*4.3. DD Network Solutions*

The accuracy of coordinates, the ambiguity fixing rate, and the accuracy of ZTD estimates with the DD network processing strategy were analyzed.

#### 4.3.1. Accuracy of Coordinates

Taking IGS daily coordinates as the reference value, the differences between the estimated coordinates and the IGS daily coordinates were calculated. The stations SOD3 (in the high latitude network), WTZR (in the middle latitude network), and SPTU (in the low latitude network) were employed to illustrate the positioning performance of GPS, GLONASS, and GPS+GLONASS. The other stations present results similar to these three stations. The coordinate error in the North (N), East (E), and Up (U) components of the three selected stations are presented in Figure 8.

**Figure 8.** The coordinate error series of station SOD3, WTZR, and SPTU estimated with DD processing.

As shown in Figure 8, the GLONASS coordinate error components of station SOD3 and WTZR, located in the high and middle latitude networks, respectively, are as steady as GPS and GPS+GLONASS results. The GLONASS error components are more fluctuated than that of GPS and GPS+GLONASS results for the low latitude station SPTU.

The RMSE of N, E, and U components, together with the three-dimensions (3D) RMSE for all the stations, were calculated and are presented in Figure 9. Figure 9 shows that in the low latitude network, the GLONASS positioning accuracy is obviously worse than that of GPS and GPS+GLONASS. The coordinates of the SALU station exhibit the worst accuracy, and the station also has poor data quality, as shown in Figure 3. Similar results can be found from the mean RMSE of coordinate estimates of each network, as shown in Table 6.

**Figure 9.** The RMSE of coordinates estimated with DD processing.


**Table 6.** The mean RMSE of coordinates of each network estimated with DD processing and their comparison among different processing modes (where R/G indicates the accuracy comparison of GLONASS and GPS results, (G + R)/G indicates the accuracy comparison of GPS+GLONASS results and GPS estimates. The positive red and negative green values indicate the percentage increment and reduction of accuracy, respectively).

The RMSE results clearly show that the accuracies of coordinates estimated with GLONASS are 13.79% and 6.35% better than that of GPS on N and U components in the high latitude network. On the E components, however, the GLONASS positioning accuracy decreased by 16.17% compared to GPS. Therefore, the 3D accuracy of GLONASS is 6.50% better than that of GPS. In addition, the GPS+GLONASS combined mode presents the best results among the three constellations, the accuracy improvements on N and U components are 24.29% and 4.66% compared to GPS, respectively, and the 3D accuracy is 6.05% better than that of GPS. The positioning performance is consistent with the analysis of PDOP in Section 4.2. The stable and good PDOP enables the high positioning accuracy of GLONASS. The PDOP of the combined constellations significantly improved over GPS; therefore, GPS+GLONASS shows the highest accuracy in the high latitude network.

The positioning accuracy of the middle latitude network reveals that GPS presents the best positioning results in E and U, as well as 3D components. GLONASS and GPS+GLONASS exhibit a slightly better positioning accuracy of 1.14% and 4.48% than GPS on the N component, respectively. The positioning accuracies of GLONASS on E and U components are 30.17% and 15.79% worse than that of GPS. In addition, the positioning accuracies of GPS+GLONASS on E and U components are 11.77% and 4.59% worse than that of GPS. Furthermore, the 3D positioning accuracy of GLONASS and GPS+GLONASS is reduced by 14.74% and 3.96% than that of GPS, respectively. As can be seen from Figure 7, GPS has the best and the most stable PDOP values in the middle latitudes when compared with the PDOP of high and low latitudes. The improvement of the GPS+GLONASS combined constellations' PDOP over GPS is weaker when compared to high and low latitudes. Besides, currently, the accuracy of GLONASS satellite ephemerides is about 3 cm, a bit lower than that of GPS, with a 2.5 cm accuracy. Hence, it can be inferred that when the stand-alone GPS has adequate visible satellites with good observation geometry, the addition of GLONASS observations shows no positive contribution to the accuracy of the coordinates. This is consistent with the conclusion of Cai and Gao [10].

The positioning accuracy of the middle latitude network shows that GLONASS presented the worst positioning accuracy among the three modes. The positioning accuracies of GLONASS on N, E, and U components are 22.13%, 62.86%, and 38.24% worse than that of GPS, and the 3D positioning accuracy is 39.93% lower than that of GPS. The positioning accuracy of GPS+GLONASS, however, is better than that of GPS, which is 1.37%, 2.39%, and 10.89% on N, E, and U components, respectively, and the 3D accuracy increases by 9.00%. The poor positioning results of GLONASS and the improvement of GPS+GLONASS

positioning performance by introducing GLONASS observations are consistent with the analysis of PDOP.
