## 3 Materials and Methods
### Design, Implementation, and Installation of a Low-Cost GNSS Monitoring System
#### 3.3.1 Hardware Technology Installed and Calibration  
In Figure 5, the location of the instruments (SAS a and SAS b) in the courtyard of the Sanctuary is shown. The set of instruments of the prototypal multisensor box installed the monitoring sites can be seen in Figure 6. The main components are:  
1. A micro PC with a Linux operating system that controls all the sensors. It is equipped with a mainboard (1 in Figure 6), four powered USB interfaces, a removable memory card on which the operating system and data are stored. The micro PC is programmable by remote control like (e.g., via Dongle Wi-Fi, 2a and 2b in Figure 6) by any Linux computer. The micro PC also manages the dialogue with the uBlox receiver (3 in Figure 6), with the accelerometer and the temperature sensor. The system must be externally powered either by a power supply (4 in Figure 6) or by a solar panel. The micro PC also manages the transmission of data to the control center, both the accelerometric (5 in Figure 6) and temperature data and the corrections in the Radio Technical Commission for Maritime Services (RTCM) format of the GNSS sensor. 2. uBlox EVK8T GNSS receiver (3 in Figure 6). It provides data on the first frequency of the three GNSS constellations: GPS, Galileo and Beidou. However, during the months in which the receivers provided their data, 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, although not yet complete, has for now only provided help in fixing the GPS phase ambiguities. 3. Garmin GA38 mass-market antenna (6 in Figure 6). As the mass market antennas have been mounted on a specially constructed aluminum plate that forms the circular ground plane. A second plate was attached to this plate with a spacer and finally a pin with a 5/8 thread that allows the antenna to be mounted on any type of topographic base. Before installing the antennas, precision measurements were carried out to calculate the position of the phase center with respect to the bottom of antenna mount but also with respect to the top of the small dome of each antenna. For deformation monitoring, this operation is not mandatory as only relative movements are required. It was useful both because the \"zero\" position was calculated with high precision with geodetic antennas and receivers, and because the dome was painted in a checkerboard pattern to be observed photogrammetrically. The coordinates of the center of the dome have been used and will, therefore, be used as a control point in these reliefs (the Ground Sample Distance-GSD-is about 6 mm). The position of the phase center was obtained with an accuracy of one mm utilizing simultaneous measurements with a geodetic receiver and calibratedantenna, using the \"antenna swapping\" technique. The planimetric phase center coincides with such precision with the physical center of the antenna. 4. A triaxial accelerometer \"4030\" (5 in Figure 6) from TE Connectivity(r), usually used for structural monitoring was mounted on the bottom of the ground plane under the GNSS antenna. This sensor, with a range of \(\pm\)2 g, provides an mV output, which can be converted into acceleration values and relative angular values. The residual noise is an angular value of 0.022\({}^{\circ}\) (0.38 mrad or 3.8 mm@10 m). The monitoring of angular values with acceleration measurements is theoretically independent of time, not being linked to integration operations as in the case of gyroscopes. This makes the accelerometric angular measurement independent of the acquisition measurement duration. The accelerometers were mounted on the lower part of the antenna ground using a compass to orient them and after having made this plane perfectly horizontal. Each sensor, before being installed, has been individually calibrated in the laboratory with the appropriate matrix described in [31].