Session A4: PNT Solutions for Space Applications

Robust Precise On Board Orbit Determination Exploiting T-RAIM for LEO-PNT
F.Menzione, C. Gioia, A. Piccolo, European Commission’s Joint Research Centre (JRC); S. Casotto, M. Bardella, University of Padua
Location: Beacon A

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 for the first time 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.

Our proposed approach introduces several innovative elements. The first is the inaugural investigation of the T-RAIM algorithm in the context of rapidly dynamic LEO scenarios. The second involves the analysis of three distinct open-loop integration architectures of the T-RAIM and P2OD building blocks. The approaches [5] appropriately identify possible outliers and or de-weight or exclude the observables prior to the P2OD phase. This assessment benefits from processing authentic GNSS data produced using the Joint Research Centre's (JRC) Hardware In The Loop (HIL) testing environment [6], which integrates simulators and Commercial Off-The-Shelf (COTS) LEO-compatible receivers. The test bench provides a comprehensive end-to-end toolchain, achieving a high level of representativeness in terms of orbiting platform dynamics, Galileo and GPS transmission and reception antenna characterizations, signal power, error calibration, and GNSS hardware measurement characterization [7]. P2OD performance analysis can be conducted under various operational conditions, including the use of E6 Signal-In-Space HAS corrections [8].
A specific LEO scenario has been crafted to trigger fault conditions within the MEO layer and to validate the T-RAIM algorithm across different geometric configurations. A series of calibrated pseudo-range ramps have been implemented to realistically simulate faults in various Galileo or GPS satellites. Multiple fault conditions have also been considered during extended-duration scenarios. The effectiveness of this method will be gauged by comparing the P2OD positioning and timing errors with and without T-RAIM pre-processing.
The paper identifies the integrated T-RAIM P2OD spaceborne architecture as a potential technological solution to mitigate synchronization errors in a disciplined LEO-PNT multi-tier architecture. The hardware-in-the-loop (HIL) test results demonstrate that selected T-RAIM algorithms are compatible with LEO applications and effectively prevents the P2OD accuracy from diverging when MEO clock faults are introduced. The precision at the decimetre level and timing accuracy in the order of nanoseconds are maintained. Furthermore, it is shown that pre-detection enables continuous operations without the necessity for a P2OD reset, which could otherwise result in a transient period lasting up to two hours.
The paper will also evaluate the unique aspects of the three proposed algorithms in terms of their computational load and complexity regarding their integration with the P2OD system. These preparatory analyses pave the way for the initial feasibility study of extending RAIM capabilities to spaceborne receivers aimed at LEO-PNT applications. The objective is to facilitate potential in-hardware implementations in current or forthcoming EC projects


[1] F. Menzione and M. Paonni, "LEO-PNT Mega-Constellations: a New Design Driver for the Next Generation MEO GNSS Space Service Volume and Spaceborne Receivers," 2023 IEEE/ION Position, Location and Navigation Symposium (PLANS), Monterey, CA, USA, 2023, pp. 1196-1207, doi: 10.1109/PLANS53410.2023.10140052.
[2]Hauschild, A., Montenbruck, O., Steigenberger, P., Martini, I., & Fernandez-Hernandez, I. (2022). Orbit determination of Sentinel-6A using the Galileo high accuracy service test signal. GPS Solutions, 26(4), 120. https://doi.org/10.1007/s10291-022-01312-5
[3] Darugna, F., Casotto, S., Bardella, M., Sciarratta, M., Zoccarato, P., & Giordano, P. (2022). Sentinel-6A GPS and Galileo Dual-Frequency Real-Time Reduced-Dynamics P2OD. 2022 10th Workshop on Satellite Navigation Technology (NAVITEC), 1–12. https://doi.org/10.1109/NAVITEC53682.2022.9847561
[4] GAlileo SPace ReceivER for Horizon 2020 IOD/IOV Mission (GASPER) under In-Orbit Demonstration and Validation (IOD/IOV) [DEFIS/2021/OP/0002]. (2021). https://etendering.ted.europa.eu/cft/cft-display.html?cftId=8112
[5] Gioia, C. T-RAIM Approaches: Testing with Galileo Measurements. Sensors 2023, 23, 2283. https://doi.org/10.3390/s23042283
[6] Piccolo, A, Cucchi, L., Gioia, C., Tegedor Navarro, J., Damy, S., Motella, B., Menzione, F., Bonenberg, L., Zoccarato, P., Picchi, O., Bonavitacola, F., Basso, M., Sgammini, M., Paonni, M., & Fortuny-Guasch, J. (2024). JRC testing and demonstration hub for the GNSS component of the EU space programme – Inventory of GNSS testing capabilities. Publications Office of the European Union. https://doi.org/10.2760/726940
[7] Piccolo, A., & Menzione, F. (2024). GNSS Test Bench Calibration for Space Service Volume Applications. 2024 11th International Workshop on Metrology for AeroSpace (MetroAeroSpace), 134–139. https://doi.org/10.1109/MetroAeroSpace61015.2024.10591570
[8] Zoccarato, P., Menzione, F., Gioia, C., Fortuny-Guasch, J., & Paonni, M. (2024, May). Monitoring and data distribution of the Galileo HAS system and user performance. Proceeding of European Navigation Conference 2024.