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A new ionospheric model called NTCM G (Neustrelitz TEC Model) is now available on the European GNSS Service Centre (GSC) website (https://www.gsc-europa.eu/news/) for correcting ionospheric delay of single-frequency Galileo signals. Investigation shows that NTCM G requires less computing resources compared to the existing reference NeQuick G model while providing a good performance to single-frequency Galileo users. NTCM G does not replace the NeQuick G, but offers an alternative with reduced computational load and complexity for certain users. The NTCM G model was developed by the German Aerospace Centre (DLR) and validated by the Joint Research Centre (JRC) of the European Commission with the support of the European Space Agency (ESA). The description of the source code and its implementation were carried out jointly between DLR and JRC, with the European Union Agency for the Space Programme (EUSPA) and ESA supporting the review and the publication of the model description. The reference implementation of the core algorithm and the auxiliary functions are available for download on the GSC website. The software package provides a portable and validated C/C++ implementation (Matlab and Simulink implementations are also available), including testing functions and testing vectors. NTCM G uses the same Az coefficients broadcast for NeQuick G by the Galileo space segment. Since the Az coefficients are computed in the Galileo ground segment a few hours before the NTCM G works as an ionosphere prediction model for its users.

This dissertation investigates a novel Multipurpose Earth Orbit Navigation System (MEONS) architecture aiming at providing a generalized GNSS based spacecraft orbit estimation kernel matching the modern navigation instance of enhanced flexibility with respect to multiple Space Service Volume (SSV) applications (Precise Orbit Determination for Earth Observation satellite, Low Thrust Low to High Autonomous Orbit Rising, formation flying relative navigation, Small Satellite Autonomous Orbit Acquisition). The possibility to address theoretical and operational solutions within a unified framework is a foundamental step for the implementation of a reusable and configurable high performance navigation capability on next generation platforms.

This work provides a possible design methodology devoted to future development of a generalized on board Earth orbit navigation system. A multipurpose dynamic model architecture is exploited in order to perform trade-off and sensitivity analysis for orbit propagation force model and structural parameters identification against a target mission scenario. The dynamic model configuration is considered also within a sequential estimation process where external trajectory observations are available. The study introduces the possibility to preliminary assess the achievable performance of the navigation system by using the Cramer Rao Lower Bound in its parametric definition for dynamic estimators. The recursive approximation around the nominal mission path is considered by updating the Fisher Information Matrix. The developed design logic is finally applied on a representative orbit determination problem belonging to high orbit navigation scenarios.

In the framework of the novel navigation scenario offered by the next generation satellite low thrust autonomous LEO-to-MEO orbit transfer, this study proposes and tests a GNSS based navigation system aimed at providing on-board precise and robust orbit determination strategy to override rising criticalities. The analysis introduces the challenging design issues to simultaneously deal with the variable orbit regime, the electric thrust control and the high orbit GNSS visibility conditions. The Consider Kalman Filtering approach is here proposed as the filtering scheme to process the GNSS raw data provided by a multi-antenna/multi-constellation receiver in presence of uncertain parameters affecting measurements, actuation and spacecraft physical properties. Filter robustness and achievable navigation accuracy are verified using a high fidelity simulation of the low-thrust rising scenario and performance are compared with the one of a standard Extended Kalman Filtering approach to highlight the advantages of the proposed solution. Performance assessment of the developed navigation solution is accomplished for different transfer phases.