# Connectivity Maintenance Based on Multiple Relay UAVs Selection Scheme in Cooperative Surveillance
## 1 Introduction  
Due to the limited capacity of single UAV, in the future information and network centric environment, multiple UAVs have been widely employed in military and civil applications [1; 2; 3; 4; 5] due to the feasibility and scalability in complex tasks in recent years, such as target tracking, wildlife monitoring, disaster rescue and so on. Serving as communication relays for remote monitoring and situation awareness [6], multiple UAVs dynamically constructing a reliable peer-to-peer communication links chain has been studied in recent years [7; 8; 9]. Multiple UAVs cooperative surveillance attracts more attention in past years, which is realized by online motion planning according to real-time detection information. Generally, surveillance region is uniformly divided into grids with the same size, and the existence probabilities of unknown targets are associated with different search maps, such as probability map [10; 11], pheromone map [12] and rate of return map [13; 14]. Based on the latest detection information gained by multiple UAVs, search map is updated according to corresponding criteria, such as Bayesian rules, pheromone update rules and rate of return update rules. Similar to rate of return map, a multiple UAVs cooperative surveillance framework with a ground station is implemented connecting with history detection information and surveillance payoff estimation in this work. While preserving connectivity, real-time surveillance information can be sent back to ground station for remote command and situation awareness, which widely exists in target monitoring,disaster rescue and remote surveillance. In [15], a novel role management concept is proposed for UAV motion control, which is realized by communication aware potential fields (CAPF) to maintain connectivity between spatial exploration agent and ground station. \(K\)-hop connectivity is a common indicator representing network topology in multi agents. The connectivity constraints are described as the gradients of \(k\)-connectivity matrix representing the network topology property of a graph, and agent motion control vectors are solved by quadratic program [16]. In [17], connectivity maintenance between a stationary agent and a remote exploring agent in a walled environment is developed, and the agent motion control is solved by bound linear programming with the derivative of Fiedler value and \(k\)-connectivity matrix constraints. However, these researches cannot effectively deal with UAV kinematic constraints, such as the minimum velocity. Detection scope expansion and effective information collection is the primary task in multiple UAVs cooperative surveillance, but network connectivity is essential for real-time data transmission and remote command. For balancing surveillance payoff and network connectivity, a dynamic communication links chain is established by UAVs with different roles, and appropriate relay UAVs selection scheme is crucial to improve the performance of communication networks. Due to different factors for characterizing the differences among UAV alternatives for preserving connectivity, the optimal relay UAV selection [18] can be considered as a multi-attributes decision making problem, which is utilized to make an optimal choice with the highest degree of satisfaction from a set of UAV alternatives. As an important aspect of decision science, there are many researches on multi-attributes decision making problems [19; 20; 21]. Considering the feature of network topology comprised of ground station and multi UAVs, four special attributes are proposed to represent UAV alternatives differences in relay communication systems, and relay UAV alternatives sequence can be obtained by fuzzy optimum selection [22; 23; 24]. In this work, multiple UAVs cooperative surveillance with a ground station for remote command and situation awareness is presented.