2.2.4. SNR Types

PBO provides multi-GNSS and multi-frequency SNR data in the observation data of receiver independent exchange format (RINEX) version 3.03. The SNR types of P387 station mainly include S1C, S1W, S2L, S2W, and S5Q for GPS; S1C and S2C for GLONASS; S1C, S5Q, S6C, S7Q, and S8Q for Galileo; S2I, S6I, and S7I for BDS; S1C, S2L, and S5Q for quasi-zenith satellite system (QZSS); and S1C and S5I for satellite-based augmentation system (SBAS). The data sampling interval is 15 s, and the specific information is shown in Table 1.

Table 1 shows that the P387 station provides SNR data of different signal types of six satellite systems. After reading the data, it is found that the QZSS and SBAS have fewer satellites and no available arc segments. Therefore, this article does not consider using QZSS and SBAS data for snow depth retrieval; instead, the SNR data of GPS, GLONASS, Galileo, and BDS are used. In the article, multi-GNSS and multi-frequency SNR data are used to retrieve the snow depth. In addition to GPS, the observation satellites will be extended to other systems, which is of great help in improving the retrieval accuracy of snow depth and expanding the observation range and time resolution.


**Table 1.** Multi-GNSS and multi-frequency SNR types and description information provided by the P387 station.

Note: When reading RINEX 3.03, BDS 1I/Q/X and 2I/Q/X can be regarded as the same as 2I/Q/X in the current RINEX standard, and the AS in Table 1 is anti-spoofing.

## **3. Experiment and Results**

*3.1. Experimental Technical Scheme*

Figure 6 shows the experimental technical scheme of multi-GNSS and multi-frequency GNSS-IR snow depth retrieval. It can be seen that the technical route of the article can be divided into three parts: (1) GNSS-IR data preprocessing is carried out, where the SNR, pseudo-random noise (PRN), satellite elevation angle, azimuth angle, and other data parameters are extracted from the observation (OBS) file and navigation (NAV) file collected by GNSS receivers; (2) the LSP method is used to analyze both the reflector height of snow-free and snow surfaces and the difference between them in order to retrieve the snow depth; (3) the multi-GNSS and multi-frequency GNSS-IR snow depth retrieval results and PBO snow depth data are compared, and then the mean fusion analysis of the multi-GNSS and multi-frequency GNSS-IR snow depth retrieval results is carried out.

Figure 6 shows that RTKLIB software is used for data processing in the article. By reading the OBS file and NAV file in RINEX 3.03 format, the corresponding elevation angle, azimuth angle, SNR, and other data can be extracted. In order to select the satellites in the four GNSS systems that have available observation arc data in 42 days of the experimental stage, this article finally sets the G10 satellite of GPS, R17 satellite of GLONASS, E12 satellite of Galileo, and C14 satellite of BDS as the experimental data source satellites. In the article, the SNR data in the rising stage are mainly selected for processing. When the retrieval results show abnormal values, the SNR data in the falling stage can be selected as a supplement for retrieval. At the same time, the minimum elevation angle threshold of 5 degrees can also be increased accordingly in order to achieve a more practical retrieval value.

**Figure 6.** The technical process of multi-GNSS and multi-frequency GNSS-IR snow depth retrieval. Among the ele is the satellite elevation angle.
