Fast GNSS Ambiguity Resolution under Frequent and Persistent Outages of Differential Data for Vessel Navigation

Jan-Jöran Gehrt, René Zweigel and Dirk Abel

Abstract:	Automatic maneuvering of vessels requires reliable and highly accurate navigation. Therefore, this publication describes the development of a tightly coupled navigation filter used as a state estimator for automated driving for vessels operating in harbors. The filter estimates 3D-position, -velocity and -orientation, bases on inertial quantities and is aided by pseudoranges, deltaranges and carrier phases. In order to achieve high position accuracy carrier phase ambiguity is estimated within the filter states and fixed by LAMBDA. For common-mode error mitigation, differential data from a reference station network is integrated, received via cellular connection. The accuracy of the ambiguity estimate correlates with the accuracy of the height estimate. In harbors, accurate height information is available in the form of sea level. Therefore, this work integrates sea level data into the ambiguity estimation of the navigation filter, and thereby significantly improves time to fix. Frequent cellular service outages can be observed in the harbor, which results in losing high accuracy temporally, or even the ambiguity fix. To enable continuous, reliable high accuracy positioning, a new Kalman filter-based local estimation of differential data is implemented, such that a continuous stream of differential data is provided. Extensive evaluation with an unmanned surface vehicle (USV) in Rostock Harbor, Germany, is performed. Time to fix is evaluated with simple GPS L1/L2C wide-lane carrier phases, as this allows sufficient constellations with more than 4 satellites for navigation. With sea level data, a nearly instantaneous ambiguity fix is achieved (time-to-fix in average 1.5 s). Results show, that the local estimation of differential data allows ambiguity fixing, even if the last received differential epoch is older than 2.5 min, which is an important achievement for autonomous navigation. Over all, a 3D-position accuracy of less than 40 cm is achieved, even with frequently cellular connection outages of several minutes. In phases where outages of only a few seconds occur, a 3D-position accuracy of less than 10 cm is achieved. The implementation bases on GPS and can be further improved in the future by adding Galileo observables.
Published in:	Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021) September 20 - 24, 2021 Union Station Hotel St. Louis, Missouri
Pages:	4116 - 4127
Cite this article:	Gehrt, Jan-Jöran, Zweigel, René, Abel, Dirk, "Fast GNSS Ambiguity Resolution under Frequent and Persistent Outages of Differential Data for Vessel Navigation," Proceedings of the 34th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2021), St. Louis, Missouri, September 2021, pp. 4116-4127. https://doi.org/10.33012/2021.18071
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