Sam Pullen, Sherman Lo, Juan Blanch, Todd Walter, Stanford University; Andrew Katronick, Mark Crews, Robert Jackson, Kevin Huttenhoff, Lockheed Martin

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GNSS augmentations that support high-integrity, safety critical applications such as Satellite-based Augmentation Systems (SBAS) and Advanced Receiver Autonomous Integrity Monitoring (ARAIM) depend upon multiple redundant, well-dispersed satellites in view to provide high availability of integrity to users. “High availability” in this context refers to a high probability over time (changing satellite geometry) that user protection levels (position error bounds valid to the probabilities required for integrity) are small enough to satisfy the alert limits (acceptable error limits for safety) of the intended operations. For example, high availability of civil ARAIM for aircraft approach-and-landing applications with tight vertical alert limits requires the simultaneous use of multiple GNSS constellations [1, 2], although this is not necessarily the case for hypothetical military ARAIM with similar alert limits [3, 4]. Unlike ARAIM, SBAS utilizes Geostationary (GEO) satellites to continuously relay measurement corrections and integrity parameters to users. The signals that provide this information can also be used for positioning, but the range errors on these signals are typically several times higher than those from GNSS satellites and thus provide limited benefit [5]. This paper examines several different approaches to and augmentations of today’s GNSS constellations provide higher availability of high-integrity GNSS-based navigation to users of either SBAS or ARAIM. The starting point is a new constellation with a variable number of inclined Geosynchronous (I-GEO) satellites in multiple planes that are evenly spaced in longitude around Earth. These satellites have capabilities similar to those of today’s GPS III satellites and make use of crosslinks to allow more rapid dissemination of ground messages being sent to users. These satellites would relay SBAS messages in the same manner as existing GEO satellites but with additional power margin and range accuracy similar to that of GPS III so that they can augment GNSS with no error degradation. They would also utilize crosslinks to relay the Integrity Support Message (ISM) required by ARAIM with a frequent update rate (e.g., multiple updates per day) to allow less conservatism in the parameters broadcast within the ISM [3, 4]. The Stanford MAAST GNSS simulation software package [6] is used to evaluate the availability of integrity for aviation LPV and LPV-200 approaches for both SBAS and ARAIM using variable I-GEO constellations on their own and combining them with existing GNSS constellations as well as with hypothetical new constellations of navigation satellites in MEO and/or LEO. Global latitude/longitude grids of military (dual-frequency M-code and/or PRS) and civil (dual-frequency L1/L5 or equivalent) user locations are considered along with three networks of global reference stations containing 12, 21, and 33 stations, respectively. Since the integrity requirements of LPV and LPV-200 approaches are met by design by SBAS and ARAIM algorithms, the results of interest are the availability of integrity for each constellation and ground-network variation compared to each other and to existing results for SBAS and ARAIM (e.g., see [2, 3]). These results show the utility of the I-GEO concept, help determine optimal I-GEO constellation arrangements, and show the non-GEO constellation types that benefit the most from the addition of I-GEO satellites. References: [1] J. Blanch, et al., "Baseline advanced RAIM user algorithm and possible improvements," IEEE Transactions on Aerospace and Electronic Systems, Vol. 51, No. 1, Jan. 2015, pp. 713-732. [2] “Milestone 3 Report of the E.U.-U.S. Cooperation on Satellite Navigation Working Group C: ARAIM Technical Subgroup,” Final Version, Feb. 25, 2016. [3] A. Katz, S. Pullen, et al., “ARAIM for Military Users: ISM Parameters, Constellation-Check Procedure and Performance Estimates,” Proceedings of ION ITM 2021, (Virtual), Jan. 25-28, 2021, pp. 173-188. [4] S. Pullen, S. Lo, et al., “Ground Monitoring to Support ARAIM for Military Users: Alternatives for Rapid and Rare Update Rates,” Proceedings of ION GNSS+ 2021, S. Louis, MO, Sept. 21-24, 2021. [5] S. Saito, N. Fujii, et al., “Solutions to Issues of GBAS using SBAS Ranging Source Signals,” Proceedings of ION GNSS 2008, Savannah, GA, Sept. 16-19, 2008, pp. 2894-2900. [6] “Matlab Algorithm Availability Simulation Tool,” Stanford University GPS Laboratory,