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in remote sensing has been reported in many works with high
performance exploring areas of different sizes, sometimes
hazardous, with assumable costs as compared to traditional
airborne or satellite systems (Jardin and Jensen, 2013). The
range of applications makes UAVs suitable tools in remote
sensing with an apparent market, which is to be consolidated,
when UAVs are widespread, which will likely be the turning
point in remote sensing, as pointed out by Esler (2010) and
Hardin and Jensen (2011) several years ago.
UAVs must navigate to perform the remote sensing mission;
for this reason they are equipped with different instruments
and sensors, such as, Global Positioning Sensors (GPS), Inertial
Navigation Sensors (INS), Micro-Electro-Mechanical Systems
(MEMS) gyroscopes and accelerometers, Altitude Sensors ( AS)
(Quinchia et al., 2013) or even camera-based sensors, among
others (Shabayek et al., 2011; Bristeau et al., 2011), where
multisensory fusion techniques are required (Oh, 2010). Obvi-
ously, UAVs are generally configured with control strategies for
autonomous navigation that must follow a previously planned
path, with the ability to make autonomous decisions. Obstacle
avoidance is also required during navigation. Ultrasonic sen-
sors (Bristeau et al. , 2011) or 3D laser scanners (Holz, 2013)
are sometimes also used for safe navigation, where they can
be used for detecting other UAVs around them. Also, dynamic
strategies for positioning, landing, and take-off, including
VTOL, as part of the full path planning, is necessary in normal
and adverse environmental conditions, where aircraft control
in wind conditions is essential. Moreover, UAVs require ad-
ditional logistic resources to be permanently operative, such
as battery recharging or refueling.
Also, in recognition of technological developments and
advances in communications, significant progress is being
made in applications involving multiple UAVs in collaboration
or even between UAVs and ground systems, including Un-
manned Ground Vehicles (UGVs) or unmanned marine Surface
Vehicles (USVs ). Collaborations between UAVs and UGVs can be
found in Martínez-de-Dios et al. (2011) and Maza et al. (2011),
and between UAVs and USVs in Sánchez-Benítez et al. (2011).
Some studies about indices of effectiveness of UAVs have been
proposed in Samkov and Silkov (2012).
From the approach of remote sensing, navigation and all
issues mentioned above, mission programming and flight
strategies are excluded and not specifically considered in
this overview, unless they are essential with high degree of
involvement in remote sensing tasks.
Navigation, communication, mission programming and
flight strategies have been widely discussed in the scientific
and industrial communities with abundant publications and
work. Being aware of this, we intentionally excluded all top-
ics related with power sources, communication, navigation
(including obstacle avoidance), path planning, flight control
systems, evasive maneuvering, landing, take-off, autonomous
flight, fueling and refueling or localization, including Simulta-
neous Localization and Mapping (SLAM). The Ground Control
Stations (GCS), required by some systems, are also excluded;
this is because these specific operations are not considered as
specific from the remote sensing point of view, although they
are absolutely necessary for conducting successful remote
sensing missions. Nevertheless, some of them could be of
interest in remote sensing because they can be considered as
the starting point for other remote sensing-based approaches.
By example, Pearre and Brown (2012) capture path gathering
information, using wireless link, from sensors on the ground
for path planning, but this method can be used for recovering
information from sensors installed on dynamical structures
or elements, such as glaciers or rivers with moving elements,
where methods above can be useful for remote sensing.
Thus, the overall goal of this paper is to provide an over-
view of remote sensing applications based on UAVs equipped
with a set of specific sensor technologies and also with
several UAVs working in concert with each other. With such
purpose, as far as it has been possible, we have collected most
recent technological advances, especially in the last decade,
where the boom has occurred. Nevertheless, apologies to au-
thors or possible references if some of them were not cited.
Different worldwide international associations and forums
have emerged related to UAVs, providing ideas, information
and opportunities for members, users, and researchers while
they cover most fields and application areas, including any re-
lated to remote sensing. Commercial benefits are also consid-
ered without ruling out the use of all available resources for
immediate humanitarian interventions protection or search
and rescue in disasters. Additional member relations, oppor-
tunities, and training are offered, where remote sensing is an
important activity (UAS Vision, 2015; UAVa, 2015; UAVc, 2015;
UAPA, 2015, UAVS, 2015; AUVSI, 2015). Also, local associations,
at the country or region level, become more or less active
from the remote sensing point of view.
In 2006, the NASA Civil UAV Assessment Team (Yuhas,
2006), defined Earth observation missions for UAVs, based
on user-defined needs to determine technologies, platform
capabilities, and a comprehensive civil UAV roadmap. Later,
Colomina et al. (2008) established some fundamental issues
of UAV-based photogrammetry and remote sensing as a para-
digm, identifying challenges and specific advantages.
Many institutions, research centers and companies world-
wide have addressed the challenge of designing and develop-
ing UAS, with the aim of performing different missions, includ-
ing remote sensing: NASA (2015a), INTA (2015), NOAA (2015),
USGS (2015a). The list can be completed through the websites,
where specific remote sensing missions can be found. Here,
flight regulations must be considered for effective use of UAVs
in different applications (Rango and Laliberte, 2012).
Regarding the state of the art of UAVs in remote sensing,
Haarbrink (2011) provided information and perspectives about
this issue. A survey is also proposed in Ma et al. (2013) estab-
lishing a framework with three levels (data acquisition, data
processing, and applications). In the data acquisition level,
flight, autonomy, and trajectory were addressed. Data process-
ing included photography, image matching, and mosaicking
and classification and finally, the applications, were catego-
rized as: environment and agriculture, terrain extraction, 3D
visualization and monitoring of hazards. Colomina and Molina
(2014) reviewed the use of UAVs in photogrammetry and re-
mote sensing (PaRS). This last work is structured according to
the following sections: (a) Introduction, where topics, names
and acronyms, pioneers, literature evolution are revised; (b)
Early developments, starting at the end of 19th century with
balloons; (c) Unmanned aerial systems and unmanned aerial
systems for PaRS, establishing the principles for classifying
the different platforms; what is considered an aircraft, ground
control station, communication to command and control
the aircraft and mission planning; (d) Regulatory bodies and
regulations, involving national and international agencies and
organizations; and (e) Navigation, orientation and sensing pay-
loads, covering autopilots, navigation and orientation systems,
sensing payloads. This last including visible-band, near-infra-
red, multi-spectral and hyperspectral cameras, thermal imag-
ing, laser scanners and synthetic aperture radar; (f) Processing,
image orientation, camera calibration and surface reconstruc-
tion; (g) Unmanned aerial systems PaRS applications and geo-
matic markets, agricultural and environmental applications,
intelligence, surveillance, reconnaissance, aerial monitoring
in engineering, and cultural heritage. Shahbazi et al. (2014)
reported on different applications including: (a) Precision ag-
riculture and rangeland monitoring with challenges and future
perspectives, including land-cover mapping and classification,
crop health monitoring, biophysical modeling attributes, soil
282 April 2015 PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING