# A Comparative Study of Signal Processing Methods for Contactless Geodetic Monitoring
## 2 Materials and Methods  
Monitoring to determine the dynamic response of the bridging structure was carried out on a pedestrian and bicycle bridge across the Drava River in Maribor, Slovenia. The dynamic response of the structure was measured using geodetic instruments and a seismograph with a triaxial geophone, which is classified as a group of physical instruments. The pedestrian bridge is a steel space bar structure, 126 m long, with three spans of 42 m each. The point at which we measured the dynamic response of the induced vibration was on the main longitudinal steel bridge girder in the middle of the first span, as shown in Figure 1.  
Vibrations for measuring the dynamic response of the bridging structure were induced by two people with a total mass of 160 kg running away from the measuring point. Geodetic measurements were performed with two Total Robotic Station (RTS) Leica TS50 and TS30. The apparatus measures the change in the horizontal and vertical angles to the geodetic target. The third measuring device was the Topcon Hiper Pro Global Navigation Satellite System (GNSS), which allows dynamic observation of a measuring point based on measured distances to satellites. Physical measurements were performed using an Instantel Minimate Plus seismograph with a triaxial geophone. The basic characteristics of measuring equipment are shown in Table 1.  
Figure 1: Location of the measuring point on the bridge over the river Drava. For the experiment, we used the Leica TS50 and TS30 robotic tachymeters shown in Figure 2. The instruments were placed on undeformable terrain, 19.5 m away from the Leica GPR121 target. The instruments automatically track the target and are capable of sampling frequencies up to 20 Hz. The TS50 and TS30 instruments have declared factory accuracies: according to ISO 17123-3 for angular measurements of 0.5 s or 0.15 mgons and according to ISO 17123-4 for spacings of 0.6 mm + 1 ppm. The TS30 recorded data at a frequency of 6 Hz, while for the TS50, we used the GeoCom/Zg add-on program, which recorded results at a frequency of 26 Hz. The software uses a GEV269 cable to connect the instrument to a laptop. As mentioned earlier, the software can measure angles, lengths, and time, or just angles and time (Figure 3). When testing the program, it was found that switching off the distance measurement gives more uniform results [31]. When determining the dynamic response parameters, it is sufficient to only record the changes along the vertical direction. Determining the position with the GNSS receiver in the first phase is based on the so-called pseudo-distance or on determining the distances to selected satellites in space. The latter have precisely defined points with perfectly known coordinates. The receiver locally creates a duplicate copy of the signal, which it receives as feedback from the satellite, and then compares the signal thus generated with the originally received signal. The comparison or adjustment of the measured point takes long enough that the receiver completely equates its locally generated signal with the received one, which is reflected in reaching the peak of the cross-correlation function between the signals. Only with full coverage is the signal decoded into a navigation message from which satellite position information can be deduced. The determination of the position in the field is performed by measuring