Real-Time Precise Absolute and Relative Orbit Determination Using Galileo High-Accuracy Services Over Varying Baselines

Samuel Y. W. Low, Simone D’Amico

Abstract:	Distributed space systems hold promise for new science and mission capabilities, enabled by improvements in the performance of real-time onboard orbit determination (OD). As these systems scale, navigation performance is envisioned as a shared network capability rather than an independent feat of an individual spacecraft. Nodes with higher-quality state knowledge can distribute this knowledge to other members, analogous to terrestrial GNSS correction networks. Realizing this vision for a network of space-based GNSS receivers requires a flight software architecture capable of precise absolute inertial-frame OD as a high-quality reference, and precise relative OD to distribute precise state knowledge over relative baselines. This paper presents an advancement of the Distributed Multi-GNSS Timing and Localization (DiGiTaL) flight software, developed at the Stanford Space Rendezvous Laboratory, and augmented with Galileo High Accuracy Services (HAS), as a first step toward this vision. In summary, the contributions are presented in three stages: (1) architecture, (2) algorithms, and (3) assessment. First, the design of the upgraded flight software architecture, based on DiGiTaL, is presented with Galileo HAS augmentation. The architecture is designed and tested to seamlessly and robustly handle real-time and realistic flight scenarios throughout various phases of a distributed space systems mission, and includes fault detection, isolation, and recovery. Computational optimizations to the architecture demonstrate fast runtimes per full navigation state update, far below the 1PPS update rate, even on legacy cube-sat on-board computers with very modest processor capabilities. Second, to ensure the flight software is capable over varying baselines, a new bias-aware and time-correlated process model for differential ionospheric path delay estimation is introduced, informed by geometry-free observables, without the need for any prior ionospheric models. Third, a full assessment of the accuracy and coverage characteristics of Galileo HAS orbit and clock corrections are conducted, analyzed, and discussed. The DiGiTaL flight software is validated across three test campaigns of increasing challenge: short- and long-baseline simulations of the VISORS and GRACE-FO missions respectively under ideal HAS coverage, followed by a long-baseline pseudo-real-time (playback) flight data campaign using real GRACE-FO onboard GNSS observables and Galileo HAS data from the internet distribution service. Results demonstrate centimeter-accurate relative OD and decimeter-accurate absolute OD achieved over the 200km GRACE-FO baseline using real observables and Galileo HAS data. Over the shorter 200m VISORS baseline, where differential ionospheric path delay errors are negligible, the relative OD performance improves to millimeter-level accuracy, which is a well-established benchmark from the prior work of DiGiTaL. Results also demonstrate that the new algorithm for differential ionospheric path delay estimation is able to accurately converge on the expected signal bias, and exhibit a periodic structure indicative of the expected diurnal variations. These results establish DiGiTaL’s capability to perform real-time onboard precise absolute and relative navigation for multiple spacecraft simultaneously, over varying baselines, as a first step towards establishing space-based navigation networks.
Published in:	Proceedings of the ION 2026 Pacific PNT Meeting April 13 - 16, 2026 Hilton Waikiki Beach Honolulu, Hawaii
Pages:	861 - 880
Cite this article:	Low, Samuel Y. W., D’Amico, Simone, "Real-Time Precise Absolute and Relative Orbit Determination Using Galileo High-Accuracy Services Over Varying Baselines," Proceedings of the ION 2026 Pacific PNT Meeting, Honolulu, Hawaii, April 2026, pp. 861-880. https://doi.org/10.33012/2026.20605
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