# Accuracy of Code GNSS Receivers under Various Conditions
## 3 Materials and Methods  
### Materials  
Measurements were performed on 4 test objects (point, line, circle, and an unobstructed sky static session) with an interval of 1 s. In order to determine the accuracy of determination of horizontal coordinates and altitude coordinates, the transformation of geographic coordinates to XYZ coordinates was carried out, and precise RTK solutions (fix, total phase indeterminacy) were adopted as error-free measurements. The obtained coordinates were transformed to the NEU topocentric system. In conditions of limited visibility, a decrease in accuracy was noticed for both code and precision receivers. It should be noted that the possibilities of simulators available on the market are quite large and low-cost solutions are also available.  
An urban park (Figure 2) with heavy forest cover, flat terrain, and paved alleys of regular shape and width was adopted as the study site. It is located in the center of Krakow (center of yellow circle on Figure 2: 50\({}^{\circ}\)03\({}^{\prime}\)45.9\" N 19\({}^{\circ}\)55\({}^{\prime}\)02.9\" E, 204 m). For testing purposes, the following were used:  
* Javad Triumph-1 (chipset 352-TFBGA, 90 nm) precision receiver operating in RTK GPS mode, with reference to a reference station located at a distance of 500 m, which eliminated the influence of the ionosphere.
* Two Garmin eTrex 30 (chipset STA8088 TESEO II) receivers allowing signals to be recorded in GPS or GPS+GLONASS mode, each receiver operating in a different mode (Table 1).
* Three Xiaomi Mi 8 (chipset Qualcomm SDM845 Snapdragon 845, 10 nm) phones running the Android 8.1 operating system capable of recording GPS, GLONASS, Galileo, and BeiDou signals. A free GPS Logger application (Geo Stats) was used to measure the smartphones. It was programmed to receive only the GPS signal, so it was not possible to set other signals, although the phones had such a possibility. All smartphones were operating in the same mode. Furthermore, the application did not record altitude. During the study, 4 software applications were tested for measuring smartphones on the move, but only 1 application positioned the device throughout the measurement, while the other 3 terminated after 1 min.  
Unfortunately, the geoid model used in the receivers we tested is a \"black box\"; thus, the authors decided to use the EGM96 model due to its widest use to date and the awareness that Garmin Etrex 30 receivers premiered in 2011. In addition, the differences between the EGM96 and EMG2008 geoid models are not significant and amount to less than the accuracy of the receivers tested, so the impact on the results is negligible.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Receiver** & **Mode** & **Marking** \\ \hline  & GPS & X\_1 \\
**Xiaomi Mi 2** & GPS & X\_2 \\  & GPS & X\_3 \\  & GPS & G\_1 \\
**Garmin eTrex 3** & GPS+GLONASS & G\_2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of receivers used for measurement.  
Figure 2: Location of measurement objects. Orange—linear object, yellow—surface object, red—point object, coordinates of the center of the yellow circle: 50\({}^{\circ}\)03\({}^{\prime}\)45.9\({}^{\prime\prime}\) N 19\({}^{\circ}\)55\({}^{\prime\prime}\)02.9\({}^{\prime\prime}\) E, 204 m (background source: Google Earth).