## 4 Results and Discussion
The RMSE were calculated \(\pm\)3 cm and \(\pm\)6 in 2D position and height components, respectively. When the CODE's COD ID products was used (Scenario 7), the same differences were found as in the previous scenario (Scenario 6). The results imply that the positioning accuracy of GPS-only PPP using IGS precise products is comparable to using CODE analyzing center's COD-ID precise products in terms of accuracy. However, if scenarios 1 and 7 are compared for the GPS-only PPP solution, the results of two different precise products (COM-ID and COD-ID), belonging to the same institution (CODE), are two times different from each other (Table 5). Both CODE COM-ID and CODE COD-ID products are produced by the institution \"Center for Orbit Determination in Europe\" that is an IGS AC. COM-ID precise products include all satellite constellations and produced in the scope of the MGEX project. However, COD-ID products only support GPS and GLONASS satellite constellations. These two products group are created by using different data sets, ground stations, and strategies. Because of that, the accuracy of these products is different and they give different PPP results for the same data and constellation. On the other hand, according to the results obtained from the quad-constellation system using the products of the GFZ (Scenario 8), mean difference of 3 cm was obtained for the 2D position and 0 cm for the height component. Using the Wuhan University products (Scenario 9), the mean values were obtained 7 cm for the 2D position and 8 cm for height. According to Figure 9 and Table 6, the RMSE values were obtained as \(\pm\)4 cm for 2D position and \(\pm\)5 cm height component for Scenario 8, and \(\pm\)8 cm for 2D position and \(\pm\)11 cm height components for Scenario 9, respectively. This result implies that, the solutions produce different levels of accuracy depending on the used different precise products. According to comparisons of the quad-constellation used with CODE's COM products (Scenario 5), GFZ's precise products (Scenario 8), and Wuhan University's precise products (Scenario 9), the best  
Figure 10: DOP values for Galileo only satellites during the kinematic test. results were obtained from GFZ products when the mean values between PPP solution and reference coordinates were considered. In terms of accuracy, GPS+GLONASS+Galileo+BDS with GFZ's precise products also present the best performance rather than the other quad-constellation alternatives. It is seen in Tables 4-6 that the quad-system using GFZ's precise products achieves a better PPP solution in terms of mean value and accuracy (RMSE) when compared to the other constellation combinations and precise products provided by different ACs. The epoch-by-epoch 2D position and ellipsoidal height differences between the PPP and PPK derived coordinates for the best multi-GNSS PPP solution (Scenario 8) are given in the Figure 11 as a 2D plot on the surveying path. Regarding the above results, the following remarks are drawn:  
* According to Figures 3 and 5, each new satellite system added to the GPS system increased the total number of satellites and decreased PDOP values. The maximum number of satellites is 27 and the minimum PDOP value is 1.0 during the kinematic PPP process of the quad satellite systems. * Considering the histograms of all calculated differences, the differences of CODE's COM ID PPP fell within the range of approximately \(-10\) cm and \(+10\) cm for both easting and northing components, respectively, and approximately \(-20\) cm to \(+20\) cm for height component (Figure 8). However, the differences in PPP solutions using other IGS Analysis Centers (ACs) precise products (Figure 9) are 2 times better than the previous solutions. * Additional constellations did not improve kinematic PPP solution results in all cases.