Typical hybrid navigation units combine high-frequency but drifting measurements, such as those provided by an Inertial Measurement Unit (IMU), with low-frequency but bounded measurements, such as the ones provided by a GNSS receiver. In the context of space navigation systems, including the new navigation unit of the VEGA-C, NAVIGA, a loosely coupled GNSS/IMU architecture is traditionally employed to combine and hybridize these signals, mainly driven by simplicity and processing constraints. In contrast, the NAVIGA Evolution (NAVIGA-EVO) navigation system upgrades the NAVIGA loosely coupled architecture to a tightly coupled hybrid solution. Furthermore, this unit integrates a multi-sensor fusion system, hybridizing attitude measurements from a star tracker (STR).

This paper presents the architecture, design and performance of the NAVIGA-EVO system, which includes new features and enhances the performances of the baseline NAVIGA unit. Envisaged for the GNC subsystem of VEGA-C and Space Rider (SR), the system is tailored to address new use cases and scenarios beyond LEO, including Geostationary Transfer Orbit (GTO) and Earth re-entry. To achieve this, the unit is capable of operating in high altitude orbits and incorporates capabilities provided by the GNSS sensor, including dual-frequency and dual-RF antenna support.

In this work, advanced processing algorithms have been developed and implemented to enable tight hybridization and STR integration, building upon a loosely coupled configuration and ensuring compatibility across all configurations. The unit processes and hybridizes the GNSS raw data together with the IMU data. The tight hybridization architecture also receives, processes and hybridizes external attitude data from a STR. The architecture is designed to maximize the exploitation of measurement data by effectively combining information from multiple sensors. To ensure robustness, the data fusion operates with a fault detection, isolation and recovery algorithms.

Finally, this paper presents a comparative analysis of the performance of the loose and tight hybridization schemes, with and without the inclusion of STR data. Navigation performance results are evaluated for high-altitude (GEO) VEGA-C missions and Space Rider trajectories, including the re-entry phase. The results conclude that the tightly coupled hybridization not only enhances system robustness system but also delivers better performance outcomes.