A Proposal for Multi-Constellation Advanced RAIM for Vertical Guidance

Abstract

The GNSS environment will experience major changes in
\nthe coming years. GPS and GLONASS are undergoing
\nmodernization phases, while Galileo and Compass are
\ncurrently in their deployment phase. When all these
\nconstellations are in their Full Operational Capability
\n(FOC) state, there will be at least three times as many
\nranging sources than today. In addition, all of these GNSS
\ncore constellations will broadcast signals in the two
\nfrequency bands, L1/E1 and L5/E5. These signals will be
\navailable for civil aviation, allowing users to cancel the
\npseudorange errors due to the ionosphere. Many studies
\nsuggest that it could be possible to achieve global
\ncoverage of vertical guidance using multi-constellation,
\ndual frequency Advanced Receiver Autonomous Integrity
\nMonitoring (ARAIM). The benefits of ARAIM would
\ninclude a reduced ground infrastructure (which would
\nreduce the maintenance costs compared to current
\naugmentation systems), a reduced dependency on any one
\nGNSS core constellation, and, in general, lessen exposure
\nto single points of failure. However, to achieve vertical
\nguidance using ARAIM, it will not be sufficient to adapt
\nthe RAIM algorithms that are used for horizontal
\nnavigation. This is due to the increased level of safety
\nrequired for vertical guidance compared to horizontal
\nguidance. Therefore, ARAIM will require a careful faults
\nand effects analysis. Because the integrity provision will
\nbe shared across service providers, it will be necessary to
\ndevelop a common understanding in at least three
\ndomains: the navigation requirements, starting with LPV-
\n200; the airborne algorithm; and the threat model,
\ncomprised of both the nominal performance of the
\nconstellations and the fault modes.
\nIn this paper, we present a concept for the provision of
\nintegrity using multiple constellations with ARAIM and
\nan Integrity Support Message (ISM). We will first
\npropose an interpretation of the LPV-200 requirements in
\nthe ARAIM context. We will then propose a typical threat
\nmodel for GNSS which includes both the nominal
\nperformance of the constellations and all the faults that
\nneed to be mitigated. These threats include both single
\nsatellite faults, multiple satellite faults, and constellation
\nwide faults, one of them being the use or broadcast of
\nerroneous Earth Orientation Parameters. We will show
\nhow the threats can be mitigated through the use of
\nground monitoring and the ISM in addition to the ARAIM
\nsubset position and residual test. Finally, we will give
\nexamples of multiple constellation configurations and
\nperformance providing worldwide coverage of LPV-200.