# 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  
This paper is focused on cooperative drone-based CUSs. These represent an instance of hybrid CUS (as defined in Section 2.3), including a cooperative set of drones. Thus,according to the definitions reported in Section 2.2, such a cooperative set represents a team or swarm of drones. For the purposes of this work, teams of mini drones (based on the NATO's UAS classification system, as shown in Figure 1, or on the classification reported in [5] and shown in Table 1) are considered as a reference subsystem of the hybrid CUS. This choice is due to the following expected advantages, which are added to the advantages of a generic multi-drone mission (described in Section 2.2):  
Mobility--mini drones show extreme mobility; it is possible to bring them closer to the malicious drones and the sensing operations could be done in the proximity of the target. So, mini drones offer a mobile proximal sensing solution for a CUS, which can improve the detection and identification phases by lowering the probability of a false alarm. Coverage expansion--it is possible to increase the coverage of the protected area. In fact, drones can be easily moved in order to circumvent obstacles and/or monitor areas that are not covered or not effectively covered by ground platforms. In other words, it is easy to extend the area protected by the CUS without increasing the number of used platforms. Nevertheless, proper bases allocation, jointly with re-charging issues, must be guaranteed. Coverage expansion can also be obtained using a single high-altitude platform, but supposedly with higher costs. Deployment flexibility--compared to other flying platforms, they are simpler and faster to use, allowing lower response times to any threat. Cooperative sensor network--the defensive team may be arranged as a cooperative sensor network, i.e., as a set of mobile sensor nodes, which may cooperatively perceive, identify and track one or more threats from different \"perspectives\". This is even more necessary for mobile proximal sensing to keep the closeness with different moving targets. Thus, a cooperative sensor network may be implemented for a distribution of the sensing tasks and a load balancing amongst the sensor nodes. Such network is expected to be reconfigurable for maintaining optimal performance. Clearly, the same concept may be applied also for mitigation purposes if the drones are equipped with the proper neutralization payloads. Team coordination--as described in Section 2.2, a swarm requires a large mass of homogeneous vehicles and the coordination of a swarm occurs by means of the emergency concept. These features are not deemed suitable for a cooperative drone-based CUS. Indeed, the homogeneity of vehicles may be incompatible with the heterogeneous tasks in a CUS. Moreover, the large mass of vehicles and the emergency of a swarm imply a non-deterministic behavior and the absence of a specific organizational structure since they are based on individuals' reactions [7]. Thus, it may be difficult to estimate the probability of success of a mission, which is generally unacceptable for a CUS. To the contrary, a drone team usually exhibits an explicit organizational structure by means of a deliberated coordination. A team may also satisfy the requirement about the heterogeneous tasks for the cooperative drone-based CUS. Automated decision-making--given the speeds involved in the operating environment for CUS systems, an automated decision-making capability is essential to aid an operator in the selection of the most proper actions to manage the given threat scenario or attack scenario and to enable the fastest possible reactions on the part of the defense system. Mini drones, with their well-known aptitudes for autonomous behaviors (also as a team), represent an ideal platform to support such capability.