Massimo Bardella

The rapid development of Low Earth Orbit Position, Navigation, and Timing (LEO-PNT) constellations presents opportunities to augment Global Navigation Satellite Systems (GNSSs) with additional signals from Low Earth Orbit (LEO) satellites, thereby improving performance and reliability for users. This research study addresses the challenges posed by the interdependency between LEO and GNSS layers, which can lead to cascading faults. By extending Receiver Autonomous Integrity Monitoring (RAIM)-like capabilities to spaceborne receivers, specifically through Timing Receiver Autonomous Integrity Monitoring (T-RAIM), this paper aims to mitigate these risks. This study validates the integration of T-RAIM with advanced Precise Real-Time On-board Orbit Determination (P2OD) techniques in LEO scenarios using a hardware-in-the-loop test environment. The findings demonstrate that the architecture with T-RAIM can maintain nominal positioning and timing accuracy even in the presence of GNSS clock faults, ensuring continuous system functionality without requiring P2OD restarts. This capability is crucial to preventing service interruptions and enhancing the robustness of LEO-PNT solutions. The proposed integration handles the computational load and complexity while accommodating the limited resources of spaceborne receivers, offering a viable and robust LEO-PNT solution. The experimental results show that T-RAIM effectively mitigates the impact of pseudorange ramp errors, maintaining stable clock bias and preserving the integrity of orbit determination and time synchronization.

In GNSS-based navigation onboard Low Earth Orbit (LEO) satellites, typical accuracy requirements are 10 cm and 0.1 mm/s for 3D position and velocity, respectively. Previous works have shown that such performance is achieved by including Galileo measurements in the estimation process. Here, we aim to evaluate the impact of employing State Space Representation (SSR) corrections, i.e., GNSS satellite orbit, clock, and biases, to be applied to the broadcast ephemerides. In this framework, the Precise Onboard Orbit Determination (P2OD) software (SW) tool developed at the University of Padua (UNIPD) is used to investigate the needs of onboard navigation. The UNIPD SW employs an Extended Kalman Filter (EKF) using a reduced-dynamics approach. The force model implemented is adapted to onboard processing, and empirical accelerations are included to take into account residual force mismodeling. Actual observation data from the LEO Sentinel-6A satellite are processed along with SSR corrections from the CNES service. Galileo-based solutions are compared to ground-based POD reference orbits. The analysis suggests that the use of SSR corrections provides sub-decimeter and below 0.1 mm/s accuracies in 3D position and velocity, respectively. Such results indicate a P2OD solution quality close to that achievable by adopting precise orbits and clocks.