# Connectivity Maintenance Based on Multiple Relay UAVs Selection Scheme in Cooperative Surveillance
## 3 Multiple Relay UAVs Selection Scheme Based on Fuzzy Optimum Selection
### Multiple Relay UAVs Selection Scheme in Cooperative Surveillance
#### 3.3.3 Multiple Relay UAVs Selection Scheme  
The multiple relay UAVs selection scheme is a centralized approach to obtain the membership degree sequence of UAV alternatives to the optimum alternative, and the detail algorithm process is described in Algorithm 1.  
Input: UAVs state information (positions \(P_{i}\) and roles \(R_{i}\)) for \(i=1,\ldots,N_{U}\), the relay UAVs number \(N_{R}\)  
Output: updated information (roles \(R_{i}\) and circling points \(P_{i,C}\)) for \(i=1,\ldots,N_{U}\).  
_// multiple relay UAVs selection stage:_  
1. Calculate \(RV(i),MH(i),RMH(i),AD(i),i=1,\ldots,N_{U}\) (Equations (10)-(13))  
2. Membership degree of UAV alternatives assessment based on fuzzy optimum selection (Equations (14)-(23))  
3. \(R_{1}\sim R_{N_{R}}\rightarrow\text{RUAV}\), \(R_{N_{R}+1}\rightarrow\text{AUAV}\), \(R_{N_{R}+2}\sim R_{N_{U}}\rightarrow\text{SUAV}\) // \(\rightarrow\) represents role allocation operation \(R_{i}\)  
_// role conversion stage: is the role of the ith UAV_  
4. if there is no SUAV in \(z_{1}\) of AUAV  
5. \(R_{N_{R}+1}\rightarrow\text{RUAV}\), \(R_{N_{R}+2}\rightarrow\text{AUAV}\), \(N_{R}=N_{R}+1\) // relay UAV supplement  
6. else  
7. if there is at least one UAV in \(z_{2}\) of the \(N_{R}th\) RUAV  
8. \(R_{N_{R}}\rightarrow\text{AUAV}\), \(R_{N_{R}+1}\rightarrow\text{SUAV}\), \(N_{R}=N_{R}-1\) // relay UAV removal  
9. endif  
10. endif  
11. Calculate the circling points \(P_{i,C}\) for \(i=1,\ldots,N_{R}\)  
**Algorithm 1.** Multiple Relay UAVs Selection Scheme  
In cooperative surveillance, all UAVs send state information to ground station to evaluate the membership degrees to ideal optimal alternative among the group via single or multi hops communications, and the multiple relay UAVs selection scheme carries out in ground station. According to relationship among UAVs depicted in Figure 4, role conversion is correspondingly performed. Subsequently, the ground station sends relevant role information to each UAV, which consists of UAV role, circling point and other messages, and relevant UAVs update roles and perform new tasks in cooperative surveillance respectively.  
Based on the multiple relay UAVs selection scheme in Algorithm 1, each UAV is allocated appropriate role over time for balancing surveillance payoff and network connectivity. It is noted that the circling points are uniformly distributed between ground station and the central of SUAVs swarm, and the interval is set as \(r_{str}\).  
**4. Multiple UAVs Motion Plan in Cooperative Surveillance**  
As network is connected, UAVs exchange information via single or multi hops communications in cooperative surveillance. Each UAV plans its optimal motion based on local neighbor UAV information. At each instant, each UAV's optimal control command sequence is calculated by decentralized receding horizon control, and the optimization metrics are different for three proposed UAV roles. Considering connection with other UAVs, the primary goal of RUAV is constructing dynamic and reliable peer-to-peer communication links between remote UAVs swarm and ground station. For the purpose of maximizing surveillance payoff, SUAVs try to gather more surveillance information and explore more unknown regions, but it is necessary that connecting with at least one UAV in its communication range. As an essential role, AUAV, which can be considered as a special SUAV, serves as a communication joint to connect remote SUAVs and RUAV, and its task is achieving tradeoff between surveillance mission and connectivity maintenance.