Combination of Ground- and Space-Based Data to Establish a Global Ionospheric Grid Model

The global ionospheric maps (GIMs) that are established using ground-based Global Navigation Satellite System (GNSS) data are important means to study the variations of the ionosphere. However, the uneven distribution of ground GNSS stations, which particularly exist at large gaps in the vast ocean areas, results to low accuracy and reliability of GIMs in the marine region and other areas that lack GNSS sites. The ocean altimetry satellite's orbit can cover most of the marine areas, and dual-frequency signals can obtained vertical total electronic content (VTEC) at the nadir track. Low-Earth-orbit occultation observations also obtain much global ionospheric uniform distribution information. The combination of the space-based ionospheric data and ground-based GNSS observation data can effectively improve the accuracy and reliability of GIM in marine areas. However, the systematic bias that exists between ionospheric data obtained by different systems must also be considered during the data combination. This paper used both ground-based GNSS data and space-based data to establish a global ionospheric model, whereas the systematic bias between the space-based ionospheric data and ground-based GNSS data can be seen as parameters to estimate. The results show that, by adding space-based data, the accuracy of GIM on the ocean areas has been improved to make up the deficiencies of the existing GIMs.