## Materials and Methods
### Data Collection  
The tests were carried out on 16 January 2020, at the same time, from 12 (noon) to 2 pm, when the planned number of satellites was maximum (8 GPS, 6-8 GLONASS, 6-8 SBAS, i.e., EGNOS, 13 BeiDou) and the PDOP (Positional Dilution of Precision) was minimum (1.9-2.1 for GPS, 2.58-2.69 for GLONASS, 2.29-2.48 for EGNOS, 1.71-2.07 for BeiDou) in the surveyed area. During the tests, these satellite receivers were set up with an NMEA (National Marine Electronics Association) serial output frequency of 1 Hz (1 position s\({}^{-1}\)). The tests were carried out using a wheeled tractor owned by the University of Palermo (Figure 2). The receivers were placed on the dashboard, while the external antenna was mounted in the upper part of the roll bar. The tractor forward speed was kept constant in all tests and equal to 0.94 m s\({}^{-1}\) during all tests. Different tests were carried out in order to evaluate the positioning accuracy of the different GNSS receivers. The tests were distinguished by the use or not of RTK differential correction data and/or an external antenna. In fact, only the differential correction data transmitted in real-time from the network of RTK ground stations (CORS) were selected, as this technique allows for better accuracy [23]. The accuracy of each GNSS receiver was evaluated by comparing the distance between the different positions logged by it during the different tests and those determined by means of the reference receiver, i.e., Stonex S7-G, used with external antenna and RTK differential correction. The longitude and latitude differences in UTM-WGS84 projection were computed. In order to evaluate the accuracy of the tested GNSS receivers, differential correction data were used: the satellite receivers were linked to the UNIPA GNSS CORS station located in the University of Palermo Campus (210 m from the geometric centre of the testing field) via NTRIP, having a frequency of 1 Hz (1 position s\({}^{-1}\)) [24]. A total of 10 tests named with the letters A, B, C, D, E, F, G, H, I, L, M were carried out using the four GNSS receivers described above, with and without RTK differential correction and/or with and  
Figure 2: Antennas (**A**) and wheeled tractor (**B**) used during the field tests. without external antenna (Table 1). Each dynamic test, carried out in RTK mode, was replicated three times in order to obtain significant results. The satellite receivers S7 and S5 determined the geodetic coordinates of the points by using or not using RTK differential correction data (tests A-B and E-F) and the external and internal antenna (tests C-D and G-H). The receiver TMMP was tested only with and without the external antenna (tests I-L), as this instrument was not able to receive differential correction data in real time. Finally, the receiver QLV2 was tested only with the internal antenna (test M). The geodetic coordinates of the points determined by the satellite receivers used in the different tests were processed by means of a GIS software. The points determined by each GNSS receiver under the different testing conditions were compared with the reference trajectory by means of QGIS software. Fifty sample points were extracted from the trajectories drawn by joining the points determined by the receivers. Since the field perimeter was 287 m, these points were 5.74 m away from each other. The shortest distance from the sample points of every test to the reference track line was calculated in QGIS by means of \"shortest_line\" Geometry Function. This returns the shortest line joining two geometries. The shortest distance from a point to a line segment is the perpendicular to the line segment. If a perpendicular cannot be drawn within the end vertices of the line segment, then the distance to the nearest end vertex is the shortest distance (Figure 3).