## 4 Results and Validations  
### Assessment of IGS-RT Orbit and Clock Products  
As the performance of tropospheric delays derived from the multi-GNSS RT precise positioning depends on the accuracy and reliability of the employed RT precise orbit and clock products, the quality and capability of those products from different IGS RTS are evaluated before applied into the multi-GNSS processing. Here, taking the GFZ final orbit and clock products as reference, we analyze and compare different multi-GNSS RT orbit and clock products (i.e., CLK01, CLK81, CLK92, GFZC2 and GFZD2) over the period from day of year (DOY) 84 to DOY 101 in 2017. It is noteworthy that the four-system (GPS + GLONASS + Galileo + BDS) RT orbits and clocks are available from CLK92, GFZC2, and GFZD2, while CLK01 and CLK81 only support GPS and GLONASS products. Figure 2 shows the RMS (root-mean-square) values of differences between the IGS-RT orbits and the GFZ final orbits for each satellite constellation in the along-track, cross-track, and radial components, respectively. The accuracy of RT GPS orbits is the best among all the orbital types in the three components, which is followed by GLONASS satellites. Limited to the insufficient number of globally tracking stations, the RT orbit accuracy for Galileo and BDS are not comparable to that for GPS or GLONASS. For the multi-GNSS orbits provided by the five ACs, the results in the radial component show a better agreement with the GFZ final orbits than those in the cross component, while the agreement in the along component is the worst. Figure 1: Geographical distribution of the multi-GNSS Experiment (MGEX) network and their supported navigation satellite constellations. For GPS satellites, the RMS values for the CLK01 orbits that fit the best with the GFZ final orbits are less than 5 cm in the radial and cross components and below 10 cm in the along component. CLK81 provides slightly worse GPS orbits than CLK01 in particular for the along and cross components,  
Figure 2: RMS (root-mean-square) values of the differences between IGS-RT orbits (CLK01, CLK81, CLK92, GFZC2, and GFZD2) and GeoForschungsZentrum (GFZ) final orbits for the four systems (i.e., Global Positioning System (GPS), Russian Global Navigation Satellite System (GLONASS), Galileo, and BDS) in the along (**a**), cross (**b**) and radial (**c**) components, respectively. where the RMS values reach up to 12 cm and 7 cm, respectively. The CLK92 GPS orbit accuracy is very similar to that by CLK81 in the along and radial components, while the RMS value for CLK92 is about 11 cm in the cross component. In addition, the GPS RT orbits derived from GFZC2 and GFZD2 reveal lower accuracy than those from CLK01, CLK81, and CLK92. The RMS values for the GFZC2 GPS orbits can reach up to 25, 17, and 8 cm for the along, cross, and radial components, respectively. The comparative results of the GLONASS satellite orbits derived from the different five ACs are similar to those of the GPS orbits mentioned above, except that they show a slightly lower accuracy than GPS orbits. For the RT GLONASS orbits, the RMS values for the CLK01 orbits are less than 20, 12, and 6 cm for the along cross, and radial, components, respectively, while the values for the GFZC2 orbits are about 22, 18, and 8 cm, respectively, in the three components. The Galileo satellite orbits, offered by the CLK92, show a better agreement with the GFZ final orbits than those from other ACs. The RMS values for the CLK92 Galileo orbits are generally below 5 cm in the radial component, and less than 10 and 12 cm in the cross and along components. The RMS value for the GFZC2 orbits can reach about 38, 19, and 12 cm in the along, cross, and radial components, respectively. For BDS GEO satellites (C01-05), the RT orbit products for the whole constellation cannot be continuously provided by CLK92, GFZC2 and GFZD2 since the corresponding global distribution of GEO tracking stations are not good and the data streams for some of the satellites are always missing, such as for satellites C01 and C03.