GNSS Augmentation for Advanced RAIM by Constellations of LEO Satellites with Degraded Performance

In previous work (Pullen, 2022; Pullen, 2023; Oak, 2023), we introduced the concept of augmenting GPS with six inclined geosynchronous satellites (I-GEOs) that would provide GPS-Block-III-quality military and civil ranging signals along with conveying Satellite-based Augmentation System (SBAS) messages to users and showed how these satellites would improve integrity using either SBAS or Advanced RAIM (ARAIM). We also examined the ARAIM integrity theoretically obtained by ranging augmentation from satellites in Low Earth Orbit (LEO) such as those in the current Globalstar, IridiumNext, and OneWeb orbits. The results for LEO satellites in particular showed the potential for dramatic improvements in protection levels and availability of integrity if LEO satellite performance parameters (range error bounds, visibility mask angles, and integrity fault probabilities) were not far below the values currently provided by GPS satellites. Because the performance of LEO satellite ranging relative to GPS is not yet known, this paper extends the study of LEO satellite augmentation by further varying the assumed range errors and integrity fault rates of LEO satellites to determine how much worse than GPS these parameters must be before the benefits of LEO augmentation are lost. For both Globalstar (~ 85-satellite) and OneWeb (~ 580-satellite) constellations, the assumed range error bounding sigmas were increased in stages up to 20 times those of GPS. Separately, the probabilities of integrity faults on individual satellites were increased by several orders of magnitude such that simultaneous but uncorrelated faults on multiple satellites could not be neglected. These new results establish the minimum levels of LEO satellite ranging performance needed to provide significant integrity improvements for ARAIM users.