## 3 BDS Precise Orbit and Clock Products Generation
### Quality of the BDS WHU Orbits
#### 3.2.1 Comparison with the Other ACs  
The most straightforward indicator for assessing orbit quality is the consistency between different ACs. BDS orbits are compared at 15 min intervals in the along-track, cross-track, and radial directions. Since the length of POD arc was 3 days, only the orbital solutions of the middle day were used for validation. The BDS WHU orbits were compared with other MGEX ACs (i.e., GBM and COM (only BDS IGSO and MEO)).  
It is clear from Figure 2 that the GEO orbits show the worst quality, while the MEO orbits show the best quality. The RMS values of orbit differences among GBM, COM, and WHU are shown in Table 3. For GEOs, RMS of orbit differences between WHU and GBM are up to nearly 0.3 m  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Item** & **Models** \\ \hline Observations Combination & Ionosphere-free code and carrier phase combination \\ \hline Carrier phase signal & Global Positioning System (GPS): L1/L2, BDS: B1/B2 \\ \hline Sampling rate & 30 s for POD, 1 s for PCE and PPP \\ \hline Elevation cutoff & 7\({}^{\circ}\) \\ \hline Observation weight & 0.002 m and 2.0 m for raw phase and code observables, respectively, and elevation-dependent data weighting \\ \hline Satellite antenna phase center & Corrected using MGEX values [22] \\ \hline Receiver antenna phase center & Corrected using GPS values [22] \\ \hline Phase-windup effect & Phase polarization effects applied [23] \\ \hline \multirow{3}{*}{Troposphere delay} & Saastamoinen model for wet and dry hydrostatic delay with Global Mapping \\  & Function (GMF), estimated as piecewise constant function with 2 h parameter for \\  & residual wet delay \\ \hline Ionosphere delay & First-order effect eliminated by forming the ionosphere-free combinations \\ \hline Station displacement & Solid Earth tide, pole tide, ocean tide loading, International Earth Rotation and \\  & Reference Systems Service (IERS) Convention 2003 [24] \\ \hline Satellite orbit & Estimated in POD; fixed in PCE and PPP \\ \hline Satellite clock & Estimated in POD and PCE, white noise; fixed in PPP \\ \hline Receiver clock & Estimated as random walk process, NNOR as reference clock \\ \hline Station coordinate & Tightly constrained for POD; fixed for PCE; estimated in epoch-wise kinematic \\  & mode for PPP \\ \hline Phase ambiguities & Real constant value for each ambiguity arc \\ \hline EOP parameters & Polar motions and UTI from IERS C04 series aligned to International Terrestrial \\  & Reference Frame (ITRF) 2008 \\ \hline \multirow{3}{*}{Attitude model} & Nominal attitude with yaw maneuver for medium Earth orbit (MEO) and inclined \\  & geosynchronous orbit (IGSO), yaw-fixed attitude mode used for geostationary \\  & Earth orbit (GEO) [13] \\ \hline Geopotential & EIGEN\_GL04C up to 12 \(\times\) 12 \\ \hline Tide & Solid Earth tide, pole tide, ocean tide, IERS Conventions 2003 \\ \hline N-body gravitation & Sun, Moon, and other planets; JPL DE405 ephemeris used \\ \hline Relativity effect & IERS Conventions 2003 \\ \hline Solar radiation & ECOM(Extended CODE Orbit Model) model 5-parameter with no initial value [25] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Observation models, dynamical models, and estimated parameters for precise orbit determination (POD), precise clock estimation (PCE), and precise point positioning (PPP) in Position and Navigation Data Analyst (PANDA) software.  
in the radial direction, whereas the values in the cross-track and along-track directions are about 2 m and 3 m, respectively. For IGSOs, the orbits of GBM, COM, and WHU agree well, with RMS values of the orbit differences less than 0.4 m, 0.3 m, and 0.1 m in along-track, cross-track, and radial directions, respectively. For MEOs, the orbits by GBM, COM, and WHU show the best agreement, with an RMS of about 0.05-0.10 m in the along-track, 0.06-0.15 m in cross-track, and 0.03-0.06 m in the radial component.