Orbit determination for Chang'E-5 mission in rendezvous and docking

In the Chang'E-5 (CE-5) mission, the orbiter and ascender are tracked by ground stations to realize high-precision orbit determination and control before reaching the handover point. Herein, the orbit determination accuracy of the CE-5 rendezvous and docking section is analyzed. The long-arc overlapping results of the orbiter show that for a 200-km-high lunar orbit, the position accuracy of 12-h orbit determination is 18 m, and the accuracy in the radial direction is 0.4 m. Furthermore, because of the frequent orbit control, short-arc orbit determination is necessary to predict the position and velocity for the next orbit-control point. The short-arc orbit determination results of each arc segment of the ascender show that using combined VLBI and ranging data for 5-min short arc orbit determination, predictions can be conducted for 4 h and the differences in the position and velocity are 3 km and 2.5 m/s, respectively. When using the ranging data of two stations for 0.5-h orbit determination, predictions can be conducted for 6 h and the differences in the position and velocity are 1 km and 1 m/s, respectively; when these two-station ranging data are combined with VLBI data, the maximum difference in the position is less than 250 m and the maximum difference in velocity is 0.223 m/s. If the arc length for the orbit determination is less than 2 h (one circle), convergence is not achieved using only range data. In this case, using VLBI data additionally, the position difference is approximately 150 m for a 3-h orbit determination prediction. Moreover, for 3.5-h (nearly two circles) orbit determination arc predictions for 6 h, the orbit difference is less than 30 m, and with the additional use of VLBI data, the orbit accuracy can be improved to 10 m. The analysis results show that the long-arc orbit determination accuracy of the Chang'E-5 200-km lunar orbit is better than 18 m, and the VLBI data significantly contribute to improving the accuracy of short-arc orbit determination and prediction results.