# Testing Multi-Frequency Low-Cost GNSS Receivers for Geodetic Monitoring Purposes
## 1 Introduction
### Overview  
In monitoring natural hazards and manmade structures, various geodetic methods may be used. Low-cost GNSS technology provides an inexpensive and flexible solution for geodetic monitoring purposes, especially when open-source software is considered [13; 15]. Moreover, the GNSS data are reliable enough to be combined with other sensors to build up an early warning system for monitoring purposes [16]. The current number of scientific research regarding low-cost GNSS receivers is very limited. Only a few papers have been published that have investigated the potential and applicability of the low-cost receivers for geodetic monitoring purposes. In general, the performance of single-frequency, low-cost GNSS receivers was tested and analyzed in open-sky environments with reduced multipath impact. Low-cost GNSS receivers were usually set up as rover stations, whereas the geodetic, low-cost receiver, or virtual reference station (VRS) were used as base stations [11; 12; 14; 17]. Studies showed that results based on low-cost GNSS receivers were promising for both static and kinematic mode [12; 17]. [PERSON] et al. [12] tested the accuracy of the single-frequency u-blox NEO-7P receivers and the results indicated that horizontal displacements of 15 mm can be detected when a geodetic receiver is used as a base station. On the other hand, the results on a basis of u-blox NEO-7P receivers only (both base station and rover) were less accurate, but still satisfactory considering that few centimeter movements are expected in monitoring natural hazards. Two-dimensional displacements in range of 25 mm and 3D displacements in range of 35 mm were detected by low-cost receivers only. [PERSON] et al. [11] analyzed the performance of u-blox EVK-6T receivers in open-sky, for short baselines, using only GPS data. The results indicated that 3D displacements on a level of few millimeters can be detected in daily sessions. Using VRS as a base station reduces baseline length and, consequently, the positional accuracy is improved. [PERSON] and [PERSON] [14] analyzed baseline with VRS as base station and low-cost u-blox EVK-5T receiver in combination with a geodetic external antenna as a rover and obtained positional accuracy on a sub-cm level. NEO-M8P, a single-frequency receiver, was used with Trimble Bullet 360\({}^{\circ}\) antenna for monitoring real-time kinematic (RTK) and post-processing mode, where GNSS data were processed with open-source software and the entire monitoring station was built with low costs [13]. RTK positional accuracy was on a level of 4 and 8 mm for the horizontal and vertical component, respectively. The results were further improved with post-processing and the accuracy for hourly sessions was estimated to be 2 and 5 mm for horizontal and vertical component, respectively [13]. The same single-frequency receiver was also tested by [PERSON] et al. [18] in RTK mode with low-cost antennae over short baseline (350 m). The results using single-base RTK solution indicate accuracy of 5.5 mm for the horizontal component and 11 mm for the vertical component. Nowadays, multi-frequency, low-cost GNSS receivers are also available at the market. This type of GNSS receiver can receive satellite signals in both L1 and L2 frequencies from all available satellite constellations. [PERSON] [10] tested multi-frequency receiver Piksi Multi and the mid-range receiver Eclipse P307 in static and dynamic applications. Both receivers performed well compared with the results obtained from the geodetic receiver, used as a reference. ZED-P9P GNSS module in combination with a patch antenna reached centimeter accuracy in 3D positioning. Similar results were obtained when an external Trimble R10 antenna was applied [19]. Lastly, low-cost dual-frequency receivers in combination with different antennas were used by [PERSON] et al. [20] for the estimation of Zenith Irpospheric Delay (ZTD).