# Evaluation of Positioning Accuracy of Smartphones under Different Canopy Openness
## 3 Results
### Influence of Canopy Openness on the Number of Observation Satellites and Signal Quality
#### 3.1.2 Influence of Canopy Openness and Equipment on Carrier-to-Noise Density  
The carrier-to-noise density (C/N0) has a significant impact on the positioning accuracy of the devices. A low C/N0 indicates that the satellite signal obtained by the receiver contains a lot of noise, which will cause a significant positioning drift probability. GPS satellites can transmit L1 and L5 frequency signals and Galileo satellites can transmit E1 and E5A frequency signals. The L1/E1 and L5/E5a frequencies of GPS and Galileo are no longer distinguished in subsequent data, collectively referred to as L1 and L5 frequencies. Comparing the average C/N0 of the L1 frequency of different devices (Figure 8), it is found that under the same measurement environment, the average C/N0 of the L1 frequency of smartphones is significantly lower than that of survey-grade GNSS receivers. The C/N0 of smartphones is generally lower than 35 dB under medium- and low-openness conditions, indicating that the data packets contain much noise. Although the smartphone K9 only supports a single-frequency, the C/N0 of L1 frequency is better than NEO with more advanced hardware. The C/N0 is not only related to the configuration of devices but also closely related to antenna design, transceiver power, and other factors. The L1 frequency C/N0o of iRTK2 is generally above 40 dB and the highest can even reach 45 dB, which is significantly higher than smartphones. L5 frequency is similar to L1 frequency and there is a large gap between the C/N0 of smartphones and survey-grade GNSS devices. Figure 6: The number of available satellites with different canopy openness. 30 dBHz. On the other hand, the C/N0 of dual-frequency smartphones at L1 and L5 frequencies is smaller when the openness is greater than 0.7 and the C/N0 is between 27.74 and 31.89 dB. Smartphones need to be in a relatively open environment to obtain high-quality satellite signals. Dual-frequency smartphone NEO may have poor signal acquisition ability due to its chip or antenna design and the C/N0 fails to reach 30 dB at L1 and L5 frequencies, especially at L5. This will also lead to low accuracy in pseudo-range single-point positioning. The C/N0 of survey-grade GNSS devices at L1/L5 frequencies and the differences in various environments are very similar. In an environment with high-canopy openness (0.5-0.9), the C/N0 is 42 dB-43 dB. Even in an environment with low-canopy openness (0.1-0.3), the C/N0 is generally higher than 41.16 dBHz. This is because IRTK2 has high hardware specifications and a strong signal-receiving ability to ensure positioning quality. Table 4 shows a very significant positive correlation between the C/N0 of each device and the canopy openness using Spearman correlation analysis with SPSS24. The correlation coefficient of the C/N0 of the L1 frequency of smartphones is significantly higher than that of the L5 frequency, while the correlation of the two frequencies is almost the same for the survey-grade device. In addition, according to the comparison of the mean, minimum, and maximum C/N0 of L1 and L5 of three dual-frequency devices (NEO, P50, and iRTK2) (Figure 9), it can be found that the C/N0 of smartphone L5 frequency is significantly lower than its L1 frequency in various open environments. This shows that the smartphone GNSS chip has a large gap with L1 frequency in using the L5 frequency signal.