## 6 Conclusions  
In this paper, we propose not only a technical solution that adopts mass-market instrumentation, but also the definition of a monitoring systems management strategy that can be adopted for GNSS taking into consideration different phases: analysis, validation, interpretation, and dissemination of obtained results. We tested the potentiality of low-cost GNSS for continuous monitoring of unstable slope and we propose a step by step methodology for semi-automatic alert messages based on continuous monitoring data. The unstable slope affecting Madonna del Sasso sanctuary represents a good case study of this scenario. According to ARPA monitoring system results, we know that the unstable sector of this area site is characterized by an annual displacement trend of 2 mm/year. The importance of this cultural heritage site has prompted in the last few decades several research groups to study the slope instability with different approaches. Nowadays, there is an important dataset that can be used for the characterization of the slope and of displacement trends. We collect and analyzed the primary outcomes of literature studies and the analysis of the non-continuous monitoring system of ARPA Piemonte in an Operative Monography, a document that helped us in the GNSS system planning and installation. The ARPA monitoring network can be considered a suitable solution for the control of the evolution of the studied area. The semiannual frequency of measurement campaigns, however, is not sufficient to detect a rapid acceleration related potentially related to a paroxysmal slope instability. The use of the proposed GNSS low-cost monitoring network can be considered an affordable solution to improve the acquisition rate. The installed low-cost GNSS receivers have shown similar performance in terms of accuracy and precision respect geodetic GNSS on the order of a few millimeters in laboratory tests ([15]. Considering that the cost is ten times lower, the precision does not decreases significantly. With the completion of the new satellite constellations (e.g., Galileo), the improvement of the processing hardware and software can be expected to reduce costs and increase monitoring accuracy. We also implemented a methodology that is based on a semi-automated procedure able to control the correct running of the system and the quality of results. In the first step, an automatized procedure checks if the obtained results are statistically correct. In the second step, the system checks if the displacement trend exceeded predefined thresholds and output light-traffic alert system. The final check done by experts that validate alert messages before publication: it is necessary to have reliable data, especially when the geological model of the unstable slope is not well known. The last phase of the proposed methodology also considers the emission of the alert bulletin. The first period of monitoring was used to test the automatic alarm thresholds, and show that the system is run adequately filtering very noised measurements and false alarms. However, further studies are needed to asses a reliable critical geological threshold on a short time interval. The first partial results on the long-term the trend of displacement registered by GNSS shows that it is comparable with topographic data in the same interval (\(\approx\)5 mm/year). With a sampling interval of a week, the rate of displacement can be calculated with millimetric precision. In the future, with more extended time series, it would be possible to understand better the behavior of unstable blocks. The results of this study show that the GNSS low-cost solution can be particularly interesting when we have to control the evolution of low rate slope instabilities that could be affected by a sudden increment of displacement that can be considered the precursory of a failure. It can also help to install a more precise and expansive monitoring system only during emergency phases.