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Session A6: GNSS Integrity and Augmentation

Baseline Advanced RAIM User Algorithm: Proposed Updates
Juan Blanch, Todd Walter, Stanford University
Location: Beacon B
Date/Time: Thursday, Jan. 27, 3:25 p.m.

Peer Reviewed

Peer Reviewed

The Advanced Receiver Autonomous Integrity Monitoring (ARAIM) concept, an evolution of RAIM to multi-constellation and dual frequency, is currently being standardized within ICAO and RTCA/EUROCAE. A reference user algorithm was part of the report describing the initial ARAIM concept developed within the bilateral US-EU Working Group C [1,2,3]. This reference algorithm has been used to evaluate the expected ARAIM performance [4] and for early prototyping [5]. Although there is no plan to make the baseline algorithm a requirement in the standards, it remains a key input for their development, because it provides an acceptable method to implement ARAIM at the user receiver and demonstrates how the integrity support data must be interpreted.
As part of this standardization process, the initial baseline user algorithm needs to be updated and refined to address recent integrity and continuity analyses [6,7], and to reduce the computational load [8]. Also, proposed default values for the Integrity Support Data are now available for all the GNSS constellations that could be used in ARAIM. These default values are expected to be valid even if no Integrity Support Message is broadcast and they may be seen as a lower bound on the expected performance. These lower bounds can be exploited to simplify the design of the algorithm and to improve the computational efficiency.
The purpose of this paper is to describe an updated baseline algorithm that integrates these latest developments, and to clarify certain design choices. After reviewing the basic elements of an Advanced RAIM user algorithm, we will focus on the following points:
1) Subset selection: in [3], the subset selection is designed to minimize the number of subsets in the fault detection mode. For algorithms where the integrity allocation among exclusion modes is not optimized (to limit computational load), it can lead to less-than-ideal subset choices. Now that we have a set of default ISD, we can define a subset selection approach that is both simpler and better in terms of performance
2) Exclusion function: after going over why we need to allocate the integrity budget to each exclusion option, we will justify the choice of exclusion options based on the continuity requirement [7]. More precisely, we will show that for the proposed ISD default values, it is sufficient to consider single fault modes (either satellite or constellation).
3) Temporal exposure integrity analysis: we will refine the Equations introduced in [6] and show how to exploit the maximum possible integrity risk contribution from a given fault mode. To do this, we will go over the impact of the faults that are not monitored on the integrity risk.
4) Methods to reduce computational load: we will integrate in the algorithm description the techniques described in [8,9]. These techniques can drastically reduce the computational load with very little performance impact, both in availability simulations and in real time implementations
5) Numerical issues: we will go over some numerical issues that may arise in the implementation of the baseline algorithm and propose mitigations. These include zero divided by zero indeterminates, as well as the evaluation of the normal gaussian tail cdf for large arguments.


REFERENCES
[1] Blanch, J., Walter, T., Enge, P., Lee, Y., Pervan, B., Rippl, M., Spletter, A., Kropp, V., "Baseline Advanced RAIM User Algorithm and Possible Improvements," IEEE Transactions on Aerospace and Electronic Systems, Volume 51, No. 1, January 2015.
[2] Working Group C, ARAIM Technical Subgroup, Milestone 3 Report, February 26, 2016. Available at:
http://www.gps.gov/policy/cooperation/europe/2016/working-group-c/
http://ec.europa.eu/growth/tools-databases/newsroom/cf/itemdetail.cfm?item_id=8690
[3] WG-C Advanced RAIM Technical Subgroup Reference Airborne Algorithm Description Document, available at:
http://web.stanford.edu/group/scpnt/gpslab/website_files/maast/ARAIM_TSG_Reference_ADD_v3.1.pdf
[4] https://gps.stanford.edu/resources/tools/maast
[5] Phelts, R. Eric, Blanch, Juan, Gunning, Kazuma, Walter, Todd, Enge, Per, "Assessing of Aircraft Banking on ARAIM Performance," Proceedings of the 31st International Technical Meeting of The Satellite Division of the Institute of Navigation (ION GNSS+ 2018), Miami, Florida, September 2018, pp. 2632-2641.
[6] Milner, Carl, Pervan, Boris, Blanch, Juan, Joerger, Mathieu, "Evaluating Integrity and Continuity Over Time in Advanced RAIM," 2020 IEEE/ION Position, Location and Navigation Symposium (PLANS), Portland, Oregon, April 2020, pp. 502-514.
[7] Joerger, Mathieu, Zhai, Yawei, Martini, Ilaria, Blanch, Juan, Pervan, Boris, "ARAIM Continuity and Availability Assertions, Assumptions, and Evaluation Methods," Proceedings of the 2020 International Technical Meeting of The Institute of Navigation, San Diego, California, January 2020, pp. 404-420.
https://doi.org/10.33012/2020.17152
[8] Blanch, Juan, Walter, Todd, Enge, Per, "Fixed Subset Selection to Reduce Advanced RAIM Complexity," Proceedings of the 2018 International Technical Meeting of The Institute of Navigation, Reston, Virginia, January 2018, pp. 88-98.
[9] J. Blanch and T. Walter, "Fast Protection Levels for Fault Detection With an Application to Advanced RAIM," in IEEE Transactions on Aerospace and Electronic Systems, vol. 57, no. 1, pp. 55-65, Feb. 2021, doi: 10.1109/TAES.2020.3011997.



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