## 4 Experiments and Results
### Contribution Analysis of the Side Signals  
The isotropic and anisotropic SWV values of inside signals were first estimated by the HFM, as listed in Figure 9. A slight consistency between the total SWV and the isotropic one can be noticed from the four sub-figures. In contrast, the anisotropic parts were inconsistent with the totals and only accounted for an average of 2.31%, 2.56%, 2.61%, and 2.25% of the corresponding total component in the four schemes, which reveals that the isotropic portion has a more marked significance than the anisotropic portion in estimating the SWV value of the inside signals. However, the latter, with a value of approximately 20 mm for certain rays, is also important. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Scheme** & **Range** & **Voxel Division** & **Total Numbers** \\ \hline Scheme East & 113.82\({}^{\circ}\)E-114.18\({}^{\circ}\)E; 22.16\({}^{\circ}\)N–22.56\({}^{\circ}\)N & 4\(\times\) 5\(\times\) 15 & 300 \\ Scheme West & 114.09\({}^{\circ}\)E-114.36\({}^{\circ}\)E; 22.16\({}^{\circ}\)N–22.56\({}^{\circ}\)N & 3\(\times\) 5\(\times\) 15 & 225 \\ Scheme South & 113.82\({}^{\circ}\)E-114.36\({}^{\circ}\)E; 22.32\({}^{\circ}\)N–22.56\({}^{\circ}\)N & 6\(\times\) 3\(\times\) 15 & 270 \\ Scheme North & 113.82\({}^{\circ}\)E-114.36\({}^{\circ}\)E; 22.16\({}^{\circ}\)N–22.40\({}^{\circ}\)N & 6\(\times\) 3\(\times\) 15 & 270 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The range of the tomographic area and the number of voxels in the four schemes. Figure 9: The slant water vapor (SWV) value of inside signals in four schemes: top for total SWV, center for isotropic SWV, and bottom for anisotropic SWV, respectively. (**a**) Scheme East; (**b**) Scheme West; (**c**) Scheme South; and (**d**) Scheme North. Furthermore, we developed a detailed analysis of the number of rays crossing from the RS-voxel columns. Figure 10 demonstrates the benefit of absorbing inside signals into the traditional tomography model. Evidently, in the East, West, and North schemes, no signals passed from the voxel from layer 1 to layer 8 in the general tomography model, while only the first four layers of voxels were not penetrated by rays in the improved model. As far as the South scheme is concerned, it is noticeable that the RS-voxel columns were all passed by GNSS rays in the optimized model, except for the voxel on the first layer. The number of penetrated signals tripled for voxels from 2 to 11 layers when adding the inside rays. The average number of effective rays and the mean signal utilization rate for the traditional model and optimized model during the tomographic period are also compared in Figure 11, where the histogram shows the number of signals and the line chart represents the signal utilization rate. The former refers to the 31-day average number of GNSS rays selected in the tomography model during the two epochs. Similarly, the latter represents the 31-day mean utilization rate of the number of rays used in the equations out of the total number of GNSS signals in each tomography epoch. Because the GNSS rays with an elevation angle below 15\({}^{\circ}\) were eliminated, the mean utilization rate of integral signals and inside ones was about 80% in the four schemes. With side signals absorbed into the tomography system, the mean effective rays increased by 32.33%, whereas the average utilization rate of GNSS signals was enhanced by 33.42%, from 47.12% to 80.54%. In particular, the number of effective signals combining inside ones remained an approximative constant for any tomographic scheme, which enhanced the stability of the improved tomography model. Figure 11: The average number of effective signals (**a**) and the mean utilization rate of signals (**b**) for the two methods during the period of DOY 213-243, 2017. Figure 10: The number of penetrated signals in the RS-voxels column, where the abscissa represents the bottom-up column voxel numbering. (**a**) Scheme East; (**b**) Scheme West; (**c**) Scheme South; and (**d**) Scheme North.