## 4 Conclusions  
With the continuous development of global positioning systems and the deepening of research, ground-based remote sensing monitoring using navigation satellite signals has become one of the most important research hotspots, of which tide level monitoring is an important part. For the current tide level inversion, GNSS-MR technology based on a signal-to-noise ratio is mostly used; the signal-to-noise ratio data are not necessary for navigation and positioning, and some receivers do not provide them. To solve this problem, this paper proposes a method of inversion of the triple-frequency band of simultaneous interpreting without the tidal geometry and ionosphere. The data are measured by the observation data of the sc02 station in Friday Harbor of the United States and compared with the traditional GNSS-MR technology tide-inversion method. Affected by the gravity of the sun and the moon, the tide level changes periodically with time. In this paper, the three carrier-phase observations of GPS L1, L2, and L5 observed by the sc02 station of Friday Harbor in the United States are linearly combined to eliminate the geometric distance and ionization measurement delay between the satellite and the receiver. The spectrum of the combined observations is analyzed to obtain the frequency value of the combined observations varying with the satellite height. The height from the phase center of different antennas to the reflector and the combined-phase frequency value are modeled once. Finally, the tide level inversion is carried out by using the combined-phase observation value and the primary model of the frequency from the antenna phase center to the reflector that is higher than the combined phase. The experimental results show that with the periodic fluctuation of the tide level, the tide level retrieved by this method can correspondingly respond, and the change trend of the retrieved tide level is the same as that measured by the tide-gauge station near Friday Harbor, which proves the effectiveness of this method. In order to evaluate the performance of the tide level inversion method in this paper, three performance indexes--RMSE, correlation coefficient R, and the number of successful inversions of the tide level--are used to evaluate the performance of the inversion results. The results show that the RMSE value of the tide level inversion method in this paper is 16.2 cm, the correlation coefficient r of the measured tide level at the same tide station is 0.977, and the total number of successful inversions of the tide level is 309. In order to further analyze the performance of this method in retrieving the tide level, this paper also uses the traditional gnss-mr technology and the observation data in the same time period to retrieve the tide level, which is compared with this method. The accuracy of traditional gnss-mr tide level inversion is 15.7 cm, the correlation coefficient is 0.981, and the total number of the retrieved tide levels is 418. The accuracy and correlation \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Method** & **RMSE/m** & **Bias/m** & \
\begin{tabular}{c} **Correlation** \\ **Coefficient R** \\ \end{tabular} &
\begin{tabular}{c} **Inversion** \\ **Points** \\ \end{tabular} \\ \hline Triple-frequency combination & 0.162 & 0.005 & 0.977 & 309 \\ GNSS-MR & 0.157 & \(-\)0.008 & 0.981 & 418 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: A comparison table of the inversion results. coefficient of this method are slightly lower than that of the traditional gnss-mr tide level inversion method. The reason is that the vibration amplitude of the combined-phase observation value is at the millimeter level and can be easily disturbed by noise. The vibration amplitude of the SNR data used for gnss-mr is usually between 10 dB and 40 dB, which has stronger anti-interference. Therefore, the traditional method is slightly better than the method in this paper, but the inversion accuracy and the correlation coefficient are similar.