# GNSS Performance Modelling and Augmentation for Urban Air Mobility +
## 2 GNSS Augmentation Strategy
Differential corrections and integrity flags are provided to the user by Satellite- and Ground Based Augmentation Systems (SBAS/GBAS) which essentially rely on external reference receivers to monitor positioning integrity and provide 'do not use' messages to the user when the estimated error exceeds the assigned limits [10]. The third augmentation layer, Aircraft-Based Augmentation Systems (ABAS) implements monitoring functionality wholly within the user receiver itself, and is independent of the other two layers. ABAS is typically dependent on the availability of redundant sensor measurements to perform consistency checks and detect faults, although a model-based approach can also be used to detect or to predict loss of integrity [8; 9]. ABAS benefits are twofold: first, the use of onboard sensors and/or models is free of dependency on external reference receivers as in the case of SBAS and GBAS; second: user-level monitoring is necessary to protect against localized error sources such as multipath, which cannot be mitigated by differential techniques like SBAS or GBAS. RAIM is one form of ABAS which relies on redundant GNSS measurements to detect and isolate faults. Owing to the capability to detect localised errors such as multipath (within a few wavelengths from the receiver antenna), ABAS assumes primary importance over SBAS and GBAS in the urban environment. Maintaining ABAS availability is therefore crucial to assure integrity in urban areas. Therefore, a UAS guidance strategy is presented that maximizes positioning accuracy accounting for propagation and user equipment -induced errors, and by extension system integrity and continuity. The approach is applicable in the strategic mission-planning phase to evaluate planned trajectories against sector-specific requirements. The overall proposed system architecture is depicted in Figure 4. UAS operators define their intended destinations or specify their preferred mission trajectory in the form of a request submitted to the UTM provider along with the equipped receiver specifications and characteristics. The augmentation module outputs the expected navigation performance over the requested trajectory, and over the local area. In order to compute the expected navigation performance, the GNSS satellite orbits are projected over the intended mission duration. Urban elevation databases are also required to model signal interactions with buildings. The expected navigation performance is output to the trajectory generator in the form of discrete datapoints that vary over the operational area and over the duration of the mission. The datapoints convey expected accuracy and the number of visible satellites at a given location and time which relate to accuracy, integrity, continuity and availability requirements. The expected navigation performance is then included as an objective to be maximized in the trajectory generation module. The output trajectory is then the result of a trade-off between conventional objectives such as minimum fuel/ minimum time and navigation performance. Figure 3: Alert limits for manned aircraft and preliminary PBN bounds for UAS. The performance requirements for the given airspace sector are included as an operational constraint. This includes accuracy specifications, maximum allowable missed detection and false alarm rates, and percentage availability. The output of the trajectory generation module can either be an optimal trajectory that meets the assigned constraints, or a rejection of the trajectory altogether owing to an assessed non-compliance.