Unmanned Aerial Vehicles (UAVs) have the potential to be used for many applications in urban environments. However, allowing UAVs to fly above densely populated areas raises concerns regarding safety. One of the main safety issues is the possibility for a failure to cause the loss of navigation capabilities, which can result in the UAV falling/landing in hazardous areas such as busy roads, where it can cause fatal accidents. Current standards, such as the SORA published in 2019, do not consider applicable mitigation techniques to handle this kind of hazardous situations. Consequently, certifying UAV urban operations implies to demonstrate very high levels of integrity, which results in prohibitive development costs. To address this issue, this paper explores the concept of Emergency Landing (EL). A safety analysis is conducted on an urban UAV case study, and requirements are proposed to enable the integration of EL as an acceptable mitigation mean in the SORA. Based on these requirements, an EL implementation was developed, together with a runtime monitoring architecture to enhance confidence in the system. Preliminary qualitative results are presented and the monitor seem to be able to detect errors of the EL system effectively.
Processes and techniques used for assessing the safety of a complex system are well-addressed by safety standards. These standards usually recommend to decompose the assessment process into different stages of analysis, so called tiered safety assessment. Each analysis stage should be performed by applying recommended assessment techniques. To provide confidence in the correctness of the whole analysis, some verification techniques, usually traceability checking, are applied between two stages. Even if the traceability provides some confidence in the correctness of the decomposition, the following problems remains How to model the system behaviours at each stage of safety assessment? How to efficiently use these stages during the design process? What is the formal relationship between these modelling stages? To tackle these problems, we propose a way to specify, formalize and implement the relations between assessment stages. The proposal and its pros & cons are illustrated on a Remotely Piloted Aircraft System (RPAS) use-case.
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