Assessment of GNSS Performance and Error Bounding for SAIL III UAS Operations

The growing use of UASs in complex operations, including Beyond Visual Line of Sight (BVLOS) operations and missions over populated areas, has increased the need for robust navigation integrity. Within this framework, a GNSS is often used as the primary source for positioning, but its reliability can be affected by various degradation sources, particularly in urban or constrained environments. This paper explores the implications of using GNSSs as an external service in SAIL III operations, with a focus on Operational Safety Objective (OSO) #13, defined in Specific Operations Risk Assessment (SORA) 2.5. A review of SORA 2.5 requirements is provided, followed by experiments involving GNSS data acquisitions in different environments using both high-end and mid-range receivers. Various performance indicators available from the receivers, such as the Dilution of Precision (DOP), Carrier-to-Noise Density Ratio (C/N0), estimated accuracy, and PLs, are examined to assess their ability to detect navigation degradations in real time. The results show that Protection Levels outperform the other indicators in detecting degradations under challenging conditions, highlighting the current limitations of GNSS-based navigation monitoring for specific category UAS operations.