# Assessment of BeiDou-3 and Multi-GNSS Precise Point Positioning Performance
## 5 Data Tests and Results Analysis
### ZTD Accuracy
Figure 9: The solutions of zenith troposphere delay (ZTD) at BRCH for 12 different processing cases on DOY 024 in 2019. In order to assess the accuracy of PPP ZTD, the ZTD products that were provided by iGMAS are used as references to calculate the absolute ZTD differences and RMS between ZTD estimated by PPP and the ZTD products. Figure 12 depicts the absolute ZTD differences between ZTD estimated by PPP and the iGMAS ZTD at BRCH for 12 different processing cases on DOY 024 in 2019. As shown in Figure 12, the absolute ZTD differences preform the maximum at the beginning. ZTD accuracy is as poor as its positioning accuracy when the convergence process is not completed because ZTD is estimated together with the receiver position parameters in an epoch and the technology of PPP needs a certain convergence time [36; 37]. The significance of this finding becomes more apparent in Figure 9. With the BDS-3 is open access, the absolute ZTD differences of BDS-2/BDS-3 is smaller than that of BDS-2. More interestingly, the trend of BDS-2/BDS-3 is slightly more stable than that of GLONASS and Galileo. Figure 13 demonstrates this interesting funding well. The ZTD RMS of GLONASS and Galileo are a little worse than that of BDS-2/BDS-3. Now, turn to multi-GNSS, multi-GNSS increases the stability of the solution of ZTD over the single-constellation and dual-constellation PPP. This viewpoint can be confirmed in both Figure 12 (GCRE) and Figure 13.  
Figure 11: Time series of the RMS between iGMAS ZTD and IGS ZTD products from 2 January 2019 to 31 January 2019. Figure 10: Distribution and 30-day RMS between the international Global Navigation Satellite System Monitoring and Assessment System (iGMAS) and IGS ZTD products of 130 IGS stations. Figure 13 indicates the RMS statistical values of ZTD and the increase rate that is based on GPS-only results for 12 different processing using real observation data collected at 16 stations over 30 days. As mentioned above, the technology of PPP needs a certain convergence time, the RMS statistical values using the ZTD bias in the second hour to last hour, in which the ZTD solution has reached stable values. We can draw the following points of view based on the results of the selected observation data. The  
Figure 12: The absolute ZTD differences between ZTD values estimated using PPP and iGMAS ZTD at BRCH for 12 different processing cases on DOY 024 in 2019. Figure 13: The RMS error of ZTD and the increase rate based on GPS-only results for GPS-only, BDS-2-only, BDS-2/BDS-3, GLONASS-only, Galileo-only, GPS/BDS-2/BDS-3, GPS/GLONASS, GPS/Galileo, BDS-2/BDS-3/GLONASS, BDS-2/BDS-3/Galileo, GLONASS/Galileo, and GPS/BDS-2/BDS-3/GLONASS/Galileo static PPP processing using real observation data collected at 16 stations over 30 days. BDS-2/BDS-3 PPP improves the ZTD accuracy by 13.16% over the BDS-2-only PPP. There are no gaps between GPS, GLONASS, Galileo, and BDS-2/BDS-3. Moreover, the GLONASS PPP and Galileo PPP are slightly poorer than the BDS-2/BDS-3 PPP in terms of ZTD accuracy. Additionally, the GPS/BDS-2/BDS-3, GPS/GLONASS, GPS/Galileo, BDS-2/BDS-3/GLONASS, BDS-2/BDS-3/Galileo, GLONASS/Galileo, and GPS/BDS-2/BDS-3/GLONASS/Galileo PPP improves the ZTD accuracy by 1.52%, 4.55%, 1.52%, 1.52%, 0.76%, and 6.06% over GPS-only PPP. The contributions of BDS and Galileo to multi-GNSS are significant.