# Design of an Autonomous Cooperative Drone Swarm for Inspections of Safety Critical Infrastructure
## 1 Introduction  
Drones are essential components of the Unmanned Aerial System (UAS) [1]. During the past decade, drones have been deployed for various real-life applications such as inspections [2; 3; 4; 5; 6], surveillance and reconnaissance [7; 8], package delivery [9; 10], search and rescue [8], agriculture [8; 11], remote sensing [8], transportation [12], emergency medical services [13; 14], and military operations [1; 8; 15]. Drones with sensing capabilities make them highly applicable for inspections in critical environments that are not easily accessible to humans. In such settings, drones bring significant benefits including reduced operational cost, time, and risk for inspectors, as well as improved inspection quality.  
The European research and innovation project Drones&Safety (D4S) strives to increase the safety of the European civil transport system by building a cooperative, autonomous, and continuously operating drone system that will be offered to railway and bridge inspection operators to inspect their transportation infrastructure accurately, frequently, and autonomously [16]. A swarm of drones addresses this ambitious objective by providing safe and cost-efficient inspection missions (Figure 1).  
To circumvent the limited flight time, drones will be integrated with the capability to navigate to and detect nearby powerline cables for recharging [17].  
Our use cases utilize high-resolution cameras with onboard fault detection algorithms to identify damages and deterioration of structural parts of bridges and the catenary system of railways. We use digital 3D reconstructions of targeted bridges or railway segments from pre-flight activities for mission planning [18]. The swarm of drones is connected with cloud services such that the ground segment, supporting the autonomous mission, is updated with the latest information concerning flight restrictions over the target assets. Furthermore, cloud services publish a coarse motion path for the swarm by disseminating waypoints defining a route from the launch location to the end location of the inspection.  
Our key contributions constitute the design of a custom-built drone swarm based on open hardware and software components integrating algorithms for obstacle avoidance, cable detection and identification, motion path planning, formation flying, and fault detection. We validate the integration of system components in a UAS operation by linking the drone system to ground infrastructure and supporting cloud services. Our innovative UAS combines the advantages of autonomous drones with algorithms for fault detection, navigation, and swarming supported by cloud computing services.  
The paper is organized as follows: Section 2 discusses work related to infrastructure inspections with drone swarms. In Section 3, we model the concept of operation for the cooperative drone swarm. Sections 4 and 5 introduce the drone hardware and the supporting cloud services, respectively. Communication aspects are described in Section 6. In Section 7, we introduce key algorithms and protocols to support autonomous inspections with a drone swarm. Section 8 gives selected examples of the validation of the system design, and Section 9 concludes the paper.