## 3 BeiDou PPP Strategy
### Static PPP  
Table 2 shows the biases of the results of the BeiDou, GPS, and BeiDou/GPS static PPP against the \"ground truth\" in the East, North and Up components on September 24, 2012. The \"ground truth\" is the mean value of the GPS static PPP coordination from September 4 to September 30, 2012. We can see that the biases in the East, North and Up components vary from station to station. Stations in the Asia-Pacific regions have better accuracy than stations in Europe like Greece and LEID. The biases of the East and North components of the stations in the Asia-Pacific region are almost within \(\pm\)1 cm except that of the East component of the LASA station, whose absolute value of the vertical bias reaches 4 cm. Meanwhile, the biases of stations in Europe, especially the GREECE station, are even worse. However, the biases of the three components of GPS PPP solutions for all the stations are almost within \(\pm\)1 cm, better than that of BeiDou. That is because the \"ground truth\" represents the average values of GPS static PPP. The horizontal accuracy of BeiDou static PPP are closer to that of GPS than the Up components. In addition, the PCV corrections of BeiDou satellites are not applied, and the PCO corrections of BeiDou satellites are not accurate enough. Furthermore, the biases between GPS and BeiDou signal phase center for the unknown PCO and PCV corrections of the receivers could introduce biases into the results. Figure 7 shows the RMS values of the daily biases for the BeiDou, GPS and BeiDou/GPS static PPP solutions of the \"BETS\" network with respect to the \"ground truth\" in the East, North and Up components from September 4 to September 30, 2012. We can see that:  
* The RMS values of the East, North and Up components also show spatial variations. The RMS values of three components of BeiDou static PPP against the \"ground truth\" are less than 1.0 cm, 1.0 cm and 3.0 cm in the Asian-Pacific in the period. However, the results get \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**SITE**} & \multicolumn{3}{c}{**BeiDou**} & \multicolumn{3}{c}{**GPS**} & \multicolumn{3}{c}{**BeiDou/GPS**} \\ \cline{2-10}  & **East(cm)** & **North(cm)** & **Up(cm)** & **East(cm)** & **North(cm)** & **Up(cm)** & **East(cm)** & **North(cm)** & **Up(cm)** \\ \hline BJF1 & –0.18 & –0.05 & –1.44 & 0.66 & –0.05 & –1.95 & –0.23 & –0.32 & –0.20 \\ CHDU & 0.87 & 0.05 & 1.50 & 0.62 & –0.31 & –0.71 & 0.69 & –0.11 & 0.62 \\ CWKJ & –0.22 & 0.51 & –3.27 & 0.43 & –0.31 & –0.87 & –0.24 & 0.10 & –0.88 \\ HRBN & 0.86 & 0.41 & –1.37 & 0.55 & –0.36 & –0.79 & 0.54 & –0.10 & –0.63 \\ LASA & 1.60 & 0.44 & –2.14 & 1.23 & –0.26 & –0.16 & 0.92 & 0.16 & –1.19 \\ WDKJ & 0.48 & 0.28 & –2.58 & 0.31 & –0.20 & 0.01 & –0.05 & 0.27 & –0.34 \\ WUHN & –0.6 & 0.37 & –2.68 & 0.35 & –0.51 & 1.19 & 0.08 & 0.08 & –1.34 \\ XIAN & 0.39 & 0.40 & –2.34 & –0.51 & –0.40 & –1.04 & –0.30 & 0.25 & –1.32 \\ LEID & –0.97 & –1.78 & 0.81 & 0.03 & –0.07 & 0.01 & –0.25 & 0.03 & 0.31 \\ GREECE & 1.04 & –0.27 & –6.22 & 0.29 & 0.07 & 1.10 & 0.58 & 0.14 & 0.63 \\ RMS & 0.79 & 0.62 & 2.70 & 0.56 & 0.28 & 0.93 & 0.45 & 0.17 & 0.81 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Biases of the results of the BeiDou, GPS, and BeiDou/GPS static PPP of the “BETS” against the “ground truth” in the East, North, and Up components on September 24, 2012.  
worse for the LEID and GREECE stations, which are far from the Asia-Pacific region. Their RMS values are less than 4.0 cm due to the fewer BeiDou satellites observed as shown in Figure 1. The average values of the RMS of the East, North and Up components for all the stations are 1.07, 0.73 and 2.21 cm, respectively. * The RMS values of the difference between the East, North and Up components of BeiDou/GPS static PPP and the \"ground truth\" for all the stations are almost all less than 1.0 cm and the average values of the RMS values are 0.53, 0.28 and 1.04 cm, respectively. Those of GPS static PPP for all the stations are 0.50, 0.29 and 0.89 cm, respectively.