A Constellation Optimization Method for Nearly Continuous Observation of Arbitrary Sites

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This study presents a new method to optimize a satellite constellation for nearly continuous observation using multi-objective genetic algorithm considering uncertainty of target sites. Many previous studies have limited the observation points to specific locations or have allowed observation gaps of several tens of minutes caused by reconfiguration, even for emergency observations. However, In the case of national security, the target sites are unlikely to be determined at the mission design phase, and observation of specific sites scattered over a wide area is urgently required. It is demanded that the uncertainty of target sites is considered at the mission design phase to achieve nearly continuous observation by optimized constellation. Constellations in this study are constructed by multiple Walker constellations with different altitudes and inclinations for achieving nearly continuous observations at both high and low latitudes. Multiobjective genetic algorithm is used for optimization. Design variables are observation-related metrics (altitude, inclination, sensor's field of regard, etc.). This algorithm allows to evaluate satellite constellations by the total number of satellites, spatial resolutions, and observation time as metrics. Analyses on observation time are repeatedly performed for each target site by randomly changing the locations. This method has been applied using Systems Tool Kit (STK) linked to MATLAB by programming interface. Design variables are input from MATLAB to STK. Multi-objective optimization is performed based on high fidelity simulation in which orbital perturbation is considered. It has been confirmed that this method is effective and flexible to design satellite constellations for improving observation capacity even under the condition that target sites are not designated clearly. In practice, the constellations having the observation rate (= observation time of target sites / scenario time period) of more than 99 % with moderate numbers of satellites and reasonable spatial resolutions over a wide area between the latitudes of -40° and +40° for a certain period of time are obtained.