# Unmanned Aerial Vehicle Routing Problems: A Literature Review
## 4 Approaches and Domains in UAV Routing in Existing Literature  
When considering the published literature both in terms of sources and keywords, it becomes clear that various approaches are used in addressing UAV routing. The contributions' approaches are inspired primarily by different variants of the VRP and traveling salesman problem (TSP). The difference between TSP and VRP is that TSP considers finding the shortest path that connects an arbitrary number of nodes whose pairwise distances are known [74; 75]. The most basic form of a VRP can be considered as a direct descendant of TSP [41; 76] in which there are one salesman and one fixed depot; the rules are to visit all customers once and only once and end the route at the depot where it started [77]. Many approaches for the VRP were inherited from the huge and successful work done for the solutions of the TSP [39]. This UAVRP is similar to the traditional TSP if the UAV can arrive at or depart from each target only once with the objective of minimizing the total cruise distance [78]. This is relevant in UAV routing if one particular considers sense-and-avoid [79], where the objective is to move to the nearest safe point in a collision-free manner [4; 80], or complete autonomy where the UAVs are self-navigating point-by-point [81]. The VRP considers a problem similar to the TSP but with a different context, as the VRP considers the problem of delivering goods located at a central depot to customers. Based on the analysis in the existing literature, VRP approaches used for routing of UAVs can be grouped mainly in a few variants of VRP. These correspond with the general classifications of VRP stated in Section 2. Table 2 categorizes the different VRP approaches used to solve UAV routing. The majority of applied VRP approaches are the general VRP, dynamic VRP, VRP with time windows, and capacitated VRP. Besides VRP approaches, the literature review also identified various other approaches. Chief among these approaches is formulating the problem as a TSP. Tables 3 and 4 presents an \begin{table}
\begin{tabular}{c c} \hline \hline
**Area/Approach** & **No of Papers** \\ \hline Wireless networks & 24 \\ Scheduling & 8 \\ VRP & 18 \\ TSP & 9 \\ Other & 13 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Distribution of papers with respect to various domains/approaches used in UAV routing. Figure 5: Analysis of keyword frequency in UAV routing. overview of contributions in the literature that utilize the VRP and TSP approaches used in UAV routing. Several contributions use a combination of the variants of both TSP and VRP. The rest of the papers found in UAV routing, other than VRP and TSP approaches, are mainly in the domain of wireless networks and scheduling. Most of these remaining contributions are related to wireless networks with a focus on contributions in the fields of ad-hoc networks and wireless sensor networks (WSN). Figure 4 presents an overview of these areas. The UAV routing taxonomy covering the four major areas of UAV routing with VRP approaches, UAV routing with TSP approaches, UAV routing in WNS, and UAV routing in scheduling problems are graphically illustrated in Figure 5.