# Contribution of Etalon Observation to Earth Rotation Parameters under a New Observation Scenario
## 3 Results
### Orbit  
In addition to global geodetic parameters, such as station coordinates, ERP, geocenter coordinates and range bias, the orbits of LAGEOS and Etalon satellites were estimated. To investigate the impact of Etalon observations on the LAGEOS orbit estimation, the orbits from the LAGEOS-only solutions and LAGEOS+Etalon solutions were compared. Figure 12 illustrates the RMS values of the differences in weekly LAGEOS-1/2 orbits in the radial, transverse and normal directions for the periods in 2018. For LAGEOS-1/2 in 2018, the RMS of the orbit error in the R component is the smallest and is generally less than 1 mm; the RMS of that in the T component is the largest and is approximately 5 mm in some weekly solutions; and the RMS of that in N is approximately 3 mm, except for on DOY 91. Figure 13 provides the orbit differences for LAGEOS-1/2 in 2019. Compared with those in 2018, similar values are obtained for LAGEOS-1/2 in 2019. However, the RMS values of the orbit error of LAGEOS-2 on DOY 090 are 8 mm and 5 mm in the T and N components, respectively, which needs to be investigated further. The mean RMSs of the orbit difference in the R, T and N parts are 0.05 cm, 0.22 cm, and 0.16 cm, respectively. Based on the above analysis, the SLR observations of Etalon in the new tracking scenario lead to an orbit difference of less than 1 cm for LAGEOS satellites. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline  & \multicolumn{4}{c}{**LAGEOS-Only**} & \multicolumn{4}{c}{**LAGEOS+Etalon**} \\ \cline{2-7}
**Parameter** & **Xp (μs)** & **Yp (μs)** & **LOD** & **Xp (μs)** & **Yp (μs)** & **LOD** (μs/day)** \\ \hline Maximum & 433.00 & 874.00 & 150.30 & 387.00 & 715.00 & 144.80 \\ Minimum & \(-710.00\) & \(-997.00\) & \(-64.90\) & \(-617.00\) & \(-560.00\) & \(-80.80\) \\ Mean & \(-29.15\)  
For Etalon-1 and Etalon-2 satellites, the precision of the orbit is evaluated by the method of overlap comparison. The middle day of the orbit solutions is compared with the first day of the corresponding solutions. Figures 14 and 15 show the orbit error of overlap comparison in the R, T, and N directions for Etalon-1 and Etalon-2, respectively. The orbit error in R direction lie in the range of \(-\)10 cm and 8 cm, for T direction in the range of \(-\)50 cm and 50 cm, for N direction in the range of \(-\)60 cm and 60 cm. Figures 16 and 17 illustrate the orbit error for two Etalon satellites in 2019. Compare with the situations in 2018, the error in the R direction is within the range of \(-\)6 cm and 6 cm for T and N  
Figure 12: RMS of orbit differences in the R, T, and N directions for weekly solutions of LAGEOS-1 (up) and LAGEOS-2 (down) in 2018 between the LAGEOS-only solutions and LAGEOS+Etalon solutions. Figure 13: RMS of orbit differences in the R, T, and N directions for weekly solutions of LAGEOS-1 (up) and LAGEOS-2 (down) in 2019 between the LAGEOS-only solutions and LAGEOS+Etalon solutions. directions within the range of \(-\)40 cm and 40 cm. Table 7 gives the statistical values of overlap comparison for two Etalon satellites in 2018 and 2019. For Etalon-2 satellite, the RMS value in R direction reduced from 1.68 cm in 2018 to 1.48 cm in 2019, that in T direction reduced from 8.61 cm to 7.15 cm, and that in the N direction reduced from 10.55 cm to 8.22 cm. The situations for Etalon-2 are similar to those of Etalon-1. Compared with the orbit in 2018, the orbit error in the R, T and N directions for Etalon-1/2 satellites can be improved by 0.2 cm, 1.5 cm and 2.9 cm, respectively. For Etalon-1, the improvement in the R, T and N directions are 0.03 cm, 1.22 cm and 2.32 cm. Under the new observation scenario, the internal precision of orbit for Etalon-1/2 satellites in N direction is improved mostly. Figure 14: Orbit overlap differences in the R, T, N directions for Etalon-1 in 2018. Figure 15: Orbit overlap differences in the R, T, N directions for Etalon-2 in 2018. Figure 16: Orbit overlap differences in the R, T, N directions for Etalon-1 in 2019.