A Proposal for Multi-Constellation Advanced RAIM for Vertical Guidance

Abstract

The GNSS environment will experience major changes in the coming years. GPS and GLONASS are undergoing modernization phases, while Galileo and Compass are currently in their deployment phase. When all these constellations are in their Full Operational Capability (FOC) state, there will be at least three times as many ranging sources than today. In addition, all of these GNSS core constellations will broadcast signals in the two frequency bands, L1/E1 and L5/E5. These signals will be available for civil aviation, allowing users to cancel the pseudorange errors due to the ionosphere. Many studies suggest that it could be possible to achieve global coverage of vertical guidance using multi-constellation, dual frequency Advanced Receiver Autonomous Integrity Monitoring (ARAIM). The benefits of ARAIM would include a reduced ground infrastructure (which would reduce the maintenance costs compared to current augmentation systems), a reduced dependency on any one GNSS core constellation, and, in general, lessen exposure to single points of failure. However, to achieve vertical guidance using ARAIM, it will not be sufficient to adapt the RAIM algorithms that are used for horizontal navigation. This is due to the increased level of safety required for vertical guidance compared to horizontal guidance. Therefore, ARAIM will require a careful faults and effects analysis. Because the integrity provision will be shared across service providers, it will be necessary to develop a common understanding in at least three domains: the navigation requirements, starting with LPV- 200; the airborne algorithm; and the threat model, comprised of both the nominal performance of the constellations and the fault modes. In this paper, we present a concept for the provision of integrity using multiple constellations with ARAIM and an Integrity Support Message (ISM). We will first propose an interpretation of the LPV-200 requirements in the ARAIM context. We will then propose a typical threat model for GNSS which includes both the nominal performance of the constellations and all the faults that need to be mitigated. These threats include both single satellite faults, multiple satellite faults, and constellation wide faults, one of them being the use or broadcast of erroneous Earth Orientation Parameters. We will show how the threats can be mitigated through the use of ground monitoring and the ISM in addition to the ARAIM subset position and residual test. Finally, we will give examples of multiple constellation configurations and performance providing worldwide coverage of LPV-200.