# Evaluation of Positioning Accuracy of Smartphones under Different Canopy Openness
## 2 Materials and Methods
### Method  
The test site is located in Baoding agricultural ecological park, Hebei Province. The environment is relatively simple and the test interference items are few. Fifty-seven test points are set according to different canopy openness levels (Figure 2). Its accurate coordinates are measured by the iRTK2 system (RTK Positioning Accuracy: plane: \(\pm(8+1\times 10^{-6}\) D) mm) in combination with FindCM CORS service (CORS accuracy: horizontal positioning accuracy 2 cm in the ideal environment). To ensure high coordinate accuracy in the test points, first, we used IRTK2 and CORS services to obtain the fixed solution in an open place as the  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Device** & **Time to** & **Operating** & \multicolumn{3}{c}{**Acceptable Signal Type of Global Navigation Satellite Systems (GNSS) Device**} \\ \cline{3-8}
**Model** & **Market** & **System** & **GPS** & **BDS** & **GLONASS** & **Galileo** & **QZSS** \\ \hline K9 & 2021 & Android 11 & L1 & B1 & L1 & E1 & L1 \\ NEO & 2021 & Android 11 & L1\(\backslash\)L5 & B1\(\backslash\)B2 & L1 & E1\(\backslash\)E5 & L1\(\backslash\)L5 \\ P50 & 2021 & Harmony & L1\(\backslash\)L5 & B1\(\backslash\)B2 & L1 & E1\(\backslash\)E5 & L1\(\backslash\)L5 \\ G138 BD & 2012 & OS2.0 & L1 & B1 & L1 & \(\backslash\) & \(\backslash\) \\ iRTK2 & 2014 & \textbackslash{}\textbackslash{}\textbackslash{}\textbackslash{}\textbackslash{} \textbackslash{}\textbackslash{}\textbackslash{}\textbackslash{}\textbackslash{} \textbackslash{}\textbackslash{}\textbackslash{}\textbackslash{}\textbackslash{} \textbackslash{}\textbackslash{}\textbackslash{}\textbackslash{}\textbackslash{} \textbackslash{}\textcontrol point of this experiment and then used two iRTK2 devices to measure the accurate coordinates of all test points in real-time difference positioning. Before data collection, all smartphones should turn on the flight mode in the setting interface to prevent mobile phone Assisting-GNSS (AGNSS) function from using the base station information to affect the positioning data. The GNSS system supports all multi-mode states and the power-saving mode is turned off during data collection. Before the test, turn on the option of \"force to enable comprehensive tracking of GNSS measurement results\" in the \"developer options\" to avoid the impact of the working cycle on the data integrity of this experiment. P50 cannot turn on the option of force to enable comprehensive tracking of GNSS measurement results due to system restrictions. The smartphone measurement program is Geo++ RINEX logger 2.1.6 (Geo++ GmbH, Garbsen, Germany), which records the original GNSS measurement data in RINEX 3.03 format. Most PC GNSS processing software can then process these formats of data. G138 BD uses its program to record the original location data and iRTK2 records the current location information through the Hi-Survey Road application. Except that G138 BD cannot change the height angle setting, all other devices are set to a height angle of 15\({}^{\circ}\), with a sampling interval of 1 s. In addition, iRTK2 is set to output the observation file in GNS format and the original file in RINEX format simultaneously. This experiment records 10 min at each test point and observed data of about 600 epochs. Figure 1: Arrangement of GNSS device. In January 2022, a Cannon EOS 50D digital camera with a sigma ex DG 8 mm fisheye lens was used to take all sky photos 2 m above the test point by hemispherical imaging technology. When taking photos, avoid the interference of surrounding shrubs and other young trees; an automatic balance bracket is used to make the camera lens horizontal and upward. The left side of the device faces north so that the direction can be distinguished during photo analysis. Choose a cloudy day with uniform clouds or set the time close to sunrise (8:00) or sunset (16:00) to avoid direct sunlight and bright spots in the photos. No wind weather shall be selected for shooting and two photos shall be taken at each point to avoid blurring and other problems.