Global Navigation Satellite System (GNSS) Radio Occultation (RO) missions, such as the Formosa Satellite-3/Constellation Observing System for Meteorology, Ionosphere, and Climate (FORMOSAT-3/COSMIC) and the upcoming FORMOSAT-7/COSMIC-2, provide valuable profiling of the ionized atmosphere for the monitoring of space weather. This study shows that the FORMOSAT-3/COSMIC and FORMOSAT-7/COSMIC-2 missions' ability to monitor highly variable ionospheric weather can be considerably extended with the help of data assimilation. The Gridpoint Statistical Interpolation (GSI) Ionosphere is a new data assimilation system designed specifically for the low-latitude and midlatitude ionosphere. The capability of the GSI Ionosphere is first demonstrated with actual FORMOSAT-3/COSMIC RO total electron content (TEC) data for January 2013. Features of the ionospheric equatorial ionization anomaly in a coupled plasmasphere ionosphere thermosphere model become more consistent with the TEC maps created with independent ground-based GPS data. The consistency has improved by assimilation of FORMOSAT-3/COSMIC RO data up to about 50% in comparison to the control simulation case without data assimilation. To evaluate the impact of future RO missions on ionospheric weather specification, comparative Observing System Simulation Experiments (OSSEs) are carried out with synthetic RO TEC data. An OSSE of FORMOSAT-7/COSMIC-2 shows that the GSI Ionosphere can improve the ionospheric specification within ±30° geomagnetic latitude by 67% over the control case, which is comparable to the improvement yielded by FORMOSAT-3/COSMIC for 2009 (61%). These results indicate a great potential for improving the monitoring of realistic ionospheric weather with the help of FORMOSAT-7/COSMIC-2 RO TEC data.