# Orbit Determination of Korean GEO Satellite Using Single SLR Sensor
## 1 Introduction  
Geostationary Earth Orbit (GEO)-Korea Multi-Purpose Satellite (KOMPSAT) 2 (GK-2) is a space program that is a part of the Korea Multi-Purpose Satellite (KOMPSAT) project by Korea Aerospace Research Institute (KARI) for meteorological and ocean monitoring by two geostationary Earth orbit (GEO) satellites. It is a follow-up mission to the GEO-KOMPSAT-1 (GK-1), which was launched in 2010 and has successfully provided high-resolution aerosol information above east Asia with the meteorological imager (MI) and the geostationary ocean color imager (GOCI) [1]. In the GK-2 program, two satellites, GK-2A and GK-2B, will be ready to be launched in 2018 and 2020, respectively. GK-2A aims at meteorological observation and space weather monitoring with an advanced meteorological imager (AMI) and the Korean Space Environment Monitor (KSEM). Meanwhile, the GOCI-II and geostationary environmental monitoring sensor (GEMS) will be mounted on GK-2B for ocean and environmental monitoring [2]. As the number of GEO satellites increases, orbit determination (OD) and monitoring of GEO satellites become essential to avoid collision between satellites [3; 4]. For GK-2B, ground-based radar is selected as a primary observation for OD, and the requirement of orbit precision is 1 km. [PERSON] et al. [5] performed the OD of GK-1 with single station antenna to yield position accuracy of 1.5 km. [PERSON] et al. [6] conducted OD by angles-only optical tracking data of GK-1 for 2-day arc to have less than 2 km maximum difference from two-line elements (TLEs). Unlike GK-2A, GK-2B will be equipped with a laser retro-reflector array (LRA) to acquire satellite laser ranging (SLR) observation data. As one of the most precise ranging techniques [7], SLR has been widely applied to the satellite OD. However, most of the previous studies about SLR-based OD focused on satellites located on low Earth orbit (LEO) and medium Earth orbit (MEO). For GEO satellites, SLR is typically applied to assess the quality of OD solution by calculating the range residual between estimated orbit and SLR data due to the limitations of SLR observations. Furthermore, only a few SLR stations that possess a high power laser transmitter can obtain SLR observations for objects at high altitude, such as GEO satellites. Because of the little transition in the ground trajectory of GEO satellites, the location of the SLR stations that can observe a specific satellite is limited. In addition to the aforementioned problems, SLR stations capable of observing GEO satellites can obtain the SLR observations only at night. These challenging situations make it difficult to acquire SLR observations of GEO satellites in contrast to the case of LEO/MEO satellites, resulting in deficiency in observation data for satellite OD. Nevertheless, by considering that the SLR observations are relatively accurate compared with other observations, SLR-based OD is still applicable to verify estimated orbit by other observations and to improve the orbit precision. Previous studies intermittently determined the orbit of GEO satellites using SLR measurements. [PERSON] et al. [8] conducted OD of high Earth orbit (HEO) satellites using SLR measurements. For the selected Japanese Quasi-Zenith Satellite-1 (QZS-1) and Chinese Compass-G1, the post-fit root-mean-square (RMS) residuals of OD were tens of centimeters. [PERSON] et al. [9] obtained orbits of BeiDou satellites with meter-level accuracy using SLR observation data. Unlike the OD of other GEO satellites, the OD of GK-2B satellite needs to impose additional constraints that were not considered in previous studies. First, in order for satellite-mounted optical instruments not to be damaged by laser during the observation, only one domestic SLR station is employed to obtain SLR observation data for GK-2B. This restriction has been previously imposed for several satellite missions such as advanced land observing satellite (ALOS); the ice, cloud, and land elevation satellite (ICESat); Lomonosov; and Sentinel-3A.