Orbit determination of China’s first mars probe Tianwen-1 during interplanetary cruise

Tianwen-1 is China’s first deep space probe to explore Mars with the goal of orbiting, landing and roving. This paper presents the orbit determination process and result of the probe in the phase of Earth-Mars trajectory in detail. Orbit determination is carried out based on the tracking data obtained by CDSN (Chinese Deep Space Network) and CVN (Chinese VLBI Network). The results show that the residual RMS of range, Doppler, VLBI delay and delay rate data are about 0.28 m, 0.10 mm/s, 0.12 ns and 0.34 ps/s, respectively. Compared with the previous missions in Chinese Lunar Exploration Program (CLEP), the accuracy of VLBI data is significantly improved. The analysis shows that the prediction error of EOP (Earth Orientation Parameter) and the planetary ephemerides lead to the difference of orbit solution. With the increasing distance between the spacecraft and Earth, the EOP prediction error has a certain influence to the orbit determination accuracy. The difference in orbit solutions due to EOPs’ prediction error before MOI (Mars Orbit Insertion) is about 300 m. Moreover, the orbit difference is about 200 m using different planetary ephemerides (DE421/DE436). Orbit overlapping results show the position and velocity error of Tianwen-1 are less than 2 km and 1 cm/s, respectively. During the approaching phase, the error ellipse in B-plane shows that σSMAA = 389.40 m, σSMIA = 140.88 m (1σ).