Monte Carlo Simulation for Impact of Anomalous Ionospheric Gradients on GAST-D GBAS

In this paper, a tool based on Monte Carlo simulation is developed for probabilistic analysis of the effects of anomalous ionospheric gradients on GAST-D GBAS approach and landing operations. While parameters associated with ionospheric gradients and satellite geometry are assumed to be at their worst-case values in the traditional certification approach, Monte Carlo simulation takes advantage of the random characteristics of these parameters. In the Monte-Carlo approach, simulation parameters are randomly chosen from the probabilistic distributions chosen for to represent the ionospheric threat model. More than 10(9) sampled events are combined into a probability of Hazardously Misleading Information (P-HMI), defined as the sum of the probabilities of missed detections (P(md)s) weighted by the probability of occurrence of the anomalous ionospheric event leading to each Pmd value. The results are represented in terms of a cumulative distribution function showing the overall probability of exceeding a given differential range error size. The maximum allowable range error due to ionospheric gradients to support GAST-D is 2.75 m according to the reformulated requirements of the Standards and Recommended Practices (SARPs). The simulation results showed that the PH-AK for differential range errors exceeding 2.75 m was well below 10-9. Furthermore, individual ionospheric events were investigated to ensure that integrity requirements were met in the traditional interpretation. The maximum differential range error in the simulation results was 2.74 m. Most of the largest differential range errors are generated from the specific geometry where the baseline direction (between the GBAS ground facility and the landing threshold point, or LTP) is closely aligned with the propagation direction of the ionospheric front.