# Testing Multi-Frequency Low-Cost GNSS Receivers for Geodetic Monitoring Purposes
## 4 Conclusions  
In geodetic monitoring, geodetic GNSS receivers and antennae are usually used as they provide high-quality results. However, in cases where there is high risk of instrument damage, these may be regarded as unsuitable. It is then more appropriate to use low-cost GNSS receivers/antenna. To date, those were usually single-frequency receivers, but recently double-frequency, low-cost receivers are also available at the market.  
In this paper, we analyzed the positional quality and the potential for displacement detection of a ZED-F9P low-cost GNSS receiver in combination with an ANN-MB-00 low-cost GNSS antenna. The survey was done in favorable conditions where a static as well as dynamic survey was performed. The results obtained in this work lead to the following conclusions regarding the used low-cost equipment:  
* Geodetic GNSS receiver with a geodetic antenna and low-cost receiver with the low-cost antenna had slightly better performance but the positional quality of used low-cost GNSS equipment is well enough and accepted in applications such as geodetic monitoring of natural hazards;
* 3D displacements in a range of 10 mm can be detected by using low-cost GNSS instrumentation with a high level of reliability in open sky;
* 2D displacements were estimated with higher accuracy than 3D and 1D displacements, used antennas are non-calibrated and this probably influences the precision of displacements' estimation, especially the vertical component;
* SimpleRTK2B board is cheap, light, and easily configurable through the freely available u-center application software. Considering these characteristics and the proven performance, it can be classified as a proper sensor in few monitoring applications.  
Figure 7: 1D, 2D and 3D differences between true and estimated displacement of point FGG1: **(a)** 1D, G-baseline; **(b)** 1D, L-baseline; **(c)** 2D, G-baseline; **(d)** 2D, L-baseline; **(e)** 3D, G-baseline; **(f)** 3D, L-baseline.  
However, it should be noted here that all tests were performed in favorable surveying conditions and with short baselines. We expect that under adverse conditions the results would deteriorate. Therefore, it would be useful to perform additional tests in the future where several factors should be analyzed. One of them is adverse surveying conditions. On the other hand, to eliminate antenna bias, a calibrated GNSS antennae should also be used, with the possibility of individual antenna calibration parameters. Low-cost GNSS instruments should also be analyzed to separately determine the performance of receiver and antenna.  
Conceptualization, V.H. and B.S.; Methodology, V.H., O.S. and B.S.; software, V.H.; validation, V.H., O.S. and B.S.; formal analysis, V.H.; investigation, V.H.; data curation, V.H.; writing--original draft preparation, V.H.; writing--review and editing, V.H., O.S., T.A., G.T. and B.S; visualization, V.H.; supervision, O.S. and B.S. All authors have read and agreed to the published version of the manuscript.  
The authors acknowledge the financial support from the Slovenian Research Agency--ARRS (research core funding No. P2-0227 Geoinformation infrastructure and sustainable spatial development of Slovenia and research core funding No. P2-0260 Structural mechanics).  
We acknowledge [PERSON] for the support in the fieldwork and [PERSON] for sharing his experience with low-cost GNSS receivers.  
The authors declare no conflict of interest.