# A Review of Counter-UAS Technologies for Cooperative Defensive Teams of Drones
## 2 Definitions and Basic Concepts for Cooperative Drone-Based Counter-UAS Systems
### Cooperative Drone-Based Counter-UAS Systems
Neutralization--if the intruder drone(s) did not immediately manifest the malicious intentions, the defensive team would act in proximity by physically chasing it and would be ready for mitigation when the threat has manifested [14], thus allowing a higher probability of success for the neutralization phase. In the literature, there have been several recent studies on the use of a drone team as a defensive tool (e.g., [15; 16]). Indeed, the features for mobile proximal sensing may be adapted also for mitigation purposes, and some defensive drones may be equipped with specific electronic or kinetic-mechanical neutralization systems in order to perform preliminary mitigation actions and to take advantage of the closeness to the moving targets. Scalability--Traditional CUSs lack scalability and are not usually able to face intrusions of drone teams or swarms. To the contrary, a defensive team of drones inherently represents a scalable solution. Indeed, the coordination mechanisms usually exhibit a scalable computational complexity with respect to the cardinality of the team, especially when decentralized approaches (i.e., without central decision points) are applied. These approaches ensure self-configuration and robustness of the team in front of individual off-nominal events (i.e., failures, communication losses, etc.) or threats (i.e., attacks to the individual defensive drones). Moreover, with the proper sizing and payload configuration, a defensive team may detect and track a team or swarm of rogue drones. The suitability of drone teams for CUS solutions has also been confirmed by state-of-the-art research works in the fields of autonomous multi-agent systems and cooperative robotics, which have proposed several applications with some similarities with CUSs' required capabilities. Indeed, examples of these applications are those related to multi-robot systems for the observation of multiple moving targets, for which different control approaches already exist, such as cooperative multi-robot observation of multiple moving targets (CMOMMT), cooperative search-acquisition-track (CSAT), multi-robot pursuit evasion (MPE) [17]. In addition, multi-drone systems have been analyzed in terms of distributed multi-agent systems for multi-target tracking problems [18]. In the end, the environmental domain presents some advanced multi-drone solutions for environmental monitoring of dynamic natural threats, such as the ones for tracking the dispersion of contaminant clouds [19]. Additionally, some current international projects are developing cooperative drone-based solutions for surveillance and situational awareness applications, such as the following European projects: ResponDrone [20], which aims at developing a multi-UAS platform for first responders to enhance their situation awareness in support assessment missions, search and rescue operations, forest fire fighting, etc. ; ROBORDER [21], which aims at developing and demonstrating an autonomous border surveillance system with unmanned mobile robots, including aerial, water surface, underwater and ground vehicles, which will incorporate multimodal sensors as part of an interoperable network; LABYRINTH [22], which proposes a road traffic surveillance by means of a multi-drone system; 5D-AeroSafe [23], which aims at developing multi-drone solutions for the monitoring of airport and waterway daily operations; Drones4 Safety [24], which aims at developing a system of autonomous, self-charging and collaborative drones that can inspect a big portion of transportation infrastructures in a continuous operation; RAPID [25], which aims at delivering a fully automated and drone-based maintenance inspection service for bridges, ship hull surveys and more. All these technologies and solutions represent a sound starting point for the future cooperative drone-based solutions in the counter-UAS domain. Besides, some recent works have already analyzed multi-agent systems for CUS, although they were focused on the single phases of the counter-UAS process, i.e., sensing or mitigation separately.