## 5 Discussion  
In this paper, we present the application of a low-cost GNSS system for continuous monitoring of the Madonna del Sasso rock cliff. In the last few decades, different authors studied the dynamic of this slope instability that nowadays is quite known. According to the kinematic model of the slope, ARPA decided to check the evolution of the two unstable blocks by six-monthly topographic campaigns. This control of displacement can be considered enough for checking the defined trend or for the detection of changes in the long-term deformation rate. The limit of this low-frequency monitoring  
Figure 12: (**A**) shows a comparison of horizontal displacement between GNNS “SAS a” based on weekly averaged measurements from December 2018 to May 2019 and topographic “T2” for the period 2006–2019; (**B**) shows GNSS “SAS a” East displacement averaged with weekly sampling and its regression line (equation \(t_{(\omega)}=0.01\mathrm{x}-634.47;\mathrm{R}^{2}=0.86\)) compared with geological thresholds (GT) of 3 mm/100 days (11 mm/year).  
solution is the possibility that the slope could change his behavior immediately after a measurement and a possible dangerous evolution of the instability cannot be detected soon. On the other hand, traditional continuous monitoring systems are not affordable.  
For this reason, a possible solution could be the use of continuous low-cost GNSS. The use of low-cost GNSS is characterized by pros and cons that can be synthetized as follows:  
Among the advantages, we can list:  
* With an adequate processing of the data and correct installation of the instrument, the accuracy and precision of the low-cost instrument are almost the same (5 mm in planimetry and 10 mm in altimetry) of traditional GNSS.
* The continuous and high frequency of measure could be able, over a long-term period of monitoring (some seasonal cycles), to better describe the process of expansion-contraction of the rock mass according to geophysical studies [22].
* The use of continues monitoring solution also allows the early detection of changes in displacement trend that could be indicative of a possible slope failure. This early warning procedure is based not only on the use of GNSS sensors but on the development of an automatized procedure that can acquire, process, and check if the displacement rate is higher than predefined thresholds.
* This procedure is fully automatized and can work without human intervention.  
On the other hand, this experimental system showed some limitations:  
* The continuity of measure and the maintenance of the system is fundamental, to have measurements' consistency when a system became operative. In our study, some problems related to lighting and benchmark repositioning have limited the period of observation (22% of days without alarm estimation). However, it is to take into account the experimental condition of our research.
* Some problem GNSS constellations: The Beidou constellation did not yet have a sufficient number of satellites to improve the robustness of the positioning. The Galileo constellation, which is more visible in Europe, was not yet complete in late 2018. Despite this, we have chosen to use these three constellations because of the future and rapid improvements in both the constellations and the processing software used.
* Even if it is a low-cost solution, the required efforts are similar or higher efforts available for periodical measurements.
* The slow-moving process needs a large time series to be understood even with continuous monitoring. This means that, if the entity of displacement is close to the accuracy of the system, the first sessions that show a trend change can be considered spikes. Only after a week can we be sure that a change in the displacement trend can be really identified and certified.