## 4 Conclusions and Future Developments  
A review of the selected research application, GNSS CORS, has been presented in this paper. Also, 15 years of experience using the GNSS CORS at the University of Palermo (Italy) were briefly reported. For instance, the GNSS CORS has found its applications in many technical and scientific applications, including network monitoring, reference frame, dam monitoring, and the performance of positional accuracy. Through the growing technical development of fully operational GNSS satellite systems (such as GPS, GLONASS, Galileo, and Beidou), in addition to those that will be implemented in the coming years (such as QZSS, KPS, and IRNSS/NavIC), CORS will be able to receive information from more than 140 satellites simultaneously. This represents an obvious advantage in terms of dilution of precision (DOP), and consequently, it will be easier to avoid electromagnetic interference and benefit from the accuracy. Several scenarios open up and constitute future fields of exploration for GNSS CORS applications. Among these are new solutions for PPP based on multi-GNSS constellations and their massive use for time series analyses; the validation of reference frames using international GNSS services; the implementation of decision support systems for managing civil infrastructure or urban transport through the integration of GNSS and Earth Observation imaging systems; as well as the use of GNSS and satellite remote sensing techniques for meteorological, climate, and hydroscience studies as hydrogeodesy; the monitoring of ground deformation geohazards; and the monitoring of ionosphere, troposphere, and atmospheric physics through GNSS-R. Finally, the integration of GNSS and InSAR to predict extreme weather events such as drought and flood. In this scenario of future improvements, however, it is evident that GNSS CORS also has limitations, such as GNSS coordinate time series noise reduction, signal extraction, thermal expansion effect correction, plate motion, common mode error, and so on, which are all important factors affecting the accuracy of GNSS coordinate time series. Figure 10: Conceptual scheme of comparisons (top left panel), confidence ellipses pre and post-removal of the outliers (dashed and continuous lines respectively in the upper central panel); pair Q-Q plots for a pair (black dots in the upper right panel, with outliers represented in white); and in the lower panels: I quartile (black dots), average (grey dots), and III quartiles (white dots) of the planimetric and plano-altimetric difference of coordinates of the differences (lower left and right panels, respectively) post outliers removal. At present, the CORS station network and GNSS measurement have been successfully applied to hydrological monitoring, such as the monitoring of land water storage changes and groundwater storage changes. In the coming years, attention should be turned to other applications, given the wide availability of data collected by CORS. In particular, further interesting applications of satellite navigation by GNSS-R emerging applications could be developed, such as remote sensing techniques of soil moisture content and SmallSats. In addition, GNSS CORS is providing crescent support for several fields of precision farming. The use of GNSS CORS for other hydrology applications could be further developed. A series of applications could be developed from the potential offered by CORS GNSS data with geographic information system software, such as monitoring electromagnetic fields, the acoustic map of the vehicular traffic of an urban agglomeration, or various environmental parameters, similar to what has been done in previous years [127; 128]. Finally, the flexibility of CORS services is opening up new research fields, such as hydrogeodesy, focusing on estimating continental-scale changes in water storage as remotely observed through gravity space missions and regional and local alterations detected using INSAR and GNSS [129]. Conceptualization C.P. and G.D.; Methodology C.P., A.M., M.L.B.