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Session B5: Trends in Future Satellite Navigation Technology, System Design and Development

Comparison of Different Parametrizations to Minimize the Ephemeris Fitting Error for LEO Satellites
Carlos Gómez, Aitor Auz, Adrián Monreal, Alejandro Muñoz, Carlos Catalán, Andrés Juez, GMV
Date/Time: Friday, Sep. 20, 8:57 a.m.

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

Concretely, this paper addresses and compares two entirely different approaches: an orbital perturbations approach, where the evolution of LEO orbital elements is modeled using linear, quadratic, and harmonic terms; and a polynomial corrections approach, where a set of Chebyshev-based polynomial corrections for the along, across, and radial components are added either on top of the classical GPS/Galileo ephemeris model or directly on top of the six basic Keplerian elements. This paper analyzes the benefits and drawbacks of each option, systematically performing the ephemeris fitting process for different sets of parameters and paying special attention to the ephemeris fitting error, which is a key factor for LEO satellites when enhancing the existing GNSS constellations. Beyond the ephemeris parametrization, other factors, such as the length of the fitting interval and the altitude of the satellite orbit, significantly influence the achievable accuracy. This paper aims to contribute to an engaging discussion about the potential standardization of a common model for the ephemeris parametrization of the LEO satellites.

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