## 3 Results and Discussion
### Impact of Network Configuration on Positioning Performance  
#### 3.3.1 Configuration without Buoys  
To analyze the contribution of the underwater subsurface buoy and sea surface buoy to user positioning, we compared the network configurations \"3 seafloor base stations  
Figure 3: User positioning indicators under various distance and height conditions. **(a)** EPG with various available stations—the colors blue, orange, yellow, purple, green, and red indicate the available station numbers of 0–5, respectively; **(b)** changes in the average, standard deviation, and maximum of PDOP—the blue box denotes the average, the black vertical line denotes the standard deviation, and the orange asterisk line denotes the maximum value; **(c,d)** spatial distribution of the ASN when the subsurface buoy height is 2.5 km, and the seafloor base station distances are 7 km and 9 km, respectively. 0 subsurface buoy + 0 sea surface buoy\" (\"3+0+0\"), \"3 seafloor base stations + 0 subsurface buoy + 1 sea surface buoy\" (\"3+0+1\"), and \"3 seafloor base stations + 1 subsurface buoy + 0 sea surface buoy\" (\"3+1+0\"). We analyzed EPGs with at least three available stations (EPG\({}_{\text{ASN}\geq 3}\)). Figure 4a shows that the EPG\({}_{\text{ASN}\geq 3}\) of the \"3+0+0\" network configuration gradually decreased as the distance increased. When the distance was greater than 17 km, the available station number was not enough for positioning. For the \"3+0+1\" network configuration with the sea surface buoy, the EPG\({}_{\geq 3}\) values were much larger than those of the \"3+0+0\" network configuration. However, when the distance was greater than 19 km, the available station number was also not enough for positioning. Figure 4b shows that, for the \"3+1+0\" network configuration, by replacing the sea surface buoy with the subsurface buoy, the EPG\({}_{\geq 3}\) values had a similar situation. Compared with Figure 2a, for the \"3+1+1\" network configuration, it can be found that the EPG\({}_{\text{ASN}\geq 3}\) values increased significantly when the sea surface buoy and subsurface buoy were configured simultaneously. Moreover, when the distance was greater than 19 km, the proportion of EPG\({}_{\text{ASN}\geq 3}\) was still greater than >21%. Figure 4: User positioning indicators of the “3+0+0”, “3+0+1”, and “3+1+0” network configurations. (**a,b**) Heat maps of the percentage of EPGs with the number of available base stations \(\geq 3\); (**c,d**) average, standard deviation, and maximum PDOP—the box denotes the average, the black vertical line denotes the standard deviation, and the asterisk line denotes the maximum value. The results for the “3+0+0” configuration and the “3+0+1” configuration are shown in subfigure (**c**), whereas the “3+1+0” configuration with a subsurface buoy height of 0–2.9 km is shown in subfigure (**d**). Figure 4c,d show the average, standard deviation, and maximum values of PDOP. PDOP outliers were removed if the PDOP value exceeded \(\pm\)1.5 \(\times\) IQR. For the \"3+0+\" network configuration, the PDOP statistics first decreased and then increased as the distance increased. For the \"3+0+1\" network configuration, the PDOP values also had similar trends, but decreased significantly with the surface buoy. When we replaced the surface buoy with the subsurface buoy, and when the height of the buoy was greater than 1 km, the PDOP values decreased first and then increased as the distance increased; when the height was less than 1 km, the PDOP values also decreased with increasing distance. When the buoy height was less than 2.5 km, the PDOP average and standard deviation of the \"3+1+0\" network configuration were greater than those of the \"3+1+1\" network configuration. Moreover, when the subsurface buoy of the \"3+1+0\" network configuration was close to the sea surface, the positioning indicators were close to the \"3+0+1\" network configuration, and there was no significant difference between the functions of the subsurface buoy and the sea surface buoy.