# Earth Rotation Parameters Estimation Using GPS and SLR Measurements to Multiple LEO Satellites
## 6 Conclusions  
A large amount of SLR observations to LEO satellites offer an opportunity for investigating the potential of LEO satellites in ERP estimation. In this study, we are devoted to recovering ERP using SLR and GPS observations from seven LEO satellites. We evaluate the impact of the LEO orbit improvement on ERP estimation, and compare the quality of the ERP solution based on different LEO satellites. The main focus is on the different performance of the single-LEO solution, and the benefit of the combination of LEO satellites. In addition, we also investigate the feasibility of the simultaneous estimation of LEO orbits and ERP, which is named the one-step method in this study. The contribution of the ambiguity-fixed orbit for ERP estimation is firstly confirmed. Compared to the ambiguity-float solution, an RMS decrease of 12 uas and 22 uas for X pole and Y pole is obtained in the ambiguity-fixed solution. Based on the ERP series of different single-LEO solutions, we find that the single-LEO ERP solution cannot reach the comparable accuracy level of the LAGEOS solution, due to the limited SLR observations or the orbit modeling deficiencies. Meanwhile, a 119-day period signal is observed in the spectrum of the Sentinel-3 solution, which is closely related to the perigee revolution of the Sentienl-3 satellite. For the SLR stations, it can be seen that the station coordinates at the accuracy level of 2-3 cm is achievable while only using one LEO satellite. For the core stations, some single-LEO solutions can achieve a comparable positioning accuracy to the LAGEOS solution. In particular, the GRACE-C and GRACE-D solutions present an improvement of 4 mm in the up component when compared to the LAGEOS solution. Then, multiple LEO solutions with different numbers of LEO satellites are explored, with three schemes of the 3-LEO, 5-LEO and 7-LEO. The best ERP estimation result is achieved by the 7-LEO solution, thanks to a large number of observations and the enhanced observation geometry. Compared to the 3-LEO solution, the accuracy of X pole, Y pole and LOD for the 7-LEO solution is improved by 57.5%, 57.6% and 43.8%, respectively. Moreover, with more satellites included, the correlation between orbit parameters and ERP is significantly reduced. It can be seen that the 119-day signal in the Sentinel-3A solution is weakened in the multiple-LEO satellite solutions. For the station coordinates, more observations from multiple-LEO satellites contribute to a more accurate and stable positioning solution, especially for the non-core station, which usually owns few observations in the single-LEO solution or LAGEOS solution. It can be seen that when the number of LEO satellites increases from three to seven, the accuracy of the non-core station coordinates is improved by 13.7%, 27.3% and 16.2% in the east, north and up components, respectively. We also jointly processed the data of LAGEOS and seven LEO satellites for ERP estimation, and this combination scheme attains an RMS reduction of 0.644 ms, 0.629 ms and 0.028 ms for X pole, Y pole and LOD, compared to the LAGEOS solution. Besides, the results for the core station can reach the accuracy level below 10 mm in the east, north and up components. In addition, we discuss the possibility of simultaneously estimating LEO orbits and ERP using GPS and SLR data. A degraded ERP and the station coordinate result is observed when the integrated processing is performed, which may be attributed to the weakened solution strength caused by introducing a large number of orbit parameters. However, it is desirable to obtain a sub-millimeter improvement for LEO satellite orbit estimation from the one-step method, thanks to the combination of GPS and SLR observations in the precise orbit determination process.