# Connectivity Maintenance Based on Multiple Relay UAVs Selection Scheme in Cooperative Surveillance
## 5 Numerical Results  
In this section, the numerical simulation results are presented to demonstrate the effectiveness of multi UAVs cooperative surveillance framework consisting of ground station for remote command and situation awareness. Particularly, the effects of proposed multiple relay UAVs selection scheme based on fuzzy optimum selection are discussed. The multi UAV cooperative surveillance consists of a ground station and six UAVs, and there are ten potential targets randomly scattering in a surveillance region. While surveillance scope expansion, relay UAVs alternatives sequence is obtained by sorting the membership degrees of UAV alternatives to optimum alternative, which is solved by a multi-attributes decision making method, namely fuzzy optimum selection. Subsequently, the relay UAVs number and communication links are dynamically adjusted based on instantaneous relationship among UAVs with surveillance scope change. According to three proposed roles in cooperative surveillance, six UAVs are allocated appropriate roles to cooperatively perform search and monitor tasks over a 4000 m \(\times\) 4000 m rectangle region, which is uniformly divided into 40 \(\times\) 40 grids with the same size. Real-time surveillance information is gathered and transmitted to ground station for remote command and situation awareness. The primary purpose of cooperative surveillance is maximizing surveillance payoff meanwhile the network connectivity should be guaranteed. According to different roles in cooperative surveillance, each UAV decides its motion plan by decentralized receding horizon control in parallel, and the optimal control sequence is solved by PSO with elite mechanism. The total simulation steps size is 300, and the receding horizon steps size is 4. UAV performance parameters \(v_{0}\), \(v_{\max}\), \(\omega_{\max}\), \(r_{comm}\), \(r_{safe}\), and \(R_{s}\) are 80 m/s, 20 m/s, 0.8 rad/s, 1000 m, 50 m, 500 m, respectively, and the role conversion threshold distances \(r_{str}\), \(r_{rts}\) are 700 m, 400 m. It is assumed that target existence probability is normal distribution without loss of generality, and the standard deviation is set as 250 m. Besides, the detection probability by one single look is defined as \(a_{j}^{m}=0.5\) for each target, \(\forall m\in U\). For communication preservation, penalty distances \(R_{Sur},R_{Rel}\) for SUAV and RUAV are set as 900 m, 650 m, and the penalty distance for collision avoidance \(R_{C}\) is 200 m. The UAVs optimal control sequence can be calculated by PSO with elite mechanism, and the corresponding parameters \(c_{1},\ c_{2}\) are 0.7, 0.6. Besides, the particle numbers of personal and global elite groups are 3, 2. Figures 5-8 represent the simulation results of multiple UAVs cooperative surveillance. For remote command and situation awareness, a ground station located at the bottom left corner of surveillance region is employed to remote command and six homogeneous UAVs cooperatively perform search and detection tasks, which are uniformly deployed at the below edge of surveillance region with 200 m interval. Figure 5: UAVs cooperative surveillance trajectories. The trajectories of multiple UAVs are depicted in Figure 5, and the black pentacles are the initial positions of UAVs. One can see that UAVs search and detect the grids with high probability of target existence, but surveillance scope expansion is moderately restricted by network maintenance with ground station. With some SUAVs away from ground station, UAV importance for communication maintenance is evaluated and relevant relay UAVs are selected to establish communication links based on fuzzy optimum selection, which circle around the predefined points corresponding to spiral trajectories near ground station in Figure 5. Correspondingly, other UAVs serve as AUAV and SUAV according to the multiple relay UAVs selection scheme. With the dynamic adjustment of communication links, the performance of cooperative surveillance mission is guaranteed.