# A Review of Counter-UAS Technologies for Cooperative Defensive Teams of Drones
## 2 Definitions and Basic Concepts for Cooperative Drone-Based Counter-UAS Systems
### Multi-Drone Missions  
Without the need for an on-board pilot, drones were usually designed to accomplish the D-cube (dull, dangerous and dirty) envelope [6], which is the set of the following mission classes: dull, i.e., monotonous or requiring high endurance for human occupants; dirty, i.e., that could pose a health risk to a human crew; dangerous, i.e., that could result in the loss of life for the on-board pilot. However, if the region of interest of a mission is large and/or the mission objectives are several, the execution of a single-drone mission may solicit a considerable amount of time and may entail poor performance in terms of mission effectiveness. Multi-drone missions may overcome this issue. They are essentially missions that engage two or more drones with some common objectives. Thus, a multi-drone mission aims at increasing the effectiveness with respect to the equivalent single-drone mission and requires a sort of collaboration amongst the involved drones. There is no common agreement about the definitions for this multi-drone collaboration and the classification of the different levels of collaboration. For the purposes of this work, the definitions in [7] are adopted and the following levels of collaborations in a multi-drone setting are considered:  
* Isolated individual--in this case, a drone independently acts. It may be piloted, or it may exhibit a given degree of autonomy for the execution of its mission on its own. * Group--a group of drones comprised of several isolated individuals, each with their own mission without coordination, i.e., collaboration is not present. * Team--a team of drones is a networked set of drones with a common mission, in which all members are assigned specialized and different tasks to accomplish the global mission. * Swarm--a swarm of drones is a uniform mass of undifferentiated drones. Thus, a swarm is typically composed of a large number of homogeneous drones, which perform a single task. According to the above classification, only drone teams and swarms envisage a significant collaboration level, which entails a cooperation within the overall system. Such cooperation should allow the achievement of more complex missions and/or effective results with respect to isolated individuals and groups of drones. Cooperation is mediated by coordination (or coordination protocol), which represents the mechanism ensuring that the activities of the single vehicles keep the desired relationships and that the collective behavior (intended as the set of individual behaviors in the system) achieve the objectives for the global system. The members of a swarm usually coordinate each other only through simple and local interactions, whereas the coordination of a team requires diverse mechanisms for the allocation of several, possibly heterogeneous, tasks. In regard to the coordination of a swarm, the emergency concept is usually adopted to indicate the ability of the swarm to achieve a collective behavior for a complex operation by exploiting limited interactions of the single vehicles, which individually accomplish simple behaviors and tasks.