## 3 Mountain-Based Test Results
### Analysis of GPS and BD Open-Loop Tracking Results  
The I and Q components of the signal have different characteristics in OL and closed-loop tracking. In the tracking process, the phases of the OL flipped by NDM and error from the OL predict model in the form of a trigonometric function on the I/Q components, which can be seen from Figure 5. The phases of the closed-loop are of huge value, with signal flipped in the I component, and the Q component is signal noise. The closed-loop tracking can detect the phase or frequency difference between the locally reproduced carrier and the received carrier through the phase detector or frequency discriminator in the carrier loop. Then, the phase alignment of the local PN code and the received code can be continuously adjusted through the code loop discriminator, so as to obtain the highest self-phase correlation peak value. Figure 3: Mountain-based location, behind our experiment team is the bottom of the mountain. Our occultation antenna (the part marked in red in the picture) is mounted on a tripod and placed facing south, and our positioning antenna (the part marked in green in the picture) is also installed on the tripod. On the tripod, place it towards the zenith. The south side of the mountain is empty, which is conducive to our experiments. The GNSS occultation receiver system used for testing is developed by Tianjin Yunyao Aerospace Technology Co., Ltd, which is compatible with GPS/BD systems. Figure 4: Installation drawing of equipment room, our equipment is installed indoors, as shown in the figure (red marking part) and verified by the company’s self-researched occultation detector. It has the characteristics of small volume and low power consumption. It is also the equipment we will use in satellite networking in the future. It is placed on the anti-static pad, and ground treatment is also carried out to prevent the occurrence of static electricity and lightning strike events on the mountain. Figure 5: Real-time results of open-loop (OL)/closed-loop signal components of In-Phase (I, in blue line) and Quadra-Phase (Q, in red line). The abscissa is relative time in second, and the ordinate represents the components’ value. The **top** plane shows the characteristics of OL tracking mode that extreme values alternately appear in I and Q components; the **bottom** plane shows the characteristics of the closed-loop tracking mode that extreme values appeared in the I component. This paper analyzes the mountain-based occultation observations from full OL tracking mode in the following ways. First, using the excess phase obtained from the observation data in the closed-loop tracking mode as the reference, the continuity and duration of the excess phases obtained in the OL tracking verifies that the full OL tracking mode of GPS and BD are more robust than the closed-loop tracking mode. Second, the refractivity profiles retrieved from observations in the OL and closed-loop tracking modes are compared to verify their consistency. We selected (part of) the data on September 7 for analysis. The data include rising and falling occultations. As shown in Figures 6 and 7, excess phases retrieved from observations of L1C/GPS in OL/closed-loop tracking mode are compared, as well as that of B1/BD. Figure 8 shows the difference of the excess phase retrieved from observations of L1C/GPS and B1/BD in OL/closed-loop tracking mode, which are within 0.2 m. From Figures 6-8, the following can be concluded:  
1. OL tracking time is longer than closed-loop, that is, for low-angle occultation events, OL has a better tracking effect. 2. The mountain-based occultation data can only detect the peak height and below. It can be seen from Figures 6 and 7 that compared to closed-loop tracking, the OL tracking can capture a lower height, and has the same result as the closed-loop tracking. Hence, the continuity of the OL is relatively robust.