# Computer Vision in Autonomous Unmanned Aerial Vehicles--A Systematic Mapping Study
## 3 Results
### RQ4: What Are the Characteristics of the Vision System?
#### 3.4.2 Concerning the Vision-Based Task, Localization, and Orientation  
With regard to the influence of the computer vision task at hand, the predominant system is again the monocular one for all the categories, followed noticeably by the stereo system, which is present in all the categories. The case of multi-camera systems is different. In fact, a 2-camera system is used in tasks related to control, tracking and sense-and-avoidance (cat. 2, 3 and 4), and a 4-camera system is employed only for navigation (cat. 1). The remaining details concerning the vision system distribution in each main category are provided in Figure 16.  
Table 6 presents the list of 144 papers grouped considering the number, localization, and orientation of the cameras. The location is _On Board_ the UAV and _On Ground_, or a combination of both in multi-camera systems. With regard to the orientation, when the vision system is on board, the most frequent setups are pointing _Downward_ (inclined towards the ground) and _Forward_ (in the direction of flight). In the case of papers that address autonomous aerial refueling, a camera pointing to the other aircraft involved in the process has also been considered. Here, the system points to the UAV that acts as a receiver (_Toward the Receiver UAV_) or to the aircraft with the fuel tank (_To the Tanker UAV_). However, in the case of ground vision systems, their orientation is always directed _Toward the UAV_. Finally, in a few cases, the vision system's orientation is modified (shown as _Depending on the test_).  
Figure 14: Vision system class distribution over the database.  
Figure 15: Annual trend per vision system.  
Lastly, some conclusions are presented on the data related to the vision systems used. (a) The results show a predominance of on-board monocular systems. This is mainly owing to the lower complexity of computer vision algorithms, and also to the ease of installation of a single camera on a UAV, with a resulting decrease in the weight added to the aircraft. (b) With regard to camera orientation, this depends to a large extent on the task for which the proposal has been designed. Visual navigation systems generally use cameras pointing toward the ground (especially in geolocation systems). Such orientation is also necessary in missions involving the tracking of targets on the ground. (c) The orientation pointing in the direction of flight is mostly used in systems for the detection and avoidance of obstacles, and when the visual reference is placed in front of the UAV. In the particular case of aerial refueling, the camera, which is installed on board the UAV receptor or tanker, points in the direction of the other aircraft in order to measure the distance between both. (d) Multi-camera systems are also interesting solutions, especially stereo systems, which have been used in a good number of papers. In this case, these systems provide information of the depth, which is a clear advantage despite being more complex. (e) The stereo systems are usually installed aboard the UAV, but some solutions also use stereo systems installed on the ground, mainly to guide fixed-wing UAVs during their landing maneuvers. In this case, the orientation of the cameras is obviously towards the UAV itself.  
Figure 16: Vision system distribution over the categories.