# GPS + BDS Network Real-Time Differential Positioning Using a Position Domain Estimation Method
## 3 Results
### Static Experiment
#### 3.1.1 Data Set I  
Figure 3 shows the position errors of the network real-time differential positioning and the number of observed satellites for station SHAT. The position errors of position domain (PD) fluctuate smaller than those of range domain (RD), even in the periods when observed satellites changed frequently. The variable number of observed satellites affects the smoothing window of RD and leads to many re-initializations, while PD performs successively with a sufficient number of observed satellites. Figure 2: Geographical distribution of the stations in the static experiment (**left**: Data set I; **right**: Data set II). Black solid triangles represent the reference stations. Black solid and hollow circles denote the user stations inside and outside the network, respectively. For both PD and RD, most of the position errors of GPS-only solution fluctuate from -1 m to 1 m in the N and E components, and from -2 m to 2 m in the U component. Obviously smaller position errors can be found in GPS + BDS solution, since the number of satellites in GPS + BDS solution are significantly larger than those in GPS-only solution. The RMS errors of the network real-time differential positioning for each user station are illustrated in Figure 4. Recall that the first two stations are inside the network. Generally speaking, the RMS errors of PD are smaller than those of RD, in other words, better positioning accuracy can be achieved using PD. Further, for both PD and RD, the RMS errors of GPS + BDS solution are smaller than those of GPS-only solution. The RMS errors of GPS + BDS solution are usually smaller than 0.3 m, 0.3 m and 0.9 m in the N, E and U components, respectively. Figure 3: Position errors of the network real-time differential positioning and number of satellites for SHAT. Left panels: Global Positioning System (GPS)-only; right panels: GPS + BDS (BeiDou Navigation Satellite System). PD: position domain, RD: range domain. Specifically, in GPS-only solution, the maximum improvement of RMS errors can be found in the N component for station SHJU, with a value of 52.8%, when replacing RD with PD. The corresponding value in GPS + BDS solution is 64.3% in the U component for station HUAD. In addition, the RMS errors of the stations outside and inside the network have little difference, whether in GPS-only or GPS + BDS solution. This may be attributed to the short distances between the outside user stations and their neighboring reference stations. Table 1 shows the RMS errors according to the types of user stations. Remarkable improvements can be found for PD compared with RD. The overall improvements are about 37.4%, 30.9% and 37.3% in the N, E and U components, respectively. For both PD and RD, the RMS errors of GPS + BDS solution are significantly smaller than those of GPS-only solution. The RMS errors of GPS + BDS solution are 0.269 m, 0.251 m and 0.753 m for RD in the N, E and U components, and 0.190 m, 0.191 m and 0.487 m for PD. The improvements of GPS + BDS solution with respect to GPS-only solution using PD are about 24%-50%, and the corresponding improvements using RD are about 30%-60%. This, in turn, implies that less accuracy decrease can be found using PD when replacing GPS + BDS solution with GPS-only