By using the same-beam VLBI technology, differential phase delay successfully monitored the lunar rover's movement during the Chang'E-3 mission when rover and lander was carrying out the tasks of separation and took photos of each other.

The sensitivity of rover motion monitoring was between 50-100mm.Furthermore, relative position between rover and lander was precisely measured by taking the use of the DPD's changing trend. Professor LIU Qing hui and his student ZHENG Xin from the Shanghai Astronomical of observatory, Chinese Academy of Sciences, obtained this result when the rover "yutu" was working on the lunar surface.

Their work, entitled "motion monitoring and analysis of Chang'E-3 rover based on same-beam VLBI technology differential phase delay", was published in SCIENCE CHINA physics mechanics &astronomy.2014, Vol 44(8).

Chang'E-3 had a successful soft landing on the lunar surface on December 14, 2013. Several hours later, the rover and lander began to separate and take photos for each other. During the whole process, the rover and lander kept transmitting X band signals to earth.

Our research devoted to make use of same-beam VLBI technology to collect the correlation phase of the middle frequency points and calculate the differential phase delay which obtains cycle ambiguity. As the lander did not move on the lunar surface, the changes of differential phase delay mainly reflected the movement of the rover and the relative movement between earth and moon.

The article's innovation lay in utilizing the high-sensitivity differential phase delay to observe its changes and analyze the movement of the rover according to the changes. Figure 1 and 2 were the differential phase delays of December 14 and 15. When the rover ran on the moon, differential phase delay correspondingly became larger or smaller quickly. When the rover took a turn, differential phase delay changed like a trajectory. Translation process was similar to running.

Differential phase delay shook when elevation changed. During the rover wriggled on the lunar surface, differential phase delay rocked back and forth. In a word, as the rover gets any movement on the lunar surface, differential phase delay would also change immediately.

The changing rates of differential phase delay are different because the baseline lengths are different. The distance between rover and lander changes, the trends of differential phase delay would also change.

The research will also play an important role in multi-target mission, such as the three period of exploration of the moon, Mars probe and Venus probe.

Zheng X, Liu Q H, Wu Y J, et al. Motion monitoring and analysis of Chang'E-3 rover based on same-beam VLBI differential phase delay ( in Chinese). Sci Sin-Phys Mech Astron, 2014, 44: 872-878, doi: 10.1360/SSPMA2014-00032