This DL discusses the vulnerabilities of Global Navigation Satellite Systems (GNSS) in Urban Air Mobility (UAM) and Unmanned Aircraft System (UAS) applications and focusses on the possible strategies for online and offline navigation performance monitoring and augmentation, which can contribute to trusted autonomy in low-altitude Air Traffic Management (ATM) operations. The concept of Avionics-Based Integrity Augmentation (ABIA) is introduced and the associated system/software architecture is presented with a focus on predictive Integrity Flag Generation (IFG) and Flight Path Optimization (FPO) functions. Successively, simulation case studies are presented, addressing respectively the synergies of ABIA with Space/Ground Based Augmentation Systems (SBAS and GBAS), and with Cooperative/Non-Cooperative Sense-and- Avoid (SAA) systems. Based on these case studies, it is concluded that the ABIA system is capable of generating both predictive and reactive integrity flags (i.e., caution and warning signals) when GNSS data are degraded or lost, and it can be successfully integrated with SBAS and GBAS to enhance integrity levels in all UAS flight phases. Additionally, in the SAA scenarios investigated and in the dynamic conditions explored, all mid-air collision threats were successfully avoided by implementing real-time software algorithms for navigation/tracking uncertainty analysis and trajectory optimisation. Current research is focusing on the development of a more detailed approach to airspace risk analysis and dynamic geofencing, which also takes into account the effects of weather (wind, clear-air turbulence, precipitation, etc.), wake turbulence, and other natural/human-induced disturbances. Prototype systems are being developed and flight test activities are being conducted on various classes of UAS to assess the potential application of this technology to next generation Mission Management Systems (MMS) and Decision Support Systems (DSS) for UAS Traffic Management (UTM), Urban Air Mobility (UAM), and operations in GNSS-challenged environments.