# Designing a Reliable UAV Architecture Operating in a Real Environment
## 1 Introduction  
Unmanned aerial platforms for special tasks often move in an environment with an increasing number of other threatening objects, including aerial platforms. That can be a source of potential danger for UAVs. It should also be assumed that the air platform, primarily used in rescue operations, will move in a hostile environment. Such an environment can be understood as flight in conditions of GPS signal interference or flight in unfavorable weather conditions. Such environments may include operations where the platform might be destroyed due to intentional human activity (i.e., mainly due to military actions, etc.). Designing a reliable UAV architecture operating in such an environment requires compliance with modern standards for the safety of the flying systems. It is insufficient to meet the requirements of a user who describes only his/her operational needs. Flight safety is the responsibility of the system builders, who must consider the guidelines for the safety of air systems that are in force in given countries. Preparing a reliable and safe system is a comprehensive activity on many levels:  
1. The development of hardware and software architecture that has the required reliability determined based on the so-called Fault Tree Analysis (FTA), in particulardesigning the physical architecture of the system that ensures redundancy of the most critical subsystems,
2. Meeting the functional requirements in the context of ensuring an appropriate level of security, usually specified in the Functional Hazard Analysis (FHA) documentation, which presents the decomposition of critical functions in the system,
3. Designing algorithms for the operation of the air platform per the principles of Model-Based Systems Engineering (MBSE), which are verifiable with the use of selected formal techniques,
4. Designing and describing algorithms that support the occurrence of emergency situations during flight, such as loss of radio link with the Ground Control Station (GCS), loss of GPS signal, avoidance of platform collisions in the air,
5. Describing and proving the correctness of specific numerical algorithms that are used during the implementation of the mission (e.g., algorithms determining the change of the platform's course after detecting the possibility of a collision in air-collision avoidance algorithms),
6. Designing system testing procedures based on mission simulators and flight tests,
7. Developing documentation rules for the most critical procedures affecting flight safety and documentation of the tests performed. The hardware architecture is first determined in designing an unmanned platform. The probability of damage to the elements is checked following the Fault Tree Analysis methodology for the developed architecture. The probability of damage to the elements is checked following the Fault Tree Analysis methodology for the developed architecture. The purpose of the activity is to check which of the elements involved in implementing a specific flight scenario is prone to failure and may lead to a potential system crash. The FTA methodology is described in literature in [1]. The general principle of proceeding in the construction of FTA trees is to arrange a series of devices that implement a given function. Then, for each of such devices, Mean Time Between Failures (MTBF) is determined, based on which the probability of failure of the entire system is verified. If the probability is above the acceptable threshold, then the system must not be allowed to operate. It is a fundamental step taken in designing an unmanned system. In a situation where the FHA shows that some system elements are too unreliable, these elements must be replaced before further design work because the platform will not meet the safety requirements. After verifying the hardware architecture to be used in the designed unmanned system, the decomposition of key processes directly impacts the platform's safety in flight should be made.