# Unmanned Aerial Vehicle Routing Problems: A Literature Review
## 2 UAV Routing with an Emphasis on the Problem Type, Transportation Mode, and Degree of Automation
### Different Types of Vehicle Routing Problems  
In its simplest form, the VRP addresses the routing of a fleet of homogeneous vehicles to deliver identical packages from a depot to customer locations while minimizing the total travel cost [14]. The VRP definition is that a set of vehicles initially located at a depot are to deliver discrete quantities of goods to a set of customers determining the optimal route used by the set of vehicles when serving the set of customers [14; 16; 36; 37; 38; 39]. The objective is to minimize the overall transportation cost, and the solution of the classical VRP problem is a set of routes that all begin and end in the depot, which satisfies the constraint that all the customers are served only once [14; 16; 36]. The transportation cost can be improved by reducing the total traveled distance and by reducing the number of required vehicles [14; 37; 38; 39; 40; 41; 42; 43; 44]. Several sub-categories of VRP exist, addressing a specific set of routing problems [16; 36; 37]:  
* Capacitated VRP (CVRP): Every vehicle has a limited capacity [38; 39]. CVRP is important when using UAVs with limited capacities in delivering goods.
* VRP with time windows (VRPTW): Every customer has to be supplied within a certain time window [40]. VRPTW is important when referencing using UAVs to deliver perishable goods.
* Multiple Depot (MDVRP) VRP: The vendor uses many depots to supply the customers [41]. MDVRP is important when using UAVs with multiple depots to transport materials to customers.
* VRP with Pick-Up and Delivering (VRPTD): Customers may return some goods to the depot [15; 40]. VRPTD is important when using UAVs with multiple pick up and deliveries of goods.
* Split Delivery VRP (SDVRP): Customers may be served by more than one vehicle [42]. SDVRP is important when using UAVs to delivering goods to customers where one vehicle can visit many customers and one customer can be visited by many UAVs.
* Stochastic VRP (SVRP): Some parameters (like number of customers, their demands, serve time, or travel time) are random [43; 44]. SVRP is important when using UAVs in delivering goods to satisfy stochastic demands.
* Time-dependent VRP with path flexibility (TDVRP-PF): Any arc between two customer nodes has multiple corresponding paths in the road network [45].
* VRP with trailers and transshipments (VRPTT): In addition to depot and customer locations, this introduces transshipment locations [46]. VRPTT is important when using UAVs along with a fleet of trucks in delivering goods and in last-mile deliveries.
* VRP with profits: A profit is associated with each customer that makes such a customer more or less attractive. Unlike to the most classical VRPs, in VRP with profits, the set of customers to serve is not given and different decisions have to be taken on which customers to serve and how to cluster the customers to be served in different routes and order the visits in each route [47; 48].  
These relate to the UAVRP as the different categories of VRP inspire the existing work in UAV routing. In solving the UAVRP, certain studies have used different VRP approaches, and these are identified in Section 4.