## 4 Results
### Overlap Comparison  
For any two adjacent three-day arc solutions shifted by one day, there are 48-h overlap orbits. The last observed day and first middle day of the adjacent solution are compared in this study. Figure 2 shows the overlap comparison algorithm for adjacent orbit solutions of a three-day arc. \begin{table}
\begin{tabular}{c c} \hline \hline
**Strategy** & **Describe** \\ \hline Observations & Double-differenced ionosphere-free code, and phase combination of B1 and \\  & B3 in deployment phase 1 \\ \hline Elevation angle cut-off & \(7^{\circ}\) \\ \hline Sampling rate & 30 s \\ \hline Data span & Three days \\ \hline Station coordinates and ERPs & Fixed using estimates from the GPS daily POD process \\ \hline Weighting & 4 mm for phase observations, 2 m for code observations and \\  & elevation-dependent weighting \\ \hline \multirow{4}{*}{Satellite antenna PCO and PCV} & BeiDou-3: PCO values are given by the Operation Control Department \\  & (with authorization), no PCV applied \\  & BeiDou-2: Obtained from [PERSON] et al. [18] \\ \hline SLR retro-reflector offsets & BeiDou-3: Given by the Operation Control Department (with authorization) \\  & BeiDou-2: Obtained from ILRS \\ \hline Attitude model & BeiDou-3: Yaw-steering attitude mode \\  & BeiDou-2: Yaw-steering and orbit normal attitude mode \\ \hline Ground antenna PCO and PCV & Not applied \\ \hline \multirow{2}{*}{Troposphere} & Dry and wet GMF mapping functions; ZTDs estimated for each station at \\  & intervals of two hours; GPS daily ZTDs used as a priori values \\ \hline Precession and nutation & IAU 2010 precession and IAU 2010 nutation model \\ \hline Geopotential & EGM2008 12 \(\times\) 12 \\ \hline Solid Earth tides, ocean tides and & IERS Conventions 2010 \\ solid Earth pole tides & \\ \hline N body & JPL DE405 ephemeris \\ \hline \multirow{2}{*}{Orbital parameters} & Six orbit elements and five ECOM SRP parameters were estimated: \\  & constants in the D, Y and X directions; periodic terms in the X direction \\ \hline Pseudo-stochastic orbit parameters & Every 12 h; constrained to \(1\times 10^{-6}\) m/s in the radial direction, \(1\times 10^{-5}\) m/s \\  & in the along direction, and \(1\times 10^{-8}\) m/s in the cross-track direction \\ \hline Ambiguity & Real constant value for each ambiguity arc \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of the precise-orbit determination strategy for BeiDou-3 satellites. ERPs, earth rotation parameters; POD, precise orbit determination; PCO, satellite antenna phase center; PCV, phase center variation, ZTDs, zenith tropospheric delays. The average daily root mean square (RMS) values of the 24-h overlap in the radial, along, and cross-track components for each satellite are shown in Table 4 and Figures 3-7. The RMS values for the along component for all BeiDou satellites are the largest among the three components, whereas the radial component has the smallest RMS values. Furthermore, BeiDou-3 MEO satellites (C33 and C34) have larger RMS values than the BeiDou-3 IGSO satellites (C31 and C32), whereas BeiDou-2 MEO satellites have smaller RMS values than the BeiDou-2 IGSO satellites. Furthermore, BeiDou-3 IGSO and BeiDou-2 IGSO satellites have identical RMS values, whereas BeiDou-3 MEO satellites have larger RMS values than the BeiDou-2 MEO satellites. In addition, BeiDou-2 GEO satellites yield significantly large RMS values among all of the BeiDou-2 satellites. BeiDou-3 MEO satellites have inferior POD accuracy compared with BeiDou-2 MEO satellites because the existing tracking network for BeiDou-3 MEO satellites is not well distributed worldwide, which implies that the tracking network does not have a good coverage for global MEO satellites, whereas the regional IGSO satellites are relatively better observed. In addition, the RMS residuals are approximately 10 cm and 25 cm in the radial and along components for BeiDou-3 IGSO satellites, respectively. For BeiDou-3 MEO satellites, the RMS residuals are approximately 40 cm and 60 cm in the radial and along components, respectively.