# A Comparative Study of Signal Processing Methods for Contactless Geodetic Monitoring
## 1 Introduction  
Nowadays, the monitoring of bridging structures is mostly based on measuring the displacements, deformations, and vibrations of bridges. Data on the static and dynamic behavior of a structure under load are usually obtained by a load test. Data on the long-term behavior of facilities are obtained by load testing and continued monitoring. This is important for heavily loaded objects on highways or railroads. The technological development of measuring equipment has also intervened in the field of geodetic instruments. The measurement can be performed in several ways. The group of alternative methods includes geodetic (optical) measurements, which can be static or dynamic. Measuring systems can be contact or non-contact. In practice, displacements are measured only at certain points with a limited set of measuring instruments, which requires a lot of time to prepare measuring points. The result of the non-contact measuring system is the movement of individual points in different places. In addition to making optical systems more accurate, measurements are more transparent and there is virtually no preparation for measurement. Automatic capture of measured values allows us to quickly and reliably determine the position of points. Because the processing of oscillation signals is conditioned by the sampling frequency, the measurement accuracy is improved by increasing the readings per second. Until recently, only vertical and horizontal displacements could be determined by geodetic measurements. Today, with the development of geodetic equipment, increasing resolution, additional sensors for faster reading, and automatic prism monitoring, it is possible, when geodetic measurements are taken together with very accurate static measurements, to perform dynamic response measurements in various monitoring [1; 2]. It should be noted that geodetic methods can be used in a very simple way to obtain quality data on thebehavior of structure in locations where this would not be possible due to inaccessibility or difficulty with other methods. The problem of determining the actual deformations of buildings under the influence of dynamic loads has been unsolved from the point of view of geodesy until recently. Systematic measurements of displacements of more flexible structureures (e.g., bridges, tall buildings) began to be carried out with the advent of advanced geodetic devices such as GNSS, which enabled the capture of a sufficient number of position measurements in a certain time interval. The system works on the principle of connection with satellites, so the condition for performing such measurements is an unobstructed view of the horizon and the presence of a sufficient number of satellites. Problems occur in narrow basins and ravines or in urban settlements, where a clear path to the satellites is difficult to ensure throughout the measurement. With the development of two-frequency systems, today, we can perform measurements according to the system of the relative method using phase (real-time kinematic--RTK), thus avoiding problems with the satellite signal. Due to the possibility of determining the 3D position of points in motion in an absolute system with the ability to read from 1 to 100 measurements per second, the GNSS method has become useful in addition to static measurements also suitable for dynamic research. Today's GNSS equipment allows us to read between 10 and 20 Hz. The development of GNSS technology today allows us to measure with a frequency of 50 Hz as well as up to 100 Hz [3; 4; 5; 6; 7; 8; 9]. The shortcomings of the GNSS method, such as multipath, poor satellite distribution, the unreliability of the elevation component, and other known shortcomings, are complemented by the introduction of additional geodetic measurements with the RTS method. The dynamic measurements of structures are mostly contaminated by dynamic noise, which is worsened by the errors caused by the GNSS monitoring techniques.