Review of the Precise Orbit Determination for Chinese Lunar Exploration Projects

China's lunar exploration program has developed and progressed for more than 13 years. Over this time, Precise Orbit Determination (POD) has guaranteed successful execution of the Chang'e missions, and is the basis for further scientific investigations using radio science data; for example, recovering the lunar gravity field to explore the inner structure and detect craters to study the evolutionary history of the moon. This study briefly reviews the Chang'e series mission orbit and radio tracking measurements. We reprocessed the tracking data and comprehensively summarized the evolution of the POD accuracy and the tracking system precision from the Chang'e 1 to Chang'e 4 missions using in-house software. Our results show a progression of the tracking station, as the first Chang'e 1 mission only reached several millimeters per second level accuracy in the Doppler measurements and several meters level accuracy for the two-way range measurements. Later on the accuracy of Chang'e 5T1 Doppler measurements reached about 0.35 mm/s and the accuracy of the Chang'e 3 two-way range measurements below 0.7 m. Moreover, the newest Chang'e 4 relay satellite achieved POD residuals Root Mean Square for the two-way range less than a meter and orbital accuracy at 200 m level, in contrast to meters level residual and the half-kilometer level accuracy achieved during the Chang'e 1 mission. We can clearly see that the POD performance of the Chang'e spacecraft continuously improved. This research study is a reference for future Chinese Lunar exploration missions, as well as Chinese Mars and asteroid explorations.