## 5 Conclusions  
Low-cost GNSS receivers have been tested in different applications for positioning and navigation purposes. Taking into account their cost and the demonstrated performance, these GNSS devices can be suitable sensors for geodetic monitoring in areas with a high risk of instrument damage. Recently, multi-frequency GNSS receivers and calibrated low-cost antennas have become available on the market, which makes them more comparable to high-end geodetic GNSS instruments. In this study, the u-blox ZED-F9P multi-frequency receiver in combination with Survey Calibrated, Survey, and Tallysman antennas was tested for displacement detection. The main objectives were to identify the range of displacements detected by low-cost devices in relative and absolute positioning modes and analyze the differences between these low-cost antennas. The results from this study lead to the following conclusions:  
* The low-cost Survey Calibrated antenna performed better in relative positioning than the non-calibrated antennas Survey and Tallysman. The smallest MAE for 1D, 2D, and 3D displacements estimated after the geodetic adjustment was obtained by the Survey Calibrated antenna, which also had the best performance in terms of the magnitude of detected displacements. * For 1D displacement detection, Survey and Tallysman antennas had similar performance. The MAE for 1D was almost equal between the two, and the size of detecteddisplacements was 9 mm. The Survey Calibrated antenna had a slightly smaller MAE, and the size of detected displacements was 6 mm. * The Tallysman antenna and Survey Calibrated antenna detected 2D displacements of 4 mm, which was not the case for the Survey antenna. The size of displacements that were detected at a high percentage by the Survey antenna was 6 mm. * For spatial displacement (3D) detection, the size of detected displacements was the same for all antennas as for 2D displacements. Displacements of 4 mm were detected by Survey Calibrated and Tallysman, while for the Survey antenna, the size of detected displacements was 6 mm. * Displacements of 20 mm or greater were detected by low-cost GNSS devices in absolute positioning mode. The performance of Survey Calibrated was also slightly better than that of Tallysman in PPP. * All three antennas were able to detect spatial displacements smaller than 10 mm in relative positioning mode and can be used to monitor many engineering buildings. In absolute positioning mode, displacements of 20 mm were detected, which is adequate for monitoring natural objects (landslides). * Geodetic GNSS instruments had better performance than low-cost instruments. The number of cycle slips and the noise of phase observations were best with geodetic GNSS instruments. However, the difference in cycle slips among them was not very large, and the noise of phase observations was at a level of a few millimeters for low-cost devices. All tests were carried out in an open-sky area and over short baselines to evaluate the antenna's impact on positioning performance and compare low-cost uncalibrated and calibrated antennas. The presented results from this study should be treated as preliminary. To better evaluate low-cost GNSS instruments, more tests will be carried out over medium and long baselines, and geodetic and low-cost GNSS instruments and different combinations thereof will be used. For a more in-depth analysis of the potential of these sensors for geodetic monitoring purposes, we will attempt to test them in harsh environments, and a more detailed analysis will be carried out, in terms of observations. A real natural object (landslide) or man-made object will be monitored, and other geodetic instrumentations may be used to evaluate the performance of low-cost GNSS devices. Conceptualization, V.H. and B.S. ; methodology, V.H., B.S. and T.A. ; software, V.H. ; formal analysis, V.H. ; investigation, V.H. ; data curation, V.H. ; writing--original draft preparation, V.H. ; writing--review and editing, V.H., B.S., T.A. and O.S. ; visualization, V.H. ; supervision, O.S.