Online ARAIM Architecture Evaluation to Support a Single GNSS Constellation

Sam Pullen and Per Enge

Abstract:	GNSS users with demanding safety or integrity requirements but without access to real-time differential corrections can utilize Advanced RAIM, or ARAIM, which now represents a well-developed and mature methodology [1,2]. ARAIM extends and improves upon traditional RAIM algorithms to detect and mitigate independent, multiple, and correlated GNSS signal faults, making it possible to support applications such as aviation precision approach without requiring repeated integrity messages within a 2-to-6 second time-to-alert. RAIM and ARAIM rely on redundant ranging measurements to detect failures, and the most of the emphasis to date has been upon using multiple GNSS constellations to provide availability similar to that obtainable from augmented GPS while detecting or otherwise mitigating correlated across a single constellation [3]. Meanwhile, the authors of this work have investigated the use of ARAIM with the GPS satellites alone for users restricted to GPS, focusing on what is expected to be possible once GPS III modernization is complete [4,5]. This work extends upon and generalizes the studies in [4,5] to propose a complete ground and airborne ARAIM architecture for single-GNSS-constellation users that is oriented toward GPS but is not limited to GPS or GPS III. In the terminology of [1,2], it is an “online” architecture in which a ground station monitoring network and Integrity Support Message (ISM) communication channel provide near-real-time updates to users regarding the achievable GNSS satellite ranging accuracy and probability of failure. It is focused on LPV and LPV-200 precision approach for aviation but also considers extensions to Category I precision approach and beyond as well as marine, land, and UAV guidance. Two design trade-offs define this architecture. The first is the content and update rate of the ISM. Depending on the bandwidth and coverage of the selected ISM communications channel, ISM updates can be provided more often than the notional one-hour minimum for “online” service from [2], and they can contain ground-system-generated satellite clock and ephemeris information that replaces what is broadcast by the satellites and thereby supports lower integrity bounds. Since the optimal choice of the ISM content and update rate depends on the resources available to it and the intended user objectives, three possibilities are considered in this work: 1) One-hour updates, replacement clock and ephemeris information 2) Ten-minute updates, replacement clock and ephemeris information 3) One-minute updates, no replacement clock and ephemeris information The ISM design should also support graceful degradation toward the performance of “offline” single-constellation ARAIM when ISM messages are missed or unavailable. The second design trade-off, which is tied to the ISM design choice, is the geographic density and complexity of the ground-station network used to monitor the satellites and update ISM values. The greater the demands on the ISM, the greater the complexity required of the ground system to, for example, compute and monitor the clock and ephemeris information transmitted to users. A minimum requirement of the ground system is to monitor satellite health and error behavior and adjust the bounding error values (URA, URE, and any biases) along with the probability of individual satellite failure (p_sat) when needed. For a single-constellation system, this role extends to insuring that the correlated satellite failure probability (p_const) is negligible for the intended operation. To support this, a probability representing correlation among individual satellite failures that does not automatically extend to the entire constellation (call this p_corr) may be needed. This work illustrates the capabilities of the three ISM options defined above (along with supporting ground systems) by simulations of the present and future GPS constellation along the lines of [4,5]. The objective is to determine where in the architecture design space the maximum performance-to-cost ratio is obtained for different classes of users (LPV and LPV-200, Category I, and marine harbor entry). References: [1] J. Blanch, T. Walter, et al, “Critical Elements for a Multi-Constellation Advanced RAIM,” Navigation, Journal of The Institute of Navigation, Vol. 60, No. 1, Spring 2013, pp. 53-69. [2] Interim Report of the E.U.-U.S. Cooperation on Satellite Navigation Working Group C: ARAIM Technical Subgroup, Issue 1.0, Dec. 19, 2012. [3] T. Walter, J. Blanch, “Airborne Mitigation Strategies against Constellation Wide Faults,” Proceedings of ION ITM 2015, Dana Point, CA, Jan. 26-28, 2015, (forthcoming). [4] S. Pullen, P. Enge, et al, “The Impact of GPS Modernization on Standalone User Performance and Integrity with ARAIM,” Proceedings of ION GNSS+ 2013, Nashville, TN, Sept. 16-20, 2013, pp. 2637-2653. [5] S. Pullen, P. Enge, et al, “GPS III Accuracy and Integrity Improvements Using ARAIM with Shorter Age of Data,” Proceedings of ION GNSS+ 2014, Tampa, FL, Sept. 8-12, 2014, pp. 3352-3362.
Published in:	Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015) September 14 - 18, 2015 Tampa Convention Center Tampa, Florida
Pages:	675 - 705
Cite this article:	Pullen, Sam, Enge, Per, "Online ARAIM Architecture Evaluation to Support a Single GNSS Constellation," Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015), Tampa, Florida, September 2015, pp. 675-705.
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