# Evaluation of Positioning Accuracy of Smartphones under Different Canopy Openness
## 4 Discussion
### Original Positioning Accuracy of GNSS Devices under the Forest  
The existing accuracy research of GNSS receivers mainly focuses on horizontal accuracy. For example, in forestry, how different forest conditions, especially canopy cover, affect horizontal accuracy is the focus of research [52]. The canopy openness has a decisive impact on horizontal and vertical position accuracy. The impact on the positioning accuracy is even more than an order of magnitude [53]. This study found a linear relationship between the canopy openness and the positioning accuracy of the device in the deciduous season. Nevertheless, in the common openness range (0.1-0.7), the horizontal accuracy of the device did not change significantly with the increase in the canopy openness. Previous leafy season tests showed an exponential function relationship between canopy openness and device positioning accuracy. When the canopy openness was less than 0.4, a small reduction in the openness would lead to a significant increase in device position error [44]. The error generally follows the rule that the vertical position error is about twice the horizontal position error [48].  
Some scholars found that the newer mobile phone models are better than the old ones in terms of the number of satellites and signal quality. However, the positioning accuracy is not necessarily better [54]. The application of dual-frequency technology in smartphones has made great progress in obtaining raw data quality [14]. Compared with single-frequency smartphones, multi-constellation and frequency receivers provide a more stable and accurate positioning solution. In this test, P50 with a dual-frequency signal has the highest positioning accuracy. However, in general, there are many outliers in the original position observation data of smartphones. In practice, it is often best to use the coordinates of the known position to compare with the measured values for accuracy testing [55]. In addition, it is necessary to study the applicability of smartphone raw GNSS data in a forest environment because the observed data are very vulnerable to multipath and other complex adverse factors.  
Many studies proved that smartphone positioning can achieve sub-meter or even centimeter accuracy under open conditions [56; 57; 58; 6]. Although the new generation of smartphones provides good accuracy in open environments, achieving the same results in forest conditions is difficult. In the forest, the error of smartphone positioning is high. In this test, the untreated real-time measurement accuracy of the common open openness environment (0.1-0.7) is best to reach only about 5 m. The average accuracy difference between the pseudo-range single-point positioning of single- and dual-frequency smartphones is small, which limits many practical applications. It may be that GPS and Galileo satellites supporting L5/E5A frequency transceivers are less distributed over China and can only receive a small number of GPS and Galileo satellite L5/E5A signals, which makesthe positioning accuracy of dual-frequency smartphones lower than the research results of European and American scholars [26]. In addition, affected by the complex canopy of the forest, even satellites with good positions may not be able to observe the whole process, which makes the satellite have a higher position dilution of precision, resulting in reduced accuracy [38].