Control Strategy of Lunar Lander-Relay Satellite Antenna Acquisition and Tracking for Chang’e-4 Lunar Exploration Mission

Chang’e-4 lunar lander and "Yutu-2" rover safely landed on the farside of the Moon on 3^rd, Jan., 2019. It is the first unmanned spacecraft to softly land on the lunar farside and carried out in-situ and rover exploration there. However, the lunar farside is beyond the communication range of the ground stations on the Earth due to the synchronous rotation of the Moon. Therefore, how to implement reliable relay communication between the Earth and the Moon is crucial for the whole mission. The relay satellite "Queqiao" was launched and is orbiting around the lunar Lagrange point 2 stably for over a year, which is to setup relay communication link between the lunar farside and the Earth. A high-gain antenna (HGA) configured on the lunar lander is utilized in Chang’e-4 lunar mission for transmitting scientific data collected on the lunar farside to the relay satellite and the data is send back to Earth via the relay communication link. Whereas errors such as the positioning and landing attitude error, pointing error of the HGA, timing difference etc. may influence the accuracy of lunar lander-relay satellite antenna acquisition and tracking thus may cause the interruption on communication link. In this paper, based on detailed analysis of the interruption items and uncertainties of HGA pointing, a control strategy of lunar lander-relay satellite HGA acquisition and tracking is proposed for reliable relay communication for Chang’e-4 lunar mission. According to different conditions, strategies of "fast scanning in large-scale" and "slow scanning in small-scale" are designed. Also criterions and basic requirements on the ground for implementation are depicted. Finally, verification results are demonstrated. The control strategy mentioned in this paper is widely used for narrow-beam HGA pointing of lunar exploration mission and is of bright prospect on application for space exploration missions in the coming future.