Frequency-Varying LEO-Assisted GNSS Positioning in Urban Environments

Songfeng Yang

Peer Reviewed

Abstract:	Global Navigation Satellite System (GNSS) signals frequently encounter blockages and multipath errors in urban canyons. In such GNSS-challenged environments, one is forced to raise the cut-off elevation angle (also referred to as the elevation mask), making it challenging to maintain sufficient satellite visibility for high-precision positioning. Commercial Low Earth Orbit (LEO) communication satellites offer a promising solution by assisting GNSS with additional measurements, providing a greater number of satellites, stronger signals, and faster geometric changes compared to GNSS. However, these LEO satellites, not originally designed for Positioning, Navigation, and Timing (PNT), often lack code measurements, primarily transmitting satellite-specific frequency-varying carrier phase signals, which complicates Integer Ambiguity Resolution (IAR). This study investigates the performance of GNSS positioning assistance using the L-band Iridium and S-band Globalstar LEO communication satellites at high cut-off elevation angles. Through the proposed LEO-assisted GNSS model and processing platform, the integer estimability of ambiguities for the frequency-varying LEO satellites is specifically ensured. Results from processing four baselines with real-world GNSS data show that both the ambiguity resolution success-rate and positioning accuracy improve with the assistance of the Iridium and Globalstar constellations compared to the standalone GPS and combined GPS/Galileo, respectively, at high cut-off elevations. We achieve the radial ambiguity-float positioning accuracy at the meter level when only using GPS satellites at 30°. The radial position errors are reduced to below 1 m with the assistance of the Iridium and Globalstar satellites and further to the centimeter level after applying partial IAR. This assistance also improves the empirical ambiguity resolution success-rates, raising them from 93.8% and 97.7% to 94.7% and 98.4% in the cases of standalone GPS and combined GPS/Galileo, respectively. Simulated short-baselines across global locations reveal similar patterns in the assistance performance with the Iridium and Globalstar constellations in terms of the number of visible satellites, radial position errors, and ambiguity resolution success-rates. This also indicates an average improvement of 44.3% and 34.8% in 1-s near real-time ambiguity-fixed positioning accuracy worldwide, compared to the standalone GPS constellation, at cut-off elevation angles of 30° and 40°, respectively.
Published in:	Proceedings of the 38th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2025) September 8 - 12, 2025 Hilton Baltimore Inner Harbor Baltimore, Maryland
Pages:	2414 - 2430
Cite this article:	Yang, Songfeng, "Frequency-Varying LEO-Assisted GNSS Positioning in Urban Environments," Proceedings of the 38th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2025), Baltimore, Maryland, September 2025, pp. 2414-2430. https://doi.org/10.33012/2025.20330
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