A Method for Establishing a Global Double-Layer Ionosphere Model Using GNSS Observations from LEO Satellites for Orbit Determination

In recent years, with the launch of numerous Low Earth Orbit (LEO) satellites, GNSS observation data used for precise orbit determination can be utilized for topside ionosphere sounding. This study integrates topside GNSS observations from LEO satellites, including the Chinese Tianmu-1 satellite constellation and COSMIC-2, with ground-based GNSS observations to jointly establish a global double-layer ionospheric model. The model employs spherical harmonic functions to fit the observations, with the two layers set at altitudes of 450 km and 1200 km, respectively, and a temporal resolution of one hour. During the modeling process, ground-based GNSS observations contribute to both the bottom and top layers, while GNSS observations from LEO satellites contribute exclusively to the top layer. To validate the model's performance, data from three months (February, May, and August 2024) during a period of high solar activity were used. The validation involved comparing the model outputs with slant total electron content (STEC) observations from over 500 global GNSS stations and vertical total electron content (VTEC) data from six ocean altimetry satellites. The results indicate that the proposed double-layer ionospheric product achieves high accuracy, outperforming traditional single-layer ionospheric models.