3.2.3. LSP Analysis Results of the Snow Surface

Based on the LSP method analysis of the SNR sequence of the multi-GNSS and multi-frequency reflection signal of DOY 024 in 2017, the results are shown in Figure 9.

Figure 9 shows that there are some differences in the results of the multi-GNSS and multi-frequency LSP analysis. Specifically, the S2L and S5Q results in GPS are 1.335 m and 1.330 m, which are consistent and close to 0.1 m compared with the S1C results. The results of S1C and S2C in GLONASS are 1.230 m and 1.285 m, and the difference is 0.055 m. The results of S1C, S5Q, S6C, S7Q, and S8Q in Galileo are 1.355 m, 1.285 m, 1.300 m, 1.315 m, and 1.305 m. The difference between the maximum and the minimum is 0.070 m, and the

deviation of S5Q, S6C, S7Q, and S8Q is slight. The results of S6I and S7I in BDS are 1.275 m and 1.310 m, respectively, and the difference is slight. The difference between the results of S6I and S2I is 0.095 m. From the above data, it can be seen that the LSP analysis results of different GNSS systems are different, and that the results of different frequencies in each GNSS system are also different, but the overall consistency is good. In the article, through the LSP method analysis, the reflector height in the multi-GNSS and multi-frequency can be obtained, and the LSP of the snow surface can be obtained.

**Figure 8.** SNR sequence data processing: (**a**) GPS S1C SNR and direct signal fitting; (**b**) reflected signal extraction.

3.2.4. LSP Analysis Results of the Snow-Free Surface

In order to weaken the difference between the multi-GNSS and multi-frequency LSP results, this article proposes that the snow-free surface reflector height is analyzed by the multi-GNSS and multi-frequency LSP results. Using the above method, the four-day accumulated data (satellites G10, R17, E12, and C14) of DOY 225–228 in 2017 were selected for processing; this period is about October and belongs to the defined bare soil period. This article selects the data of this period to process, which can reduce the influence of surface vegetation on signal propagation. The multi-GNSS and multi-frequency reference values were obtained by averaging the four-day LSP results.

Figure 10 shows that there are some differences in the results of multi-GNSS and multi-frequency LSP. There are some differences between the results of GPS S1C, S2L, and S5Q, but the results of a single frequency at 4 days are basically the same, with slight deviation. The Galileo S5Q, S6C, S7Q, and S8Q results are basically the same, and S1C has a specific difference, but the results of a single frequency at 4 days are basically the same. The deviation of the BDS S6I and S7I results is slight, and the deviation of the S2I results is significant at 225 and 228 on the annual accumulation day. The results of the GLONASS S1C analysis showed a significant variation at 227 accumulated days, and the rest showed good consistency. In the article, the snow depth is obtained through the comparison of the reflector height of different GNSS systems at different frequencies under snow and snow-free surfaces, which can better adapt to the use of multi-GNSS and multi-frequency GNSS-IR technology. The reference value of the reflector height in different GNSS systems at different frequencies is calculated, as shown in Table 2.

**Figure 10.** The snow-free surface reflector height reference value of multi-GNSS and multi-frequency LSP analysis results.


**Table 2.** Multi-GNSS and multi-frequency LSP mean values of the snow-free reflector height.

Table 2 shows that the results of LSP at different frequencies of the four GNSS systems are more than 1800 m, which is more in line with the actual situation and can be used as the initial reference value of the reflector height of the snow-free surface.

#### *3.3. GNSS-IR Snow Depth Retrieval Results*

#### 3.3.1. Multi-GNSS and Multi-Frequency GNSS-IR Snow Depth Retrieval Results

The snow depth is obtained by comparing and analyzing the difference in reflector height under snow-free and snow surfaces. The results of the different frequency retrievals of GPS, GLONASS, Galileo, and BDS are compared with the PBO snow depth data, and the results are shown in Figure 11.

**Figure 11.** Comparison between multi-GNSS and multi-frequency GNSS-IR snow depth retrieval results and PBO snow depth: (**a**) GPS snow depth retrieval results; (**b**) GLONASS snow depth retrieval results; (**c**) Galileo snow depth retrieval results; (**d**) BDS snow depth retrieval results.

Figure 11 shows that the snow depth retrieval from the multi-GNSS and multifrequency SNR data has a trend that is similar to that of the PBO snow depth data. Among them, the results of S1C in GPS are more biased than those of S2L and S5Q. The trend of the S1C results in GLONASS is worse than that of S2C. The results of S1C in Galileo are worse than those of S5Q, S6C, S7Q, and S8Q. The trend of the BDS S2I results is worse than that of the S6I and S7I results.

#### 3.3.2. Mean Fusion of Multi-Frequency Retrieval Results in the Four GNSS Systems

The results of the mean fusion of different frequencies in the four GNSS systems are shown in Figure 12.

Figure 12 shows the consistency between the retrieval results of the four GNSS systems and the PBO snow depth, where the trend is more consistent than the previous single frequency retrieval results.
