# Assessment of BeiDou-3 and Multi-GNSS Precise Point Positioning Performance
## 3 Data Sets and Processing Strategy  
The data were collected by 16 globally distributed iGMAS stations on 30 days, from 2 January 2019 to 31 January 2019. All of the selected stations can receive the observations from GPS, BDS-2, BDS-3, GLONASS, and Galileo constellations [16]. Figure 1 shows the distribution of the selected iGMAS stations. When considering that the BDS-2 is a regional navigation system, the stations in Asia and Europe are selected for BDS-2 PPP to ensure the objectivity of the evaluation. The stations that are marked with red in Figure 1 are used for the BDS-2 PPP. The observation data have a sampling interval of 30 s. The WHU orbit and clock products ([[ftp://igs.ign.fr/pub/igs/products/mpex/](ftp://igs.ign.fr/pub/igs/products/mpex/)]([ftp://igs.ign.fr/pub/igs/products/mpex/](ftp://igs.ign.fr/pub/igs/products/mpex/))) have a sampling of 15 min. and 30 s, respectively, and the time system of products is GPS time (GPST). The coordinate frame of WHU orbit and clock products is IGS14 [17]. It is worth mentioning that the BDS B1I (1561.098 MHz) and BDS B3I (1268.52 MHz) signals are used to determinate the BDS (BDS-2/BDS-3) satellite orbit and clock offset [18]. The station coordinates in the SINEX file provided by iGMAS ([[http://112.65.161.230/download/index.php](http://112.65.161.230/download/index.php)]([http://112.65.161.230/download/index.php](http://112.65.161.230/download/index.php)); [[ftp://222.240.181.170/products/](ftp://222.240.181.170/products/)]([ftp://222.240.181.170/products/](ftp://222.240.181.170/products/))) are used as the references in order to assess the positioning accuracy. It is noteworthy that the difference between the iGMAS and IGS station coordinates precision is in the millimeter level [19; 20]. Therefore, iGMAS station coordinates is sufficient to assess the positioning accuracy. Figure 1: Geographical distribution of the selected stations. In the IF PPP model, the estimated parameters include the receiver position, the zenith wet tropospheric delay (ZWD), the receiver clock offset, the inter-system bias (ISB), and the ambiguity. These selected iGMAS stations were used for static PPP solutions. The Kalman filtering algorithm is applied in PPP processing. Phase Center Offset (PCO) and Phase Center Variations (PCV) of GPS GLONASS and Galileo satellite and receiver antennas are corrected while using the ANTEX file that was provided by IGS. Regarding the BDS satellite, the ANTEX file that was provided by iGMAS Analysis Center of Shanghai Astronomical Observatory is used to correct the PCO and PCV of BDS. The correction of PCO and PCV of some receiver antennas only gives GPS and GLONASS, and it rarely updates BDS and Galileo [21; 22]. We can only use the correction value provided by GPS to correct BDS and Galileo when we make PCO and PCV corrections on the receiver antenna. In addition, the pole coordinates and UTC-UT1 value provided by the WHU ERP products correct the pole tide and the Earth rotation effects. Table 1 summarizes the detailed processing strategy for PPP. Table 2 shows that the comparison of GPS, BDS-2, BDS-2/BDS-3, GLONASS, and Galileo IF-PPP models, where m, o, n, p, and q are the number of satellites for GPS, BDS-2, BDS-3, GLONASS, and Galileo, respectively. In the figure and table below, G, C2, C, R, E, GC, GR, GE, CR, CE, RE, and GCRE represent GPS, BDS-2, BDS-2/BDS-3, GLONASS, Galileo, GPS/BDS-2/BDS-3, GPS/GLOANASS, GPS/Galileo, BDS-2/BDS-3/GLOANSS, BDS-2/BDS-3/Galileo, GLONASS/Galileo, and GPS/BDS-2/BDS-3/GLOANSS/Galileo, respectively.