Orbit and tracking data evaluation of Chang’E-4 relay satellite

Abstract

A lander and a rover were placed on the Moon in China’s Chang’E-4 Lunar exploration mission, which is the first lander on the far side of the Moon. To provide communication and telemetry between the Earth and the lander, the CE-4 relay satellite (Queqiao) was launched firstly to the Earth-Moon L2 region in May 2018. The orbit precision of the Chang’E-4 relay satellite is evaluated in this paper. The ground tracking data including Ranging, Doppler, VLBI delay and rate are processed using a batch least squares method. The results show that the residual RMS of ranging, Doppler, VLBI delay and VLBI delay rate based on S-band are about 0.53 m, 0.37 mm/s, 1.16 ns and 0.67 ps/s, respectively. Compared with the previous missions (CE-1/CE-2/CE-5T1) using S-band tracking systems, the measurement precision of the CE-4 relay satellite is significantly improved. Furthermore, the analysis shows that VLBI data can improve the precision of orbit determination and prediction of the halo orbit around the Earth-Moon L2 point: the position precision is better than 100 m and the velocity precision is better than 2 mm/s. Since the CE-4 relay satellite is less than 300 km away from the lunar surface when it flies by the Moon, the lunar non-spherical gravitation has a significant influence on the orbit determination, and the lunar gravity field need to be considered when the Earth is chosen as the central body.