# Contribution of Etalon Observation to Earth Rotation Parameters under a New Observation Scenario
## 2 Data and Methods
### Methodology

Table 1: SLR stations tracking the Etalon-1/2 satellites. tion, \(\sigma_{rb}\) is the range bias correction, \(\sigma_{com}\) is the correction of center of mass, \(\epsilon\) is noise in SLR observation. According to Equation (1), the SLR data required various corrections, such as the correction of troposphere delay, correction of range bias, correction of the center of mass, correction of the atmosphere pole tide, ocean tide and solid Earth tide, and general relativistic correction. Table 2 lists the measurement model used in the LAGEOS+Etalon solutions. The troposphere delay was corrected using the Mendes-Pavlis model [34], which calculated the signal delay caused by the troposphere by multiplying the delay in the zenith path  
Figure 1: The distribution map of SLR stations observing the Etalon-1/2 satellites. Figure 2: Number of weekly observations for Etalon-1 and Etalon-2. with a mapping function using station-specific metrological data. Range bias is another error source that should be considered in the processing of LAGEOS SLR data. Most of the stations used the information from the ILRS-recommended data handling file to correct the range bias. However, some specific stations estimated the range biases as a specific parameter of the station and satellite. The effects of ocean tides, solid Earth tides, atmospheric poles and general relativity were corrected according to the 2010 International Earth Rotation System (IERS) convention [35]. The error in the center of mass for the LAGEOS-1/2 and Etalon-1/2 satellites was corrected using station- and time-dependent correction tables provided by ILRS instead of using only one constant value for all stations [36]. In addition, the elevation angle cutoff was 3 degrees. During data processing, the rules of 2.5 sigma editing and a maximum sigma value of 30 mm for SLR residuals were applied for data screening. Based on the data quality of satellites, the weight between LAGEOS-1/2 and Etalon-1/2 was set to 1/9. \begin{table}
\begin{tabular}{c c} \hline \hline
**Type of Model** & **Description** \\ \hline Arc length & 7 days \\ Elevation angle cutoff & 3 degrees \\ Sampling interval & LAGEOS-1/2: 120 sc \\  & Etalon-1/2: 300 sc \\ Satellite weighting & LAGEOS-1/2: 10 mm Etalon-1/2: 30 mm \\  & ZTD: Mendes-Pavlis model \\  & Mapping function: Mendes-Pavlis model \\  & Not modeled \\  & Estimated for some selected stations \\  & IERS Convention 2010 \\  & Ocean tide loading: FES2004 model \\  & Earth tide loading: IERS Convention 2010 \\ Satellite center of mass & Tables of CoM values from ILRS \\  & 2.5 sigma editing, \\  & maximum overall sigma: 30 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Measurement model of the LAGEOS+Etalon solutions. Figure 3: Number of weekly observations of Etalon-1/2 satellites of 12 SLR stations. In the LAGEOS+Etalon weekly solutions, the following parameters were estimated: SLR station coordinates, geocenter coordinates, satellite orbit, range bias, and ERPs. The parameters and the corresponding strategies are listed in Table 3. The geocenter parameters and SLR station coordinates were both estimated as piecewise constants in weekly solutions. SLRF2014 [37], which is the ILRS realization of ITRF2014, is employed as a prior reference frame. The SLR core sites were selected in accordance with the recommendation of ILRS for datum definition. The minimum constraint approach was employed to select the stations for defining the terrestrial reference frame. According to the research conducted by [PERSON] [18], the minimum constraint using no net rotation and no net translation was found to be optimal for SLR data processing. Some stations whose station coordinate residuals are larger than 25 mm for at least one component were rejected from the set of core stations. The coordinates of the noncore stations were estimated with no constraints.