# Monitoring of Structures and Infrastructures by Low-Cost GNSS Receivers
## 1 Introduction  
The construction of large structures, such as dams, bridges, skyscarepers, etc., is certainly an important activity of human society. As structures are subject to degradation of their mechanical characteristics, their maintenance and safety become fundamental issues for both avoiding damage to the population and for the economical losses implied by a structural collapse. Quantitative changes of a number of measurable indexes can be monitored in time to program actions of restoration or, in extreme cases, for demolition of the structure. Geodetic monitoring, which is a traditional tool of this kind, focuses on the positioning of the body of the structure with respect to the environment (e.g., detecting an anomalous inclination) and on the changes to its geometric figure, namely its deformation (or strain). This task has been performed in the past by classical geodetic instrumentation (such as theodolites, distance meters, levels, etc.) or by special tools derived from the former; nowadays a new tool has entered into the geodetic arsenal, namely Global Navigation Satellite System (GNSS) receivers, which, for the Global Positioning System (GPS) system only, date back to 1978, the year of the launch of the first experimental GPS satellite. Synthetic Aperture Radar (SAR) observations are nowadays also an important tool for structure monitoring, which, however, have different characteristics than GNSS monitoring. The integration of the two is an interesting subject of research. Moreover, with the introduction of new satellite systems such as GLONASS, BEIDOU and GALILEO, GNSS tools have enormously improved their performance in terms of positioning accuracy. It is a characteristic of GNSS observations to be based on time-shift measurements of two kinds of signals: the code and the carrier phase. Since the measurement is based on aligning the received signal with a copy generated inside the receiver, the accuracy of the measurement depends crucially on the quality and stability of the local oscillator. Thus, there are nowadays a large variety of receivers with accuracy ranging from dozen of meters down to the sub-millimeter-level. The most precise instruments, also called geodetic receivers, can reach the lower extreme of the accuracy scale, so it was natural that they could be applied to structural monitoring; early examples are, for instance, Refs. [1; 2; 3]. Of course many diverse applications are present in the literature: e.g., Refs. [4; 5] deals with dam monitoring, Refs. [6; 7; 8; 9] deals with bridge monitoring, Refs. [10; 11] also deals with bridge monitoring by combining GNSS with accelerometers, Ref. [12] deals with cultural heritage monitoring, and [13; 14; 15] deals with monitoring a bridge in a landslide area and also integrating SAR observations. On this subject, one can consult as well the interesting paper [16]. In recent years, however, new receivers have been developed with much lower cost and electronic noise that is not too high, although they are not at the same level as geodetic receivers. Nevertheless, the noise is \"white\" enough to be averaged over a large number of measurements and to provide the estimate of the position of a network of receivers with the millimeter-level required to get useful information for structural monitoring. For several years, GReD (Geomatics Research and Development, a spin-off of Politecnico di Milano university), in cooperation with Algowatt SpA, has been providing a monitoring service called GeoGuard [17; 18; 19; 20] to deploy suitable stations providing GNSS measurements on a network of receivers installed on structures, infrastructures and territory. The corresponding data are collected and analyzed to identify the continuous network deformation in time.