# Precise Orbit Determination of FY-3C with Calibration of Orbit Biases in BeiDou GEO Satellites
## 3 FY-3C POD Results and Analysis
### Distribution of FY-3C BeiDou Observations  
Dual-frequency BeiDou observation data at 30 s sampling interval measured by the positioning antenna of GNOS receiver equipped on the FY-3C satellite throughout July 2017 were used in this study. The GNOS receiver is capable of tracking pseudo-range and carrier-phase on frequency B1 and B2. The amount of available BDS data is very impressive even though the GNOS receiver can only track up to six BeiDou satellites. Figure 5 shows the functional observation number of each BeiDou satellite and each BeiDou satellite type throughout the whole month. The right panel indicates that GEO satellites contribute \begin{table}
\begin{tabular}{l l} \hline \hline
**Item** & **Description** \\ \hline POD arc length & 24 h \\ \hline Observation selection & Ionosphere-free linear combinations of space-borne dual-frequency phase and code \\  & observations of FY-3C during July 2017 \\ \hline BeiDou orbit and clock products & GBM products generated by GFZ \\ \hline BeiDou transmitter antenna phase center & Adopt antenna phase center patterns in IGSI4\_1967.4 tx [28] \\ \hline Phase wind-up & Correct using the model published in [29] \\ \hline Observation process interval & 30 s, use observations recorded at 0 s and 30 s of each minute in GPS time system \\ \hline Cut-off elevation & Accept all available observations including those with negative elevation angles \\ \hline Observation weighting strategy & Equal weight for phase and code respectively, and the weight ratio of phase to that of \\  & code is 40,000:1 \\ \hline Earth gravity & EIGEN-8C model with degree 100 and order 100 [30] \\ \hline Solid Earth tides & IERS2010 [31] \\ \hline Ocean tides & FES2012 model with degree 30 and order 30 [32] \\ \hline Relativistic effects & IERS 2010 \\ \hline Atmospheric drag & Box-wing model, atmospheric density adopts DTM94 model [33] \\ \hline Solar radiation pressure & Box-wing model \\ \hline Third body gravity & Consider the gravitational attraction of celestial bodies, use celestial ephemeris named \\  & DE421 published by Jet Propulsion Laboratory (JPL) [34] \\ \hline FY-3C satellite attitude & Nominal attitude mode \\ \hline  & Receiver antenna phase center coordinates in \(Z\) direction estimated as a constant \\  & Receiver clock offset estimated as white noise \\  & Phase ambiguity estimated as a float constant during each satellite tracking arc segment \\ Estimated parameters & Satellite initial position and velocity in the inertial reference frame estimated as constants \\  & Atmospheric drag coefficient estimated as piece wise constant every 6 h \\  & Amplitudes of periodic empirical accelerations acting on the along-track and cross-track \\  & of FY-3C orbit estimated as piece wise constants every 6 h \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Concepts of reduced dynamic orbit determination for FY-3C using BDS observations. to 32.1% of the valid observations, so it is not reasonable to exclude the GEO satellites during data processing. Several BeiDou satellites tracked by the FY-3C GNOS receiver on 14 July 2017 are presented in Figure 6, which illustrates the sky view containing varying patterns of elevation and azimuth for each selected satellite in the FY-3C positioning antenna reference frame. There are no apparent BeiDou signal interruptions and discontinuities [35] seen during the timeframe when observed satellites with approximately azimuth 90\({}^{\circ}\) or 270\({}^{\circ}\) change from the forward hemisphere to back hemisphere in the antenna view field. It is because the satellites including all range observations recorded as zero in the observation data file are also plotted in Figure 6. No obvious differences of satellite distribution can be found for BeiDou GEO, IGSO, and MEO. One tracking pass always begins with the satellite entering the field in elevation around 30\({}^{\circ}\) and ends with the satellite disappearing when the lock on satellite signal is lost in negative elevation, which can reach a minimum of \(-30^{\circ}\).