# Orbit Determination of Korean GEO Satellite Using Single SLR Sensor
## 4 Compass-G1 Orbit Determination for Strategy Verification  
As the obtained OD solution of GK-2B is based on numerical simulation, the proposed OD remains to be validated for the OD in the real world. For instance, SLR data might include unexpected bias or errors caused by the local weather condition or unusual observation characteristics. Furthermore, the dynamic model utilized in the simulation does not accurately \"simulate\" the real-world dynamics. As an experimental verification of the proposed OD strategy, the OD based on actual observation is performed. The SLR observation data of Compass-G1 from the Changchun SLR station, which is the combination used in the observation analysis described in Section 2, are used as the verification target. As the arc with the most observation data in 2017 was from 21 October to 24 October, the OD is performed for that period. International Terrestrial Reference Frame (ITRF) 2014 coordinates are applied to the station coordinate system of Changchun. The other dynamic/observation model and estimation parameter settings are the same as those of GK-2B OD simulation.  
Figure 11 describes the position uncertainty from the filter/smoother in the Compass-G1 OD process. The position uncertainty is reduced when the sufficient observation data are obtained, while it continuously increases without observations for radial and in-track directions. After the smoothing process as shown on the right-hand side of Figure 11, the position uncertainty peaks mostly disappear for the whole  
Figure 10: Estimated orbit uncertainty and orbital errors for Scenario 2 in radial (**upper left**), in-track (**upper right**), and cross-track (**lower**) directions.  
arc. The RMS of position uncertainty in the filter is 3.18 m, 10.01 m, and 11.38 m, for radial, in-track, and cross-track directions, respectively, and these decrease to 1.33 m, 8.00 m, and 11.20 m, respectively, after the smoothing process.  
The estimated orbits are now verified by external comparison. As Compass-G1 is a navigation satellite, the orbital ephemeris is regularly published online by IGS ACs. German GeoForschungsZentrum (GFZ) AC obtains precise ephemeris solution called the final orbit of Compass-G1. In general, the precision of the final orbit from GFZ is known to be 30-80 cm [18]. Figure 12 and Table 7 present the comparison between GFZ final orbit and the estimated orbit from this study in radial, in-track, and cross-track directions. The GFZ publishes the final orbit of Compass-G1 with 5 min intervals. Therefore, the orbit overlap comparison is performed every 5 min for the whole four-day arc. The errors in the radial direction are the smallest while the errors in in-track and cross-track directions are about 22 m. From these comparative analyses, it can be concluded that the OD of GEO satellites based on a single SLR station is applicable under normal observation environments. The orbit precision is enhanced to tens of meters, which is a vast improvement compared with the OD with other observations such as optical tracking or angles-only data.  
Figure 11: Position uncertainty of filter (**left**) and smoother (**right**) from Compass-G1 orbit determination (OD).  
Figure 12: Orbit overlap differences between the estimated orbits and GeoForschungsZentrum (GFZ) final orbits.