# Precise Orbit Determination of FY-3C with Calibration of Orbit Biases in BeiDou GEO Satellites
## 3 FY-3C POD Results and Analysis  
### Strategies of FY-3C POD  
The final precise orbit of FY-3C is generated using PANDA (Positioning And Navigation Data Analyst) software developed at GNSS Research Center of Wuhan University [27]. Each arc length of the POD is limited exactly to 24 h which is identical to the time period covered by daily GBM products delivered. This ensures that the orbit biases of one GEO are approximately similar, otherwise the deficient BeiDou orbits at the end-of-day of the last 5 min are propagated using high precision dynamic orbit fitting and integrating method. The BeiDou satellites in the attitude switch period are directly excluded in daily data processing because of their inferior orbit products.  
The nominal attitude mode is used for FY-3C since the measured attitude data cannot been obtained in practice. The FY-3C satellite body fixed axes are regarded as parallel to the instantaneous orbit along-track, cross-track, and radial directions. The positioning antenna associated with the GNOS receiver is mounted on the top plane of the FY-3C body, and the antenna surface is parallel to the supporting plane. The axes of the antenna reference frame are parallel to the satellite body axes. Moreover, the origin of antenna reference frame is the antenna mounting point, the \(X\), \(Y\), and \(Z\) axes in antenna frame are parallel to the \(Y\), \(X\), \(-Z\) axes in the satellite body frame.  
The ionosphere-free linear combinations of BeiDou code and phase are adopted as the basic observations. By considering the quantity of available dual-frequency BDS observation data, every six hours we estimate the atmospheric drag coefficients and the amplitudes of the periodic empirical accelerations acting on FY-3C orbit along-track and cross-track, but the empirical accelerations acting on the orbit radial are ignored. A more detailed configuration description is listed in Table 2.  
Incidentally, the FY-3C reference orbit derived by onboard GPS is produced according to the strategies listed in Table 2 completely, except for the following points: The daily arc duration of POD begins at 21:00 of the previous day and ends at 03:00 of the next day, which lasts 30 h in order to implement the 6 h orbit overlap comparison, while it lasts 24 h for BeiDou. The FY-3C GNOS receiver positioning antenna phase center variations (PCVs) for dual-frequency GPS ionosphere-free linear combination observations have been calibrated based on the posterior phase residuals approach and corrected as the initial PCVs in the next POD process within three iterations, and the final precise orbit  
Figure 4: Phase observation residual series of five BeiDou GEO without (**top**) and with (**bottom**) calibrating orbit errors on 14 July 2017.  
is produced in the fourth GPS-based orbit determination. However, the FY-3C PCVs for BDS are not estimated iteratively and considered as zero values in POD.  
This paper designs three different POD solutions to handle the BeiDou GEO observations in FY-3C POD:  
Solution A: all onboard observations of BeiDou GEO are regarded as equivalent to those of IGSO and MEO, and ignore the special errors in the GEO orbit products.  
Solution B: all onboard observations of BeiDou GEO are excluded in the general POD procedure, hence the special errors in BeiDou GEO orbit products will not affect the final FY-3C orbit, but the orbit solution can be weakened because fewer observations are available.  
Solution C: utilize all onboard observations of BeiDou GEO and estimate daily constant biases in orbit along-track and cross-track for each BeiDou GEO together with the POD process.  
The precisions of the three different kinds of FY-3C orbits determined by solution A, B, and C respectively are assessed by comparing with the GPS-based orbit. The phase observation residuals are also analyzed to evaluate the performance of the current method for mitigating errors in BeiDou GEO orbit products.