# Unmanned Aerial Vehicle Routing Problems: A Literature Review
## 3 UAVRP Current State  
Limited contributions have been presented [68; 69] in the area of UAV routing in 3D environments. What has been accomplished in the field has focused on UAV routing for transporting materials and surveillance [11] without considering the stochastic conditions in weather and non-linear fuel consumption models [10]. Table 1 shows the top seven subject areas in UAV routing literature, where the majority of contributions are seen in engineering and computer science domains.  
A literature review aims to map and evaluate the body of literature and identify potential research gaps highlighting the limitations of knowledge [70; 71]. The search was conducted for the context keyword \"UAV routing,\" using the \"article title, abstract, keywords\" search in the Scopus database. Through the exhaustive search, we initially identified 396 papers published in UAV routing, and these papers were analyzed to identify the areas covered in addressing the UAV routing problem.  
The first published research we were able to identify on the topic stems from 1998, and this work contains a Reactive Tabu Search (RTS) heuristic within a discrete-event simulation to solve routing problems for unmanned aerial vehicles (UAVs) [72]. The next contribution was in 1999 and proposes a variation of standard VRP that arises in routing UAVs in the presence of terrain obscuration, thus introducing visibility-constrained routing of UAVs [73]. From the timeline presented in Figure 3, it is apparent that the UAV routing theme is gaining increasing attention, especially from 2005 and onward, with a steadily increasing number of publications per year. After the year 2000, an increasing trend is visible with a focus on wireless sensor networking and ad-hoc sensor networking. The top journals  
\begin{table}
\begin{tabular}{c c} \hline
**Subject Area** & **No of Papers** \\ \hline Engineering & 173 \\ Computer Science & 83 \\ Mathematics & 41 \\ Physics and Astronomy & 25 \\ Social Sciences & 24 \\ Materials Science & 18 \\ Decision Sciences & 14 \\ \hline \end{tabular}
\end{table}
Table 1: Top seven subject areas.  
Figure 2: Overall hierarchical representation of the systems related to the UAV routing problem (UAVRP) [14].  
and conferences contributing to UAV routing are identified in Figure 4 where publication sources with more than three contributions are included. From Figure 3, The International Society for Optical Engineering Conferences, IEEE Military Communications Conferences, and Journal of Intelligent and Robotic Systems have topped the list with the majority of contributions.  
To limit the detailed study of the literature to understand the current state of UAV routing, we chose to focus on all the relevant published journals and the top 10% cited conference papers. This list includes 51 journal papers and 21 conference papers and indicates that the clear majority (70%) of contributions on UAV routing to date have been disseminated through conferences rather than journals.  
For the identified 72 main contributions, the keywords have been recorded, and Figure 3 shows a rich image of these and their prevalence. The illustration of the keywords presents the different areas of technologies, industries, and research areas linked to UAV routing. Most commonly used keywords are presented in larger fonts and we can see the various areas linked with UAV routing in Figure 5.  
Figure 4: Top journals/conferences contributing to the area of UAV routing [14].  
Figure 3: Publishing trend in UAV routing as identified using Scopus [14; 70].