# Assessment of BeiDou-3 and Multi-GNSS Precise Point Positioning Performance
## 4 Service Area  
In this paper, the Position Dilution of Precision (PDOP) is used to explore the differences between GPS, BDS-2, BDS-2/BDS-3, GLONASS, Galileo, and multi-GNSS in service areas. The global region is divided into grids of \(2.5^{\circ}\times 5^{\circ}\). Subsequently, the PDOP values of each grid are calculated using the broadcast ephemeris (brdm0240.19p) on day of year (DOY) 024, 2019. During the computation, the cut-off elevation is set to \(7.5^{\circ}\) and the time resolution is 10 min. For every grid, one calculation result of the PDOP is obtained at each epoch, which generated a PDOP sequence. Afterwards, we could get the mean value from this sequence for every grid.  
Figure 2 shows the global mean PDOP distribution for GPS, BDS-2, BDS-2/BDS-3, GLONASS, Galileo, GPS/GLONASS, GPS/Galileo, GPS/BDS-2/BDS-3, BDS-2/BDS-3/GLONASS, BDS-2/BDS-3/Galileo, GLONASS/Galileo, and GPS/BDS-2/BDS-3/GLONASS/Galileo constellations. It can be seen from Figure 2 (BDS-2) that the BDS-2 service area is mainly centered in the Asia-Pacific region, where its geostationary Earth orbit (GEO) satellites and inclined geosynchronous orbit (IGSO) satellites are mainly concentrated, which results in users in other regions not receiving the signal of BDS-2 or only receiving a small amount of the signal of BDS-2. This is an important reason why BDS-2 does not have global navigation service capability all over the world. When compared BDS-2 and BDS-2/BDS-3 PDOP, the global PDOP of BDS navigation system has significantly decreased with the opening of the global basic navigation service of BDS-3. BDS has full navigation service capability all over the word and it has been transformed from a regional to global navigation system. The performance of BDS will be better when BDS-3 completes the constellation [29]. When compared with GPS, BDS-2/BDS-3, and GLONASS global PDOP, they have basically the same global service performance, except individual regions of the world. From the global PDOP distribution of the Galileo, Galileo has full navigation service capability all over the word, the as same as BDS-2/BDS-3, GPS, and GLONASS. Although the global coverage of each navigation system is already excellent, the problems of the low satellite numbers and the poor satellite constellation structure occur in some special regions of the world when using a single navigation system for positioning and the multi-GNSS is a good way to solve that. The variation ranges of the global mean PDOP of GPS/GLONASS, GPS/Galileo, GPS/BDS-2/BDS-3, BDS-2/BDS-3/GLONASS, BDS-2/BDS-3/Galileo, GLONASS/Galileo, and GPS/BDS-2/BDS-3/GLONASS/Galileo are below 1.5.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **G** & **C2** & **C** & **R** & **E** & **GC** \\ \hline Observed quantity & 2 m & 2 o & 2 o + 2 n & 2 p & 2 q & 2 m + 2 o + 2 n \\ parameter & m + 5 & o + 5 & o + n + 5 & p + 5 & q + 5 & m + o + n + 6 \\ Redundancy & m - 5 & o - 5 & o + n - 5 & P - 5 & q - 5 & m + o + n - 6 \\ \hline  & **GR** & **GE** & **CR** & **CE** & **RE** & **GCRE** \\ \hline Observed quantity & 2 m + 2 p & 2 m + 2 q & 2 o + 2 n + 2 p & 2 o + 2 n + 2 q & 2 p + 2 q & 2 m + 2 o + 2 n + 2 p + 2 q \\ parameter & m + p + 6 & m + q + 6 & o + n + p + 6 & o + n + q + 6 & p + q + 6 & m + o + n + p + q + 8 \\ Redundancy & m + p - 6 & m + q - 6 & o + n + p - 6 & o + n + q - 6 & p + q - 6 & m + o + n + p + q - 8 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of GPS, BeiDou navigation satellite system (BDS)-2, BDS-2/BDS-3, GLONASS, Galileo, and multi-Global Navigation Satellite System (GNSS) PPP models.  
Figure 2: _Cont._  
Figure 2: The global mean Position Dilution of Precision (PDOP distribution for GPS, BDS-2, BDS-2/BDS-3, GLONASS, Galileo, and multi-GNSS constellations on DOY 024, 2019, with the elevation cutoff of 7.5\({}^{\circ}\).