## 4 Verification and Accuracy Analysis of Single-Frequency Occultation Processing
### Accuracy Analysis of Refractive Index Products  
The vertical profile of the refractive index is important in occultation data processing. In the ionosphere, the refractive index is directly related to the electron density [36; 37]. In a neutral atmosphere, the refractive index is a function of pressure, temperature, and water vapor density [38; 39]. In this section, the accuracy of refractive index products measured by single-frequency occultation of BDS signal is analyzed. The reference atmospheric refractive index product is the ERA5 reanalysis field data provided by the European Centre for Medium-Range Weather Forecasts. Specifically, the pressure, temperature, and humidity data in ERA5 reanalysis field data are converted into a refractive index, and then as a function of geopotential height, the consistency between the refractive index of single-frequency occultation inversion and dual-frequency inversion, as well as the deviation between the refractive index of ERA5 reanalysis field data and the refractive index of dual-frequency inversion, are compared through the average deviation index and the root mean square error index, so as to evaluate the accuracy of single-frequency inversion products. The single-/dual-frequency refractive index inversion results of the BDS B1I and B3I signal from FY3E in the first two weeks of January 2022 are selected. The ERA5 reanalysis field data product is used as the reference value for accuracy statistics. The results are shown in Figure 8. Figure 8A compares the single-frequency and dual-frequency refractive index accuracy in the first week of 2022, and Figure 8B compares the single-frequency and dual-frequency refractive index accuracy in the second week of 2022. The solid line represents the average deviation bias between the inversion result and ERA5 data, the blue solid line represents the percentage deviation of the inversion result of the single-frequency reconstruction, and the black solid line represents the percentage deviation of the dual-frequency inversion result. It can be seen that between the geopotential heights of 10 km and 40 km, the average deviation index of the refractive index produced by single-frequency and dual-frequency inversion is less than 1%, and both are distributed around 0, with a very high degree of consistency. The average deviation index of the refractive index near the Earth's surface is 2-5% for both single-frequency and dual-frequency inversions, and it quickly converges to 0. The dotted line represents the distribution of root mean square standard deviation between the inversion results and ERA5 data, the red dotted line represents the percentage standard deviation of the inversion results of single-frequency reconstruction, and the blue dotted line represents the percentage standard deviation of the dual-frequency inversion results. It can be seen from the figure that the single-frequency and dual-frequency refractive index inversion results are consistent with the standard deviation root mean square of ERA5 data. From the surface to 25 km, the root mean square error index is highly consistent. The root mean square error index of the single-frequency and dual-frequency inversion results of the part with the geopotential height greater than 25 km has some slight deviation, but the deviation does not exceed 1%, and the standard deviation from ERA5 data is generally less than 6%. The reason for the slight deviation of the single- and dual-frequency inversion processing results may be that it is necessary to assume the symmetry of the large weather balloon and use the Abel integral transform [40; 41] to determine the refractive index from the bending angle in the inversion of the refractive index.