Uncrewed aerial vehicles (UAVs), also known as drones, are ubiquitous and their use cases extend today from governmental applications to civil applications such as the agricultural, medical, and transport sectors, etc. In accordance with the requirements in terms of demand, it is possible to carry out various missions involving several types of UAVs as well as various onboard sensors.
1. Introduction
Uncrewed aerial vehicles (UAVs) or drones
[1] belong to the large family of connected objects. After being utilized extensively in military operations
[2[2][3],
3], drones have remained out of reach for civilians
[4] due to their high price. With recent innovations in micro-controllers and sensors, drone prices have been reduced to become affordable. Currently, they are widely used for commercial purposes such as surveys, photography, and cinematography. UAVs, in general, are outfitted with on-board sensors that allow them to collect geospatial information about their environment and are remotely controlled from a ground control station
[5]. From this station, the operator can plan and supervise the evolution of the mission. Their use in the civil and industrial sector
[4] has allowed us to optimize industrial processes or to carry out missions in hostile environments that are partially or entirely inaccessible to humans
[6,7,8,9][6][7][8][9]. For example, in the agricultural field, farmers are now facing various problems that impact the quality of their crops. Drones are an efficient and cheap means to collect information on ecosystems and their variations due to, e.g., climate change, soil erosion, water availability, and meteorological extreme events. They are, for example, also used in spraying plantations, which saves time and optimizes yields
[10]. Goodrich Payton et al.
[11] have demonstrated the efficiency of drones in precision agriculture. From the collected data, they reconstructed a 3D cartographic representation of the plots to better analyze the density of vegetation and soil heterogeneities.
Drones provide a broad variety of purposes in the health sector, including delivering medical supplies to remote or hard-to-reach areas, e.g., transporting blood samples and lab results
[12]. In the field of transportation, drones can be used for package delivery
[13[13][14][15],
14,15], traffic monitoring
[16], and infrastructure inspections
[17]. Drones have also been used to map volcanoes’ terrain and to detect volcanic activity. Thiele et al.
[5] used drones equipped with thermal cameras, gas sensors, and other instruments to measure temperature, gas concentrations, and other indicators of volcanic activity. This information can be used to predict eruptions, to elaborate rescue operations, for photogrammetry and infrastructure monitoring, or even for delivery services. UAVs can also be used to study the geology of a volcano and its surrounding area, providing valuable information for volcano research. The use of UAVs in this field can greatly improve the efficiency, accuracy, and safety of operations, as well as decrease costs by reducing the need for human intervention in dangerous areas
[5]. However, it implies several challenges related to communication services, such the range, security system, and communication architecture
[18].
The communication architecture of drones relies on a Flying Ad-Hoc Network (FANET)
[19], without the requirement for a fixed infrastructure. FANET is a decentralized ad-hoc network that enables communication between the ground station and flying vehicles, such as drones and aircraft
[20]. As shown in
Figure 1, FANET inherits from both Vehicular Ad-Hoc Network (VANET)
[21] and Mobile Ad-Hoc Network (MANET)
[22] networks. It is a subclass of MANET that is an extension to highly mobile devices such as smartphones and laptops. These three types of ad-hoc networks share the ability to form and maintain a network connection dynamically
[23]. Combined, they are an effective tool in creating a wide-area network that links UAVs, vehicles, and communication devices. They can also be used for applications such as collision avoidance
[24], traffic jam prevention, and intelligent transportation systems
[25].
Figure 1.
Different types of wireless Ad-Hoc Networks: FANET [19] vs. VANET [21] and MANET [22].
As the need for UAVs grows, there is an increasing requirement for systems that can coordinate several UAVs in a dispersed environment
[1,26][1][26]. This is where multi-UAV systems are relevant. Multi-UAV systems, commonly called swarms, are composed of multiple UAVs that are coordinated to collaborate in order to accomplish a shared purpose
[27]. These systems have the potential to coordinate mission tasks across multiple UAVs in a parallel manner. They have several possible applications, including rescue missions
[7,28][7][28], surveillance and reconnaissance
[6,26[6][26][29][30],
29,30], environmental monitoring
[17], and even payload carrying
[13,14,15][13][14][15]. One of the primary benefits of swarms is their capacity to operate cohesively, allowing them to cover large regions quickly and make choices collectively, making them more robust to failure than individual UAVs
[31]. Swarms operate according to the design of a multi-UAV system architecture that is controlled by a central system
[6] or through decentralized algorithms
[1,32,33][1][32][33].
To work in a coordinated manner, a multi-UAV system requires the following key components. (1) The UAVs themselves: these are the drones that make up the swarm. (2) A ground control station: this is the central point of control for the drone swarm. It oversees the transmission of commands to the drones, as well as the retrieval of data from them. It can be a single computer or a cluster of computing nodes. (3) A communication system: it consists of devices and antennas that enable communication between the drones and the ground station via a common protocol. The communication system can use a variety of technologies, including MANET
[22], FANET
[19], and VANET
[21]. (4) A navigation system: this is the system that enables the UAVs to fly and locate themselves in the environment. It includes sensors such as GPS and Inertial Measurement Units (IMUs). (5) A control system: it allows the ground station to control the drones and to coordinate their actions. It consists of software for mission planning, decision making, and swarm behavior. (6) Finally, a data processing system: this is the system that allows the ground station to process the data in real time and feed the data back to the control system. It can include tools for image processing, data analysis, and machine learning. All these components work together to allow the swarm to function as a cohesive unit, with the ground station providing overall command and control, while the UAVs collaborate to attain a common aim.
Depending upon the operations’ nature and mission requirements, the architecture of a multi-UAV system might be centralized
[6] or decentralized
[32,33][32][33]. Because of its complexity and the very dynamic environment of operation, there are several challenges related to using multi-UAV for highly mobile networks operating in large-area missions:
- Communication: maintaining communication between the drones in a swarm can be challenging, especially in highly mobile and wide areas. The drones must be able to maintain communication even when they are moving at high speeds or when they are far apart from each other.
Coordination
: coordinating the actions of the UAVs is complex, especially in elaborate missions. To reach a shared purpose, the drones must be able to successfully collaborate.
-
Autonomy: the drones must be able to operate autonomously without human intervention. This requires collaborative actions for decision making, navigation, and swarm behavior. Theses challenges include spatial awareness, maintaining a distance from each other, and communicating potential threats to other drones, such as heavy wind gusts, rain, and obstructions.