IEEE Geoscience and Remote Sensing Magazine - March 2022 - 255

for the exploration of shallow glaciers (depths <500 m). The
second radar, called snow radar, works at higher frequencies
(a center frequency of 5 GHz, with a band of up to 6 GHz)
to measure the thickness of the snow layer on ice (depths
<40 m). The use of a Ku-band radar was also proposed to
evaluate the ice-surface topography and shallower internal
layering (depths <10 m) [52]. Further airborne sounders for
ice exploration were reported in [53]-[60].
As a part of the OASIS project funded by NASA, a
50-MHz airborne GPR was designed to estimate ice thickness
and detect aquifers in arid areas. An experiment conducted
over the Kuwait desert in 2011 proved the capability
of detecting a water table at a depth of approximately
50 m [61]. A VHF airborne sounder, with a bandwidth of
145-185 MHz, was developed by the research consortium
Consorzio di Ricerca su Sistemi di Telesensori Avanzati
(CO.RI.S.T.A.) in the framework of a research project funded
by Italian Space Agency [62].
Airborne GPRs have attracted considerable attention for
subsurface target detection and under-foliage imaging applications
[63], [64]. A popular example in this field is the
Coherent all Radio Band Sensing (CARABAS) system developed
by SAAB AB [65].
Recently, GPR ability to penetrate ice has also triggered
the development of satellite-borne radar systems for planetary
exploration. For example, SHARAD [66], [67], MARSIS
[68], and JAXA's SELENE radar instruments [69] are
low-frequency (e.g., few MHz) sounders installed on satellite
platforms in orbit around Mars and the moon. The aim
of these instruments is to investigate the planet's subsoil.
In this regard, an indirect proof of the presence of liquid
water on Mars has recently been achieved by processing
MARSIS data [70].
DRONE AND UNMANNED AERIAL
VEHICLE-BORNE GPRs
An emerging research trend in contactless GPR deals with
the exploitation of drones and unmanned aerial vehicles
(UAVs). The impetuous development in this field is favored
by several factors, such as accurate autonomous
flight capability, the possibility of multiple-sensor integration,
innovative avionic systems and energy-saving
batteries, and a significant reduction of costs and maintenance
operations.
The first studies on drone-based radar imaging were reported
in [71] and [72]. Later, early tests were carried out by
means of UAV-borne radar systems operating at the C [73],
P [74], and X [75] bands. Although these high-frequency radars
did not allow for significant penetrating capabilities,
they did demonstrate the possibility of using imaging radars
on UAVs.
The first feasibility study of a multifrequency GPR for rotary
UAV platforms was reported in [76] and supported by
simulated scenarios. The study aimed at assessing the performance
of a GPR operating at 500 MHz and 4 GHz by analyzing
the signatures of targets with different sizes and shapes.
MARCH 2022 IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE
An experimental test involving a C-band pulsed radar
mounted on a consumer microdrone in DL modality
was proposed in [77]. In this work, the imaging performance
of the system was assessed by considering the
measurement results involving different physical and
man-made targets.
Later, a GPR system using two log-periodic dipole array
(LPDA) antennas and mounted on a mini-UAV for archaeological
prospections was presented in [78]. The system
consisted of a light and portable frequency-modulated continuous-wave
(FMCW) sensor operating at 745 MHz with
510 MHz of bandwidth. The radar was tested in a controlled
environment, showing the ability to detect a buried object
at a 0.4-m depth.
A custom-designed, lightweight GPR based on software-defined
radio technology mounted on board a
minidrone was proposed in [79] for landmine detection.
This GPR was lighter than conventional GPR systems and
was capable of real-time reconfiguration. As described in
[79], the system was able to detect landmines at up to a
0.2-m depth in semiwet terrains (at humidity levels of approximately
70%).
A prototype combining a light and compact radar onboard
a commercial minidrone and data processing based
on an MWT approach was proposed in [80]. Specifically,
the P440 pulsed radar module (Humatics Corporation) operating
at 3.95 GHz with 1.7 GHz of bandwidth was employed.
A proof-of-concept flight experiment was carried
out to assess the imaging capabilities of the prototype and
demonstrate the possibility to build up cost-effective UAVbased
radar-imaging solutions.
A wideband FMCW GPR operating from 1 up to 4 GHz
in a bistatic configuration was presented in [81]. In this
case, the radar was designed to balance among weight, size,
power consumption, and penetration depth. The system
was successfully integrated with horn antennas. A preliminary
flight test, with the radar on-board a mini-UAV, demonstrated
the system's ability to detect corner reflectors.
Following the encouraging preliminary results, the use
of UAV-borne radars has become more attractive in the
international scientific community. Recently, an ultrawideband
snow sounder was presented in [82]. The system
was designed with a focus on low payload weight to fulfill
mini-UAV mounting and high-range resolution requirements
for snow measurements. The sensor is a multipurpose
microwave device working in the frequency range of
0.1-6 GHz, capable of operating as a microwave imaging
system, short-range radar, digital network analyzer, and
time-domain reflectometer. In the same context, an analogous
drone-borne GPR system was proposed in [83] for soil
moisture mapping and precision agriculture applications.
The system consists of a handheld vector network analyzer
(VNA) working as SFCW radar, and its weight is approximately
1.5 kg. The system is equipped with a lightweight
hybrid horn-dipole operating in the 250-2,800-MHz frequency
band.
255
IEEE Geoscience and Remote Sensing Magazine - March 2022

Table of Contents for the Digital Edition of IEEE Geoscience and Remote Sensing Magazine - March 2022
