can hold without slipping of the subject is nine, so the majority
of vest with pipes experiments have been performed with nine pipes. A
summary of results follows for both polarizations of the radar where the recorded
average returns are sorted and compared in decibels and where each color
represents a different human subject.

Determining a reliable detectable is the immediate concern. Still, should this
application prove feasible at these closer distances, an antenna must be
developed to accommodate ranges up to and including 50 m, which is the ultimate
goal. One proposed design is that of the Gregorian Confocal Dual Reflector
(GCDR) which is illustrated in Figure 7.

Vertical Polarization

L1

Max Return

R1

18

R3

16

Figure 7 Gregorian Confocal Dual Reflector
The principal of the GCDR system is that a smaller aperture, by means of a subreflector, shown in the upper-rightmost portion of Figure 7, and a main reflector,
shown in the lower-leftmost portion of Figure 7, a smaller aperture may be
magnified in direct proportion to the ratio of the focal length of the main reflector to
the focal length of the sub-reflector. The sub-reflector may be optimized in order
to achieve constant path length, and thus phase. However, previous research by
Jose Martinez has shown the gains made by sub-reflector shaping and
optimization are negligible and thus standard parabolic reflectors may be utilized.

14
Magnitude of Return (dB)

Testing Equipment
In order to achieve a beamwidth capable of illuminating a single human torso at
large distances while still maintaining a feasibly-sized aperture, one requires a
small wavelength. This can be seen mathematically in the equations for halfpower beamwidths for a rectangular aperture of area a x b where b is the
dimension parallel to the electric field vector [1].

3dB

Horizontal Polarization

Magnitude of Return (dB)

Human with vest

Align multiple signals
to common reference

Human w/o vest

FFT -> scattered power
for each range bin

In view of this, the approach to testing has been to vary the amount of metal worn
by human subjects in the supplied vest and to compare these with the returns
from human bodies without body-worn IED simulates, i.e. cylindrical metal pipes.

Three-Level Diagram
Syste
ms
Areas

Collect multiple radar
return signals in time

Chamber back wall

The aim of this research is to examine the feasibility of using millimeter-wave
(MMW) radar to detect body-worn IEDs at distances up to 50 meters. In order to
achieve a beamwidth capable of illuminating a single human at 50 meters, which
in this case would be approximately 0.01 radians, while still maintaining a practical
aperture size, we require a wavelength on the millimeter scale. The radar made
available for testing, provided by Raytheon, operates at 77GHz. At a wavelength
of 3.89mm, this radar provides, at a testing distance of 10.1 meters, an adequate
simulation of a human-torso-sized beamwidth at 50 meters. This research also
examines the role of the Gregorian Dual Confocal Reflector antenna in achieving
smaller beamwidths from apertures of limited size.

In order to process the data shown in Figure 2 into a usable result, one must
normalize and average it. It must be normalized because internal actuation
causes the entire data set to shift amplitude periodically. It must be averaged
because of the obvious variation shown. A diagram of the further data processing
is shown below.

Background

Amidst the manifold threats currently afflicting public welfare, that of body-worn
explosives is significant if not altogether paramount. Commonly referred to as
suicide bombers, the bearers of body-worn, improvised explosive devices (IEDs)
enter crowded public areas in order to detonate the IDE, inflicting lethal damage to
themselves and surrounding individuals. Constructed of non-standard parts and
veiled under layers of clothing, these body-worn IEDs go frequently undetected.

For this research, a radar has been provided by outside sources. This radar was
originally developed for automotive intelligent cruise control technology [2]. This
radar proved an attractive fit for suicide bomber detection research in that it
operated at 77GHz and thus provided a beamwidth in the millimeter wave region,
the region in which we can expect to work if we hope to achieve long-range radar
detection. It should be noted that although in this presentation and other
documentation we refer to our operating frequency of 77GHz as existing in the
millimeter wave region, according to IEEE standards of 1984, we are technically
operating at the lower end of the W band [3].

Human with jacket

Abstract

50.6

b/

3dB

68.8

a/

12
10
8
6

References

4
2

Figure 2 Sample of Data Returned from a Human Subject Target

0
Subjects

Subjects with
Empty Vest

4
5
Pipes Pipes

9
Pipes

11
Pipe
s

Figure 5 Compiled Results For Human Subjects at Vertical Polarization

[1] Balanis, Constantine A., Antenna Theory: Analysis and Design, 3rd Ed., John Wiley &
Sons Inc., Hoboken, NJ, 2005.
[2] Russell, M. E., Crain, A., Curran, A., Campbell, R. A. and Drubin, C. A., IEEE
Transactions on Microwave Theory and Techniques, vol. 45, n. 12, pp. 2444-2453, 1997.
[3] Skolnik, Merrill L., Introduction to Radar Systems 3rd Ed., McGraw-Hill Companies, Inc.,
New York, NY, 2001.
[4] Angell, Amanda & C. Rappaport, Computational Modeling Analysis of Radar Scattering
by Metallic Body-Worn Explosive Devices Covered with Wrinkled Clothing, p. 2.

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