Session A1: Alternatives, Backups, Complements to GNSS

An Overview of ESA’s LEO-PNT In Orbit Demonstration Mission
M. Cordero, P. Giordano, R. Sarnadas, Florin-Catalin Grec, M. Anghileri, and R. Prieto-Cerdeira, European Space Agency
Location: Beacon A

Global Navigation Satellite Systems (GNSS) play a crucial role in providing Positioning, Navigation, and Timing (PNT) services across a wide range of applications being the only global technology providing such services for free. Although GNSS has demonstrated exceptional performance, there are still a number of cases in which it is not enough. These include the operation under harsh conditions like urban environments, indoor environments and under the presence of radiofrequency interferences. The reduction of the time to convergence to centimetre-level accuracy is also a case in which there is still a gap between the performance achieved by GNSS and the user expectations.
In response, multiple initiatives are exploring PNT solutions utilizing signals from Low Earth Orbit (LEO) constellations, referred to as LEO-PNT, with the goal of creating a future multi-layer PNT system of systems. The characteristics of these orbits can be exploited to offer new signals that can complement those of GNSS in higher orbits. Being closer to the Earth the signal propagation losses are smaller which can enable the use of higher frequency bands for which the ionospheric delay is lower and there can potentially be wider bandwidths leading to more accurate and robust PNT signals. The lower propagation losses can also enable uplink signals for smaller user terminals which can be exploited for a number of complementary applications including two-way services. The rate of change of line-of-sight for LEO satellites is much faster than for MEO which leads to better decorrelation of atmospheric and multipath errors and then enabling faster decorrelation times [1]. However, there are a number of technical challenges which need to be tackled including the higher Doppler of lower orbits and higher frequency bands which pose a challenge for signal acquisition, the higher dynamic stress and oscillator phase noise for higher frequency bands, the shorter duration of satellite passes…
In this context, and following a set of successful previous studies, the European Space Agency kicked off two parallel contracts in March 2024 for a LEO-PNT In-Orbit Demonstration mission within the FutureNAV programme, aiming at demonstrating how LEO-PNT systems can complement GNSS in providing more performing and robust PNT services as well as experimenting with different enabling technologies and techniques to derive lessons learned for a potential future operational system. As part of this mission, a pair of CubeSats (Pathfinder A satellites) are planned to be launched at the end of 2025 to a quasi-polar orbit at around 550-km altitude in order to de-risk the critical technologies and implementation of the payload as well as the Test User Receivers and ground control segment, and to carry out the first experiments which do not need more than one satellite. Following them, a set of 4 satellites per contract (the so-called Pathfinder B satellites) will be launched to similar orbits than the previous ones to have a mini constellation deployed by 2027. With such mini-constellations, experiments in the PVT domain for different use cases will be carried out. The results obtained with this limited number of satellites can then be extrapolated via simulations in order to assess the achievable performance with a full constellation.
Among the experiments, Pathfinder A satellites will transmit signals in E5 and S bands (legacy RNSS bands) while Pathfinder B satellites will also transmit signals in UHF and C band. They will also include a two-way payload. In addition, experiments will be carried out on real-time onboard Orbit Determination and Time Synchronization (ODTS) including the onboard timing subsystem.
This contribution will provide a description of the LEO-PNT IoD mission, focusing on the system architecture and system concepts, key enabling technologies and the experiments to be carried out, as well as the expected outcomes.
References:
[1] R. Orus, et al. Mixing Real and Simulated Observables to Assess the Performance of Hybrid GNSS/LEO-PNT Precise Positioning, ION GNSS+ 2024.