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Abstract: The first Korean multi-mission geostationary satellite, Communication, Ocean, and Meteorological Satellite (COMS) will be launched in 2010. The missions of this satellite will be Ka-band communications, ocean color monitoring, and meteorological imaging. The satellite was designed with only one solar array on the south panel. This novel configuration will keep imaging instruments on the north side from heating up. Asymmetry of the spacecraft configuration requires twice-a-day thruster-based Wheel Off-Loading (WOL) operations to keep the satellite attitude for imaging and communication. Thruster firings during the WOL operations cause the satellite orbit to change two times a day. Weekly East–West Station-Keeping (EWSK) and North–South Station-Keeping (NSSK) maneuver operations are planned for the COMS satellite in order to maintain the satellite in ±0.05° box at 128.2°E longitude. The EWSK maneuver is planned to be performed two days after the NSSK maneuver to correct small side effects in East–West direction due to plume impingement against the solar array during NSSK maneuver. Normally thruster firings during the WOL operation affect the satellite orbit only in the North–South direction, but there are also small perturbations in the East–West direction. In this paper, EWSK maneuver strategy for the COMS satellite is presented. This strategy takes twice-a-day thruster-based WOL operations and plume impingement effect of the NSSK maneuver into account. And an iterative process to calculate the velocity changes and maneuver time for the EWSK maneuver is applied. One year of the EWSK and NSSK maneuver simulations are performed with twice-a-day WOL to verify the proposed strategy. The proposed strategy showed that the mean longitude of the satellite is controlled at 128.2°±0.007° which is three times tighter than that of non-iterative process. Required Delta velocity (ΔV) for EWSK maneuver is also slightly smaller for iterative process. Tightly controlled mean longitude guarantees a better quality of satellite communications and Earth imaging when the spacecraft attitude is well maintained by on-board control. [Copyright &y& Elsevier]

Abstract: In this paper, Global Positioning System-based (GPS) Orbit Determination (OD) for the KOrea-Multi-Purpose-SATellite (KOMPSAT)-2 using single- and double-differenced methods is studied. The requirement of KOMPSAT-2 orbit accuracy is to allow 1m positioning error to generate 1-m panchromatic images. KOMPSAT-2 OD is computed using real on-board GPS data. However, the local time of the KOMPSAT-2 GPS receiver is not synchronized with the zero fractional seconds of the GPS time internally, and it continuously drifts according to the pseudorange epochs. In order to resolve this problem, an OD based on single-differenced GPS data from the KOMPSAT-2 uses the tagged time of the GPS receiver, and the accuracy of the OD result is assessed using the overlapping orbit solution between two adjacent days. The clock error of the GPS satellites in the KOMPSAT-2 single-differenced method is corrected using International GNSS Service (IGS) clock information at 5-min intervals. KOMPSAT-2 OD using both double- and single-differenced methods satisfies the requirement of 1-m accuracy in overlapping three dimensional orbit solutions. The results of the SAC-C OD compared with JPL’s POE (Precise Orbit Ephemeris) are also illustrated to demonstrate the implementation of the single- and double-differenced methods using a satellite that has independent orbit information available for validation. [Copyright &y& Elsevier]

Abstract: A GPS-based orbit determination system for the KOMPSAT-2 has been developed. Two types of orbit determination software such as operational orbit determination and precise orbit determination are designed and implemented. GPS navigation solutions from on-board the satellite are used for the operational orbit determination, and raw measurements data such as C/A code pseudorange and L1 carrier phase are processed for the precise orbit determination. Operational concept, architectural design, and performance test are described. [Copyright &y& Elsevier]