## Theoretical Basis  
The task of studying the effectiveness of a UAV control system is formulated as follows:  
* There are a number of functional (target) tasks that this system must solve, for example, cargo delivery, monitoring of important infrastructure facilities, environmental monitoring, agriculture, etc. ;
* Requirements for their achievement are set, for example, in the form of a condition that the probability of solving the target problem should not be less than a given value;
* For each of these tasks, there is a specific set of UAVs designed to solve it and having known characteristics, including reliability;
* It is necessary to determine one or more UAV types that best meet the specified requirements for solving a specific target task. To solve this problem, there is the following approach. In the first stage, only one UAV is considered and its particular efficiency criterion is determined. Such an efficiency criterion should meet the following basic requirements [18; 19]:  
* To characterize the object as a whole;
* To provide the possibility of obtaining its quantitative assessment with the required reliability;
* To have clearly defined boundaries of the area of its change. With regard to a UAV, the most informative particular criterion is its reliability, which is numerically characterized by the probability of failure-free operation [20; 21]. The second stage involves the division of the process of solving the target task that the UAV must perform into certain stages. For example, if the target task is the delivery of goods, then such stages are takeoff, finding a given route to the recipient of the cargo, landing at the place of unloading, unloading itself, taking off, reaching a given route and landing at a given place. Each of these stages is controlled by the control system; however, all of them are subject to the influence of destabilizing environmental factors (wind, rain, birds, electromagnetic effects, artificial and natural obstacles, etc.). The influence of these factors leads to the fact that, at a certain stage, UAVs may fail as a result of their influence and the target task will not be fulfilled, i.e., there is a non-zero probability of system failure at a certain stage. Therefore, the probability of solving the target problem is a multiplicative convolution of the probabilities of successful completion of all stages of UAV flight. Since the reliability of a UAV is a probabilistic value, and the probabilities of performing individual stages can be considered to be independent of each other, it is advisable to use the following multiplicative convolution as a global criterion for the effectiveness of a UAV flight control system [18; 19; 22; 23]:  
\[W=p_{g}\prod_{i=1}^{n}p_{i}, \tag{1}\]  
where \(p_{i}\) is the probability of completing the \(i\)-th stage; \(p_{g}\) is the UAV reliability; and \(n\) is the number of steps that the UAV must complete. The global criterion (1) has a simple structure and a clear physical meaning (the global criterion is the product of the particular criterion), but there are the following significant disadvantages:  
* The criteria are indifferent to the probability distribution \(p_{i},\ \left(i=\overline{1,\ldots,\ n}\right)\) over the flight stages, but depend only on the value of their product. In other words, if we have some set of values \(\left\{p_{1},\ p_{2},\ldots,p_{n}\right\}\), then with any distribution of this set over stages, the value of the criteria will not change;
* The criteria are indifferent to the relative changes in the probabilities \(p_{i},\ \left(i=\overline{1,\ldots,\ n}\right)\), i.e., the relative increase or decrease in any of them is compensated by exactly the same relative decrease or increase in any of the others. We consider, in more detail, the disadvantages of a criterion of the form of (1) using the classical example of two particular criteria:  
\[W=W(p_{e},p_{g})=p_{e}\cdot p_{g}, \tag{2}\]  
where \(p_{e}=\prod_{i=1}^{n}p_{i}\) is the probability that an absolutely reliable UAV will solve its target problem.