LEO navigation systems have gained significant attention in recent years as an opportunity to enhance and complement existing GNSS systems relying on MEO satellites. To achieve these goals, the LEO satellites shall broadcast their ephemeris to allow the users locating them in the space at any point of time. These ephemerides are a parametrisation of the underlying orbits, and the nature of the LEO orbits is substantially different from the MEO one. Therefore, although the GPS/Galileo ephemeris model is a good starting point, it needs to evolve to cope with LEO orbit dynamics. This paper addresses the selection and justification of parameters to be included in the LEO navigation message, in order to ensure high-accuracy ephemeris.

To identify the relevant ephemeris parameters, the temporal evolution of the instantaneous LEO orbital elements has been analysed, characterising this evolution into linear, quadratic and harmonic trends, complemented with a Fourier analysis to characterise the harmonic frequency and power. Additionally, a software tool has been developed for LEO ephemeris computation. This tool is capable of fitting ephemeris to a given input orbit for different subsets of parameters, allowing to systematically test the most convenient combinations to minimise the ephemeris fitting error.

If the adequate parameters are added on top of the GPS/Galileo ephemeris model, the fitting error tends to reduce. Beyond the ephemeris parametrisation, other factors, such as the length of the fitting interval, significantly influence the achievable accuracy. As shown in the figure below, for a reference orbit at an altitude of 510 km, the best results in terms of Signal-In-Space Ranging Error (SISRE) at Worst-User-Location (WUL) are in the order of 1 cm for a 21-parameter set and a fitting interval of 10 minutes, or even close to 1 mm for a 5-minute interval and a 19-parameter set.

Multipath interference significantly degrades GNSS positioning in automotive applications, posing challenges for the safety and reliability of autonomous driving and ADAS systems. Employing a patented supercorrelation algorithm, the S-GNSS receiver [1], developed by Focal Point Positioning under an ESA NAVISP project, offers a unique solution to multipath interference in challenging automotive environments by filtering incoming signals by angle of arrival in software processing alone. This ensures only multipath-free line-of-sight signals are processed in the navigation solution.

In this work, we will present initial experimental results in urban and under foliage scenarios with an S-GNSS receiver, areas where traditional processing struggles to provide high accuracies.

Preliminary analysis reveals significant improvements in GNSS robustness and accuracy, hinting at the full potential of supercorrelation technology in a final ADAS solution. Ongoing investigations will quantify these gains under different conditions, with the full results to be presented at the conference. These findings pave the way for reliable GNSS navigation in the toughest automotive environments, ultimately propelling the safe and efficient deployment of autonomous vehicles.

[1] Garcia, J.G.; van der Merwe, J.R.; Esteves, P.; Jamal, D.; Benmendil, S.; Higgins, C.; Grey, R.; Coetzee, E.; Faragher, R. Development of a Custom GNSS Software Receiver Supporting Supercorrelation. Eng. Proc. 2023, 54, 9. https://doi.org/10.3390/ENC2023-15423