# Earth Rotation Parameters Estimation Using GPS and SLR Measurements to Multiple LEO Satellites
## 2 Data Set and Processing Strategy
The GPS orbit, clock products, and wide-lane satellite biases products from the Centre National d'Etudes Spatiales/Collecte Localisation Satellites (CNES/CLS) are used to enable the single-receiver ambiguity fixing [25]. Table 2 summarizes the force model for the POD of the LEO satellites. The N-body gravitation of LEO satellites refers to the gravitational attraction from the sun, the moon, and other planets. The solar radiation pressure of LEO satellites is computed based on the box-wing model with simplified geometric model and panel information. In terms of atmospheric drag, the atmosphere density is computed by the NRLMSISE00 model and a drag scale coefficient is additionally estimated as a piecewise constant parameter to compensate for the mismodeled atmospheric force. The interval of this coefficient varies with the specific satellite, i.e., 90 minutes for GRACE-FO and Swarm satellites, and 100 minutes for Sentinel-3 satellites. Additionally, in order to absorb the unmodeled or mismodeled force errors, a set of cycle-per-revolution empirical accelerations in the along-track and cross-track directions are introduced. Figure 1: Flowchart of two-step method and one-step method. In the step of ERP estimation, we process the available SLR observations that were collected by all the SLR stations. In the SLR observation modelling, the relativity effects, due to the presence of the Earth's gravitational field, are taken into consideration [28]. We use the zenith delay model proposed by [PERSON] and [PERSON] [31], as well as the mapping function FCULa proposed by [PERSON] et al. [32], to correct the range delay caused by the troposphere. In addition, the azimuth- and elevation-dependent range correction patterns of laser retroreflector arrays (LRA), derived from [PERSON] [33] and Montenbruck and Neubert [34], are adopted for the four-prism arrays of GRACE-FO and Swarm, and the seven-prism array of Sentinel-3, respectively. The estimated parameters that are related to the SLR observations from the LAGEOS and LEO satellites are illustrated in Table 3. We use SLRF2014 (ILRS [35]) to provide a priori reference frame. The no net rotation minimum condition is imposed in order to define the datum. The station coordinates and range biases are estimated as one set of parameters for each three-day solution, with priori constraints of 1 m and 100 m, respectively. It should be noted that in the LAGEOS-involved solutions, only range biases for selected stations are estimated according to the ILRS data handling file. For the ERP, a piecewise linear (PWL) parametrization is applied in order to achieve a better estimation [36]. Since UT1-UTC is correlated with the node of the satellite, it is not possible to independently determine UT1-UTC from any satellite tracking technique, such as SLR or GNSS measurements. Hence, we adopt a similar strategy to that used by the ILRS Analysis Standing Committee, in which UT1-UTC are strongly constrained to the finals2000A.data series published by IERS, and a constraint of 1 m is imposed on the remaining ERPs. In addition, for the LAGEOS-involved solutions, we also estimate the LAGEOS orbit apart from the ERP and station-specific parameters.