## 4 Conclusions and Prospects  
In this paper, multi-GNSS developments for Earth observation and related applications have been presented, including GNSS navigation and positioning, GNSS RO from FY-3, CYGNSS, FS7/COSMIC-2, PlanetiQ, KOMPSAT-5, PAZ, ZTD for meteorology, ionosphere monitoring, GNSS-R, and various applications. One of the most important results of these studies concerns the definite strengthening of the GNSS technique involved in the field of environmental remote sensing when compared to traditional positioning applications. Over the next few years, future applications will certainly adopt these strategies. Furthermore, Artificial Intelligence (AI) will be further applied in Earth observations and applications associated with GNSS positioning, navigation, timing and remote sensing, in addition to data processing with the use of Machine Learning for the purposes of ground-based and space-borne Earth observation as well as other uses involving Autonomous Navigation Systems. Also, the development of GNSS-based products, in the context of services such as drones beyond line-of-sight (BVLOS), will play an important role: such drones can work  
Figure 13: Biases (**a**,**b**) and RMSEs (**c**,**d**) of the retrieved wind speed for (**a**,**c**) cyclone-free and (**b**,**d**) cyclonic conditions (Reprinted with permission from Ref. [54]. 2023 [PERSON]). over greater distances than those limiting traditional ones, even in those areas not easily accessible to humans, or in emergency situations arising due to natural events such as floods or earthquakes. Also, another challenge comes from indoor positioning systems (IPS), which can currently achieve accuracy levels matching with GNSS-RTK (which is used only outdoors), as computer technologies involving wireless access, 5G networking, and 3D mapping of the surrounding environment can meet the requirements of predictive positioning when GNSS data are not forthcoming. Among the most recent applications are Advanced Satellite Earth Observing Technologies for Weather and Climate Resilience, Decision Support Systems for Civil Infrastructure with GNSS Technology, Land Deformation and Engineering Structural Health Monitoring Using Geo-Spatial Technologies, Multi-GNSS PPP, and GNSS for Urban Transport Applications, which will also contribute to increasing the use and potential of GNSS systems. The progress towards multi-GNSS methods and techniques for Earth observation as well as its emerging applications, are reviewed and presented in details. GNSS technology is not only designed for conventional positioning purposes, and it could also be used more widely in the context of remote sensing. Furthermore, GNSS technology is developing towards high precision, global coverage, multi-frequency and multi-system compatibility. For example, dual-frequency and triple-frequency GNSS receivers have improved anti-interference ability and positioning accuracy, and centimeter-level positioning accuracy is becoming more and more popular. The integration of low-orbit satellites and GNSS, and the deep integration of 5G communication and intelligent driving will also promote the use of GNSS in new application fields, particularly low-altitude economy. With the continuous development of automobile and UAV industry, assisted driving and automatic driving technology have been developed rapidly, and GNSS technology has been extensively deployed in this field. The combined positioning technology of GNSS and INS allows to exploit the advantages of both, enhancing positioning accuracy, stability and reliability, especially in environments where GNSS signals are interfered with or cannot be received. GNSS/INS fusion provides high-precision and continuous positioning for vehicles and drones. Especially in urban environments, INS can maintain stable navigation performance in cases in which GPS signals are blocked by buildings. The development of new sensors, such as quantum INS, to achieve higher accuracy and greater stability of navigation will be a significant upcoming research topic.