# Contribution of Etalon Observation to Earth Rotation Parameters under a New Observation Scenario
## 1 Introduction  
As a space geodetic technique, satellite laser ranging (SLR) is indispensable for deriving SLR station coordinates, geocenter coordinates, Earth rotation parameters (ERPs) [1; 2; 3; 4; 5], the scale parameters of the International Terrestrial Reference Frame (ITRF) [6], the standard gravitational constant parameter (GM), the low-degree coefficients of Earth's gravity field [7; 8; 9] and many other parameters [10; 11]. The International Laser Ranging Service (ILRS) [12] is the main institution responsible for collecting and processing SLR observations of various satellites and distributing products to users for scientific research. Because of their spherical shape, high altitude and area--mass ratio, laser geodynamics satellites (LAGEOS) and Etalon satellites are the main tracking targets processed by ILRS to estimate geodetic parameters. However, the number of SLR observations of the Etalon satellites is approximately one-tenth of that to the two LAGEOS satellites [13]. Therefore, the calculations of ILRS are mainly based on the two LAGEOS Satellites. High-precision applications, such as sea level change monitoring, require the ITRF to be accurate at the level of 1 mm in terms of station position and 0.1 mm /year in terms of station velocity [14], which is a task of the global geodetic observing system (GGOS) [15], founded by the InternationalAssociation of Geodesy [16]. To fulfill the requirements of GGOS, ILRS has coordinated analysis centers operated by analysts of various international institutions to carry out extensive research [17; 18; 19; 20], where employing more SLR observations of satellites with different orbital characteristics and extending the SLR tracking network are two feasible approaches. The potential of passive geodetic satellites for ITRF generation, such as Starlette, Stella, AJISAI and laser relativity satellite (LARES), was studied by some researchers. [PERSON] et al. estimated the station coordinates and Earth orientation parameters (EOPs) using SLR observations of the Starlette, Stella and LAGEOS-1/2 satellites for the period between 2002 and 2005. The results showed that good quality SLR station coordinates and EOPs can be achieved by combining observations of the Starlette, Stella with LAGEOS-1/2 satellites [21]. [PERSON] et al. assessed the impacts of Starlette, Stella and AJISAI Sak observations on SLR-derived products and studied the factors of the arc lengths and orbit parameterization of low Earth orbit (LEO) satellites in detail. The results showed that the quality of polar coordinates and the scale parameter and the repeatability of the estimation of the horizontal components of station coordinates are improved when observations of the Starlette, AJISAI, Stella with LAGEOS-1/2 satellites are combined [2]. [PERSON] et al. analyzed the quality of SLR station coordinates using SLR data of LARES satellites. The results reveal that the stability of SLR station coordinates ranges from 9 mm to 46 mm, while that of LAGEOS solutions from 5 mm to 15 mm. The SLR data of some active satellites, including GNSS satellites and Earth observation LEO satellites, were used for the estimation of station coordinates, ERPs and other geodetic parameters [22]. [PERSON] et al. investigated the contribution of the SLR observations of a multi-GNSS constellation to the estimation of geodetic parameters, using 55 GNSS satellites for the period from 2014.0 to 2017.4. The data analysis showed that the quantity of weekly solutions for some SLR sites can be improved by 41%, that the root mean square (RMS) of LOD can be reduced by 65% and that the mean offsets of the LOD are decreased from \(-\)81.6 \(\upmu\)s/d to 0.5 \(\upmu\)s/d when LAGEOS and GNSS satellite observations are combined [23; 24]. [PERSON] et al. checked the possibility of using SLR observations of Sentinel-3 satellites for determining the global geodetic parameters using data from between July 2016 and July 2018.