Global Navigation Satellite Systems: A review from the creation of a monitoring network to the data analysis and the interpretation of the relative results.

During the last 20 years, we observed a growing number of permanent GPS networks developed for monitoring the evolution of the deformation at different spatial scales in active tectonics and volcanic areas. Thanks to the continuous instrumentation's development, the frontiers of the studies on the deformation have been pushed also at different temporal scales, $i.e.$, from years to seconds (and even below) for observing and modeling earthquake sources or dike intrusions and for early warning applications. The capability to observe a target geophysical phenomenon strongly depends on the effort for planning the monitoring network and on the characteristics of the chosen instrumentation. Further steps are represented by the data analysis and the interpretation of the results. Different GPS data analysis strategies can be adopted for monitoring the deformation of a given target (PPP, DD, RTK). All these strategies have their pros and cons to know, in terms of accuracy, for an actual, aware and rigorous interpretation of the results. A review of some science-driven GPS monitoring networks, of data analysis strategies and their relative accuracies and results will be shown.