## 4 Experiment and Analysis
### Accuracy Analysis of RDSS Time Delay  
Assume that the out-station beam number of an RDSS four-way range is \(i\), the instation beam number is \(j\), and the channel number of the in-station beam is k. Then, the time delay number of the RDSS data is 240 \(\times\) (\(i-1\)) + 24 \(\times\) (\(j-1\)) + \(k\). Since BDS-2 has 10 out-station beams, 10 in-station beams, and 24 channels in one in-station beam, the time \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{2}{c}{**POD Methods by RDSS and SLR Data**} \\ \hline Satellite & 8 GEO: C01, C02, C03, C04, C05 in BDS-2 system \\  & C59, C60, C61 in BDS-3 system \\ \hline \multirow{4}{*}{Stations} & Seven RDSS calibration stations deployed in Beijing, Sichuan, Hainan, Southeast China, and Northeast China, and two in Xinjiang \\  & One SLR station: Beijing \\ \hline \multirow{2}{*}{Arc length} & 3 days, sampling interval: RDSS data: 60 s \\  & SLR data: 1 s \\ \hline \multirow{2}{*}{observation} & RDSS data in Seven RDSS calibration stations \\  & SLR data in Beijing station \\ \hline \multirow{2}{*}{Estimated parameter} & GEO Initial orbit of the satellite, solar radiation \\  & pressure parameters and RDSS equipment time \\  & delay parameters \\ \hline \multicolumn{2}{c}{Parameter estimation method} & Least squares algorithm \\ \hline \multicolumn{2}{c}{Gravitational field model} & EGM 2008 12 \(\times\) 12 \\ \hline Sun, Moon gravity and the gravitational force of the other planets & Jet Propulsion Laboratory Development Ephemeris \\ \hline \multirow{2}{*}{Solar radiation pressure (SRP)} & An empirical SRP model which is similar to \\  & BERNESE ECM 9 parameter model \\ \hline Solid tides, ocean tide perturbation & IERS Convention 2003 \\ \hline \multicolumn{2}{c}{Precession and nutation} & IAU2000R06 \\ \hline \multicolumn{2}{c}{EOP parameters} & Constraints to the International Earth Rotation and \\  & Reference Systems Service (IERS) C04 model \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Strategies for the combined POD methods by RDSS and SLR data. delay number ranges from 1 to 2400. For satellite C01, the out-station beam number \(i\) is 1 or 2, and the in-station beam number \(j\) is 1 or 2. If C01 is used as the out-station and in-station satellite simultaneously, the time delay value is within the following four ranges. \[num= \begin{cases}0\text{ to }24&i=1,j=1\\ 24\text{ to }48&i=1,j=2\\ 240\text{ to }264&i=2,j=1\\ 264\text{ to }288&i=2,j=2\end{cases}  \tag{5}\]  
The out-station beam number \(i\) for satellite C02 is 3 or 4, and the in-station beam number \(j\) is 3 or 4. Therefore, the time delay number of satellites C01 to C05 can be obtained when they are both out-station and in-station satellites. For C01 to C05 satellites, the time delay number of the RDSS data is as follows:  
\[num= \begin{cases}0\text{ to }48&\text{and}&240\text{ to }288&for\text{ C01}\\ 528\text{ to }576&\text{and}&768\text{ to }816&for\text{ C02}\\ 1056\text{ to }1104&\text{and}&1298\text{ to }1344&for\text{ C03}\\ 1584\text{ to }1632&\text{and}&1824\text{ to }1872&for\text{ C04}\\ 2112\text{ to }2160&\text{and}&2352\text{ to }2400&for\text{ C05}\end{cases}  \tag{6}\]  
The SLR observations of satellites C01 to C05 were carried out in Beijing. The values and standard deviation (std) of the RDSS combined time delay solved in the POD based on SLR and RDSS data from 1 to 3 July are shown in Figure 3. In the figure, the horizontal axis represents the combined time delay number. As the figure indicates, some of the combined time delay parameters cannot be estimated because there are no corresponding RDSS observation data. For example, as satellite C04 does not have in-station and out-station data, the combined time delay parameters of this satellite have no solution. The std of most of the combined delay is less than 1 ns, and the std of some time delay is greater than 1 ns but less than 1.5 ns because of insufficient observation data, which means the internal consistency of the time delay is better than 1 ns.