## 2 Description of the Observation Setup  
### Precise Inclination Sensors  
Inclination sensors measure angular tilt with respect to an artificially-generated horizon (_i.e._, a steady liquid surface), and they are used in a wide spectrum of applications, including land (or platform) leveling, aircraft flight controls, automobile security systems, and many civil engineering applications. Two specifications of these instruments, namely, precision and number of axes (which are usually, but not always, orthogonal), determine whether an inclination sensor is appropriate for a specific application [22]. In this study, two Leica NIVEL20 digital inclination sensors were employed [27]. These are precise dual-axis instruments with \(\pm\)0.001 mm/m (\(\pm\)0.001 mrad) measuring accuracy within an effective range of \(\pm\)1.5 mm/m and capable of measuring inclines and declines along the orthogonal directions (described as X and Y axes, intersect each other at an origin that defines a coordinate system). This axis definition is displayed on the sensor for the user's reference, and thus, the directions of inclination can easily be identified (see Figure 1). The advantage of a dual-axis instrument is that it can observe the tilt of a surface and thus be able to interpret the movements of the observed object when it is deflected from the vertical. Figure 1: Functional principles of the inclinometer: **(a)** Horizontal position. **(b)** Tilted position. **1** The light from the LED, **2** LED, **3** Container with liquid inside, **4** Biaxial position detector [27]. In the design of the sensor, the horizon (tilt-sensitive element) is provided by a liquid in a closed container. The liquid's surface is perpendicular to the vertical, independent of the sensor's orientation. The inclination of the horizon relative to the sensor is measured by opto-electronic means. All the optical components used in the sensor are fastened to the underside of a flat glass plate. The plate also forms the bottom of the container with the transparent liquid. Through opto-electronic components, the luminous surface of a Light-Emitting Diode (LED) is imaged on a position-sensitive photodiode. The light from the LED is guided from below through the flat glass and the liquid, is totally reflected at the liquid's surface, and then passes again through the liquid and the flat glass. Finally, the photodiode detects the position of the impinging light spot relative to the origin, which was adjusted and calibrated precisely in the horizontal configuration of the sensor (see Figure 1) [27]. The flat glass plate, serving simultaneously as a component and as the support for the actual sensor element, is clamped onto a trough-shaped metal base. The sensor is set up on three hardened and ground circular support surfaces on the underside of this base. The support surfaces have through holes for the screws in the center. The sensor is covered with a plastic hood that serves primarily as a thermal insulator, which is meant to prevent external heat effects from causing non-uniform thermal expansion in the interior of the sensor and consequent measuring errors (see Figures 2(b) and (c)). A temperature sensor is also installed to monitor the sensor's temperature. A bubble level on the sensor allows the operator a rough determination of the horizontal. More precise adjustment of the horizon is done electronically by the help of software before starting measurement with the instrument. The values of inclinations in the X and Y directions (in mrad, mm/m, or arc-min) and the temperature (in \({}^{\circ}\)C) are available as measurement values, and the observed data is visualized and stored using commercial software in a terminal computer. The inclinometer has an RS-232 or RS-485 serial interface, the latter of which has the capability of operating up to thirty-two sensors on the same network. Some specifications of the measuring capacity of the sensor are summarized in Table 1[27]. Figure 2: **(a)** Mounting elements of the NIVEL20 sensor.