IEEE Geoscience and Remote Sensing Magazine - June 2022 - 71

easier and also enables the UAV to
use the high-accuracy georeferring
system for navigation, without the
need for duplicating these sensors.
UAV systems that incorporate stateof-the-art
sensors or require ad hoc
flight operation modes are usually
implemented following this approach
[43].
The prototype of a UAV-based
Wi-Fi Antenna RTK Antenna
GNSS Antenna
External Compass
UAV Controller
RTK Module
GPR system is shown in Figure 3,
where the main elements and subsystems
are indicated. In this prototype,
two helix antennas with reverse
handedness are used for the GPR
subsystem [44].
ARCHITECTURES OF UAV-BASED
GPR SYSTEMS
Air-launched GPR systems can be
classified into three main groups
according to the orientation of
the GPR antennas with respect to
the ground, as illustrated in Figure 4. As UAV-based GPR
systems are a subgroup of air-launched GPR systems, the
same classification applies to them.
◗ Forward-looking GPR (FLGPR): GPR antennas are pointed
toward the ground with oblique/near-grazing incidence.
This helps in reducing reflections from the air-
ground interface [32], [45], [46]. The main advantages
of FLGPR systems are 1) the clutter due to reflection on
the ground is minimized as it is scattered away from the
incident angle and 2) in the case of an FLGPR mounted
on a ground vehicle [Figure 1(b)], a safe distance between
the scanned area and the vehicle can be kept,
which is significantly important in the field of explosive
threats detection.
R/C Receiver
Radar Module
Helix Antennas
FIGURE 3. An example of a UAV-based GPR system depicting the main subsystems and
components [44].
[52]. As GPR antennas are pointed downward, in-band
interferences to and from other radio-frequency sources
within the vicinity of the UAV are minimized with respect
to FLGPR and side-looking GPR architectures.
The choice of the GPR architecture to be placed on board
the UAV depends on the kind of scenario to be surveyed.
For example, in the case of shallow targets, FLGPR or sidelooking
GPR architectures could provide better detection
capabilities than those of DLGPR as the air-soil clutter is
minimized. However, for deeper targets, DLGPR is likely
to provide better results than FLGPR because of its greater
dynamic range. A comparison of the imaging capabilities
◗ Side-looking GPR: This is similar to FLGPR, as the GPR
antennas are also tilted with respect to the air-soil interface
to minimize specular reflection. However, unlike
FLGPR, the vehicle where the GPR is assembled does
not move in the same direction at which the antennas
are pointing toward. Instead, it moves laterally [47] or
describes a circular path [that is, circular SAR (CSAR)]
[48]-[50], as illustrated in Figure 4.
◗ Down-looking GPR (DLGPR): In this architecture, GPR
antennas are pointed perpendicular to the soil surface
[32], [33], [51] [Figure 1(c)]. As the antennas are usually
closer to the inspected area, this results in smaller freespace
propagation losses than in FLGPR systems but at
the expense of a stronger reflection of electromagnetic
waves at the air-soil interface. Depending on the range
or depth resolution of the DLGPR system, the reflection
at the air-soil interface might mask shallow targets.
An additional advantage of DLGPR systems is related
to electromagnetic interferences, a concern stressed in
JUNE 2022 IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE
FLGPR
DLGPR
SLGPR
CSAR
FIGURE 4. An illustration of UAV-based GPR-scanning architectures.
FLGPR: forward-looking GPR; DLGPR: down-looking GPR;
CSAR: circular SAR.
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IEEE Geoscience and Remote Sensing Magazine - June 2022

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