# Orbit Determination of Korean GEO Satellite Using Single SLR Sensor
## 2 SLR Data Generation  
The orbit of GK-2B has been fixed to GEO of 128.408\({}^{\circ}\) east longitude to satisfy its primary purpose of monitoring meteorological and marine environments around the Korean peninsula. Figure 2 shows the LRA flight model of GK-2B. The diameter of each of 84 corner cube retro-reflector (CCR) is 33 mm. Total reflective area for the LRA is set to 718 cm\({}^{2}\) and the weight is about 9.54 kg, which is relatively large compared with that of common LRA for LEO/MEO satellites because of the long distance to ground stations. Geochang station was constructed at Gamak Mountain by Korea Astronomy and Space Science Institute (KASI) as the second Korean SLR system. It is in pilot operation and calibration stage and is expected to operate normally in 2019. Figure 3 shows the overview of Geochang station. Unlike the first Korean SLR system located in Sejong, the laser intensity of Geochang system is high enough to obtain SLR observations for high-altitude satellites. The system also aims to perform debris laser tracking (DLT) along with SLR, and the beam diameter of transmitting telescope for the laser system is 100 cm, which is ten times larger than that of Sejong station. Also, adaptive optics (AO) is applied to the Geochang system to obtain satellite images larger than 10 magnitudes [10]. Figure 4 shows the ground trajectory of GK-2B and the location of Geochang SLR station. Figure 1: Orbit determination procedure. SLR—satellite laser ranging; GFZ—GeoSronschungsZentrum. Figure 2: Laser retro-reflector array of Geostationary Earth Orbit (GEO)-Korea Multi-Purpose Satellite (KOMPSAT)-2B (GK-2B). Table 1 presents the detailed mission parameters of GK-2B. The mass of the satellite decreases from 1925 kg at the beginning of life to 1492 kg at the end of life because of fuel consumption. The satellite position and velocity state vectors in this paper are set on 28 June 2020 with true-of-date (TOD) coordinate system. Table 2 shows the station coordinates of Geochang in the geodetic coordinate system. For simulating SLR observations, Orbit Determination Toolkit (ODTK) version 6 of Analytical Graphics Inc. (AGI, Washington, DC, USA), is utilized. Once the orbit of the satellite and SLR ground station is provided, the program generates simulated range-only SLR observations by adding Gaussian random noise into the true range between the target satellite and the SLR station. The one sigma error of SLR observations is set to 4 mm. Figure 4: Ground trajectory of GK-2B (red cross) and Geochang station (blue dot). Figure 3: Geochang SLR station on Mt. Gamak. As mentioned in the Introduction, obtaining SLR observations for GEO satellites is limited. For the observation data simulation, considering all these factors and figuring out how many actual SLR observations of GK-2B will be available in the future, statistical analysis is performed on the SLR observations of a GEO satellite in operation. The selected operational GEO satellite is Compass-G1 of BeiDou Navigation System (BDS). Figure 5 shows the ground trajectory of the satellite and the location of SLR stations that were successful in observing Compass-G1 in 2017. Six stations, including three Chinese stations, obtained SLR data of Compass-G1 in this period. \begin{table}
\begin{tabular}{c c} \hline
**Parameter** & **Value** \\ \hline Development period & July 2011–September 2019 \\ Year of launch & 2020 \\ Mission lifetime & 10 years \\ Primary mission & Meteorological observation \\ Satellite mass & Space weather monitoring \\ Satellite cross section & 1925 kg @BOL 1, 1492 kg @EOL 2 \\ Satellite reflectance & 22.68 m2 \\  Surface reflectance & 0.41 \\ Epoch orbit & \(r=[-42,\)160.2970 \(-613.6082\) \(6.9611]\) km \(\sigma=[0.044454\) \(-3.07440\) \(-0.00107]\) km/s \\ \hline \end{tabular} \({}^{1}\) Beginning of life; 2 end of life  
\end{table} 
Table 1: Geostationary Earth Orbit (GEO)-Korea Multi-Purpose Satellite (KOMPSAT)-2B (GK-2B) mission parameters [11]. UTC—Coordinated Universal Time.