Abstract: Aiming at the optimization problem of in-orbit backup scheme of medium orbit Walker navigation constellation, the optimization design method of in-orbit backup satellite orbital position was proposed firstly. Considering that backup satellites and working satellites provide services jointly during the constellation operation, orbital position optimization model was established with selecting PDOP value and number of visible satellites as optimization indexes, and based on the NSGA-II algorithm, the simulation analysis was carried out on the improvement of the service performance of the navigation constellation by backup satellites under different orbital positions. Then, based on the optimization results of backup satellites orbital position, the orbit maneuver model of replacement of in-orbit backup satellite was established, and the backup satellites replacement scheme with the minimum replacement time as the optimization objective was determined by considering the velocity increment and replacement time. The results show that in-orbit backup scheme proposed in this paper can effectively meet the design requirements of backup satellites, enhance the service performance of the navigation constellation, realize the rapid replacement of the failed satellite, and provide reference for the construction of the backup satellite of the navigation constellation. 

Key words: Walker navigation constellation, inorbit backup, backup satellite orbital position, NSGAII, replacement scheme