The emergence of Low Earth Orbit Position, Navigation, and Timing (LEO-PNT) satellite constellations is set to transform the landscape of PNT services, providing Global Navigation Satellite System (GNSS) users with supplementary signals from an extensive network of LEO satellites. These additional signals are intended to significantly enhance both performance and reliability. Numerous initiatives are taking shape globally, with some leveraging existing broadband LEO constellations through a fused approach, while others are creating dedicated LEO infrastructures [1]. Regardless of the approach, all LEO-PNT endeavours offer cost-effective solutions by adopting multi-tiered architectural designs. Within these architectures, the LEO-PNT services depend on spaceborne GNSS receivers. These receivers function as autonomous systems for Orbit Determination and Time Synchronization (ODTS), capable of generating on-board ephemeris and other necessary corrections that are disseminated to the final LEO-PNT end-users. The driving technology behind this opportunity is the innovative onboard Precise Orbit Determination (P2OD) capability, bolstered by Precise Point Positioning (PPP)-like correction signals in space, similar to those offered by Galileo High-Accuracy Services (HAS). The goal is to achieve accurate real time decimeter-level reconstruction of spacecraft orbits and, more critically, to provide unbiased nanosecond-level time synchronization by using GNSS Pulse Per Second (PPS) events. In the LEO-PNT multi-tier solution, the spaceborne GNSS PPS is used to maintain the clock stability and discipline the transmitter generating the navigation signals. This synchronization mechanism significantly affects the precision of the corresponding LEO-PNT observables, such as pseudorange and carrier phase. This framework creates a strong dependency between the Medium Earth Orbit (MEO) constellations and the LEO layer, meaning that any faults in the upper layer can propagate to the lower one generating a cascade effect detrimental for the final end-user. This phenomenon is thoroughly explained in [1], which also suggests that adopting Receiver Autonomous Integrity Monitoring (RAIM)-like capabilities within the spaceborne receivers could be the best recovery. This paper investigates the possibility to extend to spaceborne receivers T-RAIM solutions generally proposed for ground and envisage the possibility to integrate it with the state of art of P2OD HAS based techniques ( [2], [3] ). This work builds upon and enhances the ongoing European Commission (EC) activities [4] , which focus on the development of innovative algorithms and technologies aiming to achieve unparalleled accuracy and reliability in the LEO Space Service Volume (SSV). Although the challenges of on-board RAIM in space extend to both positioning and timing accuracy, our research intentionally begins with the temporal aspect. Within the LEO PNT multi-tier architecture, ephemeris errors can be mitigated by projecting the most recent valid ephemeris, thus affording a grace period for users. However, timing errors cannot be similarly compensated, as they immediately result in undetectable delays in LEO-PNT signal generation. We contend that to ensure terrestrial integrity when using LEO-PNT observations, the timing requirements for P2OD should serve as a mission-critical threshold for the T-RAIM algorithm.
MENZIONE Francesco;
GIOIA Ciro;
PICCOLO Andrea;
CASOTTO Stefano;
BARDELLA Massimo;
2025-02-17
Istitute of Navigation
JRC141129
978-0-936406-40-4 (online),
2330-3646 (online),
https://www.ion.org/publications/browse.cfm?proceedingsID=168,
https://publications.jrc.ec.europa.eu/repository/handle/JRC141129,
https://doi.org/10.33012/2025.19963 (online),
