## 6 Conclusions  
In local monitoring, for example, a landslide, geodetic GNSS receivers are usually adopted: they provide very accurate and near-real-time results. However, geodetic instrumentation is very expensive: the initial cost, and the risk of damages and losses can discourage this approach. The objective of this research was to assess the accuracy and reliability of a monitoring system based on low-cost GNSS receivers. For the testing, two experiments were planned and performed. Firstly (Milano Test), we analyzed a week of acquisitions on a single baseline between a geodetic reference station and a LEA-4T u-blox. This test provided a first accuracy assessment and the comparison between Bernese GPS SW and LGO package. Out of 168 results, five and four outliers were present in the results, respectively for BSW and LGO. The other hourly sessions provided accuracies well below the cm level, even for the vertical component. The two packages provided similar results. Then (Como Test), a simulation of a local network for displacements monitoring was set up: the experiment involved three receivers. Two receivers (a geodetic receiver and a NEO-7P u-blox) acted as reference stations, another u-blox acted as rover: it was installed on a device that allowed to move its antenna by known displacements in horizontal and in vertical directions. Starting from a zero position, the rover moved 10 cm by steps of 5 mm in both directions. Each position was occupied for (at least) two hours. In total, the experiment lasted about 15 days. In this case, all of the processing was performed by LGO. Firstly, the baseline between the geodetic reference and the rover was processed by hourly sessions. The estimated displacements were compared with the imposed ones. In this test, just one blundered session, with float results, was excluded. The other hourly sessions presented residuals smaller than 10 cm and provided accuracies at the sub-cm level in horizontal and at the cm level in vertical. They are worse than those of Milano: indeed, while Milano site presents ideal environmental conditions, Como site is more representative of typical monitoring sites, with some obstructions around. In this regard, the results are completely satisfactory. Moreover, significance and congruence tests were applied to Como time series, in order to investigate the capability of the system to:  
1. correctly alarm in case a displacement occurred, without false alarms (significance); and
2. correctly identify the imposed displacements (congruence). Horizontal results are completely satisfactory: the system is able to detect displacements after three movements (15 mm) in most of the cases and the classification errors are almost always within three classes. Vertical tests are weaker: after five displacements (25 mm), only 38% of right answers are given and classification errors exceed five classes in many cases. Then, the same approach was applied to process and analyze the baseline between the two u-blox. The results are slightly worse, but not significantly, and confirm previous conclusions. Therefore, we can state that movements with a dominant horizontal component can be detected by local networks of low-cost GNSS receivers with sub-cm accuracy. The data processing can be performed by simple, user-friendly software and the results can be obtained by processing hourly sessions: therefore, they can provide a quasi-real-time monitoring of the phenomenon. In particular, we experimentally tested that these accuracies can be achieved even in a configuration entirely based on low-cost receivers. **Acknowledgments:** The data of Milano Test were acquired and courtoeously provided by Riccardo Barzaghi and Mirko Reguzzoni, Department of Civil and Environmental Engineering, Politecnico di Milano. In Como test, the slide device was lent by Department of Environment, Land and Infrastructure Engineering of Politecnico di Torino, the Tallysman antenna was lent by Stefano Caldera of G-Red.