# Design of an Autonomous Cooperative Drone Swarm for Inspections of Safety Critical Infrastructure
## 2 Related Work  
Infrastructure inspections have traditionally been performed as ground-based visual inspections. This method is laborious and suffers from low coverage of difficult-to-access areas. The main added value of inspections with drones is their ability to cover inspection targets efficiently and accurately often in areas inaccessible by foot or manned vehicles. To reduce the cost of labor, a variety of autonomous platforms have been introduced in the literature. These platforms cover unmanned ground vehicles, fixed-wing drones [19], rotor-based drones [2], autonomous robotic blimps [20], and even hybrid [6] or cooperative solutions [21]. Table 1 compares different unmanned platforms for infrastructure inspections. Figure 1: Drones4 Safety applies a drone swarm with a self-charging capability for railway or bridge inspections [16]. Image processing allows autonomous fault detection. The mission operation is supervised through a set of cloud services. Despite the low operational risk of inspections with ground-based vehicles, we focus on autonomous Unmanned Aerial Vehicles (UAVs) in our study. Our proposed UAS design is based on the multi-rotor drone platform as it offers a good trade-off between inspection quality, maneuverability, flexibility, and cost-efficiency. The multi-rotor drone can pass over terrain that may be impossible to cross for ground vehicles and ground personnel, hovering at specific locations and acquiring high-resolution inspection images. For long-duration inspection, a recharging scheme or a drone replacement scheme is needed [22]. However, this topic is beyond the scope of this paper. The concept of using cooperative drones as part of a UAS has been discussed in the literature [23; 24]. In a countermeasure UAS, drones can identify and neutralize multiples malicious drones through cooperation [23]. The authors of [23] review the available technologies for sensing, mitigation, and command and control systems. Another recent review of the state-of-the-art of swarm-based UASs provided a set of recommendations on the specific types, modes of communication, architectures, and swarm patterns for particular applications of a swarm-based UAS [24]. Common to these studies is the lack of motivation for specific and chosen technologies to instantiate the UAS. However, while several studies in the literature point to specific technologies for UAS design, there is a gap in knowledge concerning design aspects when taking into account the UAS as a whole. Several papers address the application of drones for bridges [2; 25] and railway inspections [26; 27]. [PERSON] et al. [25] studied details of drone-enabled inspections and presented considerations to obtain optimum data acquisition. The authors recommended a five-stage bridge inspection methodology using a commercial drone. The method involves a bridge information review and site risk assessment analyzing information about the inspection target and the surrounding area. A drone pre-flight setup is used to conduct a thorough inspection of the drone before the first flight. The following drone-enabled inspection and the damage identification stages define the core objectives of the inspections. [PERSON] et al. [26] gave an overview of sensors and platforms for railway inspections and highlighted several benefits of using UAVs for these inspections [26]. Recommendations from the survey pointed toward the need for lightweight sensor development and robustness in the design of Artificial Intelligence (AI) components. Another important aspect was the needed collaboration between railway companies and the robot manufacturer to ensure a successful system design. [PERSON] et al. [27] discussed the emerging applications of drones in the railway domain. The authors point out that drone-based monitoring can be advantageously integrated into existing security surveillance systems for railway infrastructures.