## 2 Materials and Methods
### Experimental Setting  
In order to be able to carry out the measurements simultaneously, a holder was built for the 10 smartphones and the geodetic GNSS receiver. A 1.5 m long aluminium bar was mounted on a tripod. The two-channel Trimble Geo7x GNSS receiver's external antenna was installed in the middle, precisely above the survey point. Attached to the bar were smartphone holders with an exact distance of 14 cm. The experimental design was aligned consistently east to west, and smartphones were placed with a known defined offset in an east-west orientation. Thus, the position of every smartphone could be derived relative to the known survey point. The smartphones were arranged vertically in the holders. The tripod was aligned precisely with the centre of the survey point using a plumb bob. The height above the ground varied somewhat due to the settings and the micro-relief, but it was approximately 1 m.  
Figure 3 shows the arrangement used in these experiments, including the names of the smartphone models used. The experimental setup was placed exactly over a terrestrially surveyed geodetic survey point where the exact absolute position was known. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**ID** & **Manufact.** & **Type** & **Model** & **NF 1** & **RY/HY/PAPI 2** \\ \hline Mi8 & Xiaomi & Mi 8 & Mi 8 & 2 & 2018/2018/10/29 \\ Mi8 pro & Xiaomi & Mi 8 Pro & 4 & 2 & 2018/2018/10/29 \\ Mi10 & Xiaomi & Mi 101 ight & M20020P6 & 2 & 2020/2019/10/29 \\ P20 & Huawei & P20 & EMI-L29 & 1 & 2018/2016/10/29 \\ P40 & Huawei & P40 & ANA-N30 & 3 & 2020/2018/10/29 \\ S5 & Samsung & S5 & SM-GO0F0 & 1 & 2014/2013/6/23 \\ A7 & Samsung & Xover 4 & SM-A750 FN & 1 & 2018/2016/10/29 \\ XC4s\_a & Samsung & Xover 4s & SM-G396 FN & 1 & 2017/2015/9/28 \\ XC4s\_b & Samsung & Xover 4s & SM-G396 FN & 1 & 2019/2016/10/29 \\ Trimble & Trimble & Geo7x & TrimbleGe7x & 2 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Overview of the smartphones and GNSS receivers used in the empirical experiments (1 Number of Frequencies; 2 Release Year/GNSS NMEA Hardware Year/Android Platform/API-Level). Figure 2: Overview of the different types of smartphones used and their ability to use the different GNSS constellations and frequencies. The studies took place in Germany in the federal states of Bavaria, Savony, and Baden-Wurttemberg. In total, measurements were carried out at 15 different study sites. The tree species, the relief, the degree of canopy closure, and the foliage conditions varied among the study spots. Only sites under a canopy and with official terrestrial survey points of which the absolute accuracy was known were used. Table 2 shows the characteristics of the different study sites. The locations of the survey points in the forest were acquired from the official survey authorities of the respective federal states. It was a challenge to find points in the forest under a canopy with sufficient accuracy and suitability. In addition, a wide range of real forest structures should be represented. This included both old and young stands in hardwood, softwood, and mixed wood. Only 8% of the potential points were used for the study. According to the surveying offices of the various federal states, the absolute accuracies of the used survey points were less than 5 cm. The coordinates of the permanently marked survey points were determined in each case as local terrestrial geodetic connection measurements of fixed trigonometric points. The coordinates are available in the official reference system for the federal states of Baden-Wurttemberg and Bavaria (ETRS89/UTM zone 33N; EPSG: 25832), as well as for the federal state of Savony (ETRS89/UTM zone 33N (EPSG: 25833). The GNSS coordinates of the smartphones and the geodetic receiver were available in WGS 84 (EPSG: 4326) and were converted into ETRS89/UTM zone 33N (EPSG: 25832) for further processing and analysis by transformation method (ETRS89 to WGS 84 (1); EPSG: 1149) using a Python script (package ESRI arcpy).