## 4 Conclusions and Prospects
The advantage of GNSS/INS fusion can reach continuous and high-precision positioning in dynamic, complex, and signal-constrained environments. In the future, with the advancement of technology, these applications will be more extensive [59]. GNSS radio occultation technology also shows great potential in atmospheric and ionospheric monitoring, especially in meteorology and space weather monitoring. It is well known that various GNSS signals are influenced by the Earth's atmosphere, which includes both the troposphere and ionosphere, and these effects have been represented through intricate mathematical models [60]. While atmospheric delays significantly impair the precision of GNSS positioning, a wealth of useful data can be extracted from the GNSS signals that traverse the atmosphere, and can then be utilized in areas such as natural disaster monitoring, weather forecasting, and other domains. GNSS technology is playing an increasingly important role in monitoring extreme weather events, such as typhoons and sea ice monitoring [61]. In typhoon monitoring, GNSS-Reflectometry provides information valuable for disaster warnings, with its significant advantages in obtaining information about typhoons. Errors occur in GNSS signals as they travel through the atmosphere, and these inaccuracies are significantly linked to the level of atmospheric humidity. Through the examination of this delay, researchers are able to gauge the volume of water vapor present in the atmosphere, allowing for predictions regarding the intensity and rainfall associated with typhoons. Additionally, the monitoring of rainstorms gains from the advancements of GNSS technology. GNSS offers crucial data on atmospheric humidity, which is vital for evaluating the intensity and precipitation of rainstorms. By integrating GNSS observational data with precipitation radar information, a more precise estimation of precipitation intensity and the rainstorm can be accomplished, thereby enhancing the reliability of early warning systems. In monitoring extreme weather phenomena, such as cold waves and droughs, GNSS technology provides valuable data by monitoring changes in humidity and changes in atmospheric structure. These data help scientists to better understand the formation and development of extreme weather phenomena. In general, the application of GNSS technology in extreme weather monitoring has improved the accuracy of weather forecasting and the timeliness of disaster warnings, which are of major relevance for dealing with climate change and protecting public safety. An ionospheric model has been created utilizing GNSS occultation data, and information regarding the electron density profile is acquired by detecting the delay effect evident in satellite signals traversing the ionosphere. These data can be used to construct accurate ionospheric models to help analyze ionospheric variations and improve the accuracy of navigation, communication, and space weather warnings [62; 63]. Hence, it is crucial to thoroughly explore the impact of the atmosphere on multi-GNSS positioning applications on the ground and in space (such as BDS, Galileo, GLONASS, GPS, IRNSS, and QZSS) and enhance the ZTD and TEC potential values. Finally, the GNSS has been extensively implemented in many fields, showing excellent performance in global positioning, navigation, timing, and so on. It plays a key role in transportation, agriculture, fishery management, emergency rescue, and natural disaster monitoring. Especially in precision agriculture, unmanned driving, intelligent logistics, and other aspects, the high-precision GNSS positioning and timing functions have been widely used [64; 65; 66]. In the future, the application scope of GNSS will be further expanded, covering the frontier fields of the Internet of Things, the smart city and 5G technology, so as to help the global digital landscape and enhance intelligent development. Conceptualization, S.J. and G.D.; methodology, S.J. and X.M. ; validation, S.J. and X.M. ; formal analysis, S.J. and X.M.