## 3 Validation with Data from International GNSS Service
### Performance of Positioning  
The PPP and POP integer ambiguity resolution on the four stations CAS1, DAV1, OHI3 and MCM4 are performed. The position differences of the float and fixed solutions calculated with PPP and POP approaches regarding the IGS estimated coordinates for station CAS1 are shown in Figures 5 and 6, respectively. The average RMS values of the station coordinates over 14 days are calculated for each station to assess the performance of the PPP and POP float, as well as fixed solutions; the statistical results are shown in Figure 7.  
Figure 5 gives us a visual impression that the POP approach has, on average, a better performance than PPP for GE, GC and GREC solutions. Some fluctuations appear in the PPP GE position differences; these fluctuations are possibly caused by the short-term fluctuations in the Galileo satellite clocks. The influence of BDS orbits and clocks on PPP GC and GREC kinematic positioning is significant. Some large positioning errors appear between 14:00 and 16:00. The fixed solutions can improve the accuracy in the horizontal directions, and the RMS value is reduced by 8% and 17% for the east and north components, respectively. However, in the POP solutions, most of the clock the orbit biases are compensated by the clock parameters, the positioning errors caused by the remaining orbit errors are below 5 cm for all components, as shown in the lower half panel in Figure 5. What is more, the POP ambiguity resolution can further improve the positioning performance in the east component and the  
Figure 4: The fixing percentages of PPP and POP AR with 14 days of single-, dual- and four-system observations. By doing a chi-squared distribution test with a 0.05 level of significance, it is shown that the p-value that indicates the significance-level for the differences between PPP and POP calculated with GLONASS, Galileo, BDS, GR, GE, GC and GREC observations is 0.062, 0.078, 0.0030, 0.25, 0.09, 0.0036 and 0.019, respectively, with respect to GPS. We can see there are significant differences between the PPP and POP results with BDS, GC, and GREC observations. accuracy is improved by 14%. The differences of PPP and POP positioning results are not significant with GPS and GR observations, which is mainly due to their stable orbit and clock quality derived from globally well distributed stations. Figure 6 shows the average RMS values of PPP and POP kinematic positioning results regarding the IGS reference over the 14 days. The positioning results on stations CAS1 and DAV1 indicate that the accuracy of the GC and GREC float PPP solutions are relatively poor, and even worse than the corresponding GPS-only solution. This result is mainly caused by the poor quality of BDS orbits. The GE solution shows almost no improvement because of the limit number of available Galileo satellites. Meanwhile the GR solution represents an average improvement of 31.3, 33.1 and 18.5% compared to the GPS-only solution for the east, north and up components, respectively. The PPP fixed solutions can improve the accuracy in the east and north components, i.e., an average improvement of 13.1, 9.5, 16.9, 29.7 and 20.0% with GPS, GR, GE, GC and GREC observations, respectively. Figure 5: The single-, dual- and four-system PPP (upper half) and POP (lower half) float and fixed solutions with respect to the IGS nominal position for station CAS1 on 1 January 2018. We can see better performance of POP than PPP with BDS involved solutions. The POP single-, dual- and four-system float solutions are more stable and can achieve higher accuracy than the corresponding PPP float solutions on the four stations, i.e., the average improvements are 12.7, \(-\)2.4, 21.0, 28.1 and 6.3% with GPS, GR, GE, GC and GREC observations for the up component.