Instrumentation & Measurement Magazine 25-3 - 37

Measurements of Micro-Doppler
Signals Induced by Acoustic
Stimulation
James Kennedy, David Green, Javier Ortiz Castro, Anthony A. Faust, Yue Ma,
Pedram Ghasemigoudarzi, Matthew Circelli, and Michael D. Henschel
T
he detection of concealed objects is a concern in multiple
domains, especially in areas of defense and
security where hidden threats pose an ever-present
danger. The measurement and analysis of micro-Doppler signals
from concealed targets is a promising approach to this
challenge. In this paper, a new detection technique is presented
which measures the micro-Doppler signature of object
vibrations that are actively induced using acoustic energy. This
would permit the detection of stationary threats, such as improvised
explosive devices. An exploratory system is set up
and measurements are performed and assessed on non-concealed
target objects. The results show a clear detection of
targets for certain acoustic frequencies, along with favorable
noise characteristics. This promising outcome has spurred the
continuing development of a next generation system.
Challenge
Countering explosive hazards (EH)-such as unexploded
ordnance (UXO), landmines, explosive remnants of war,
and improvised explosive devices (IEDs)-is a high priority
capability for allied forces in order to maintain freedom
of movement while minimizing casualties in a wide range of
operations: protection of fast moving tactical or logistical convoys;
more deliberate EH defeat operations during route or
area clearance; and in static roles such as checkpoints, critical
infrastructure protection, and support to public security.
Experiences of allied forces over the last two decades have
dramatically changed the way our military forces live and
fight in the battlefield. Although various techniques have been
developed for EH detection [1], adversaries have shown themselves
to be highly adaptable, using low-cost technologies to
rapidly develop increasingly sophisticated EH designs, and
mitigating their use in urban environments has proven more
challenging still due to underground infrastructure and the
presence of significant electromagnetic traffic.
In light of the evolving nature of the EH threat, the Canadian
Armed Forces (CAF) posed a challenge to industry,
through the Innovation for Defence Excellence and Security
(IDEaS) program of the Department of National Defence, for
May 2022
the proposal and development of novel solutions to increase
the distance for stand-off detection of concealed explosive
hazards and to improve the speed at which detection occurs.
Conventionally, the requirement for stand-off detection
has limited solutions to those exploiting electromagnetics or
electro-optics; however, acoustics-based techniques offer additional
opportunities.
Acoustic excitation of EH targets has been investigated
previously, specifically for the detection of buried anti-tank
landmines. Here, the target is excited seismically through direct
mechanical coupling to the ground, or via seismic-acoustic
coupling when employing an audio source. The resulting target
excitations are then measured directly using a radar [2] or
indirectly, using a laser source to probe variations in the vibrations
at the surface of the overburden soil resulting from the
buried target [3].
The challenge in the work presented here is in the application
of the technique to a more general class of threats: buried
and not buried, obscured and not obscured. In these scenarios,
the optimization of the acoustic source, the predicted reaction
of the target, and the optimization of the interrogating radar
are all subject to different constraints and opportunities than
in the buried landmine case.
Micro-Doppler Signals
A typical radar system transmits electromagnetic energy into
a region of space and measures the return signal that has been
reflected from targets in the environment. The characteristics
of the return signal carry information about the physical
properties of the reflecting target. For example, if the target
is moving with respect to the radar, then the returning signal
will have a different frequency than the transmitted signal.
The magnitude of this phenomenon, known as a Doppler shift,
is proportional to the radar transmitting frequency and the
speed of the target in the direction of the radar.
In addition to the bulk displacement of the target object, the
motion of the target's constituent components, or a repetitive
micro-motion of the target itself, results in a special Doppler
signature known as the micro-Doppler signal [4]. Familiar
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