# Initial Positioning Assessment of BDS New Satellites and New Signals
## 2 Satellite Visibility and PDOP Availability  
With the availability of BDS-3 satellites, global users would benefit from the PNT service from BDS as more satellites are visible all over the world. To investigate the service area with the current BDS constellation, we will assess the satellite visibility, PDOP value and availability in this part. Using the broadcast ephemeris on January 1, 2019, we explore the global satellite visibility and PDOP value comparing BDS-2 and the current constellation of BDS-2 and BDS-3. The latitude and longitude are divided into a \(2^{\circ}\times 2^{\circ}\) grid and the satellite elevation mask is set as \(10^{\circ}\). As for PDOP, we adopt the conventional concept that a PDOP value less than 6 is necessary to provide basic PNT service [8]. To assess the normal performance, the result is calculated on an hourly basis and the global average result is plotted, which is depicted in Figures 1 and 2, and the minimum, maximum and average satellite number are listed in Table 1.  
Figure 1: Average visible satellite number comparison of BDS-2 (**upper**) and BDS-2+BDS-3 (**lower**) for an elevation angle mask of \(10^{\circ}\) on 1 January 2019. Figure 2: Average PDOP value of BDS-2 (**upper**) and BDS-2+BDS-3 (**lower**) for an elevation angle mask of \(10^{\circ}\) on 1 January 2019. Only value of PDOP \(<6\) is counted. It can be observed from Figure 1 and Table 1 that for BDS-2 constellation, the maximum visible satellite number is about 13.8, which is located in Southeast Asia, where the equatorial region is at the longitude of about 110\({}^{\circ}\). For the opposite part of the Earth, i.e., the Americas, the average satellite number would less than 4, which is challenging for PNT service. With the contribution of BDS-3 satellites, Southeast Asia would still be the \"hottest\" area of BDS, with 20 satellites that can be tracked at most. As for the Americas, a minimum number of 5.7 satellites is visible, which indicates that a global PNT service is possible using the current BDS-2 and BDS-3 constellation. Comparing with BDS-2, the global average visible satellite number increases from 5.1 to 10.7. As for PDOP, it shows a similar performance. For BDS-2, the minimum PDOP is 1.8, while for BDS-2+BDS-3, it would decrease to 1.2. Outside the Asia-Pacific region, PDOP of BDS-2 would dramatically increase. Nevertheless, combining BDS-2 and BDS-3, the PDOP value changes gradually and it is still able to provide normal PNT service in the Americas, with a maximum PDOP value of 3.3. For the above PDOP statistics, only values of less than 6 are counted. However, for a navigation system, the percentage of time to provide a specified level of PNT service, or the availability, is also important [20]. For the case of BDS-2 PDOP performance, although some areas outside the Asia-Pacific region could experience normal PDOP, most of the time the PDOP is larger than 6, which is not shown in Figure 2. Therefore, it is necessary to assess PDOP availability. In this paper, we define the PDOP availability as the percentage of time that the PDOP value is less than 6 [8]. Figure 3 plots the global overall PDOP availability of BDS-2 and BDS-2+BDS-3 constellation and the minimum, maximum and average value are listed in Table 1. It can be seen that for BDS-2, the average of global PDOP availability is only 35.25%, which is mostly around the Asia-Pacific region, while for BDS-2+BDS-3, it could reach the level of 98.84%, which fulfills the target of PDOP availability of BDS open service performance standard [8]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Property**} & \multicolumn{3}{c}{**BDS-2**} & \multicolumn{3}{c}{**BDS-2+BDS-3**} \\ \cline{2-7}  & **Min** & **Max** & **Average** & **Min** & **Max** & **Average** \\ \hline Satellite Number & 0.9 & 13.8 & 5.1 & 5.7 & 20 & 10.7 \\ PDOP Availability & 0\% & 100\% & 35.25\% & 79.17\% & 100\% & 98.84\% \\ \hline \hline \end{tabular}
\end{table}