## 4 Satellite Precision Orbit Determination and Accuracy Analysis
### Analysis of Orbit Accuracy
The differences in coordinates between SLR2008 and SLRF2020 files are a combined result of various factors, which may include crustal movement, mass redistribution, station maintenance, and other causes. Since SLRF2020 is the latest version, incorporating updated station positions and velocities, this study utilizes the SLRF2020 model for SLR range validation. Data quality varies from SLR station due to the different surroundings and technical hardware. Therefore, we not only tally all SLR validation results but also select a number of high-quality SLR stations (called core stations) based on the Monthly GLOBAL report cards published by ILRS to provide more reliable SLR validation results. The Normal Point (NP) data, published by the ILRS, are generated following the conclusion of the laser pass and are typically transmitted to the ILRS within hours of observation. NP data undergo preprocessing, which includes the removal of obvious erroneous data and the exclusion of outliers caused by equipment malfunctions or other anomalies. These core stations (including Yarragadee and Mount Stromlo stations in Australia; Matera MLRO stations in Italy; Potsdam and Wettzell stations in Germany; Graz stations in Austria; Herstmonceux stations in the United Kingdom; Haleakala, Greenbelt, and Monument Peak stations in the United States; and Zimmerwald Station in Switzerland) provided 66% of NP data. The statistics of the HY-2B/2C/2D satellite observation data relative to the global SLR station are shown in Figure 8. Station 7090 in Australia provided the most NP data, and observations were made every day during the experiment. During DOY349-355 in 2020, 15 SLR stations tracked the HY-2B satellite, with a total of 876 NP data, 7090 and 7237 stations have observation data every day, and these two stations have a total of 392 NP data, 7403 station has the least amount of NP data. There are 15 SLR stations tracking the HY-2C satellite, with a total of 978 NP data, 7090 and 7237 stations have observation data every day, and these two stations have a total of 410 NP data, and 1834 station has the least NP data. During the period of DOY263-269 in 2021, 19 SLR stations tracked the HY2D satellite, with a total of 1456 NP data. The HY2D satellite was continuously observed by stations 7090, 7810, 7825, and 7941. Table 8 shows the SLR validation results. If only the core station observation data are used, the RMS values of the HY-2B/2C/2D reduced-dynamic orbit SLR validation are 2.34 cm, 2.40 cm, and 2.31 cm, respectively. If NP data from all stations are used, the RMS values are within 3.5 cm. The experimental results show that the \"HY2_Receiver\" demonstrates stable performance and can reliably receive high-quality data, effectively meeting the requirements for high-precision orbit determination. As can be seen from Table 9, SLR three-dimensional validation results are similar to CNES orbit intercomparison results, and 3D RMS are greater than SLR core station inspection errors, but smaller than all SLR station inspection errors. This is because most of the stations with more than 50 NP data are core stations. The SLR three-dimensional validation method can evaluate orbit accuracy more directly. Figure 8: NP data quantity of SLR station. \begin{table}
\begin{tabular}{c c c} \hline
**Satellite** & **Core Stations** & **All Stations** \\ \hline HY-2B & 2.34 & 3.25 \\ HY-2C & 2.40 & 3.24 \\ HY-2D & 2.31 & 3.34 \\ \hline \end{tabular} \
\begin{tabular}{c} (4) SLR 3D validation \\ \end{tabular} \\ \end{tabular}
\begin{tabular}{c} (5) Since the SLR validation results can only reflect the distance error and cannot evaluate the orbit accuracy from the three directions of R, T, and N, this study obtained the orbit accuracy evaluation in the three directions based on the SLR validation and the error propagation law [42]. SLR stations with more than 50 NP data are selected for experiments, and the results are shown in Table 9.  
\end{table} 
Table 8: SLR validate residuals at core stations and all stations (cm).