# Precise Orbit Determination of FY-3C with Calibration of Orbit Biases in BeiDou GEO Satellites
## 3 FY-3C POD Results and Analysis
### Statistics of Observation Residuals  
Residuals of the ionosphere-free linear combination for BeiDou carrier-phase observations are composite responses of receiver measurement noises, quality of BeiDou orbit and clock products,  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Category** & **Along-Track** & **Cross-Track** & **Radial** & **3D** \\ \hline Solution A & 45.7 & 26.6 & 17.7 & 56.9 \\ Solution B & 6.4 & 3.3 & 2.6 & 7.8 \\ Solution C & 5.0 & 3.0 & 2.0 & 6.2 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Average daily RMS of orbit differences in along-track, cross-track, radial, and 3D with three strategies for FY-3C POD using only BDS (unit: cm).  
Figure 7: Daily RMS series of orbit differences in along-track, cross track, radial, and 3D between BDS-derived and GPS-derived FY-3C orbit, the top, middle, and bottom subgraphs give the solution A, B, and C, respectively.  
precision of FY-3C orbit and clock, etc. The maximum, minimum, and average values of daily RMS series of BeiDou phase observation residuals for GEO, IGSO, MEO, and all BeiDou satellites obtained from the three solutions are listed in Table 4. In Table 4 we can see that phase residuals from solution B and C are significantly smaller than those from solution A. The residuals from solution C which achieves the best orbit quality are slightly larger than those from solution B.  
Average daily RMS of BDS phase residuals are 45.8, 6.2, and 7.9 mm for solution A, B, and C, respectively. The average daily RMS of residuals derived in solution B and solution C are all less than one centimeter. This demonstrates a good matching attribute between the BeiDou observation models and the FY-3C orbit dynamic models. Residuals at the centimeter level obtained in solution C also prove that it is a practical method for eliminating most of the errors in BeiDou GEO orbit products by modeling systematic errors as daily constants in along-track and cross-track of the GEO orbits. The results are consistent with the precision evaluation of calibrated GEO orbits in Section 3.5.  
The residuals derived in solution B and C should be compared and analyzed much more meticulously since they both obtained one centimeter level residuals while solution A achieved half-decimeter level residuals. Figure 8 shows the daily RMS values of phase residuals for GEO, IGSO, MEO, and overall BeiDou satellites in FY-3C POD for solution B and C during July 2017. Both Figure 8 and Table 4 indicate that the daily RMS of GEO phase residuals mostly range between 8 and 10 mm when the GEO orbit biases are in-flight calibrated correctly in LEO POD. Daily RMS of IGSO and MEO mostly range between 5 and 9 mm for solution B and C. In addition, the RMS values of IGSO and MEO for solution C are slightly larger than those for solution B.  
Several reasons can explain the phenomenon that the daily RMS series of phase residuals from solution B are slightly less than those from solution C. First, stronger constraints in the BeiDou observation models could be achieved with an increase in the redundant observations of solution C containing GEO compared to those of Solution B. Second, observations are contaminated by GEO orbit and clock errors that cannot be modeled completely by using the calibration method of constant biases existing in along-track and cross-track of GEO orbits. Third, the quality of the BeiDou GEO products cannot reach the same accuracy level with respect to that of IGSO and MEO even though the systematic errors existing in GEO orbits can be estimated completely. It should be noted that the orbit quality of solution C is the best even though the average daily RMS of the residuals is slightly larger than that of solution B by about 1.7 mm.