## 4 Propagation Loss Calculation Using the Improved TWPE Model
### Effects of Different Land Covers  
The previous section shows (assuming that the LC is grassland and water bodies) the PL trend and numerical range of PL at the antenna height layer when mixed ducts and tropospheric turbulence occur on the plain area. The accuracy of the results of the TWPE model with tropospheric turbulence was verified. The PL situation covered by different LCs will be discussed in this section. Figure 17: The propagation loss with tropospheric scattering, IGBP LC, and DEM considered on Shanghai–Wuxi. (**a**) PL from Shanghai–Wuxi using TPWE and IGBP LC. (**b**) PL from Shanghai–Wuxi using TPWE, WAT, and GRA LC. (**c**) PL from Shanghai–Wuxi using AREPS and IGBP LC. (**d**) PL at the duct-trapping layer of 60 m of different LCs. In this section, three types of LCs, barren or sparsely vegetated (BSV), urban and built-up (URB), and savannas (SAV), were selected, respectively. The over-the-horizon propagation was studied on the Shanghai-Wuxi and Jiaxing-Wuxi links considering the joint effect of tropospheric turbulence and the duct. Figure 19 shows the PL distribution on Shanghai-Wuxi link. The parameters are the same as in the previous section, except that the LC dielectric parameters use the values corresponding to Table 3, respectively. We used MSE to compare the PLs on Shanghai-Wuxi under three hypothetical LC types with the PLs on the S-W link under real IGBP LCs; see Figure 17a. The results are shown in Figure 20.  
Figure 18: The Propagation loss with tropospheric scattering, IGBP LC, and DEM on Jiaxing–Wuxi. (**a**) PL from Jiaxing–Wuxi using TPWE and IGBP LC. (**b**) PL from Jiaxing–Wuxi using TPWE, WAT, and GRA LC. (**c**) PL from Jiaxing–Wuxi using AREPS and IGBP LC. (**d**) PL at the duct-trapping layer of 60 m of different LCs. To clearly compare the influences of the PLs of the transmission signal in the ducting layer of the Shanghai-Wuxi link under the backgrounds of several different LC types, we compared the PLs of the link at a height of 60 m in the ducting layer, as shown in Figure 21.  
According to the impedance boundary conditions applicable to irregular terrain, it is assumed that the LCs are BSV/URB/SAV. Figure 19 shows the spatial distribution of the PL on Shanghai-Wuxi link when the three types of hypothetical single LCs are distributed. Figure 20 shows the spatial distribution of the mean square error (MSE) obtained by assuming different LC types and the real LC classified by IGBP on the Shanghai-Wuxi link. It can be seen that different LCs have a great influence on the PL accuracy, particularly at the antenna propagation height near the surface within the duct trapping layer. From the PL distribution of the antenna height layer shown in Figure 21, it can be seen that on the Shanghai-Wuxi link, the difference between the PL coverage of different LCs is mainly reflected in the front section and the rear section. The 20-40 km and 70-100 km sections both show that the IGBP LC results are significantly larger than the PL covered by a single LC type. The research in [29] shows that terrain fluctuation has a great influence on the PL distribution of the signal's over-the-horizon propagation. The results in this section show that the changes of different LCs may increase the PL, especially in the section with sudden and large terrain fluctuations, the actual situation may be a sudden change of LC, so it is more necessary to consider the coverage of real terrain. Figure 22 shows the over-the-horizon PL distribution of different LCs on the Shanghai-Wuxi link when the receiving antenna height is 40 m. Different from the results obtained with the receiving antenna height of 60 m in Figure 21, there is a significant difference on the range-dependent PL in various LCs at 40 km; that is, in the middle of the link. However, under the backgrounds of the distribution of real LCs, the calculated PL still exceeds the sensitivity threshold of the receiving antenna at a height of 40 m at range of 45 km, blocking the long-distance propagation of the signal.