# Research on the Positioning Performance of GNSS with a Low-Cost Choke Ring Antenna
## 2 Materials and Methods
### Research Methodology  
The usefulness of the constructed cover of the choke ring antenna for RTK measurements was assessed by analyzing the measurements of satellite signals of the GNSS antenna in two cases: with the dedicated antenna and the choke ring antenna on a previously prepared test stand (Figure 6).  
All measurements of satellite signals were performed based on the same reference station at identical points. Two long-term RTK measurements were conducted at reference points using the static technique. The points were chosen to ensure unfavorable measurement conditions (numerous horizontal curtains: the vicinity of trees, a building, and working machines equipment causing possible signal reflections). Moreover, to obtain the reference coordinates, a two-hour static session of RTK measurements was performed after prior checking of the possibility of performing RTK measurements. The results of the location of the above-mentioned points were anonymized due to the military terrain. We also assume no PCV pattern correction of GNSS in the study.  
Figure 3: Attaching the antenna to the base.  
Figure 2: The designed cover for the GNSS antenna: (**a**) front view: 1—pipes, 2—ring-shaped plate, 3—base, 4—screw assembly, 5—GNSS antenna, (**b**) rear view.  
NMEA (National Marine Electronics Association) sentences were logged in the measurements with a sampling frequency of 10 [Hz]. The receiver is configured to minimize the number of necessary NMEA sentences (headers). For this purpose, two NMEA headers were used: $GPGGA (Global Positioning System Fix Data) and $GPGSA (Global Positioning DOP and active satellites) [37; 38]. The $GPGGA sentences enable logging of all parameters necessary for localization, i.e., latitude, longitude, and latitude, while the $GPGSA sentences logging of statistical DOP coefficients. The information from these sentences is necessary for the subsequent conversion of the coordinate system, positioning errors, analysis of the DOP coefficient values (Section 2.3), and accuracy measures (Section 2.4).  
Figure 4: Mounting the housing to the antenna: 1–4 (**a**) step 1, (**b**) step 2, (**c**) step 3, (**d**) step 4.  
Figure 5: Choke ring antenna (**a**) cover, (**b**) cover with the antenna.  
Conversion to a metric coordinate system is necessary for the subsequent determination of positioning errors. ECEF (Earth-Centered/Earth Fixed) is an example of the mentioned coordinate system. The WGS-84 reference system was adopted for data analysis, which contains a set of parameters defining the size and shape of the Earth and the properties of its gravitational potential. The aforementioned system defines an ellipsoid used to create maps, used, inter alia, in the ECEF system [1,2,4].  
The knowledge of the reference position of the GNSS antenna allows for determining the errors of the antenna positioning in the geographic coordinate system and, after conversion, in the ECEF system. In the research on the accuracy of positioning, the determination of errors in the geographic coordinate system was omitted from the determination of errors in metric systems, such as ECEF or UTM. It results from difficulties in interpreting positioning results expressed in geographical degrees.