## 1 Introduction
In landslides monitoring, the high cost of geodetic receivers (several thousands of USD) represents a problem at least at two levels: firstly, the initial setup of the control network is very expensive, especially in case many monitoring sites are needed on the landslide; secondly, in the case of a landslide event, the receivers can be damaged or lost forever, and this means a significant budget deficit. This clearly could discourage this monitoring technique in many cases. Nowadays, GNSS receivers exist that cost few hundreds of USD and output single frequency phase observations: the aim of this paper is to evaluate their performances in local monitoring applications. Some of the early works in this direction involved monitoring of the Aggenalm landslide situated in the Bavarian Alps, Germany. In general, the final accuracy of the solution is influenced by baseline length, satellite visibility, nearby environment and can be even better than 1 cm in perfect working conditions [14]. Furthermore, two experiments carried out in the Alps, provided a first, satisfactory guess of the accuracy of low-cost, single frequency receivers in landslide monitoring [15]. A recent research is presented also in [16]: in this case, the accuracy of low-cost receivers is assessed for local monitoring by processing daily sessions: accordingly to the baseline length, the final accuracy range from 1 mm (very short baseline) to 1 cm (30 km long baseline). The present research continues the experimentation of low-cost receivers in local monitoring. The first objective is to test their accuracy by processing short (e.g., hourly) sessions, that provide a quasi-real-time monitoring. Secondly, their reliability in detecting displacements at the sub centimeter level will be tested. This implies two features:  
(1) the capability to correctly detect displacements occurrence, without false alarms (significance); and the ability to correctly identify at least the order of magnitude of a displacement (congruence). In monitoring, a possible approach is the adoption of a two-level monitoring network: geodetic receivers are used as reference stations while low-cost receivers are installed on the monitored area. This is already cost effective, because one reference station could suffice and it does not risk any damage or loss. The other possible scheme implies the installation of low-cost receivers both in reference and in monitored sites and is even more cost effective. Therefore, a comparison between these two alternatives is interesting. In general, scientific packages provide the best accuracies in GNSS data processing because they implement the state of the art in modeling data and relevant physical phenomena. However, scientific packages require highly specialized and well trained users because they allow a wide selection of processing parameters. On the contrary, several commercial packages exist that implement more simplified models but are certainly more easy to use: at least at the local scale, it is interesting to check their results. A comparison between scientific and commercial packages is then useful. We presented a first experimental test in [17]: a horizontally-controlled movement was imposed to a low-cost rover, and the estimated displacements were compared with the imposed ones. These results were very promising. Therefore, we want to complete this first experiment with a longer dataset and checking displacements both in horizontal and vertical directions: rigorous significance and congruence tests will be applied to address the above questions, following a methodology firstly presented in [18]. Firstly, we analyze a week of acquisitions on a single baseline: this is referred to as Milano Test and is aimed at a preliminary assessment of accuracy and comparisons between GNSS processing packages. Then, a simulation of a local network for displacements monitoring is set up. One geodetic and two u-blox receivers are used.