# Designing a Reliable UAV Architecture Operating in a Real Environment
## 2 Related Works
Depending on the obstacle type identified and the information available about the obstacle, the algorithm determines the time when the UAV should start avoiding the obstacle. When the UAV reaches the point where obstacle avoidance begins, the algorithm starts the avoidance operation for a specified period of time. The length of the time window in which the UAV modifies the flight direction, is flexible and depends on the size and distance from the obstacle. After completing the obstacle avoidance maneuvers, the algorithm searches for new route points that will allow the UAV to return to the planned route as quickly as possible. The work [20] also contains extensive literature review on collision avoidance algorithms. [PERSON] et al. [21] present a geometric model for UAVs where horizontal maneuvers are only performed by varying the speed of the UAV. The direction of flight remains constant. The model predicts the separation to be achieved by a horizontal collision avoidance maneuver. In addition, they calculate the effects of the speed change time and autopilot response on the horizontal miss distance and the reserved time to the nearest potential collision point. The second group of methods is methods based on optimization, in particular, methods based on the principles of optimal control. Their description is out of the scope of this article due to their complexity. The reader interested in the theory of optimization is referred to the works of [PERSON] [22] and [PERSON] [23], who present direct and indirect methods in optimal control. In the case of optimal control, it is worth emphasizing that the task to be solved consists in determining the UAV flight trajectory between the starting and ending point, assuming the knowledge of the air platform flight dynamics model. The dynamics model is represented in the task in the form of state equations, which are subject to discretization during the determination of feasible solutions using one of the aforementioned methods. [PERSON] et al. [24] describe the collision avoidance problem as an optimal control task. The goal is to find a combination of safe maneuvers between the two UAVs that guarantees the longest minimum separation distance among possible escape maneuvers. The optimal control problem was formulated with quadratic performance criteria. Many authors include among the set of algorithms that can be used in the process of determining the separation of platforms, UAV route planning algorithms based on the general VRP displacement planning task. For an example of the third group algorithm, see [25]. It seems, however, that the use of integer methods for this purpose does not guarantee a short time of obtaining the result, because the general VRP problem belongs to the NP-hard problems class. The authors of the article also believe that the use of heuristic methods in the form of Genetic Algorithms or Particle Swarm Optimization (a good introduction can be found in the article [26]) for this purpose, is also not a perspective path. For heuristic algorithms, it is not possible to determine convergence, which makes their use in control in the event of emergency situations very risky. In the case of system certification, it should rather be assumed that the certification authority may prevent the operation of the system with implemented algorithms of this class. At this point, it is also worth noting that for several years there have been attempts to formalize the methods of testing models based on languages such as UML or SysML, as described, among others, in [8]. However, testing according to these methods is a design task to which a separate team should be assigned. This topic is not covered in the article. The interested reader is referred to the papers described in the article [8]. The final stage of the design work is the design of system testing procedures based on mission simulators and air tests. This includes the development of documentation rules for the most critical procedures affecting flight safety and the documentation of the tests performed.