# GNSS Performance Modelling and Augmentation for Urban Air Mobility +
## 2 GNSS Augmentation Strategy  
Navigation system performance has traditionally been quantified and evaluated through four metrics under the Performance Based Navigation (PBN) framework: accuracy, integrity, continuity and availability. An accuracy requirement places an upper limit on the maximum allowable difference between the estimated position and the true position. Accuracy requirements are specified as the maximum allowable 95% percentile of the position error. Integrity relates to the level of trust that can placed in the output of the navigation system, and is typically specified as a maximum allowable probability of the system providing hazardous and misleading information to the user. Integrity includes the ability of a system to provide timely and valid alerts to the user. Integrity requirements are described by the following metrics:  
* Horizontal Alert Limit (HAL): Radius of a circle in the local horizontal plane (which is tangent to the ellipsoidal earth model) with its centre being at the true position, that describes the region that is required to contain the computed horizontal position with the required probability for a given navigation mode. * Vertical Alert Limit (VAL): Half the length of a vertical segment, with its centre being at the true position, that describes the region that is required to contain the computed vertical position with the required probability for a given navigation mode. * Time To Alert (TTA): The maximum allowable time measured from the onset of a positioning failure to the annunciation of the alert to the autopilot/mission-planner. * Integrity risk: The probability of computing a position that is out of defined bounds without warning the autopilot/mission-planner within the TTA. Continuity relates to the availability of the navigation system for a given phase of flight, and is typically specified as an allowable false alarm rate. Availability specifications are typically expressed as a minimum percentage of flight time for which the accuracy, integrity and continuity requirements must be met. Limiting values for each of these metrics are rigorously defined by ICAO for manned aircraft for different phases of flight. However, these standards cannot be directly applied to UAS since separation minima for unmanned aircraft do not currently exist. Aviation standards are in the process of evolving to accommodate UAS operations, and as such, performance requirements are not yet formally specified for this use-case. Even the standard approach RNP 0.3 (1823 ft/556 m) and minimum RNP 0.1 (607 ft/185 m) are too large for UAM and UAS platforms considering their intended applications. In a recent NASA study [22], a quantitative framework to adopt RNP to UAS applications based on operational risk was developed. An alternative source has been adopted in another NASA study in defining preliminary limits for navigation performance. The proposed limits are based on specifications derived by the Federation Aeronautique Internationale (FAI) through years of practice and assessment of various model aircraft classes. Based on the FAI-derived safety boundaries, the preliminary standard horizontal PBN specification for UAS is set at 15 m, and the preliminary precision horizontal specification is set at 3 m. Nominal values of 30 m (standard) and 15 m (precision) were chosen for vertical PBN. These limit values are used in lieu of standardized alert limits in this paper, and form a volume bound within which positioning errors are to be contained. This is illustrated in Figure 3, comparing the assigned limits with the alert limits for manned aircraft. Integrity requirements in civil aviation are met through a fault detection system that is either implemented onboard the mobile platform itself, or external to the mobile platform. The most common strategy is to employ a three-layered approach in assuring integrity.