Topside Total Electron Content Retrieval and Validation Using FY-3G GNOS II Observations

The Fengyun-3G (FY-3G) satellite equipped with a global navigation occultation sounder II (GNOS-II) is capable of tracking BeiDou Navigation Satellite System (BDS) and Global Positioning System (GPS) dual-frequency signals by its zenith precise orbit determination (POD) antenna. Currently, most GNSS-related studies derived the topside total electron content (TEC) only by GPS signals, while there are relatively few studies using BDS signals. Additionally, as FY-3G satellite operates at a lower orbit with an altitude of 410 kilometers, it is closer to the pivotal regions of the ionosphere, particularly the F2 layer.This grants it the capability of capturing dynamic changes and intricate details of the ionosphere with enhanced resolution and sensitivity. In this paper, we derive the topside TEC from the six months of FY-3G GNOS-II data and analyze the performance of differential code bias (DCB). The final retrieved topside BDS and GPS TEC has also been evaluated. The internal assessment of the FY-3G receiver DCB is conducted by evaluating the long-term stability, which shows that the standard deviation (STD) of the FY-3G receiver DCB reaches 0.86 TECU for the GPS C1C-C2W observation type and 0.83 TECU for BDS B2I-B6I type. The FY-3G topside slant TEC (STEC) shows a high agreement with the Space Weather and Atmospheric Response Mission (SWARM) TEC product, with a mean bias of 0.836 TECU and STD of 2.044 TECU. A comparative analysis in FY-3G STEC between BDS and GPS satellites reveals no significant systematic bias, and the TEC retrieval performance of BDS-3 is superior to that of BDS-2. Moreover, the ionospheric characteristics of the monthly average FY-3G topside vertical TEC (VTEC) map is consistent with the International GNSSService (IGS) Global Ionospheric Map (GIM) product in latitude, season, and local time. Above all, this research shows that FY-3G GNOS-II can provide high-quality topside TEC observations, which can serve as a valuable dataset for research in ionospheric and plasma sciences.