###### Abstract
The main drawbacks of such monitoring systems were essentially the necessity of favorable atmospheric conditions, specialized personnel and expert surveyors, as well as the long duration of both the measurements and processing procedures [12]. Starting from the 1960s, total electronic motorized stations, digital levels and, in general, the introduction of automatic data reading, acquisition, recording, and telemetric transmission allowed for continuous monitoring, thus providing denser long-term data series and subsequently enhancing the capability of analyzing deformation patterns [13; 14]. As happened with remote sensing techniques [15; 16; 17; 18], the application of methods based on Global Navigation Satellite Systems (GNSS) is now consolidated, where GNSS networks of points are measured with reference to proximal CORS (Continuously Operating Reference Station) or other stations established on purpose in nearby stable areas [19; 20; 21; 22]. More recently, automatic systems based on GNSS positioning have been developed to be integrated into early-warning systems for dam safety management [23; 24]. Furthermore, recent research proved that low-cost and mass-market GNSS equipment can also be properly used for structural monitoring purposes, and adequate accuracies and system integrity can be reached without geodetic level instrumentation [25; 26; 27; 28], making the GNSS approach even more attractive and widespread. In fact, by reducing the overall cost of the GNSS monitoring systems, more CPs can be instrumented, and a more comprehensive and broad reconstruction of the dam displacement/deformation over time and space is attainable. In the framework of GNSS dam monitoring, the typical setup consists of a (or more) reference station installed on a stable platform located some hundred to thousands of meters apart from the dam body, several monitoring stations on the dam crest, and in the case of earth embankments, on the downstream shell. Relative positioning is performed between the reference (considered fixed) and the monitoring points, and on the basis of 24 h continuous measurements, daily positioning solutions are provided [29; 30; 31]. The estimated positions are commonly framed in a previously established local reference frame with one of the horizontal axes parallel to the dam crest and an upward vertical axis. Three-dimensional CP displacements are typically considered independent of each other, and the corresponding temporal series represent the time-dependent position variations along the most meaningful directions, i.e., the dam crest, the reservoir up/downstream, and the altitude. Such temporal series suffer from measurement noise, and they are generally smoothed, filtering out the high-frequency variations by means of interpolation techniques [32]. The resulting 3D time series describes the dam displacement in correspondence with the monitored points. It is known that environmental factors and operation activities influence the displacements, and therefore, they should be modeled in order to analyze the residual data looking for unexpected trends or behaviors. Air/water temperature and reservoir water level are the main factors to be taken into account for computing the so-called HTT (Hydrostatic Temperature Time) model of displacements [33; 34; 35; 36]. In this paper, a statistical approach for dam monitoring based on the use of GNSS techniques is described and detailed, taking as reference the experience of a real case of an embankment dam monitored with such a methodology. Despite the fact that results will be presented and discussed with reference to a real case, the goal of the manuscript is more associated with the accurate description of the methodological approach of these analyses, from the preliminary processing to the final interpretation, so as to set up a general and effective procedure for structural monitoring.