This paper proposes an effective approach for modelling and assessing the risks associated with unmanned aerial vehicles (UAVs) integrated into national airspace system (NAS). Two critical hazards with UAV operations are considered and analyzed, which are ground impacts and midair collisions. Threats to fatalities that result from the two hazards are the focus in the proposed method. In order to realize ground impact assessment, a multifactor risk model is designed by calculating system reliability required to meet a target level of safety for different UAV categories. Both fixed-wing and rotary-wing UAVs are taken into account under a real scenario that is further partitioned into different zones to make the evaluation more precise. Official territory and population data of the operation scenario are incorporated, as well as UAV self-properties. Casualty area of impacting debris can be obtained as well as the probability of fatal injuries on the ground. Sheltering factors are not neglected and defined as four types based on the real scenario. When midair collision fatality risk is estimated, a model of aircraft collisions based on the density of civil flight in different regions over Chinese airspace is proposed. In the model, a relative collision area and flying speed between UAVs and manned aircraft are constructed to calculate expected frequency of fatalities for each province correspondingly. Truthful data with different numbers of UAVs is incorporated in the model with the expected number of fatalities after a collision is included. Experimental simulations are made to evaluate the ground impacts and midair collisions when UAVs operate in the NAS.

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The path protection with reliability guarantee is investigated under multiple constraints. The key challenge of providing ample multi-constrained loop-less paths to the detouring as candidates is addressed by a modified relaxation principle of dominated path. The loops in the new extended sub-paths are explored with an elastic depth, and additive criterions derived from the path information are adopted to obtain more accurate identification of feasible paths. Our hybrid detouring gives priority to the end to end disjoint backup paths and uses the residual space of inter-media nodes to store the potential local disjoint subpaths. The obtained candidate backup paths are selected by appropriate cost functions according to the combined reliability and the consumed resources. Simulation results demonstrate its superiority in restoration path-finding capability leads to a significant improvement in the success rate of the QoS path protection.