Modelling and Simulation of GNSS Observables for Spacecraft Navigation

GNSS-based navigation plays a central role in space applications, supporting both absolute and relative navigation. New mission concepts are targeting tight formations with baselines as short as 5-13 m, where accurate characterisation of GNSS observables becomes critical during early algorithm development. The GNSS Environment and Measurements Simulator (GEMS) is being developed at Politecnico di Milano and is meant to support the development and early prototyping of (relative) navigation algorithms. It features a modular architecture and simulates raw GNSS measurements received onboard. GEMS models key error sources, such as clock biases, ionospheric delay, and thermal noise, and integrates real data from the International GNSS Service (IGS), including precise products and ionospheric maps. Validation against flight data from the Sentinel-3 and Sentinel-6 missions shows that GEMS achieves simulation errors at the metre level, while successfully reproducing the temporal behaviour of GNSS observables. These results demonstrate that GEMS provides a reliable and cost-effective platform for early-stage algorithm development and Model-in-the-Loop testing, bridging the gap between simplistic error models and high-fidelity hardware simulators. Copyright (c) 2025 The Authors. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/)