# 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
For example, reference [14] proposed a network of defense drones, which is capable of self-organizing its formation to intercept malicious drones. However, this work specifically focused on formation management algorithms to realize intercept and capture formations for the mitigation of drone intrusions, without considering neutralization aspects. Instead, reference [26] proposed a multi-drone framework for the autonomous detection of rogue drones in a defined airspace and detailed the preliminary development of a hardware and software testbed, based on commercial systems. Other ongoing research activities are developing a cooperative drone-based CUS covering all the phases of the counter-UAS process. For example, the SWADAR (Swarm Advanced Detection And Tracking) project [27] has been awarded the Defense Innovation Prize 2020 ([[https://www.edrmagazine.eu/defence-innovation-prize](https://www.edrmagazine.eu/defence-innovation-prize)]([https://www.edrmagazine.eu/defence-innovation-prize](https://www.edrmagazine.eu/defence-innovation-prize)), last accessed on 13 February 2022) (20.RTILPRZ.080, \"Innovative Solutions/Technologies for the Countering of Swarms of UAVs, specifically on the Protection of Static and Dynamic Land Facilities and Platforms\"), assigned by the European Defense Agency (EDA). SWADAR builds an intelligent drone-based network for mobile proximal sensing, tracking and neutralization of intruder swarms, as shown in Figure 2. Based on sensing and tracking data, SWADAR autonomously assesses the behavior of the rogue swarm by evaluating instantaneous and variational swarming metrics (i.e., cohesion, segregation, etc.) that can help in identifying the attack scenario and predicting the course of action of the swarm attack. Such information supports the selection of optimal neutralization actions to suppress the enemy swarming behavior. Moreover, SWADAR relies on on-board sensors, like LiDAR (light detection and ranging), optical and infrared sensors, etc., which are typically available on the market. In the US, the DARPA is funding the Aerial Dragnet program that seeks to perform persistent wide-area surveillance of multiple small drones in urban terrain on a city-wide scale. This innovative sensor array should be mounted on tethered drones, enabling a non-line-of-sight (NLOS) tracking and identification of a wide range of slow, low-flying threats [28]. Worthy of note is also the European project JEY-CUAS (Joint European System for Countering Unmanned Aerial Systems) [29], which will pave the way for the development of a joint European counter-UAS capability by developing a new-generation C-UAS system based on a modular and flexible plug'n'play architecture to include the emerging challenge of mini drones, increasingly used for defense purposes. The solution will contribute to an improvement of the situational awareness and reaction engagement by reducing the minimum reaction time. However, not everything can be achieved through the use of mini drones. In some cases, ground systems are required--if we consider the case of early detection, it would be inefficient to keep drones in flight permanently to check for the presence of malicious drones in the area to be protected. Furthermore, with mini drones, we should consider SWaP constraints, and consequently, not all of the operations necessary to fight the threat could be performed via these platforms. For example, it is not possible to use strong  
Figure 2: SWADAR concept for assessing swarming metrics (**a**) and for multi-UAV cooperative tracking (**b**) [27]. The left part shows some examples of useful metrics to measure the swarm behavior of intruder drones. Some of these metrics are: the cohesion, the segregation, the presence of hierarchical structures and clusters and the f-divergence (i.e., the temporal variation of the spatial distributions of the swarm). neutralizers (like high-power electromagnetics or high-power laser), which cannot be integrated on mini drones due to obvious SWaP constraints.