## 4 Results and Validations
### Assessment of IGS-RT Orbit and Clock Products
For BDS inclined geosynchronous orbit (IGSO) (C06-10) satellites, the orbits provided by GFZD2 reveal the best accuracy in all three components. The CLK92 orbits show the lowest accuracy in the along component, and the GFZC2 orbit accuracy is the worst in the cross and radial components. For BDS medium earth orbit (MEO) (C11-14) satellites, CLK92 and GFZD2 offer the RT orbits with similar accuracy, better than the GFZC2 ones. The RMS values for the CLK92 orbits are within 23, 15, and 7 cm in the along, cross, and radial components, respectively, which are no more than 38, 19, and 14 cm, respectively for the GFZC2 orbits. Figure 3 shows the averaged RMS values of the differences between IGS-RT orbits and GFZ final orbits for GPS, GLONASS, Galileo, and BDS satellites in the along, cross, and radial components, respectively. It can be seen that CLK01 provides the highest accuracy of both GPS and GLONASS satellite orbits. The averaged RMS values for the CLK01 GPS orbits are about 4.00, 2.12, and 1.34 cm in the along, cross, and radial components, respectively, which are about 9.15, 4.30, and 2.99 cm for the CLK01 GLONASS orbits. The GPS and GLONASS orbits offered by CLK81 show similar accuracy with those offered by CLK92, and they are both less accurate than the CLK01 orbits. The GPS and GLONASS orbits obtained from GFZC2 show the lowest accuracy. The averaged RMS values for GPS are 15.13, 12.92, and 5.61 cm in along, cross, and radial components, respectively, and for GLONASS they are 9.50, 13.67, and 5.85 cm. Figure 3: _Cont._  
The Galileo orbits from CLK92 show a better accuracy of 8.31, 4.66, and 2.68 cm in the three components, respectively, compared to those from GFZC2 and GFZD2. For BDS satellites, the orbit accuracy of BDS GEO satellites is much lower than that of IGSO and MEO. This could be due to the fact that the GEO satellites do not move significantly with respect to the ground stations, resulting in a rather weak geometrical constellation. Meanwhile, low elevation of the satellites' signals tracked at several stations far away from Asia-Pacific region and constant multipath error also have negative impact on orbit accuracy. Currently, the RT GEO orbits derived from any ACs are not reliable. The GFZD2 offer the best orbit accuracy of BDS IGSO satellites, where the averaged RMS values are about 24.98, 27.54, and 10.48 cm in the along, cross, and radial components, respectively. The accuracy of BDS MEO satellite orbits offered by CLK92 is comparable to that by GFZD2. The CLK92 MEO orbits reveal averaged accuracies of 17.13, 9.68, and 4.48 cm in the three components, respectively, while the accuracies for the GFZD2 MEO orbits are about 34.05, 17.23, and 7.23 cm, respectively. The averaged RMS values of the differences between IGS-RT orbits and GFZ final orbits for the four systems are also summarized in Table 3.  
Figure 3: Averaged RMS values of the differences between IGS-RT orbits and GFZ final orbits for GPS, GLONASS, Galileo, and BDS satellites in the along (**a**), cross (**b**), and radial (**c**) components, respectively. Afterwards, the GFZ final clock products are employed as reference to assess the quality of the multi-GNSS RT clocks offered by CLK01, CLK81, CLK92, GFZC2, and GFZD2. In general, the standard derivation (STD) is equal to the RMS since the removed mean biases can be absorbed by ambiguity items. Here, the STD value will be taken as the indicator for evaluating the clock quality. The STD values of the differences between the IGS-RT clocks and the GFZ final clocks for the four systems (i.e., GPS, GLONASS, Galileo, and BDS) are illustrated in Figure 4.  
As can be seen from Figure 4, the RT GPS satellite clocks agree well with the GFZ final clocks, in particular for those provided by CLK92, whose STD values are generally less than 0.15 ns. The clock accuracies of GPS satellites are around 0.2 ns for the CLK01 and CLK82, while GFZC2 and GFZD2 provide the RT GPS clocks with the accuracy of about 0.3 ns.