IEEE Geoscience and Remote Sensing Magazine - December 2021 - 174

spotting of the hazard before the vehicle drove over it. By pointing
the antenna array to look ahead of the vehicle, FL-GPR
systems are able to achieve a reasonable lead detection time
before reaching the actual explosive hazard location.
However, performance of an FL-GPR system is highly
impacted by rough surface clutter (see, e.g., [13]). Depending
on the soil conditions and degree of surface roughness,
the returns from the ground interface can dominate the
radar measurements and obscure the target response. This
leads to significant uncertainty in the assessment and interpretation
of the attained radar images.
Compared to its downward-looking counterpart, whereAPPROACHES
BASED
ON CONTROLLED
EXPERIMENTS, THOUGH
COMPLICATED AND COSTLY,
ARE VALUABLE IN
UNDERSTANDING
PHENOMENOLOGY AND
CAN PROVIDE REAL
SCATTERING DATA.
in the antennas are either coupled or very close to the ground
surface (see, e.g., [3], [4], and [14]-[16]), an FL-GPR system
employs oblique and near-grazing incidence sensing to enable
target detection from a
safe standoff distance. In this
case and depending on the
roughness profile of the illuminated
surface, most of the
energy would be forward-scattered
along the specular direction,
yielding reduced returns
from the air-ground interface.
In practice, however, even
if the backscattered echo from
the rough surface is relatively
weak, the intensity of the signal
returns from concealed
targets can also be quite low.
This renders target detection
and localization challenging, especially in the case of nonmetallic
objects. Therefore, proper design of both imaging
and detection approaches becomes fundamental to improving
the performance of the FL configuration. To compensate
for the loss of energy due to the signal bounce at the
ground interface, synthetic aperture radar (SAR)-based focusing
is typically employed [4], [17]-[21], wherein coherently
combining the returns at multiple antenna positions
focuses the energy to an image pixel, thereby improving
weak target representations.
Several approaches based on electromagnetic modeling
and statistical detection analysis have been proposed for
FL-GPR (see, e.g., [19], [22], and [23] and the references
therein). In this article, we focus on array-based FL-GPR systems
and provide a comprehensive review of the state-of-theart
radar imaging and detection methods, highlighting their
advantages and limitations. We attempt to group advances
in FL-GPR based on the nature of the data employed, system
prototyping, properties of the imaging scene assumed, and
principal signal processing algorithms undertaken.
SOLUTION TO THE FORWARD PROBLEM
A controlled solution for the forward-scattering problem can
be a powerful tool to assess and characterize the ground interface
contributions, predict the target signature, and design
174
and validate image formation methods, including clutter
mitigation approaches. This would require determining the
scattered field from the illuminated scene that essentially
comprises the targets buried in a dielectric half-space with
a rough surface profile. For simplicity and without loss of
generality, the involved media are assumed homogeneous.
Under known materials and imaging geometry, in the
presence of a flat ground interface and considering targets
represented by canonical simple shapes, the underlying scattering
problem can be analytically characterized by solving
Maxwell's equations (see, e.g., [12], [24], and [25]). However,
in most cases of practical interest, those assumptions
and prior information do not hold, and more realistic and
flexible approaches are needed. These approaches call for
implementing a full-wave solution of the scattering problem
numerically (see, e.g., [26] and the references therein) or
collecting experimental data. In the next sections, we summarize
key FL-GPR systems used in data collection and also
discuss the numerical approaches used for data modeling.
FORWARD-LOOKING GROUND-PENETRATING RADAR
PROTOTYPES AND EXPERIMENTAL APPROACHES
Approaches based on controlled experiments, though complicated
and costly, are valuable in understanding phenomenology
and can provide real scattering data. This, however,
requires the availability of specific facilities and preparation
of experimental campaigns. Toward this end, different
prototypes and radar systems have been developed for data
measurements, which are summarized as follows.
In [27]-[29], a prototype of a high-resolution GPR system
was designed and deployed by SRI International. This
system is a stepped-frequency, fully polarimetric radar, operating
over the 0.3-3-GHz frequency band. The prototype
was conceived to operate as an FL SAR system, providing a
ground-surface resolution of about 5 cm. The experimental
activity was originally designed to define optimal FL-GPR parameters
and support image processing for the standoff target
detection of concealed antitank mines. Reference [27] was the
first publication reporting experimental data collection and
processing with an FL SAR system for GPR applications. It
provided insights into the signal-to-clutter ratio (SCR) of shallow-buried
targets as well as key features of clutter statistics.
Time-frequency analysis was applied in [30] using an
FL-GPR system to detect plastic targets buried under a
rough ground surface. Different quadratic time-frequency
distributions were considered to characterize and interpret
the scattering from both the targets and the rough surface.
This work employed experimental data described in [27]
and proposed a target detector based on the signal ambiguity
function, which showed superior detection performance
over a conventional detector.
In [31], an FL-GPR operating from 0.76 to 3.8 GHz was
developed by Planning System Incorporated (PSI). This FLGPR
system is a broadband, stepped-frequency, continuous
wave (CW) system performing digital phase detection
of the CW echoes on a fixed number of receiving channels.
IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE DECEMBER 2021
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