Instrumentation & Measurement Magazine 25-3 - 38

Fig. 1. A simulated radar signal from a moving helicopter.
examples of micro-Doppler sources include the swinging arms
of a person walking, the rotating blades of a flying helicopter,
or the rattling of a car engine, all of which create a characteristic
micro-Doppler signal. Analysis of this characteristic signal
can be used to identify and classify targets. For example, it
has been used to discern humans and animals [5], identify helicopters
[6], and distinguish ballistic warheads from decoy
material [7].
To illustrate, Fig. 1 presents a simulation of the Doppler and
micro-Doppler signal from a flying helicopter. As the helicopter
approaches the radar position and then passes overhead, its
bulk motion produces a slowly varying Doppler signal which
changes from a positive frequency to a negative frequency.
The rotation of the helicopter's rotor blades imparts an oscillating
micro-Doppler signal centered on the helicopter body.
The speed of the helicopter can be measured from the Doppler
signal, and the number and speed of the rotor blades can be
measured from the micro-Doppler signal.
The processing and analysis of micro-Doppler signals is
an active area of research. A wide set of tools has been applied
such as the short-time Fourier transform (STFT), the wavelet
transform [8], empirical mode decomposition (EMD) [9], and
many others. The suitability of a given tool depends on context
and analysis requirements, such as the necessary time-frequency
resolution for target detection and identification, the
processing load and limitations thereof, and the complexity
of the micro-Doppler signal. In the case that a constant radar
transmitter frequency is used, a frequency-demodulation can
be applied to the radar signal to detect the target micro-Doppler
signature.
Active Detection of Concealed Objects
This paper presents the first step in a novel approach to detect
concealed objects. The concept is to induce vibrations in a
target using acoustic stimulation from a nearby loudspeaker
and then measure the corresponding micro-Doppler signature
of the target using a continuous-wave (CW) radar. This
new approach has two key requirements: a radar signal capable
of transmission through a concealing barrier; and sufficient
38
coupling between the acoustic waves and the target, known
as the vibro-acoustic coupling. The radar component introduces
a trade-off: the amplitude of micro-Doppler signatures
increases with higher transmission frequencies, whereas penetration
through concealing barriers favors lower transmission
frequencies. While frequencies as high as 24 GHz have been
demonstrated in through-the-wall mapping [10], Doppler
detection of people moving behind walls [11] and through-thewall
detection of heartbeats via their micro-Doppler signal [12]
use several GHz or less. The vibro-acoustic coupling requirement
is necessary to induce target vibrations large enough to
be detected through their micro-Doppler signal. This requirement
is best achieved using a tunable acoustic system that
can adjust the frequency and volume on a target-by-target basis.
The system shall also provide exceptional performance at
lower acoustic frequencies, where there is a greater chance of
encountering a natural resonance frequency of small to moderately
sized targets.
The measurements presented in this paper demonstrate a
correlation that can be observed between the micro-Doppler
signal from a target and the acoustic frequency of a nearby
loudspeaker. The clear detection of vibro-acoustic coupling via
the micro-Doppler signal is a foundational step towards a fully
developed detection system. While the targets in this experiment
were not concealed behind a barrier, the measurements
employed a radar frequency suitable for through-the-wall target
detection. The validation of the detection principles and
assessment of the measurement data anticipate future measurements,
which will include concealing the target.
Measurement Setup
The measurement system comprises two components: a CW
radar system and an audio system. The radar system uses
separate Aaronia HyperLOG 7040 log-periodic broadband
antennas to transit and receive the radar signals. An Agilent
N5183A signal generator is used create a radar carrier signal
between 750 MHz and 3.3 GHz that is fed into a power amplifier
before transmission. The received signal is fed into a
low-noise amplifier and mixed down to a 1 kHz intermediate
frequency (IF), chosen to avoid 1/f noise and spurious signals
at low frequencies. The IF data are sampled at 20 kHz by
a PicoScope 4224 data acquisition module and stored for offline
analysis.
The audio system uses an Android tablet to create a lowfrequency
tone. This signal is fed into a Traynor SB115 bass
amplifier/speaker cabinet combo. The full system setup is
shown in Fig. 2, along with a water bottle target.
Micro-Doppler Measurements
Micro-Doppler measurements of acoustically induced target
vibrations were performed, initially using a vertically
mounted metallic plate, and then using various household objects
as representative targets, such as cans and plastic bottles,
both with and without water. The assortment of targets provides
a comparison of different target electrical conductivities,
masses, and mechanical properties which should impact the
IEEE Instrumentation & Measurement Magazine
May 2022
Instrumentation & Measurement Magazine 25-3

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