## 3 Mountain-Based Test Results
### Comparison of Closed-Loop Tracking and Open-Loop Tracking  
In order to verify the effectiveness of the OL tracking algorithm, the comparison of the OL and closed-loop methods is used for verification, and the OL and closed-loop channels are used to capture and track the same satellite. Compare the start capture angle and end capture angle of the two to verify the capture effect. We selected 7 days of observation data for statistics, and the selected data are shown in Table 1:  
From Table 1, it can be seen that the full OL tracking method can effectively capture occultation events at lower angles. As shown in Table 1, observations from an occultation of GPS (PRN 30) at 1 September 2020 0:10:43 UT showed that its OL end pitch angle reached \(-1.87\) deg, which is 0.88 deg smaller than its closed-loop end pitch angle. At low elevation angles, it is difficult to track in the closed-loop strategy stably, and problems such as drastic changes in the signal-to-noise ratio and loss of signal lock will occur. The fully OL tracking method effectively solves the above problems. We have made statistics on all the data to evaluate the effect of OL tracking. One calculation method is to calculate the angles of the whole occultation tracking, and mark it as strategy \(A\); the other is to calculate the negative angles of the occultation tracking, and mark it as strategy \(B\). For strategy \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \multirow{2}{*}{**NO**} & **GPS** & \multirow{2}{*}{**Data/UTC**} & **CLIPA/\({}^{o}\)** & **CLEPA/\({}^{o}\)** & **OLIPA/\({}^{o}\)** & **CLEPA/\({}^{o}\)** & **SA/\({}^{o}\)** & **EA/\({}^{o}\)** \\ \hline
**1.** & 30 & 2020-9-1 0:10:43.20 & 4.98 & \(-0.99\) & 4.98 & \(-1.87\) & 201.34 & 195.87 \\
**2.** & 5 & 2020-9-1 1:46:43.5 & \(-0.91\) & 4.99 & \(-2.54\) & 5.02 & 212.47 & 175.49 \\
**3.** & 28 & 2020-9-1 2:37:45.60 & 4.97 & \(-1.23\) & 4.98 & \(-2.13\) & 175.49 & 173.51 \\
**4.** & 13 & 2020-9-1 3:46:13.2 & \(-1.11\) & 4.98 & \(-2.87\) & 5.02 & 184.44 & 151.35 \\
**5.** & 17 & 2020-9-1 4:27:38.30 & 4.97 & \(-0.97\) & 4.98 & \(-1.96\) & 151.35 & 152.57 \\
**6.** & 19 & 2020-9-1 5:75:12.70 & 4.97 & \(-2.18\) & 4.98 & \(-3.08\) & 156.96 & 157.57 \\
**7.** & 24 & 2020-9-1 6:43:14.70 & \(-1.93\) & 4.99 & \(-2.63\) & 5.02 & 183.66 & 149.86 \\
**8.** & 2 & 2020-9-1 7:53:44.20 & 4.98 & \(-1.35\) & 4.98 & \(-2.34\) & 149.86 & 151.07 \\
**9.** & 17 & 2020-9-2 4:23:38.40 & 4.97 & \(-1.51\) & 4.98 & \(-2.45\) & 151.37 & 152.65 \\
**10.** & 19 & 2020-9-2 5:33:70 & 4.97 & \(-1.91\) & 4.98 & \(-3.07\) & 159.96 & 157.57 \\ \hline \multirow{2}{*}{\(\vdots\)} & \multirow{2}{*}{\(\vdots\)} & \multirow{2}{*}{\(\vdots\)} & \multirow{2}{*}{\(\vdots\)} & \multirow{2}{*}{\(\vdots\)} & \multirow{2}{*}{\(\vdots\)} & \multirow{2}{*}{\(\vdots\)} & \multirow{2}{*}{\(\vdots\)} \\ \cline{1-2} \cline{5-5}
**57.** & 1 & 2020-9-7 18:39:1.50 & \(-1.53\) & 4.99 & \(-2.6\) & 5.03 & 181.63 & 177.46 \\
**58.** & 4 & 2020-9-7 19:36:49:00 & 4.96 & \(-1.37\) & 4.96 & \(-2.52\) & 200.61 & 195.33 \\
**59.** & 22 & 2020-9-7 20:12:19.00 & \(-1.67\) & 4.99 & \(-2.84\) & 4.99 & 151.94 & 144.3 \\
**60.** & 3 & 2020-9-7 20:54:48.60 & \(-1.51\) & 4.99 & \(-2.68\) & 5.03 & 156.9 & 146.99 \\ \hline \end{tabular}
\end{table} 
Table 1: List of the closed-loop initial pitch angle (CLIPA), closed-loop end pitch angle (CLEPA), open-loop initial pitch angle (OLIPA), open-loop end pitch angle (OLEPA) and start/end azimuth (SA, EA) from selected occultation events. Figure 11: Difference of refractivity profiles retrieved from observations of BI/BD and LIC/GPS in OL/closed-loop tracking mode. Left panel, BI/BD; Right panel, LIC/GPS.