Further Analysis of Sensitivity of RAIM Performance to Integrity Support Message Deviation

Abstract

The Advanced Receiver Autonomous Integrity Monitoring (ARAIM) concept based on dual-frequency multiconstellation (DFMC) Global Navigation Satellite System (GNSS) has the potential to support LPV-200 approach (Localizer Performance with Vertical guidance with a decision height as low as 200 ft) without the need for barometric vertical guidance. The ARAIM Technical Subgroup (TSG) created by the leadership of the EU-US Cooperative Working Group (WG)-C has performed a feasibility study of ARAIM and concluded that ARAIM has the potential to achieve significant operational benefits worldwide. However, the TSG also identified challenges that must be resolved before enabling ARAIM services. One major challenge to enabling ARAIM service is determining the parameter values in Integrity Support Messages (ISMs) broadcast to user equipment. ISM is used to periodically update statistical characterization of the core GNSS constellation performance as a priori information to the airborne ARAIM algorithms. The selection process on what ISM values to broadcast requires a compromise based on solid analyses. If the ISM parameter values are not conservative enough, ARAIM may fail to detect hazardously misleading information (HMI) with the required probability; if they are too conservative, ARAIM may not achieve as high availability and coverage as desired. Moreover, the process to determine the broadcast ISM values must be agreed upon globally. However, Constellation Service Providers (CSPs) and Air Navigation Service Providers (ANSPs) in different countries may have opposing views in selection of ISM parameter values. While CSPs may insist on broadcasting ISM values that are not fully supported by service history or have not yet been fully validated, ANSPs may not readily accept such values. Toward the goal of developing systematic procedures to resolve this issue, we began in our previous paper to analyze the sensitivity of ARAIM performance to the deviation between the broadcast ISM parameter values and the ISM values representing the actual core constellation performance. This paper comprehensively addresses these issues. We used two methods in combination under a variety of scenarios: one is a brute-force method developed in the earlier paper; the other method is a first-order Taylor polynomial (FOTP) method newly developed in this paper. An analytic expression derived with the FOTP method used with the other method reveals several important findings including the following. Sensitivity of Pr{HMI} to ISM deviation widely varies depending on which ISMs deviate, which constellation that the deviated ISMs are associated with, and what the broadcast ISM values are. In particular, this study reveals that depending on the broadcast ISM values, Pr{HMI} hardly degrades when ISM deviates for majority of the ISM parameters. Therefore, ISM parameter values to broadcast need to be determined precisely only for the other few. With this finding, the time and energy in monitoring the ISM parameter values on the ground may be focused on those ISMs to which the ARAIM performance is sensitive to. Also, in case of the ISM parameters whose deviations cause significant Pr{HMI} degradation, our study finds that it is advisable to select conservative ISM parameter values without concern of appreciable increase in PL. Such a selection would cause insignificant increase in PL and minimal reduction of availability. Insights gained with this analysis would be useful in developing globally harmonized procedures to select ISM parameter values to broadcast in overall ISM generation and ISM verification process.