Bounding GPS L1 Antenna Group Delay Variation for GNSS Landing System Integrity

Matt Harris, Matt Miltner, Tim Murphy, Anurag Raghuvanshi, Frank van Graas

Peer Reviewed

Abstract:	RTCA DO-301 requires a limit on the group delay variation for an L1 GNSS antenna. An antenna that barely meets these DO- 301 limits could contribute ranging errors up to 75 centimeters at low elevations and 20 centimeters above 45 degrees elevation [1]. For GNSS landing system integrity bounding, the allocation for airframe multipath error bounding was believed to also bound antenna group delay variation since the two error sources were not separable during the flight test campaigns used to set these bounds. The DO-301 limits are in the one-to-two standard deviations range for the multipath error bounding equations for GNSS landing systems in DO-253, DO-316, and DO-229 [2,3,4]. It has since been demonstrated through antenna measurements that dual-frequency antennas can have much lower group delay variation than L1-only antennas, which can be minimally or marginally compliant with the DO-301 limits. Furthermore, the multipath error allocation is based on flight testing that was accomplished using a dual frequency GPS antenna and may have outperformed the DO-301 specification, thereby underestimating the worst group delay contribution. It has also been noted that, on a single approach, both multipath and group delay variation errors will be bias-like, and a bias over bound could be applied to both. A bias over bound method would require changes to fielded aircraft equipment and would have an unacceptable impact on availability due to its conservatism. This paper validates that even with a minimally compliant L1 GPS antenna, the current multipath error model is a sufficient bound for multipath and group delay. Electromagnetic models for airframe multipath for multiple tracking correlator types are combined with a model of antenna group delay, and the range domain and position domain bounding levels are assessed. Also, hundreds of hours of flight test data from a large, fixed-wing aircraft using an L1 antenna were analyzed and summarized to show that position domain bounding is achieved at the required level for precision approach using the current multipath error model. Rationale that justifies application of the current multipath model to Category III GNSS Landing Systems is also provided.
Published in:	Proceedings of the 2017 International Technical Meeting of The Institute of Navigation January 30 - 2, 2017 Hyatt Regency Monterey Monterey, California
Pages:	591 - 605
Cite this article:	Harris, Matt, Miltner, Matt, Murphy, Tim, Raghuvanshi, Anurag, van Graas, Frank, "Bounding GPS L1 Antenna Group Delay Variation for GNSS Landing System Integrity," Proceedings of the 2017 International Technical Meeting of The Institute of Navigation, Monterey, California, January 2017, pp. 591-605. https://doi.org/10.33012/2017.14878
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