## 3 Recent Developments and Progress
### GNSS Ionospheric Sounding  
The global ionospheric map was derived from the global international GNSS service (IGS) observation data. [PERSON] et al. [37] compared the precision of global ionospheric maps produced by different IGS analysis centers with observational datasets from the MGEX and the IGS. The institutions and sources of data involved in this examination included the Chinese Academy of Sciences (CAS), the European Space Agency (ESA), the multi-mode and multi-frequency GNSS observations (MMG), the Jet Propulsion Laboratory (JPL), the Polytechnic University of Catalonia (UPC), and Wuhan University (WHU), while the ionospheric maps from the Center for Orbit Determination in Europe (CODE) serve as the benchmark standard [38]. Furthermore, the variability of the TEC was examined based on the discrepancies observed between the maps produced by the Ionospheric Associated Analysis Centers (IAACs) of the IGS and the altimetry-derived TEC obtained from an extensive dataset covering the period from 2005 to 2021 [39]. The equatorial plasma bubble (EPB) was observed in the temporal range of 2019-2023 based on the FS7/COSMIC-2 dataset, as to both equatorial and near low-latitude regions when considering a proposed hypothesis on seasonal versus longitudinal variability [40]. [PERSON] et al. [41] found that the occurrence rate of EPBs was increased with increasing geomagnetic activity based on the data statistics from the Swarm satellite from 2013 to 2019. [PERSON] et al. [42] analyzed the dependence of equatorial plasma bubbles on solar and geomagnetic activities and seasons by using the long-term global observation data of GNSS TEC monitors in low latitudes. The results showed that the incidence of equatorial plasma bubbles in each longitude-based region of the world was significantly positively correlated with solar activity. High-precision ionospheric monitoring and modeling is of great significance for satellite communication, satellite navigation, space weather monitoring, and so on. Nowadays, using GNSS signals to invert the ionospheric TEC is an important technical means for space weather monitoring [39], such as constructing a 2-D or 3-D global ionospheric model through GNSS technology. The precision of the worldwide ionospheric model is primarily affected by two factors: firstly, the source of observational data, which encompasses the quantity of puncture points, their spatial distribution, and the quality of the observational data; secondly, the modeling approach, which includes methods for data extraction, estimation techniques, algorithm reliability, and other related elements. The carrier-phase smoothing pseudorange method is a simple STEC extraction method [43]. With the frequency expansion of multi-GNSS, this method has also been applied to multi-frequency observations. In order to capture higher-precision ionospheric information, the use of PPP technology to extract STEC has been confirmed. In ionospheric modeling, the ionospheric TEC is typically expected to be on a thin layer at a fixed height, but this affects the accuracy of the model. [PERSON] et al. [44] established a two-layer ionospheric model (Figure 7) using IGS station data through three-dimensional ionospheric model constraints, and compared the results with the global ionospheric map (GIM). It was found that the TEC of the model was increased by about 20-27% with respect to the GIM model. [PERSON] et al. [45] constructed a three-dimensional electron density model (Figure 8) using COSMIC GNSS occultation data and showed a good performance by using a variety of independent data validation. With the increase of multi-GNSS observational data, real-time ionospheric simulation has become more and more popular. [PERSON] et al. [46] carried out real-time ionospheric modeling based upon ground-based and space-borne data sources. In addition, there have also been some interesting advances in space-based monitoring of disaster-related weather by analyzing the characteristics of ionospheric TEC disturbances.